thermal, kinetic and morphological studies of available

Thermal, kinetic and morphological studies of available and

synthesized pyrotechnic/propellant compositions and their

ingredients

A Thesis Submitted to the Department of Chemical Engineering, School of

Chemical and Materials Engineering (SCME), NUST, Islamabad, in the

partial fulfillment of the requirements for the degree of

Doctor of Philosophy (PhD)

In

Energetic Materials Engineering

Submitted by

Name: Zaheer-ud- din Babar

Reg No: 2011-NUST-TfrPhD-EM-E-78

Supervisor: Dr. Abdul Qadeer Malik

School of Chemical and Materials Engineering (SCME)

National University of Sciences and Technology (NUST)

H-12, Islamabad, Pakistan

List of Publications

Journal Papers (Published)

1. Zaheer-ud-din Babar, Abdul Qadeer Malik, Thermal and kinetic comparison of various

oxidizers used in propellant /pyrotechnic compositions, Caspian Journal of Applied Sciences

Research 2(7), pp. 63-69, 2013.

2. Zaheer-ud-din Babar, Abdul Qadeer Malik, Synthesis of micro porous barium nitrate with

improved ignition reliability as a reliable pyrotechnic oxidant, Journal of Saudi Chemical

Society, 18, pp.707-711, 2014.

3. Zaheer-ud-din Babar, Abdul Qadeer Malik, Kinetics of thermal decomposition of nano

magnesium oxide catalyzed ammonium perchlorate, Journal of Chemical Society of Pakistan,

36(6), 2014.

4. Zaheer-ud-din Babar, Abdul Qadeer Malik, An investigation of thermal decomposition

kinetics of nano zinc oxide catalyzed composite propellant, Combustion Science and

Technology, 187: 1295–1315, 2015.

5. Zaheer-ud-din Babar, Abdul Qadeer Malik, Thermal decomposition, ignition and kinetic

evaluation of magnesium and aluminum fuelled pyrotechnic compositions, Central European

Journal of Energetic Materials, 2015, 12(3), 579-592.

6. Zaheer-ud-din Babar, Abdul Qadeer Malik, Thermal decomposition and kinetic evaluation

of composite propellant material catalyzed with nano magnesium oxide. NUST Journal of

Engineering Sciences, Vol. 7, No. 1, 2014, pp. 5-14.

7. Zaheer-ud-din Babar, Abdul Qadeer Malik, Investigation of the thermal decomposition of

magnesium-sodium nitrate pyrotechnic composition (SR-524) and the effect of accelerated

aging, Journal of Saudi Chemical Society (Article in press).

Under Review

8. Zaheer-ud-din Babar, Abdul Qadeer Malik Accelerated ageing of SR-562 pyrotechnic

composition and investigation of its thermo kinetic parameters. (Journal of Fire and Materials)

International Conference

9. Zaheer-ud-din Babar, Abdul Qadeer Malik, Thermal decomposition and kinetic evaluation

of composite propellant material catalyzed with nano magnesium oxide. 3rd ASEAN-Pakistan

Conference on Materials Science (APCoMS-3) held on 25-27 November 2014 at Islamabad,

Pakistan.

i

Abstract

The work presented in this thesis is focused on the thermal, kinetic and morphological

studies of various pyrotechnic/propellant compositions and their ingredients. A lot of

research work has been carried out in the field of explosives; however, there is a lack of

theoretical understanding and experimental work concerning the reaction kinetics of the

pyrotechnics. The published work in the field of pyrotechnics presents some individual

studies concerning different aspects such as thermal behaviour, kinetics and aging of the

pyrotechnic compositions and their ingredients. The present work is a concerted effort to

provide an insight into the thermal behaviour, kinetics, aging and morphological aspects

of pyrotechnics/propellants. For this purpose, differential scanning calorimetry,

differential thermal analysis and thermogravimetery have been mainly used along with

scanning electron microscopy and X-ray diffraction for accomplishment of the present

work.

The comparison of thermal cum kinetic behaviour of five different oxidizers that are

commonly used in various pyrotechnic/propellant compositions was carried out. Next,

modified barium nitrate with micro porous structure was synthesized using three different

vesicants to make it more reliable as a pyrotechnic oxidant. The pyrotechnic composition

formulated with the modified oxidizer ignited at a lower temperature as compared to the

one formulated with pure barium nitrate. The ignition behaviour of the vesicant modified

barium nitrate has not been reported earlier to the best of our knowledge. Moreover,

thermal and kinetic behaviour of ammonium perchlorate has been improved by

catalyzing it with a small amount of nano magnesium oxide catalyst. The results indicate

that the two distinct decomposition stages of the pure ammonium perchlorate merged

with each other and reduced to a single stage. Furthermore, the reaction rate constant of

the catalyzed AP also increased significantly.

Pyrotechnics and propellants are inherently associated with some potential safety hazards

and are therefore required to be investigated for their stability and decomposition

kinetics. The thermal, kinetic and ignition behaviour of three pyrotechnic mixtures has

ii

been investigated in detail to elucidate the mechanism of ignition of these fuel oxidizer

mixtures and to assess the thermal stability and reactivity.

Temperature and humidity are amongst the important factors that influence the shelf life

and ignition behaviour of the pyrotechnics. The effect of aging on two commonly used

military pyrotechnics has been studied. The investigated compositions include SR-524

and SR-562 pyrotechnic compositions. The results indicate that aging of the pyrotechnic

compositions at extreme conditions of temperature and humidity changed their thermal

behaviour, kinetics, chemical composition and the surface features.

The last part of the thesis describes in-depth kinetic analysis of three different versions of

the composite solid propellant. Magnesium oxide and zinc oxide nano particles were used

as catalysts to alter the performance of the composite propellant. The kinetic analysis has

been carried out by Kissinger method, Flynn–Wall–Ozawa method, Friedman method

and Kissinger-Akahira-Sunose method. The results indicate lowering of the

decomposition temperatures in the catalyzed versions of propellant. The kinetic analysis

showed increased reactivity of the catalyzed versions of the propellants.

In a nut shell, the work presented in the thesis provided new insight into the thermal,

kinetic and morphological aspects of propellants and pyrotechnics vis-a-vis their

enhanced reactivity through incorporation of vesicants and nano catalysts, coupled with

aging studies, to help design formulations for specific requirements wherever required.

iii

DEDICATED TO MY

Dearest Family

My Mother, Wife and Daughters

iv

ACKNOWLEDGMENTS

All Commendations to Almighty Allah, The Most Beneficent, The Most Merciful, Who

blessed me with intellect, resoluteness and determination to accomplish the present

work. All the respects to His Holy Prophet Hazrat Muhammad (Peace be upon him),

who enabled us to recognize our Creator.

I owe my deepest and sincere gratitude to my supervisor Dr. Abdul Qadeer Malik. He

guided me with patience, wisdom and professionalism throughout my research work. I

can never forget his benign guidance, illustrative advice and keen interest. My thanks are

also due to my co-supervisor Dr. Hasnat Malik for his able guidance and technical

support. My special thanks are due to all the members of GEC for their step by step

guidance for the accomplishment of this work. Worthy members of the GEC include Dr.

Arshad Hussain (HOD Chemical Engineering Department), Dr. Habib Nasir and Dr.

Nazre Haider.

I would like to express my thanks to my friend Muhammad Ahsan for helping me

throughout these years with his software skills and giving me the moral support and hope

whenever I was depressed and frustrated. I shall also thank the admin officer Mr. Amjad

Khan for helping me in every possible manner for the conduct of experiments in different

labs. I also applaud the nice company of my class fellows i.e. Farooq, Muddassar, Sajid

Ali, Mukhtar and Sajid Nawaz. I always cherish the happy moments spent with them.

I pay my homage and sweet sensation of love and respect to my family including my

mother, my wife and my sweet daughters who prayed for me all the time and helped me

in every possible manner. It would not have been easy to complete this work without

their cooperation and prayers.

(ZAHEER-UD-DIN BABAR)

v

List of Contents

Chapter 1: General Introduction

1.1 Energetic Materials 1

1.1.1 High Explosives 1

1.1.2 Low explosives 1

1.1.3 Pyrotechnics and their applications 2

1.1.4 Distinction between propellants, explosives and pyrotechnics 3

1.2 Main Ingredients of pyrotechnic compositions 3

1.3 Important aspects concerning the performance of pyrotechnics 5

1.3.1 The fuel to oxidant ratio 5

1.3.2 Effect of particle size 6

1.3.3 Heat output of pyrotechnics 6

1.3.4 Ignition reliability and safety 7

1.4 Propellants 7

1.4.1 Liquid propellants 8

1.4.2 Liquid Monopropellants 9

1.4.3 Liquid Bipropellants 9

1.4.4 Solid propellants 10

1.4.5 Double based solid propellants 10

vi

1.4.6 Modified double based propellants 11

1.4.7 Composite solid propellants 11

1.5 Similarity of ingredients for composite propellants and pyrotechnics 12

1.6 Motivation for the present work 12

1.7 Analytical techniques and kinetic methods 14

1.8 Scope of the present work 15

References 19

Chapter 2: Experimental Techniques, Materials and Methods

2.1 Experimental Techniques 22

2.1.1 Thermal analysis 22

2.1.2 Different methods for thermal analysis 23

2.1.3 Differential Scanning calorimetry (DSC) 24

2.1.4 Differential Thermal Analysis (DTA) 26

2.1.5 Thermogravimetery (TG) 28

2.1.6 X-ray diffraction (XRD) 29

2.1.7 Scanning electron Microscopy (SEM) 31

2.1.8 Particle size analysis (PSA) 32

2.2 Materials Used 33

vii

2.2.1 Oxidizers 33

2.2.2 Fuels 34

2.2.3 Pyrotechnic Compositions 34

2.2.4 Composite solid propellant compositions 35

2.3 Kinetic evaluation methods. 36

2.3.1 Importance of Kinetics for solid state reactions 36

2.3.2 Kissinger method 37

2.3.3 Ozawa Method 39

2.3.4 Horowitz and Metzger Method 40

2.3.5 Flynn–Wall–Ozawa Method 41

2.3.6 Friedman Method 42

2.3.6 Kissinger-Akahira-Sunose Method 42

References 43

Chapter 3: Thermal and Kinetic comparison of commonly used oxidizers

3.1 Introduction 45

3.2 Experimental Conditions 47

3.3 Thermal and kinetic behaviour of Ammonium perchlorate 48

3.4 Thermal and kinetic behaviour of Ammonium Nitrate 51

3.5 Thermal and kinetic behaviour of Potassium Nitrate 54

3.6 Thermal and kinetic behaviour of Potassium Permanganate 57

3.7 Thermal and kinetic behaviour of Barium Nitrate 59

3.8 Comparative Analysis 61

3.9 Conclusions 64

viii

References 65

Chapter 4: Modification of barium nitrate and ammonium perchlorate

4.1 Synthesis of micro porous barium nitrate with improved ignition reliability 68

4.2 Introduction 68


4.4 Results and Discussion 70

4.4.1 SEM Analysis 70

4.4.2 Bulk density 73

4.4.3 XRD Analysis 74

4.4.4 Thermal Analysis of binary pyrotechnic mixtures 78

4. 5 Conclusions 79

4.6 Improvement in the thermal and kinetic behaviour of ammonium perchlorate using

nano magnesium oxide catalyst 80

4.7 Introduction 80


4.8.1 Kinetic Methods 83


4.9.1 Analysis of Nano particles 84

4.9.2 Analysis of Pure AP 84

4.9.3 Analysis of Catalyzed AP 91

4.9.4 Comparison of thermal and Kinetic Parameters 95

4.10 Conclusions 97

References 98

ix

Chapter 5: Thermal and kinetic evaluation of some magnesium and aluminum

fuelled pyrotechnic compositions

5.1 Introduction 101

5.2 Experimental conditions 103


5.3.1 Thermal analysis of the pyrotechnic ingredients 105

5.3.2 Thermal analysis of the pyrotechnic compositions 110

5.3.3 Variation in peak temperatures at multiple heating rates 113

5.3.4 Kinetic evaluation of the mixtures 116

5.3.5 Estimation of critical ignition temperature 119

5.3.6 Comparative analysis of the kinetic data and ignition temperatures 120

5.4 Conclusions 123

References 124

Chapter 6: Thermal decomposition of the magnesium fueled pyrotechnic

compositions and the effect of accelerated aging


6.2 Thermal decomposition kinetics and effect of accelerated aging on SR-524

pyrotechnic composition 130


6.4 Results and discussion 131

6.4.1 Thermal analysis of the fresh and aged composition 131

6.4.2 Kinetic Analysis of fresh and aged compositions 136

x

6.4.3 SEM Analysis of the fresh and the aged compositions 138

6.4.4 XRD Analysis 140

6.4.5 Comparison of fresh and aged compositions 142

6.4.6 Evaluation of the pyrotechnic thermal stability 143

6.5 Conclusions 144

6.6 Accelerated aging of SR-562 pyrotechnic composition and investigation of its

thermo kinetic parameters 145


6.8 Results and discussion 146

6.8.1 Thermal Analysis of the fresh and aged compositions 146

6.8.2 Kinetic Analysis of the fresh and aged composition 150

6.8.3 SEM Analysis of fresh and aged compositions 152

6.8.4 XRD Analysis of fresh and aged compositions 154

6.8.5 Comparison of fresh and aged compositions 156

6.8.6 Estimation of thermal stability of the pyrotechnics 157

6.9 Conclusions 157

References 158

Chapter 7: Thermal decomposition kinetics of nano zinc oxide and nano magnesium

oxide catalyzed composite solid propellant



7.3 Kinetics of thermal decomposition 163


xi

7.4.1 Characterization of nano particles of ZnO 165

7.4.2 Characterization of nano particles of MgO 167

7.4.3 Analysis of Propellant composition A 168

7.4.4 Analysis of Propellant composition B 176

7.4.5 Analysis of Propellant composition C 184

7.5 Estimation of critical ignition temperature 191

7.6 Conclusions 192

References 193

Chapter 8: General Conclusions 196

xii

List of Tables

Table 2.1: The mass ratio of fuel and oxidizer in pyrotechnic mixtures 34

Table 3.1: Comparative data of the oxidizers 62

Table 4.1: Bulk density of pure and modified barium nitrate 74

Table 4.2: Lattice parameters of pure and modified barium nitrate 76

Table 4.3: Effect of heating rate on decomposition peaks 87

Table 4.4: Summary of kinetic data of non-catalyzed ammonium perchlorate 89

Table 4.5: Summary of kinetic data of catalyzed ammonium perchlorate 95

Table 5.1: Summary of experimental thermal data 109

Table 5.2: Effect of heating rate on the decomposition peak temperature of the

pyrotechnic mixtures 113

Table 5.3: Kinetic parameters of pyrotechnic mixtures 116

Table 6.1: Kinetic parameters of SR-524 pyrotechnic composition calculated by

Kissinger method 133

Table 6.2: Kinetic parameters of aged SR-524 pyrotechnic composition calculated by

Kissinger method 134

Table 6.3: Kinetic parameters of SR-562 pyrotechnic composition calculated from the

heat flow data 149

Table 6.4: Kinetic parameters of aged SR-562 pyrotechnic composition calculated from

the heat flow data 149

Table 7.1: Kinetic parameters of composite solid propellant calculated from DSC data by

Kissinger Method (Composition A) 172

xiii

Table 7.2: Activation energy data corresponding to different degrees of conversion for

propellant composition A (calculated from TG data) 173

Table 7.3: Kinetic parameters of composite solid propellant catalyzed with nano zinc

oxide calculated from DSC data by Kissinger Method (Composition B) 178

Table 7.4: Activation energy data corresponding to degree of conversion for propellant

composition B (calculated from TG data) 181

Table 7.5: Kinetic data obtained by Kissinger Method for MgO catalyzed composite

propellant from DSC data (Composition C) 187

Table 7.6: Activation energy data corresponding to different degrees of conversion for

MgO catalyzed composite propellant calculated from TG data 189

xiv

List of Figures

Figure 1.1: Schematic diagrams depicting the main ingredients of pyrotechnic

compositions 4

Figure 1.2: Flow chart showing the general classification of propellants 8

Figure 1.3: Flow chart showing the road map for the accomplishment of present work 15

Figure 2.1: DSC Instrument model number DSC823e 25

Figure 2.2: The diamond TG/DTA Instrument 27

Figure 2.3: Main components of the TG system 29

Figure 2.4: Main components of the XRD instrument 30

Figure 2.5: Scanning electron microscope JSM- 6490 LA 32

Figure 2.6: Particle size distribution analyzer LA-920 33

Figure 3.1: TG/DTA curve of ammonium per chlorate 49

Figure 3.2: Graph used for calculation of activation energy of Ammonium

Perchlorate 50

Figure 3.3: TG/DTA curve of ammonium nitrate 52

Figure 3.4: Graph for calculation of activation energy of Ammonium Nitrate 53

Figure 3.5: TG/DTA curve of potassium nitrate 55

Figure 3.6: Graph for calculation of activation energy of Potassium Nitrate 56

Figure 3.7: TG/DTA curve of potassium permanganate 58

Figure 3.8: TG/DTA curve of barium nitrate showing the heat flow and mass loss 59

Figure 3.9: The kinetic plot of barium nitrate representing the Horowitz Method 60

Figure 4.1: SEM micrographs of pure barium nitrate 71

xv

Figure 4.2(a): SEM micrographs of barium nitrate modified with potassium carbonate 72

Figure 4.2(b): SEM micrographs of barium nitrate modified with sodium bicarbonate 72

Figure 4.2(c): SEM micrographs of barium nitrate modified with ammonium

perchlorate 73

Figure 4.3: XRD spectra of pure barium nitrate 75

Figure 4.4 (a): XRD spectra of barium nitrate modified with ammonium perchlorate 75

Figure 4.4 (b): XRD spectra of barium nitrate modified with ammonium perchlorate 76

Figure 4.4 (c): XRD spectra of barium nitrate modified with sodium bi carbonate 77

Figure 4.5: Heat Flow curves of aluminum with pure and modified barium 78

Figure 4.6: SEM micrograph of nano sized MgO powder used for catalyzing AP 84

Figure 4.7: XRD spectra of the nano sized MgO powder 85

Figure 4.8: TG/DTA curve of pure ammonium per chlorate 86

Figure 4.9: Effect of heating rates on the thermal decomposition of pure ammonium

perchlorate 87

Figure 4.10 (a): Kissinger Plot for Low stage decomposition of pure AP 88

Figure.10 (b): Kissinger plot for high stage decomposition of pure AP 89

Figure 4.10 (c): The Ozawa plot for low temperature decomposition stage of pure AP 90

Figure 4.10(d): Ozawa plot for high temperature decomposition stage of ammonium

perchlorate 91

Figure 4.11: TG/DTA Curve of AP + 4 Percent MgO 92

Figure 4.12: Effect of heating rates on thermal decomposition of AP + 4 Percent MgO 92

Figure 4.13(a): Kissinger Plot for decomposition of catalyzed AP 93

Figure 4.13(b): Ozawa Plot for the decomposition of catalyzed AP 94

xvi

Figure 5.1: Particle size distribution of aluminum powder (Davg= 20µm) 104

Figure 5.2: Particle size distribution of magnesium powder (Davg= 24µm) 104

Figure 5.3: TG curve of magnesium powder showing the mass gain due to oxidation 105

Figure 5.4: XRD spectra of the residue obtained after oxidation of magnesium powder

showing typical MgO peaks 106

Figure 5.5: TG curve of aluminum powder showing the mass gain due to oxidation

in air 107

Figure 5.6: TG/DTA curve of barium nitrate showing the heat flow and mass loss

curves 108

Figure 5.7: TG/DTA curve of Al + Ba (NO3)2 pyrotechnic composition showing the mass

loss and heat flow curves 110

Figure 5.8: TG/DTA curve of Mg + NH4ClO4 pyrotechnic composition showing the

mass loss and heat flow curves 111

Figure 5.9: TG/DTA curve of Mg + KMnO4 pyrotechnic composition showing the

mass loss and heat flow curves 112

Figure 5.10(a): Effect of heating rate on thermal decomposition of mixture containing Al

and Ba (NO3)2 114

Figure 5.10(b): Effect of heating rate on thermal decomposition of mixture containing

Mg and NH4ClO4 115

Figure 5.10(c): Effect of heating rate on thermal decomposition of mixture containing

Mg and NH4ClO4 115

Figure 5.11 (a): Kissinger plot for determination of activation energy of pyrotechnic

composition containing Al + Ba (NO3)2 117

xvii

Figure 5.11 (b): Kissinger plot for determination of activation energy of pyrotechnic

composition containing Mg + NH4ClO4 118

Figure 5.11 (c): Kissinger plot for determination of activation energy of pyrotechnic

composition containing Mg + KMnO4 118

Figure 6.1: The heat flow curves of fresh and aged SR-524 pyrotechnic compositions 131

Figure 6.2 (a): Effect of multiple heating rates on the heat flow curve of Fresh SR 524


Figure 6.2(b): Effect of multiple heating rates on the heat flow curve of aged SR 524


Figure 6.3: The mass loss curve of fresh and aged SR 524 pyrotechnic compositions 135

Figure 6.4(a): The Kissinger graph for determination of kinetic parameters of fresh SR

524 pyrotechnic composition 136

Figure 6.4(b): The Kissinger graph for determination of kinetic parameters of aged SR

524 pyrotechnic composition 137

Figure 6.5 (a): SEM micrographs of the fresh SR-524 composition showing smooth and

solid surfaces 139

Figure 6.5(b): SEM micrographs of aged SR-524 composition showing micro cracks 140

Figure 6.6: XRD spectra of fresh and aged SR 524 pyrotechnic composition 141

Figure 6.7: Comparison of the DTA curve of fresh and aged SR-562 pyrotechnic

composition 146

Figure 6.8: Effect of heating rate on the DTA curve of fresh SR-562 Pyrotechnic

composition 147

xviii

Figure 6.9: Effect of heating rate on the DTA curve of aged SR-562 pyrotechnic

composition 148

Figure 6.10 (a): Kissinger graph for calculation of Kinetic parameters of fresh SR-562


Figure 6.10 (b): Kissinger graph for calculation of Kinetic parameters of aged SR-562


Figure 6.11(a): SEM micrographs of fresh SR 562 pyrotechnic composition showing

solid and even surface 153

Figure 6.11(b): SEM micrographs of aged SR 562 pyrotechnic composition at showing

micro sized cracks 153

Figure 6.12(a): XRD spectra of fresh SR-562 pyrotechnic composition showing the

presence of magnesium, sodium nitrate and calcium oxalate 154

Figure 6.12(b): XRD spectra of aged SR-562 pyrotechnic composition showing the

presence of magnesium hydroxide as the reaction product of aging 155

Figure 7.1: XRD pattern of nano sized zinc oxide used as catalyst (Miller indices

marked) 165

Figure 7.2: SEM micrograph of zinc oxide nano particles 166

Figure 7.3: SEM micrograph of magnesium oxide nano particles 167

Figure 7.4: XRD spectra of the MgO nano particles 168

Figure 7.5: Effect of heating rate on the DSC curve of composition A representing un-

catalyzed CSP 169

Figure 7.6: Mass loss pattern of propellant composition A at multiple heating rates 170

xix

Figure 7.7: Graph representing the Kissinger method for determination of kinetic

parameters of composition A from DSC data 171

Figure 7.8(a): Graph representing the Flynn–Wall–Ozawa method for determination of

kinetic parameters of composite solid propellant from TG data 174

Figure 7.8(b): Graph representing the Friedman method for determination of kinetic

parameters of composite solid propellant from TG data 174

Figure 7.8(c): Graph representing the Kissinger-Akahira-Sunose (KAS) method for

determination of kinetic parameters of composite solid propellant from TG

data 175

Figure 7.9: Plot showing the activation energies versus extent of conversion for

composition A by three different methods (calculated from TG data) 175

Figure 7.10: Effect of heating rate on the heat flow curve of composition B containing

2% nano zinc oxide as a catalyst 176

Figure 7.11: Mass loss curve of propellant composition A and B 177

Figure 7.12: Mass loss pattern of propellant composition B at multiple heating rates 178

Figure 7.13: Graph representing the Kissinger method for determination of kinetic

parameters of composition B from DSC data 179

Figure 7.14(a): Graph representing the Friedman method for determination of kinetic

parameters of nano zinc oxide catalyzed composite propellant from TG

data 181

Figure 7.14(b): Graph representing the FWO method for determination of kinetic


data 182

xx

Figure 7.14 (c): Graph representing the KAS method for determination of kinetic

parameters of zinc oxide catalyzed composite propellant from TG

data 182

Figure 7.15: Plot showing the activation energies versus extent of conversion for

composition B by three different methods (calculated from TG data) 183

Figure 7.16: The DSC curves for MgO catalyzed composite propellant at multiple heating

rates 184

Figure 7.17: The TG curves for the MgO catalyzed propellant at multiple heating

rates 185

Figure 7.18: The Kissinger graph for catalyzed version of the propellant

(Composition C) 186

Figure 7.19(a): Representative graph of Friedman method for kinetic evaluation of the

propellant composition C 189

Figure 7.19(b): Representative graph of Flynn–Wall–Ozawa method for kinetic

evaluation of the propellant composition C 190

Figure 7.19(c): Representative graph of Kissinger-Akahira-Sunose method for kinetic

evaluation of the propellant composition C 190

Figure 7.20: The “E” versus “α” curve of the composite propellant composition C 191

xxi

List of Acronyms and Symbols

α Degree of conversion also called extent of conversion

A Frequency factor also known as pre-exponential factor (s-1

),

ABOL All Burn on Launch

AP Ammonium perchlorate

Heating rate (oC min

-1)

BLO Boiled Linseed Oil

CSP Composite solid propellant

DBP Double Base Propellants

DSC Differential Scanning Calorimetry

DTA Differential Thermal Analysis

E Activation energy (kJmol-1

)

Activation energy (kJmol-1

) corresponding to a specific degree of

conversion „α‟

GAP Glycidyl Azide Polymer

Enthalpy of activation (kJmol-1

)

HMX High Melting Explosive (Cyclotetramethylenetetranitarmine)

HTPB Hydroxy-terminated polybutadiene

ICTAC International Confederation for Thermal Analysis and Calorimetry

xxii

k Reaction rate constant (s-1

)

n Order of reaction

PBAN Polybutadiene acrylonitrile

R Gas constant (Jmol-1

K-1

)

RDX Royal Development Explosive (Cyclotrimethylenetrinitramine)

SEM Scanning Electron Microscope (SEM)

STA Simultaneous Thermal Analysis

T Sample temperature (oC)

Temperature (oC) corresponding to a certain degree of conversion „α‟

Critical temperature of thermal explosion (oC),

Onset temperature (oC) corresponding to → 0

Temperature of decomposition peak (oC)

TG Thermogravimetery

TMOs Transition Metal Oxides

XRD X-ray Diffraction

1

Chapter No. 1

General Introduction

1.1 Energetic Materials

“Energetic materials” is a soft term commonly used to refer propellants, explosives and

pyrotechnics. These are the chemicals or materials that possess a huge amount of energy

that is released at a very high rate when these materials are suitably ignited. These

materials are commonly used for various military and commercial applications. Energetic

materials are generally classified into two different categories i.e. the high explosives and

the low explosives. A brief introduction to these categories of the energetic materials is

obligatory before the actual account of the present work is narrated.

1.1.1 High Explosives

These are the class of explosives that undergo combustion by the detonation

phenomenon. The detonation means that the chemical reaction is very fast and violent

associated with extremely high temperatures and pressures [1]. The detonation reaction is

so fast that it proceeds faster than the speed of sound. The velocity of detonation for

commonly used high explosives ranges between 3 and 10 km s

-1. A very simple and

interesting comparison of velocity of detonation with the speed of sound will give a better

idea regarding the magnitude of the detonation velocity. The speed of sound in air is

nearly 330m s-1

and the detonation velocity of HMX is nearly 9.1km s-1

which means that

this explosive detonates with the speed that is nearly 28 times higher than that of the

sound.

1.1.2 Low explosives

The low explosives do not undergo detonation rather they decompose by the deflagration

process. These reactions proceed at much lower burning rates as compared to the

detonation. The flame front in the deflagration reactions proceeds with a subsonic

2

velocity [2]. Contrary to the high explosives where the oxygen required for the

combustion is present within the explosive molecule, low explosives generally are a

mixture of a fuel and an oxidizer. The low explosives generally include propellants and

pyrotechnics such as flares and sparklers.

1.1.3 Pyrotechnics and their applications

Pyrotechnics are materials or chemicals which are capable of producing some special

effects like heat, light, smoke and sound [3]. When these compositions are appropriately

initiated, they undergo combustion and release heat. Heat is the essential output in all the

cases and the other effects are also associated in some way or the other with the heat

output [3]. For instance, light output of the pyrotechnics depends largely on the amount

of heat released and the production of high temperatures that are required to give light

effect. Similarly, production of smokes for screening or signaling applications depends

on the heat released by the pyrotechnic material or composition. The use of chemicals in

the art of “fireworks” started thousands of years ago in countries like India and China.

Later on these energetic chemicals were used for military applications in different parts

of the world. For past several decades now, the pyrotechnics are being used very

scientifically and their special effects are being used in various civil and military

applications to achieve the desired outputs. Some of the applications of commonly used

pyrotechnics are listed below:

(a) Signaling and smoke screening

(b) Tracking, illumination and different types of decoys

(c) Incendiary compositions

(d) Different pyro-mechanisms

(e) Igniters and delay systems

After a brief and general introduction to the pyrotechnics, chemical composition of the

pyrotechnic will be discussed.

3

1.1.4 Distinction between propellants, explosives and pyrotechnics

The propellants, high explosives and pyrotechnics are all energetic materials and when

suitably initiated release a huge amount of energy that can be channelized for different

applications. It is important to clearly differentiate between the high explosives,

propellants and the pyrotechnics on the basis of the type of their combustion. The

combustion of propellants and pyrotechnics takes place by the deflagration phenomena.

Deflagration means fast and rapid burning due to layer by layer and systematic

combustion of the pyrotechnic explosive composition. However, combustion takes place

by the detonation phenomena in case of high explosives. Detonation takes place due to a

shock wave moving towards the unreacted explosive at a super-sonic speed. The

detonation is associated with extra ordinarily high temperatures and pressures. The

propellants and pyrotechnics both undergo decomposition by the deflagration process;

however, they can be differentiated on the basis of their specific role.

1.2 Main ingredients of pyrotechnic compositions

Most of the pyrotechnic compositions are heterogeneous mixture of a fuel and some kind

of oxidizer [4]. The oxidizer provides the oxygen required for the combustion of the fuel

and eliminates the dependence of the pyrotechnic composition on the external oxygen.

Some other additives are also added to the pyrotechnic composition to produce the

different kind of special effects. It is interesting to mention here that some of the

materials do not undergo combustion but still they are considered to be pyrotechnics due

to the fact that they produce some kind of special effect. A typical example of such a case

is the titanium tetrachloride. It does not undergo combustion but gives a smoke by

reaction with the water vapours present in the air. The pyrotechnics make use of a large

variety of chemicals to produce a specific composition with desired properties. The fuels

are mostly metallic in nature; however, the non-metallic fuels are also used. The major

role of the fuel is to provide high energy when it is combusted in the presence of the

oxygen. Aluminum, magnesium, iron, manganese, chromium, titanium, zirconium and

tungsten are some of the metallic fuels used in different pyrotechnic compositions.

4

Whereas, the nonmetallic fuels used in the pyrotechnic mixtures include boron, carbon,

silicon, phosphorous and sulphur. There is a wide variety of oxidizers which are being

used in the pyrotechnics to provide oxygen necessary for the combustion of the fuel.

These oxidizers include chlorates, perchlorates, nitrates, chromates, dichromates, oxides

etc. Some of the oxidizers do not provide the oxygen rather they act as oxidizing agent

such as halocarbons. The main components of pyrotechnic compositions have been

depicted in Figure 1.1.

Figure 1.1 Schematic diagrams depicting the main ingredients of

pyrotechnic compositions

Another important ingredient used in the manufacture of pyrotechnics is the binder. It is

used to bind and consolidate the fuel and oxidizer present in the pyrotechnic mixture.

Binders also protect the metal powders used as fuel from reacting with the atmospheric

air or moisture. A large number of binders are used in the field of pyrotechnic and some

of them are paraffin wax, lithographic varnish, boiled linseed oil and different types of

5

resins. The pyrotechnic compositions are generally very sensitive to friction and the

binders also help in reducing the sensitivity to friction.

Sometimes it is necessary to slow down the actual burn rate of certain specific

compositions. For this purpose some retardant materials are used in the pyrotechnic

compositions along with the fuel and oxidizer It is important to highlight here that

addition of any material other than the stoichiometric amounts of the fuel and oxidizer

bears an effect on the burn rate of the pyrotechnic compositions. Sometimes additives

which are endothermic in nature are added to reduce the overall heat evolved when the

composition is ignited. We have already discussed the role of binder in the preceding

paragraph and even the addition of binders to the pyrotechnic composition alters the

burning rate.

1.3 Important aspects concerning the performance of pyrotechnics

It is essential that the performance of pyrotechnic composition is above mark during its

operation and it must be able to give the desired output. The overall performance of

pyrotechnics depends on many factors some of which have been discussed briefly.

1.3.1 The fuel-to-oxidant ratio

Fuel-to-oxidant ratio is one of the most important factors that influence the performance

of the pyrotechnic compositions [3]. The pyrotechnic compositions are generally mixture

of a fuel and the oxidizer. The oxidizer provides the oxygen when it is decomposed and

the fuel reacts with the oxygen to release the heat energy. It is therefore necessary to mix

the correct proportion of the fuel and the oxidizer to get complete combustion of the fuel

for optimum performance of the pyrotechnic composition. The chemical reaction

between the fuel and oxidizer needs to be balanced by selecting the correct stoichiometric

ratio of the reactants. This will ensure that both the reactants will be consumed

completely and convert into the desired products. The pyrotechnic compositions designed

with correct stoichiometric ratios of the reactants are likely to provide the best possible

results. The appropriate fuel-to-oxidant ratio will also help in attaining the maximum

possible burn rates due to the fact that the heat output of the stoichiometric mixture will

be the maximum. If the pyrotechnic mixture is rich or lean in fuel, it is likely to affect the

6

overall performance of the pyrotechnic composition in terms of its heat output and burn

rate. In certain specific cases the pyrotechnic compositions during their use are bound to

be exposed to the atmospheric oxygen. In such cases, the stoichiometric ratios will not

give the best results but a fuel rich mixture will give better performance.

1.3.2 Effect of particle size

The particle size of the fuel and the oxidizers is one of the important factors for all the

reactive systems such as energetic materials including propellants, explosives and

pyrotechnics [5-7]. It is essential to consider the particle size of the materials used in the

pyrotechnic composition especially where the high reactivity is desired. The maximum

temperature that is achieved by the combustion of fuel oxidizer mixture depends on the

heat of reaction. Researchers working in the field of energetic materials have found that

the heat of reaction increases with decrease in the particle size. The small particle size

means a large surface area and it becomes easy for the reaction to take place. The

reaction proceeds faster with the smaller particles as compared to the large ones in case

all other parameters are same. Metals such as aluminum, magnesium, iron and

manganese are generally used as pyrotechnic fuel. These metal powders are generally

available in different particle sizes. The particle sizes are measured in terms of microns

(µ). It is also called as a micrometer and is the one millionth part of a meter. The particle

sizes of the metals powders being used in the pyrotechnics are generally in the range of

5µ to 1000µ. It is not always a good idea to use a very small and same size of the

particles in the entire composition. In some applications such as the composite

propellants, oxidizer particles having different sizes are used to increase the loading

density. The appropriate particle size of the fuel and oxidizer particles is mandatory to

achieve better reactivity of pyrotechnic compositions. It would not be out of place to

mention the importance of homogeneity of the ingredients for getting the desired

reactivity for not only pyrotechnics but also for propellants and explosives.

1.3.3 Heat output of pyrotechnics

Heat is produced whenever a pyrotechnic composition is ignited. This heat is the primary

output of the pyrotechnics and it contributes to the other effects also such as light smoke

7

etc. The heats of reaction for the pyrotechnic compositions are therefore important for

determination of the specific application in which these compositions will be used. The

heat of reaction is measured by the calorimetric methods. The units for the heat of

reaction are calories per gram or calories per mole. “Calorie” is the unit of heat and it is

defined as the amount of heat that is required for raising the temperature of one gram of

water by one degree Celsius. By applying the basic concept of a calorimeter, the heat

released by the combustion of the pyrotechnic mixture or any energetic composition can

be calculated. It is generally a matter of intended application of the energetic composition

that determines the requirement of either high or low heat of reaction.

1.3.4 Ignition reliability and safety

One of the most desirous properties of the pyrotechnic composition is that they must

undergo the ignition reliably. Safety is another concern and therefore the good

pyrotechnic composition will be the one that has reliable ignition and it is completely safe

during the storage, handling and operation. There have been some incidents where the

pyrotechnic compositions have been inadvertently ignited and in other cases they have

failed to initiate at all. There could be many reasons for the above mentioned anomalies

but one of the main reasons is the source of ignition as to how these compositions are

ignited. The proper ignition system is a must for the pyrotechnics.

1.4 Propellants

Propellants are chemicals that are used for the propulsion of different objects including

missiles, rockets, gun bullets etc. They are explosives in nature but they are designed to

burn smoothly in a controlled manner to deliver propulsive energy. The high pressure

gases produced as a result of propellant combustion are expelled through some opening

or nozzle to achieve the propulsion. Propellant generally comprise of two main

components including fuel and the oxidizer. The oxygen liberated by the oxidizer reacts

with the fuel to provide high energy in the form of hot gases. Propellants give us an

effective and easy way of attaining the propulsion. From the view point of battlefield

weapons, there are two different categories of the propellant i.e. liquid propellant and

solid propellant. The weapon systems make use of one of these two types of propellants

8

based on the operational requirements for which they are used. A brief introduction to

both types of propellants is considered essential to differentiate the two from each other.

The flow chart showing the general classification of propellant is shown in Figure 1.2.

Figure 1.2 Flow chart showing the general classification of propellants

1.4.1 Liquid propellants

These propellants are in the liquid form as specified in their name itself. The propulsion

systems that make use of the liquid propellants have the fuel and the oxidizer stored

separately. The fuel and oxidizer are mixed together in the combustion chamber when

ignition of the fuel is required. Such systems are generally referred as rocket engine [8].

The liquid propellant based systems are intricate in their operation as compared to the

solid ones. The main advantage of these propellants is that the range of the missile can be

controlled. Moreover, they have better performance in terms of the specific impulse

9

which is a figure of merit to evaluate the performance of the missile systems. Two types

of liquid propellants are popular for use in the rocket motor.

1.4.2 Liquid Monopropellants

These propellants are made up of a single material that contains both the fuel as well as

the oxidizer part in itself. These materials decompose exothermically and evolve heat and

gases. These propellant systems have the advantage of the simplicity of design and

operation. They may be ignited by using, pressure, thermal effects or by using catalysts.

They suffer from the disadvantage of low performance as compared to other types of

propellants. Therefore, their role is limited to low power applications e.g. gas generators

and attitude control thrusters. Some of the monopropellant may offer better performance

like nitro methane, however, it is very difficult to initiate. Some of the commonly used

monopropellants are hydrogen peroxide, hydrazine, nitrous oxide and steam under

external heating.

1.4.3 Liquid Bipropellants

The liquid bipropellants employ two different components, one of which is a fuel and the

other one is the oxidizer. The fuel and oxidizer are stored in two different and isolated

tanks so that they are not mixed with each other till they reach the chamber where

combustion is required to produce thrust. The bipropellants are preferred over the

monopropellant because they are safe and their specific impulse is also higher. Different

methods are used to ignite the bipropellants and some of which are pyrotechnic igniters

and spark plugs. There is special class of bipropellants that is known as hypergolic and

they do not require any external source of ignition fuel and the oxidizers spontaneously

ignite when they come in contact with each other [9]. But this is not a very safe method

of ignition and warrants special care in design to avoid accidental mixing of fuel and the

oxidizer due to leakage or hardware failure. Some examples of the of the bi propellants

combinations include red fuming nitric acid and kerosene, red fuming nitric acid and

unsymmetrical di-methyl hydrazine, liquid oxygen and liquid hydrogen, liquid oxygen

and ethanol.

http://en.wikipedia.org/w/index.php?title=Red_fuming_nitric_acid_(RFNA)&action=edit&redlink=1

http://en.wikipedia.org/wiki/Kerosene

http://en.wikipedia.org/w/index.php?title=Unsymmetrical_dimethyl_hydrazine_(UDMH)&action=edit&redlink=1

http://en.wikipedia.org/wiki/Liquid_oxygen

http://en.wikipedia.org/wiki/Liquid_hydrogen

http://en.wikipedia.org/wiki/Liquid_oxygen

http://en.wikipedia.org/wiki/Ethanol

10

1.4.4 Solid propellants

These propellants are in the solid form after their ingredients have been processed. The

fuel and the oxidizer are both part of the solid propellant and unlike liquid propellants

they are not kept separately rather they are mixed together to form a solid grain. These

propellants are therefore safe from the storage and operational point of view. The rocket

motors are filled with the solid propellant and the combustion takes place as soon as the

motor is suitably initiated. These propellants burn very fast evolving large amount of heat

and gas which are vented through the nozzle to produce thrust. Burning surface area

plays a very important role in the performance of the solid propellants. It is therefore very

essential that the geometry of the burn surface is carefully designed. The solid propellant

grains are manufactured in various geometrical forms like cigarette shape, star centered,

slotted cylinder, multi perforation etc. The range of solid propellant based missiles cannot

be controlled during operation. Once ignited, the propellant will burn completely and

continue to produce thrust till it is fully consumed. Solid propellants can be further sub

divided into different categories such as double based propellants and composite

propellants.

1.4.5 Double based solid propellants

They are called as double based propellants because two different energetic materials

form the base and propellant contains nitrocellulose and nitroglycerin as the active

ingredients. The oxygen required for the combustion of fuel is present inside the same

compound [10]. Stabilizer is another main component of the double based propellants. It

is used to increase the storage life of the propellant by removing the oxides of nitrogen. If

not removed, these oxides may be detrimental to the propellant composition and degrade

it considerably. Based on the manufacturing process, double based propellants are further

classified as cast double based and extruded double based solid propellants. They are

sometimes called as homogenous propellants and their specific impulse is generally not

very high and typically around 210 seconds. In certain cases where the propellant grain is

designed to provide a considerably large burn area, the specific impulse of close to 230

seconds can be achieved. A well-known example of such a case is the double based

11

propellant used for all burn on launch (ABOL) rocket motors. The double based

propellants are also used for some secondary functions like for jettisoning of parts and

separation of different stages. It would not be out of place to mention that the double base

propellants have the advantage of less visible exhaust plume and therefore they are

suitable for use in tactical weapons where the launch location is required to be

concealed.

1.4.6 Modified double based propellants

As discussed above, the double base solid propellants suffer from the limitation of low

specific impulse and therefor their use is also limited for high performance applications.

These propellants can be effectively modified to improve their performance significantly

by the addition of some modifiers. Such propellants are then called as modified double

based propellants. Generally some oxidizer such as ammonium perchlorate is added as a

source of oxygen improves their oxygen balance. Also some energetic fuel such as the

aluminum metal powder is also added to achieve higher heat release. These additions are

known to have a great influence on the propellant to increase its performance in terms of

the specific impulse. Some high explosives such as “RDX” and “HMX” are sometimes

also added to further improve the performance of these propellants [11].

1.4.7 Composite solid propellants

The composite propellants are different from double based propellants mainly because

they contain separate materials to act as fuel and oxidizer. They are mixture of powdered

metallic fuel mostly aluminum and some oxidizers like ammonium perchlorate. The

oxidizer constitutes major part by mass of the propellant and in some cases it may be as

high as 90 percent. The fuel oxidizer mixture is held in the matrix of a polymeric binder.

Some of the commonly used binders are hydroxy terminated polybutadiene (HTPB),

polybutadiene acrylonitrile (PBAN) and glycidyl azide polymer (GAP)[12-14]. The

binder is not only used as a polymeric matrix to hold the ingredients of composite

propellant but it also acts as a fuel during the combustion process. A very small amount

of different additives is also mixed in the composite propellant to improve its burn rate

12

and mechanical properties. The composite propellants can be given any desired shape or

geometry before they are finally dried up. The propellants are classified on the basis of

the binder that is used to hold the ingredients. The hydroxy terminated polybutadiene and

polybutadiene acrylonitrile are the common choices for majority of compositions

although quite a few other binders are also being developed and used. The density,

burning rate and the specific impulse of the PBAN based composite propellants is better

as compared to the HTPB based formulations. However, the former has difficulties in

mixing, processing and requires high temperatures for curing. HTPB based propellants

offer ease of processing and therefore preferred in most formulations. In fact both these

binders are being used in a variety of applications as they have good mechanical

properties and overall decent performance. Some of the other additions used in composite

solid propellants include the plasticizers and the curing agents. All the ingredients have a

one or more different role that is necessary to get a good quality of final product with

desired properties.

1.5 Similarity of ingredients for composite propellants and

pyrotechnics

We have discussed the main ingredients of pyrotechnic compositions and different types

of propellants. The comparison of the ingredients shows that some of the main

components of the pyrotechnic compositions and composite solid propellant are

somewhat similar. For instance, in both the cases, metallic fuel, inorganic oxidizers and

binders are used. Moreover, the pyrotechnic and propellants are both regarded as low

explosives and they decompose or ignite by burning/deflagration and do not undergo

detonation like high explosives. Although, our prime focus in the present work has been

on the pyrotechnics but we have also investigated and modified some of the composite

propellant compositions.

1.6 Motivation for the present work

A brief introduction to the energetic materials especially to pyrotechnics has been

presented above to build up the basis for the motivation and the scope of the present

13

work. The present work is focused on the thermal, kinetic and morphological analysis of

various pyrotechnic and propellant compositions and their ingredients. Main role of the

pyrotechnics has traditionally been limited to the art or the display of fireworks. Ever

since my childhood I have been fascinated by the fireworks and display of beautiful

colors in the sky. For most of us pyrotechnic may be merely a spectacular event at the

start of a football world cup or at the closing ceremony of Olympics. Things are not that

simple and over a period of time the pyrotechnics have transformed into the science and

technology of energetic materials. They are being designed to perfection for various

military applications to meet the stringent requirements of the defence forces. The

chemistry of pyrotechnics and propellants is an important area in the field of energetic

materials engineering i.e. explosive engineering. The fundamental principles and

associated mechanisms involved in the pyrotechnic reactions offer a large scope for the

scientific investigation in this area.

The literature survey revealed that a lot of research work has been carried out in the field

of explosives and propellants; however, there is lack of theoretical understanding and

experimental work concerning the reaction kinetics of the pyrotechnics. The same

concern has been mentioned in a recent review paper on the pyrotechnics [15]. The

authors stated that:

“Compared to the fairly exact, well-defined science of propellant and explosive

technology, there is a lack of theory and understanding factors which influence the

kinetics of pyrotechnic reactions”.

The present work has been dedicated to the thermal, kinetic and morphological studies of

various pyrotechnic compositions and the composite solid propellant. The rational for

selecting the pyrotechnic and the composite propellant is that the main ingredients in both

the cases are somewhat similar to each other. The aim is to systematically evaluate the

thermal behaviour and associated kinetics of some of the main ingredients i.e. oxidizers

and fuels and then extend the analysis to the actual pyrotechnic and propellant

compositions.

14

1.7 Analytical techniques and kinetic methods

Thermal analysis is a set of most important tools that is being effectively used around the

world for the investigation of energetic materials. Pyrotechnic studies based on thermal

analysis are aimed at investigation of the thermal stability, kinetic and combustion

mechanism [16,17]. Most common techniques that have been used in this regard include

differential scanning calorimetry (DSC), differential thermal analysis (DTA),

thermogravimetery (TG) and modulated temperature differential scanning calorimetry

(MTDSC). All these techniques play a valuable role in the analysis of energetic materials

and can be used in a variety of applications. DTA, DSC and TG have been mostly used

for the accomplishment of the present work along with some other techniques which

include X-ray diffraction technique (XRD), scanning electron microscope (SEM) and

particle size analyzer. The set of above mentioned thermal techniques provides

information regarding different thermal events such as thermal decomposition, solid state

phase transformations, melting and various other phenomena.

TG has become very popular technique during the past one decade for the researchers

working in the field of energetic materials. It is being used to study the oxidation process

of the pyrotechnic fuel in the presence of air or oxygen atmosphere. Thermal

decomposition of the pyrotechnics and propellants has been effectively studied in this

work by using this technique.

Thermal cum kinetic evaluation of energetic materials is essential to assess their thermal

stability and reactivity. The thermal data obtained from TG, DTA and DSC experiments

is used to calculate the kinetic parameters for elucidation of the reaction mechanism. The

kinetic analysis in the present work has been carried out by using six different methods

including Kissinger method, Ozawa method, Horowitz and Metzger method, Friedman

method, Flynn–Wall–Ozawa method and Kissinger-Akahira-Sunose method. The main

strength of the kinetic methods based on thermal analysis is that both the analytical data

and kinetic parameters are obtained from the same experiments and in a relatively shorter

time. The detail of these kinetic methods is presented in Chapter 2.

15

1.8 Scope of the present work

A comprehensive literature survey has been carried out to identify different research

areas in which the pyrotechnic and propellant research work has been focused in the

recent past especially using the thermal analysis techniques and employing different

kinetic evaluation methods. Detail of the literature review pertaining to different research

areas is given in the introduction section of the respective chapters. Figure 1.3 presents

the road map used to accomplish the present work. The details of the literature survey and

the work done in each specific area will be described chapter wise in the next part of this

thesis; however, a brief and general introduction to these work areas has been discussed

in the following paragraphs.

Figure 1.3 Flow chart showing the road map for the accomplishment of present work

The first part of our research work has been presented as Chapter 3 and it includes the

comparative thermal and kinetic analysis of five different oxidizers that are most

commonly used in various energetic compositions. The investigated oxidizers include

16

ammonium perchlorate (NH4ClO4), ammonium nitrate (NH4NO3), potassium nitrate

(KNO3), barium nitrate Ba(NO3)2 and potassium permanganate (KMnO4). These

oxidizers have been studied by using simultaneous thermal analysis including differential

thermal analysis and thermogravimetery. All the experiments were performed under

identical reaction conditions for better comparison of the results. The thermal

decomposition, phase transformations, kinetics, and the amount of free oxygen provided

by the each oxidizer have been compared. Although, some of the data in this regard has

already been published for individual oxidizers, but the aim here was to compare and

contrast different aspects in one place and under similar conditions [18,19]. It is

important to mention here that the thermal behaviour and the kinetic parameters may be

different under different experimental conditions.

After carrying out the detailed comparison of the various oxidizers, two of them i.e.

barium nitrate and ammonium perchlorate were selected for further investigation and

modification and the work done in this regard has been presented in Chapter 4 of the

thesis. Modified barium nitrate with micro porous structure has been synthesized to

increase the reliability of its ignition as a pyrotechnic oxidant. The motivation for this

work was to promote the use of barium nitrate as a suitable replacement of potassium

chlorate as a pyrotechnic oxidizer. There are different ways to modify the ignition

behaviour of the barium nitrate. One way is to make use of some thermal decomposition

catalyst to sensitize the oxidizer so that it becomes easy to ignite and more reliable when

ignited [20,21]. The method that has been used in the present work is to make use of

some inorganic vesicants for modifying the performance of the oxidizer to increase its

sensitivity and reliability of ignition by creating micro porous structure [22,23]. The

vesicants that have been used for the modification of barium nitrate in this case are

sodium bicarbonate (NaHCO3), potassium carbonate (K2CO3) and ammonium

perchlorate. The ignition behavior of the pure and the vesicant modified barium nitrate

has been compared by formulating two pyrotechnic mixtures with aluminum fuel and the

results have been reported for the first time.

The catalytic effect of nano sized magnesium oxide (MgO) on the thermal decomposition

and kinetic parameters of ammonium perchlorate (AP) has also been included in Chapter

17

4. The catalytic activity of the nano metal oxides is considered to be better than

conventionally used micro sized particles due to their relatively large surface area

[24,25]. The catalytic effect of MgO on AP decomposition has been reported earlier by

Guorong Duan et al. however, the authors have not carried out the kinetic analysis in

their work [26]. The earlier work reports that the thermal decomposition of AP gets

effected by the addition of different percentages of the catalyst and the two distinct

decomposition stages reduce to a single stage by the addition of 4 percent MgO. Our

work in this regard has been focused to see the changes in the kinetic parameters of the

catalyzed version of AP as compared to the pure one and this aspect has not been

reported in the former work.

The work presented in Chapter 5 describes the results of thermal and kinetic analysis of

pyrotechnic compositions containing magnesium and aluminum fuels. The thermal

stability and reactivity of three pyrotechnic mixtures has been investigated. These are

both very important factors concerning the storage safety and operational reliability of the

pyrotechnics. Metal powders are generally used as pyrotechnic fuels. Magnesium (Mg)

and aluminum (Al) metals are preferred as a fuel in pyrotechnic compositions where a

very high heat and light output is desired. These two effects are most commonly

employed in pyrotechnics that are used as flare and tracer compositions [27,28]. Three

different pyrotechnic mixtures have been formulated by using aluminum and magnesium

as a fuel. The oxidizers that have been used are barium nitrate, potassium permanganate

and ammonium perchlorate. Both the fuel and the oxidizer play an important role in the

combustion of pyrotechnic mixtures and influence their ignition reliability [29]. The fuel

and oxidizer present in the pyrotechnic mixture undergo redox reaction. Thermal analysis

provides a set of powerful tools to investigate such reactions.

The effect of aging i.e. high temperatures and elevated humidity levels on two very

commonly used military pyrotechnics has also been investigated and the results are

presented in Chapter 6. The investigated compositions include SR-524 and SR-562

pyrotechnic compositions. The aging of both the composition was carried out at a

temperature of 70oC and relative humidity of 70 percent for 30 days. The thermal,

kinetic and morphological analysis of fresh and the aged compositions were carried out

18

and the results were compared to identify the changes caused by the aging process. The

thermal behaviour has been studied using simultaneous TG/DTA and the Kissinger

method has been used for evaluation of the kinetic parameters.

Temperature, humidity, thermal shock and impact are amongst the important factors that

influence the performance of the pyrotechnic compositions [30]. The stability of some

materials including pyrotechnics, propellants, polymers etc. is seriously affected by high

temperatures and humidity. The effect of aging on pyrotechnics and propellants,

especially the effect of high temperatures and high humidity, has been investigated by the

researchers in the past as well [31-33].

It is significant to find out different physical and chemical changes caused by the aging

because they may have some undesirable effect on the ignition behaviour of the

pyrotechnic compositions. These changes may result in poor performance of the

pyrotechnics and they may not produce the required output or even fail to ignite [34].

Some of the changes caused by the aging may cause the accidental initiation of

pyrotechnic compositions.

The last part of the thesis describes the in-depth kinetic analysis of composite solid

propellant based on HTPB binder and AP. The three different version of the composite

propellant have been evaluated to check the catalytic effect of magnesium oxide (MgO)

and zinc oxide (ZnO) nano catalysts on the thermal decomposition and kinetic behaviour.

The transition metal oxides are frequently used as catalysts for thermal decomposition of

composite propellants and to improve their performance by increasing their burn rates

[24,35]. The percentage of the catalyst and surface area of the particles are important

factors that influence the catalytic activity [36]. It is reported that nano sized catalysts are

preferred due to their small size and large exposed area and improve the catalytic activity

[37]. The nano catalysts have been characterized by using analytical techniques such

SEM and XRD. The kinetic analysis has been carried out by using four different methods

including Kissinger method, Flynn–Wall–Ozawa method, Friedman method and

Kissinger-Akahira-Sunose method.

The comparison of thermal data and kinetic data parameters was carried out for the

catalyzed and un-catalyzed versions to quantify the effect of catalyst. The differential

19

scanning calorimetry (DSC) and thermogravimetery (TG) has been used for the thermal

analysis.

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[23] X. Zhang, X. Chen, M.H. Feng, Z.F. Zheng, G.P. Pan, H.P. Lv, Advanced

Materials Research 550 (2012) 27.

[24] P.R. Patil, V.e.N. Krishnamurthy, S.S. Joshi, Propellants, Explosives,

Pyrotechnics 33 (2008) 266.

[25] I.P.S. Kapoor, P. Srivastava, G. Singh, Propellants, Explosives, Pyrotechnics 34

(2009) 351.

[26] G. Duan, X. Yang, J. Chen, G. Huang, L. Lu, X. Wang, Powder technology 172

(2007) 27.

[27] C. Lăzăroaie, S. Eşanu, C. Său, R. Petre, P.-Z. Iordache, G. Staikos, T. Rotariu, T.

Zecheru, Journal of Thermal Analysis and Calorimetry 115 (2014) 1407.

[28] J.J. Granier, M.L. Pantoya, Combustion and Flame 138 (2004) 373.

[29] S.G. Hosseini, S.M. Pourmortazavi, S.S. Hajimirsadeghi, Combustion and flame

141 (2005) 322.

[30] L. Wang, X. Shi, W. Wang, Journal of Thermal Analysis and Calorimetry 117

(2014) 985.

[31] W. de Klerk, E. Krabbendam-LaHaye, B. Berger, H. Brechbuhl, C. Popescu,

Journal of thermal analysis and calorimetry 80 (2005) 529.

[32] D. Sorensen, A. Quebral, E. Baroody, W. Sanborn, Journal of thermal analysis

and calorimetry 85 (2006) 151.

[33] M.D. Judge, Propellants, Explosives, Pyrotechnics 28 (2003) 114.

21

[34] S. Brown, E.L. Charsley, S. Goodall, P.G. Laye, J.J. Rooney, T.T. Griffiths,

Thermochimica acta 401 (2003) 53.



[36] K. Kishore, V.P. Verneker, M. Sunitha, AIAA Journal 15 (1977) 1649.

[37] S. Chaturvedi, P.N. Dave, Journal of Saudi Chemical Society 17 (2013) 135.

22

Chapter 2

Experimental Techniques, Materials and

Methods

2.1 Experimental Techniques

Thermal analysis has been used mostly for the conduct of the present work. It is an

effective technique for investigating energetic materials including propellant and

pyrotechnics. The thermal data is very useful for the evaluation of the kinetic parameters.

Different analytical techniques that have been used for the accomplishment of this work

are listed below:

1. Differential Thermal Analysis (DTA).

2. Thermogravimetery (TG).

3. Differential Scanning Calorimetry (DSC).

4. X-ray Diffraction (XRD).

5. Scanning Electron Microscope (SEM).

6. Particle Size Analysis (PSA)

2.1.1 Thermal analysis

It is one of the most widely used techniques for the investigation of energetic materials

primarily because the analysis requires very minute quantity of the sample making it

safe for the analysis of explosives, propellants and pyrotechnics [1-3]. The changes in

the physical and chemical properties of the material are observed by exposing it to high

temperatures. The material temperature is raised in a systematic and controlled manner.

23

For this purpose, both the sample and an inert reference are heated together and the

temperature difference between them is monitored. The methods of thermal analysis are

not limited to explosives rather they can be used for the investigation of a wide variety

of materials. The behaviour of pure compounds and different mixtures is studied by

thermal analysis. The analysis provides some meaningful data such as melting points,

boing points, solid state phase transformations, decomposition temperatures and glass

transition temperatures. Different events are identified as exothermic or endothermic in

nature by measuring the heat flow. Moreover, the main strength of these methods is their

ability to provide information regarding the kinetic parameters such as activation energy,

frequency factor and rate of reaction. The thermal analysis finds application in many

fields including the following:

1. Characterization of different polymers [4].

2. Investigation of energetic materials.

3. Pharmaceutical drug development [5].

4. Evaluation of soils [6].

5. Study of thermal properties of foods [7].

2.1.2 Different methods for thermal analysis

Thermal analysis refers to a broad analytical technique and it includes many different

methods. Each method is based on the measurement of some specific properties of the

material with respect to the variation in the sample temperature. Some of the methods of

the thermal analysis are described below [8].

Differential Thermal Analysis

This method measures the temperature difference between the sample and the inert

reference.

Thermogravimetery

The method measures mass loss or mass gain in the sample as a consequence of

heating.

http://en.wikipedia.org/wiki/Differential_thermal_analysis

24

Differential Scanning Calorimetry

It measures the difference in the heat flow of the sample and the inert reference

material.

Thermo Mechanical Analysis

It is an important thermal method by which the mechanical properties of materials

are measured.

Dynamic Mechanical Analysis

It measures the properties such as mechanical stiffness and damping.

Thermomagnetometry

The magnetic properties of materials are measured in this method.

Thermoparticulate Analysis

The evolution of particles is measured with this method of thermal analysis.

It is obvious that all these methods provide a powerful set of tools to analyze different

material properties. The choice of any specific method depends upon the type of the

material being investigated and the information required. Thermal methods including

Differential Thermal Analysis (DTA), Thermogravimetery (TG) and Differential

Scanning Calorimetry (DSC) have been used in the present work and are discussed in

detail.

2.1.3 Differential Scanning calorimetry (DSC)

DSC is an analytical technique that is used to investigate different thermal events such

as decomposition temperatures, phase transformations, melting and boiling points and

for subsequent processing of the thermal data for kinetic evaluation. It is commonly used

for the thermal cum kinetic analysis of energetic materials including explosives,

http://en.wikipedia.org/wiki/Differential_scanning_calorimetry

http://en.wikipedia.org/wiki/Thermomechanical_analysis

http://en.wikipedia.org/wiki/Dynamic_mechanical_analysis

http://en.wikipedia.org/w/index.php?title=Thermo-optical_analysis&action=edit&redlink=1

25

propellants and pyrotechnics [9-11]. The thermal stability and reactivity of energetic

materials are two important aspects concerning the safety and reliability of the energetic

compositions. DSC provides a very safe and effective method for the study of such

compositions. In the present work, DSC has been used to carry out the thermal studies of

different versions of composite propellant at multiple heating rates. The obtained

experimental results have been used to calculate the kinetic parameters using the

Kissinger method. The DSC instrument by Mettler Toledo model number DSC823e has

been used in the present work and it is shown in Figure 2.1.

Figure 2.1 DSC Instrument model number DSC823e

The DSC is used to measure the difference in the heat flow of an inert reference and the

material under investigation. Both the sample and reference are simultaneously

subjected to a common heating program and under exactly similar experimental

conditions. The principle of operation in this technique is to keep both the reference and

the sample material at the same temperature while they are being heated at some

required heating rate. The amount of energy supplied to keep the reference and the

sample at the same temperature is then plotted against the temperature or time. When the

26

sample undergoes some physical or chemical change, the heat is either evolved

(exothermic change) or the heat is absorbed (endothermic change); the heat flow is

adjusted accordingly.

There are two distinct sensors in the DSC instrument that are used for the sample and the

reference separately. Two identical pans are used for the sample and the reference and

the reference pan is generally kept empty. Some inert gas is also used for the conduct of

experiments unless the experiment is intentionally designed to be conducted in the air or

some other reactive atmosphere.

2.1.4 Differential Thermal Analysis (DTA)

This technique is quite similar to the DSC; however, it mainly differs from DSC in its

operational mechanism. The sample and the reference are both subjected to a same

heating program and the difference in the temperature between the sample and the

reference is then measured and plotted against temperature. The DTA is also very

powerful analytical method and is used to investigate the changes such as melting,

decomposition, solid state phase transformations, glass transitions, crystallization,

sublimation etc. [12-14]. The DTA has been extensively used to carry out the present

work. It has been used for the thermal analysis of various oxidizers used in the field of

propellants and pyrotechnics. It has been used for the thermal cum kinetic analysis of

different pyrotechnic compositions and their ingredients. The changes caused by the

aging process to some commonly used military pyrotechnics related to their thermal

stability and kinetic parameters have been identified on the basis of the experimental

results obtained from DTA. The Diamond TG/DTA instrument manufactured by Perkin

Elmer has been used in this work and shown in Figure 2.2.

It is pertinent to mention here that the reference material has a strong influence on the

experimental results of the DTA and therefore it should have the following properties.

It should be thermally inert during the entire course of heating and cooling.

There should be no reaction between the reference material and any other

hardware component like pan holders and thermocouples.

27

The most popular choices of the reference material for the analysis of inorganic

materials are alumina and carborundum. There are some other important factors as well

that bear an effect on the results of the DTA. These factors may be either sample related

or instrument related.

Figure 2.2 The diamond TG/DTA Instrument

Factors related to the sample

Nature and the characteristics of the sample are considered to effect the DTA curve.

These factors include but are not limited to:

The mass of the sample

The size of sample particles

Heat capacity and the thermal conductivity of the sample

The packing density of the sample

Swelling/shrinking of the sample

28

Factors related to the instrument

Sample heating rate

Material of the sample holding pan

Geometry of the sample holding pan

The reaction atmosphere

Type of the furnace used for heating

Position of thermocouple related to the sample

The response and specifications of the instrument

It would not be out of place to mention here that the similar factors also influence the

experimental results of TG and DSC.

2.1.5 Thermogravimetery (TG)

Thermogravimetery (TG) or the thermal gravimetric analysis (TGA) is one of the most

important methods of the thermal analysis. The mass gain or the mass loss of the sample

is measured in this technique when the sample is systematically heated in a controlled

manner. There are materials that may lose weight on heating due to some physical or

chemical change occurring at high temperatures e.g. decomposition of the sample and

liberation of the gaseous products will cause the sample to lose its mass. Similarly, the

sample can gain mass at higher temperatures due to reaction with the surrounding

atmosphere and the typical example of such a case is the oxidation of metals in the air

atmosphere. It is pertinent to mention here that both the mass loss and mass gain provide

significant information concerning the sample.

The technique is very useful to determine the changes in mass of a sample and then

correlating those changes with the chemistry of the material to identify the thermal

events that have taken place in some specified temperature range. It provides vital

information regarding the thermal decomposition and stability of the material under

investigation in the form of mass vs. temperature curve. It finds extensive utilization in

http://en.wikipedia.org/wiki/Weight

29

the field of energetic materials [15-17]. It has been widely used in the present work for

the thermal and kinetic evaluation of propellants/pyrotechnics and their ingredients.

Measurement principle of TG

There are two beams i.e. sample and reference beams that are supported by the driving

coils. Whenever there is some change in the mass of the sample, it is detected by an

optical position sensor with the help of a reference slit. The balance circuit reception

signals from the optical sensor and it supplies feedback current to the driving coils so that

the slit returns to its original position. Since the feedback current supplied by the balance

circuit is proportional to mass gain or mass loss of the sample, therefore the change in

mass of the sample with respect to a reference is detected. Some of the main components

of the TG system are visible in Figure 2.3.

Figure 2.3 Main components of the TG system

2.1.6 X-ray diffraction (XRD)

X-ray diffraction is a commonly used technique for finding the atomic or molecular

structure of crystalline materials. The incident X-rays are scattered in different

30

directions due to interaction with the atoms present inside the material. The scattered X-

rays may undergo either constructive or destructive interference. For the constructive

interference, the diffraction of x-rays is described by the Bragg‟s law:

nλ = 2dsinθ

Where, n is an integer, λ represents the wavelength of incident X-rays, d is the spacing

between the planes and θ represents the angle between the incident X-rays and set of the

crystal planes.

The x-rays are electromagnetic radiations having their wavelength in the range of 0.5-

2.5 Ao. This range of the wavelength makes them very suitable for identifying the crystal

structure at atomic level. It is no doubt one of the most powerful characterization tools

being used in a variety of disciplines including material science and solid state

chemistry. It is also used by researchers working in the field of energetic materials [18-

20]. We have used this technique for the characterization of micro porous barium nitrate,

identification of the residue after thermal decomposition and for investigating the

reaction products of aging. In the present study, XRD instrument manufactured by

STOE, Germany has been used. The main components of this instrument are shown in

Figure 2.4.

Figure 2.4 Main components of the XRD instrument

31

The basic components of the XRD instrument are listed below:

1. X-ray source (Monochromatic)

2. The sample holder

3. Data collector (scintillation counter in this case)

4. The x-rays used in this case are Cu Kα.

The following important information can be obtained from the XRD analysis of a

material.

Identification of phase composition

Determination of unit cell lattice parameters

Crystal structure

Texture/orientation

Crystallite size

2.1.7 Scanning electron Microscopy (SEM)

It is an electron microscope that makes use of a focused beam of electrons to scan the

sample. The electrons beam interacts with the sample and generates some signals that are

used to get the information regarding the composition and structure of the investigated

samples. The scanning electron microscope is far more superior in performance than the

ordinary microscope. It can achieve a resolution of less than one nanometer due to very

short wavelength of the electrons. The wavelength of the white light is relatively very

high as compared to the electrons. The high wavelength is a limitation due to which the

resolution of the modern microscope is not more than 250nm.

The scanning electron microscopy is a versatile technique and being used around the

globe by researchers working in many areas of science and technology [21,22]. The

technique has been used in the present work for the characterization of nano particles,

investigation of porous structures and for identification of effects of aging on the

morphology and surface features of the pyrotechnic compositions. The scanning electron

32

microscope model number JSM- 6490 LA manufactured by JEOL, Japan has been used

in this work and shown in Figure 2.5

Figure 2.5 Scanning electron microscope JSM- 6490 LA

2.1.8 Particle size analysis (PSA)

It is an analytical technique which is used to measure the particle size distribution of both

the powders and liquids. There are numerous methods by which the particle size analysis

is carried out. The particle size is known to play important role in different applications in

the field of chemical engineering, materials engineering, reaction kinetics, agriculture and

mining and many more. In all the above mentioned fields, there are application specific

reasons due to which the particle size measurement is considered important. The particle

size distributions are reported as means, medians or modes depending on the nature of

distribution and the information required to be presented. In the present work, laser

scattering particle size distribution analyzer (LA-920 by HORIBA) was used for the

particle size analysis of metal powders that were subsequently used as fuels in the

pyrotechnic formulations. The particle size analyzer used in this work is shown in Figure

33

2.6. The technique is often used by the researchers working in the field of energetic

materials to specify the particle size distribution of sample.

Figure 2.6 Particle size distribution analyzer LA-920

2.2 Materials Used

2.2.1 Oxidizers

Analytical grade potassium nitrate, ammonium nitrate, potassium permanganate,

barium nitrate, and defense grade ammonium perchlorate with high purity have been

used for this analysis. Due to high purity level of the compounds, no further purification

was carried out. Ammonium nitrate and potassium nitrate used in this work were

purchased from Merck. Barium nitrate and potassium permanganate were purchased

from Scharlau (Spain) and ammonium perchlorate was supplied by National

Development Complex of Pakistan. All these oxidizer were ground finely and sieved

through a 300 mesh before the analysis. For the comparison purpose, the reaction

conditions of all the experiments conducted on these oxidizers were kept same. The

sample mass was kept as 10 mg and the heating rate of 10ºC/min was used. Nitrogen

gas was used to provide the inert atmosphere during the conduct of experiments.

34

2.2.2 Fuels

Analytical grade magnesium powder and aluminum powder were purchased from

Scharlau (Spain) for the present work. The average particle size of the aluminum and

magnesium powder was nearly 20µm and 24µm respectively. Laser scattering particle

size distribution analyzer LA-920 by HORIBA was used for the analysis. The sample

mass of 3 mg was used to study the oxidation of these fuels in the air atmosphere. The

heating rate of 10ºC/min was used. The experiments were carried out in the air

atmosphere to monitor the oxidation process of the metal powders. The thermal

behaviour and oxidation process was investigated by using thermogravimetery.

2.2.3 Pyrotechnic Compositions

The pyrotechnic compositions investigated in the present work include pyrotechnic

mixtures prepared by us by mixing stoichiometric fuel oxidizer ratios and some

commonly used military pyrotechnic compositions purchased from Pakistan Ordnance

Factories. The detailed composition of all these pyrotechnic mixtures/compositions is

given below. Table 2.1 represents the fuel to oxidant ratio of the pyrotechnic mixtures

containing aluminum and magnesium fuel.

Table 2.1 The mass ratio of fuel and oxidizer in pyrotechnic mixtures

The military pyrotechnic compositions investigated in the present work and the mass

ratio of their ingredients is shown below.

S. No Fuel Oxidizer Mass Ratio

1 Aluminum Barium Nitrate 17/83

2 Magnesium Ammonium perchlorate 34/66

3 Magnesium Potassium permanganate 18/82

35

SR 524 Pyrotechnic composition

The composition is mainly based on magnesium fuel and sodium nitrate oxidizer. It is

used as a flash mixture for military applications. The exact composition of the ingredients

is given below:

Magnesium 58%

Sodium Nitrate 38%

Boiled Linseed Oil 4%

SR 562 Pyrotechnic composition

The other military composition that has been investigated in this work is SR-562. It is

used as a flare in military applications. The exact composition of the ingredients is given

below:

Magnesium 50%

Calcium oxalate 11%

Sodium Nitrate 35%

Lithographic varnish 4%

2.2.4 Composite solid propellant compositions

The composite solid propellant based on HTPB binder fuel and ammonium perchlorate

oxidizer has been investigated in this work. The propellant was manufactured by National

Development Complex, Pakistan and the specifications are given below:

Composition 1

HTPB binder fuel 15%

Ammonium perchlorate 80%

Miscellaneous additives 5%

36

Composition 2 and 3

The propellant composition 2 and 3 are similar to composition 1 except that they have

been catalyzed with two percent of nano magnesium oxide and nano zinc oxide

respectively.

2.3 Kinetic evaluation methods.

2.3.1 Importance of Kinetics for solid state reactions

Investigation of the kinetic behaviour in solid state reactions provides a useful insight

into these processes. The thermal and kinetic evaluation of energetic materials is essential

to assess their reactivity and thermal stability. The TG, DTA and DSC data obtained

under non-isothermal conditions can be used to evaluate the kinetic parameters. The

information pertaining to the reaction kinetics of the energetic materials is important in

many aspects. The kinetic data not only describes the reaction pathway but also helpful in

determining the stability and compatibility of the energetic compositions. The thermal,

kinetic and stability parameters are indeed very helpful in the manufacturing process for

the formulation of new compositions and for estimating the safety aspects during storage,

handling and operation. Thermal methods such as TG, DTA and DSC are versatile

methods for studying the thermal decomposition and kinetics of a wide variety of

materials including energetic materials. A variety of kinetic methods based on thermal

data have been developed and used for evaluation of different materials. The main

strength of the kinetic methods based on thermal analysis is that both the analytical data

and kinetic parameters are obtained from the same experiments and in a relatively shorter

time.

Broadly speaking, there are two types of kinetic methods based on the non-isothermal

kinetic data, i.e., they are either based on single heating rate experiment or the multiple

heating rate experiments. The non-isothermal methods offer several advantages over the

conventional isothermal methods [23,24]. Some of these are:

(a) These methods provide easy and rapid temperature scan of the material over

the desired range of temperature.

37

(b) The kinetic evaluation is continuous in the temperature range of interest.

(c) The isothermal methods have a non-isothermal part as well. The sample is

required to be raised to the desired temperature non-isothermally and the

sample may undergo some reaction during this time. There is no such issue in

non-isothermal methods.

(d) These methods require less experimental data and shorter time. Although,

multiple heating rates are preferred, however, the kinetics can be calculated

even from a single heating rate experiment.

The non-isothermal methods of kinetic analysis have been used in the present work.

There are six different kinetic methods including the three isoconversional methods that

have been used. These methods include:

(a) Kissinger method

(b) Ozawa method

(c) Horowitz and Metzger method

(d) Friedman method

(e) Flynn–Wall–Ozawa method

(f) Kissinger-Akahira-Sunose method

2.3.2 Kissinger method

The Kissinger method has been extensively used in the present work for non-isothermal

kinetic evaluation of different pyrotechnic mixtures and composite propellant

compositions. A brief description of the Kissinger method is presented below. Starting

from the following equation:

⁄ (2.1)

In this equation (1), “α” presents conversion degree, “β” is the heating rate, “A”

represents the frequency factor or pre exponential factor, “E” is the activation energy,

“n” is the reaction order and “R” represents the gas constant.

38

∫

∫ (

)

(2.2)

(2.3)

Kissinger has proposed different ways to calculate the kinetic parameters by using the

thermal data [25].

(

)

(2.4)

In equation (2.4), is the peak temperature, is the heating rate, “A” represents

frequency factor, “R” is the gas constant, “T” represents sample temperature, “n”

represents reaction order and “E” represents activation energy in kJ/mol . For reaction

order other than one, the kinetic parameters can be calculated from equation (2.5)

[26,27].

(

) (

)

(2.5)

The above equation means that the energy of activation can be calculated from (

)

and

without changing the reaction order. The slope of the line obtained by plotting

above mentioned graph gives the value of activation energy. When the “E” has been

calculated, the frequency factor can be calculated by using equation (2.6) given below

[28,29].

[ (

)]

(2.6)

39

And when both E and A are known, the reaction rate constant can be found by using the

famous Arrhenius equation.

k = A (

) (2.7)

An important thermodynamic parameter known as the enthalpy of activation can also be

calculated using activation energy value and employing the under mentioned relationship

[30].

(2.8)

2.3.3 Ozawa Method

Another kinetic evaluation method used in the present work is the Ozawa method. This

method is based on the assumption that for any heat flow curve, the degree of reaction is

independent of the heating rate at the peak temperature (Tp). The final equation of the

Ozawa method is given below.

(

( )) (

) (2.9)

Experiments are performed at multiple heating rates and the peak temperatures are

determined in each case. In this method, kinetic plot of log β is plotted against reciprocal

peak temperature. The slop of the resulting plot is then used to determine the value of

activation energy using the following relationship.

Ea = - 2.19 R (d log β/ d Tp-1

) (2.10)

In equation (2.10), “Ea” is the activation energy in kJ/mol, “β” is the heating rate and “Tp”

is the peak temperature corresponding to a specific heating rate and “R” is the universal

gas constant. After calculating the activation energy, the frequency factor can be

40

determined from the intercept of the log β axis. For majority of kinetic models, ( ) 1

and this assumption enable the determination of A from the intercept [31]. The rate

constant is then determined using equation (2.7) described previously.

2.3.4 Horowitz and Metzger Method

The main strength of this method is that it affords to calculate the activation energy by a

single heating rate experiment. We have used this method sparingly in the present work

and most of the kinetic calculations have been done by multiple heating rate experiments.

The method has been briefly discussed in the following paragraphs.

First of all, a reference temperature is determined and it is denoted as Ts. This is the

temperature where the rate of decomposition is the maximum. The reference is calculated

from the TG data by the relationship given below.

Ts = Wt / Wo = 1/e (2.11)

Where, “Ts” is the reference temperature, “Wt” is the weight of the sample at any given

temperature T, “Wo” is the total initial weight of the sample and “e” is the exponential.

When the reference temperature is known, the temperature difference ө (theta) is

calculated by using equation (12)

ө = T- Ts (2.12)

Where, “T” is the sample temperature at any weight “Wt”. The “ө” is then plotted against

lnln Wo/ Wt. The gradient or the slop of a nearly straight line obtained from the above

graph gives the value of activation energy[32].

Ea = Slop 2.303(RTs2) (2.13)

Where, Ea is the activation energy and R is the Universal Gas Constant and Ts is the

reference temperature.

41

2.3.5 Flynn–Wall–Ozawa Method

Three isoconversional methods have been used in this work to carry out the thorough

kinetic evaluation of the composite solid propellant. These methods allow the calculation

of activation energy at several degrees of conversion and the variation of activation

energy can be found as the reaction proceeds. The methods are not very specific and are

suitable for the kinetic investigation of variety of materials. They offer model free

calculation of activation energies corresponding to different degrees of conversion. The

Flynn-Wall-Ozawa method is an integral kinetic method. The method assumes that the

kinetic equation follows the Arrhenius behaviour and the calculations have been

simplified by using the famous Doyle approximation [33]. The final and simplified form

of the Flynn Wall Ozawa method is given in equation (2.14).

(

) (

) (2.14)

Where, “β” represents heating rate at which the experiment was conducted , “A” is the

pre- exponential factor commonly known as the frequency factor , “α” represents the

degree of conversion, “E” is the energy of activation and “R” is the universal constant.

Multiple heating rate experiments are required to plot the kinetic graph of

versus lnβ

for different degree of conversion of conversion (α). The gradient of the above mentioned

graph can be used to calculate “E” for each degree of conversion.

2.3.6 Friedman Method

Friedman proposed a kinetic evaluation method and it is very popular amongst the

researchers working in the field of energetic materials. It is a differential method that

enables model free kinetic evaluations. It provides a large amount of activation energy

data at several degrees of the conversion [34]. The Friedman method of kinetic evaluation

and its final equation has been presented in equation (2.15).

42

(

) [ ]

(2.15)

Where, “β” represents the heating rates used for carrying out the experiments, “α” is

conversion degree of the sample, “A” is the frequency factor, “E” represent the activation

energy and “R” is the gas constant.

It is clear from equation (2.15) that the plots of (

) versus the reciprocal

temperatures at different heating rates are required to determine the value of activation

energy corresponding to different degree of conversion.

2.3.6 Kissinger-Akahira-Sunose Method

The Kissinger-Akahira-Sunose method is somewhat similar to the original Kissinger

method; however, the main difference between the two methods is the value of

temperatures used in the kinetic evaluation. Kissinger-Akahira-Sunose method uses the

sample temperatures corresponding to each degree of conversion α (Tα) instead of peak

temperatures (Tp) used in the basic Kissinger method. The Coats-Redfern approximation

concerning temperature integral has been used to simplify this method [35]. The method

has been described below in equation (2.16).

(

) (

)

(2.16)

Where, is the temperature corresponding to conversion (α) , represents

multiple heating rates used during the experiments , “E” denotes the activation energy ,

“A” denotes pre-exponential factor and “R” is the gas constant.

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[15] X. Kang, J. Zhang, Q. Zhang, K. Du, Y. Tang, Journal of thermal analysis and

calorimetry 109 (2012) 1333.

[16] J. Luman, B. Wehrman, K. Kuo, R. Yetter, N. Masoud, T. Manning, L. Harris, H.

Bruck, Proceedings of the Combustion Institute 31 (2007) 2089.

[17] Q.S. Kwok, R.C. Fouchard, A.M. Turcotte, P.D. Lightfoot, R. Bowes, D.E. Jones,

Propellants, Explosives, Pyrotechnics 27 (2002) 229.

[18] G. Harding, A. Harding, Counterterrorist Detection Techniques of Explosives

(2007) 199.

[19] L. Meda, G. Marra, L. Galfetti, F. Severini, L. De Luca, Materials Science and

Engineering: C 27 (2007) 1393.

[20] S.H. Ba, Z. Zhang, M.H. Yan, Z.X. Sun, X.P. Teng, Applied Mechanics and

Materials 217 (2012) 669.

[21] R.C. Dujay, Encyclopedic Dictionary of Pyrotechnics:(and Related Subjects)-

B&W 11 (2012) 27.

[22] J.R. Verkouteren, Journal of forensic sciences 52 (2007) 335.

[23] S. Maitra, S. Mukherjee, N. Saha, J. Pramanik, Cerâmica 53 (2007) 284.

44

[24] S. Vyazovkin, C.A. Wight, International Reviews in Physical Chemistry 17

(1998) 407.

[25] H.E. Kissinger, Analytical chemistry 29 (1957) 1702.

[26] B. Lehmann, J. Karger-Kocsis, Journal of Thermal Analysis and Calorimetry 95

(2009) 221.

[27] T. Hatakeyama, F. Quinn, Thermal Analysis (1994).

[28] S. Vyazovkin, C.A. Wight, Thermochimica acta 340 (1999) 53.

[29] M. Sunitha, C. Reghunadhan Nair, K. Krishnan, K. Ninan, Thermochimica acta

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(2013) 10162.

[35] B. Janković, Chemical Engineering Journal 139 (2008) 128.

45

Chapter 3

Thermal and kinetic comparison of

commonly used oxidizers

3.1 Introduction

Oxidizers are usually oxygen rich compounds which are used to provide oxygen for the

burning/ignition of the fuel in various energetic formulations including pyrotechnic and

propellant compositions [1]. The nature of the oxidizer and its oxygen balance play

important role during the ignition of energetic compositions. Composite solid propellants

and pyrotechnic compositions utilize a large quantity of oxidizers [2-4]. These are

generally inorganic solids and the commonly used oxidizers are chlorates, perchlorates,

nitrates, chromates etc. Pyrotechnic compositions mainly comprise fuel and oxidizer and

a small amount some of other additives. They produce some special effects after they are

properly ignited and the effects include the production of heat, light, sound and smoke.

The pyrotechnics are heterogeneous mixture of fuel and oxidizer and they may be either

inorganic or organic in nature [5].

The thermal behaviour and kinetics of the oxidizer has a strong influence on the thermal

decomposition and the ignition of the propellants as well pyrotechnics [6]. It is therefore

considered essential to investigate the thermal cum kinetic behaviour of the oxidizer

before it is used in any energetic composition. The present work describes the thermal,

kinetic and parametric comparison of five oxidizers that are commonly used in various

pyrotechnic and propellant compositions. These investigated oxidizers include

ammonium perchlorate, ammonium nitrate, barium nitrate, potassium nitrate and

46

potassium permanganate. In the reported literature, most of these oxidizers have been

investigated individually. However, the focus in the present work was to carry out the

thermal and kinetic analysis of all these oxidizer under exactly similar reaction conditions

so that the comparisons are realistic and conclusions are meaningful. The reaction

conditions bear an effect on the experimental results in the field of thermal analysis and

the results may be different under different conditions. Therefore, it has been considered

essential to carry out the thermal cum kinetic investigation of these oxidizers afresh and

under identical reaction conditions.

Ammonium perchlorate has been extensively used as an oxidizer for composite

propellants and it is still very popular [7]. An extremely large quantity of this oxidizer is

utilized in the propellants that are used for the space shuttles [8]. The thermal behaviour

and kinetic parameters of ammonium perchlorate have been investigated by many

researchers working in the field of energetic materials [9-11]. Ammonium nitrate finds

use in both the military and commercial applications. It is used in mining operations as an

industrial explosive and the composition is generally known as ammonium nitrate fuel oil

mixture (ANFO) [12]. The main advantage of this oxidizer is that its decomposition

products are chlorine free and therefore it is considered to be a clean oxidizer. With the

increasing concern for the protection of environment the work is now being focused to

develop eco-friendly energetic composition [13]. Potassium permanganate has also been

used in wide variety of pyrotechnic compositions such as antimony/ potassium

permanganate [14]. Potassium nitrate is frequently used in various formulations and

considered to be safe as well. Moreover, it can also produce colored flame compositions.

Barium nitrate is an important oxidizer which is used in various pyrotechnic and

explosive formulations [15]. It is used for the production of green colours in different

pyrotechnics. It is also used as an additive in the thermite compositions. It is also used in

combination with TNT to make an explosive composition known as the “Baratol”.

However, it suffers from the difficulty of ignition and some work has been reported to

eliminate this drawback [16].

The investigation of thermal decomposition of the oxidizers is important to determine the

decomposition temperatures and evaluation of kinetic parameters. Activation energy is

47

perhaps the most important parameter in this regard to find out the amount of energy that

is required to be supplied to decompose any specific oxidizer. The non-isothermal

temperature scan of the oxidizer gives valuable information regarding its melting,

decomposition and the solid state phase transformations. These phase changes are very

important in determining the thermal stability. The phase changes are often associated

with the volume change due to the changes in the crystal structure.

Therefore, all the five oxidizers have been investigated by simultaneous thermal analysis

to determine the thermal events that take place until the oxidizer is finally decomposed.

The results have been compared on the basis of different parameters such as the

decomposition temperature, activation energy, phase transformations and the oxidizing

ability of the oxidizers. Most of the above mentioned oxidizers have been used in

different pyrotechnic/propellant compositions investigated in the subsequent chapters. It

is therefore necessary that the thermal and kinetic behaviour of these oxidizers is known

before they are added to different fuels to make energetic compositions. The knowledge

about different parameters of the oxidizers investigated in the present work has helped in

carrying out the main work concerning different aspects of pyrotechnics/propellants.

Simultaneous Thermal Analysis (STA) has been used to carry out the work presented in

this chapter. This method of thermal analysis is used to observe the physical or chemical

changes in the sample when it is systematically heated as per the temperature program

[17]. Thermal Analysis is being widely used by researchers around the globe for analysis

of all kinds of materials including energetic materials [18-22]. The experimental data has

been used to calculate the kinetic parameters by using Horowitz and Metzger method for

this part of the work. The main strength of this method is that it affords to determine the

activation energy in a single heating rate experiment.

3.2 Experimental Conditions

The information regarding the source from where these oxidizers were purchased has

already been described in detail in chapter 2 and therefore only the experimental

conditions have been discussed here. Simultaneous Thermal Analysis (STA) including

Differential Thermal Analysis (DTA) and Thermogravimetery (TG) has been used to

48

accomplish the work presented in this chapter. The sample mass in each case was kept

close to 10 mg and all the experiments were conducted at a heating rate of 10ºC to ensure

better comparison of results. The thermal behavior of all the five oxidizers was analyzed

to identify different thermal events such as melting, decomposition and phase

transformation. For this purpose, the samples were subjected to a thermal scan from room

temperature to the temperature where the decomposition of the sample takes place. The

experimental data obtained by thermogravimetery (TG) has been processed to calculate

the activation energy of all the oxidizers by using Horowitz and Metzger Method. The

kinetic computations have been carried out by using curve fitting software based on the

Horowitz and Metzger method to obtain the kinetic parameters of all the oxidizers.

The reaction atmosphere interferes with the sample and therefore the results may be

affected. Therefore, the nitrogen gas has been used to provide an inert atmosphere during

all the experiments. The flow rate of the nitrogen was maintained at 100ml/min till

completion of the experiment in each case. The crucibles made up of aluminum metal

were used to hold the sample and as a reference material for ammonium perchlorate,

ammonium nitrate and potassium permanganate. The alumina crucibles were used for the

analysis of barium nitrate and potassium nitrate. It is re-emphasized here that the sample

mass, heating rate and the nitrogen gas flow rate was kept same during the conduct of all

experiments for better comparison of results. The Diamond TG/DTA instrument by

Perkin Elmer has been used for the conduct of experimental work reported in this

chapter. The technique is also known as simultaneous thermal analysis because a single

sample is subjected to TG and DTA in the same experiment.

3.3 Thermal and kinetic behaviour of Ammonium Perchlorate

The investigation of the thermal behavior of ammonium perchlorate has been carried out

to monitor the thermal events including its decomposition. The DTA curve of ammonium

perchlorate shows three distinct peaks. Two peaks are exothermic and one peak is

endothermic in nature as shown in Figure 3.1. The first peak in the DTA curve is

endothermic and it appears at a temperature of approximately 242ºC. It represents the

solid state phase transformation of ammonium perchlorate and the crystal structure of the

49

AP changes from orthorhombic to cubic due to this phase change [23]. The ammonium

perchlorate after this phase change is generally called as cubic AP. This phase change

takes place at a sufficiently higher temperature and therefore does not affect the oxidizer

during storage and operation. The next peak is exothermic and appears at a temperature

of 309ºC. This peak represents the low temperature decomposition (LTD) stage of

ammonium perchlorate.

Figure 3.1 TG/DTA curve of ammonium perchlorate

Last and the most prominent exothermic peak appears at 388ºC and it denotes the high

temperature decomposition (HTD) stage of ammonium perchlorate. Most of the

researchers have reported this peak to be exothermic; however, some have reported that

the high temperature decomposition peak of ammonium perchlorate may be either

exothermic or endothermic depending on the reaction conditions [24]. In our case and

under the experimental conditions mentioned in this work the high temperature

decomposition peak of AP appears as exothermic.

The mass loss or the TG curve for ammonium perchlorate is also presented in Figure 3.1.

The curve clearly shows that the thermal decomposition of ammonium perchlorate takes

place in two distinct mass loss stages. The first mass loss stage begins near a temperature

of 280ºC and it indicates a mass loss of approximately 20 percent up to a temperature of

320ºC. Second and the more significant mass loss stage starts from 340ºC and completes

50

near 400ºC. This mass loss stage depicts almost complete mass loss of ammonium

perchlorate as shown in Figure 3.1.

The mass loss data has been used for the determination of kinetic parameters of the

ammonium perchlorate by Horowitz and Metzger method. The graph used for calculation

of activation energy of the AP is shown in Figure 3.2 below. The energy of activation for

the high temperature decomposition stage of AP calculated in this work is 155.3 kJ/mol.

Another associated thermodynamic parameter is the enthalpy of activation. For AP the

activation enthalpy has been found to be 152.2 kJ/mol. The thermal decomposition

reaction of ammonium perchlorate is reported by Conkling and Mocella in their book

“Chemistry of Pyrotechnics” [1].

2NH4ClO4 → N2+ 3H2O+2HCl+2.5O2 (3.1)

Equation (3.1) shows that the decomposition products of ammonium perchlorate include

nitrogen, oxygen, hydrochloric acid and water. The equation shows that the two moles of

ammonium perchlorate i.e. 235 g produce two and a half moles of oxygen i.e. 80 g.

Therefore, the oxygen released per gram of this oxidizer is 0.34 g.

Figure 3.2 Graph used for calculation of activation energy of ammonium perchlorate

51

The DTA results of AP show that it undergoes a solid state phase transformation near

242oC and its crystal structure changes from orthorhombic to cubic. The decomposition

of AP takes place in two stages i.e. low temperature decomposition stage and a high

temperature decomposition stage. The TG results confirm that the AP decomposition

takes place in two stages with a minor mass loss in the first stage and a major mass loss

in the second stage. The kinetic results show that the activation energy of high

temperature decomposition of ammonium perchlorate is nearly 155kJ/mol. The oxygen

balance of the AP is nearly 34 percent which is quite good. The oxygen balance of an

oxidizer is an important factor for assessing its suitability concerning utilization in

propellant or pyrotechnic compositions.

The AP is perhaps the most widely investigated oxidizer around the globe due to the fact

that it is often used in the composite solid propellant compositions and in pyrotechnics.

The catalysts are known to play an important role in modifying the thermal

decomposition of the AP and a lot of research work is being carried out in this regard.

The nano MgO catalyzed thermal decomposition of AP and changes in its kinetic

behaviour have been investigated in detail in the present work and reported in the next

chapter.

3.4 Thermal and kinetic behaviour of Ammonium Nitrate

Ammonium nitrate (AN) has been heated from room temperature to its complete

decomposition. The DTA curve of ammonium nitrate exhibits four major peaks as shown

in the Figure 3.3. All the peaks are endothermic in nature. The first peak appears at a

temperature of 56ºC. This peak represents the solid state phase transformation of

ammonium nitrate and its crystal structure changes [12]. The commonly encountered

operational and storage temperature ranges between -40oC to 70

oC in different parts of

the world. The phase change of ammonium nitrate is therefore considered to be

undesirable because it takes place within the operational and storage limits of the

propellants/pyrotechnics. The ammonium nitrate is not considered to be thermally very

stable due to this phase transformation.

52

Figure 3.3 TG/DTA curve of ammonium nitrate

The next endothermic peak appearing at 132ºC represents the second phase

transformation of ammonium nitrate. The next peak is very sharp and depicts the melting

of the ammonium nitrate at a temperature of 170ºC. The last peak is very broad it appears

at a temperature of 283ºC. This peak represents the thermal decomposition of ammonium

nitrate.

The TG curve of ammonium nitrate is also shown in Figure 3.3. The curve shows that the

thermal decomposition of ammonium nitrate takes place in a single stage. The TG curve

remains stable with no mass change till 200ºC after which the thermal decomposition of

ammonium nitrate starts. Almost all the mass is lost during the thermal decomposition of

the oxidizer.

The decomposition is rather slow initially but it is quite fast when the temperature

increases beyond 250ºC and results in the complete mass loss. The thermal

decomposition of ammonium nitrate is complete near 300ºC.

53

The graph representing the Horowitz and Metzger method is shown in Figure 3.4. The

energy of activation of ammonium nitrate calculated using TG data is found to be

98.8kJ/mol. The enthalpy of activation of ammonium nitrate has been calculated from its

activation energy and it is 96.5kJ/mol.

Figure 3.4 Graph for calculation of activation energy of Ammonium Nitrate

Gunawan et al. studied the kinetics of ammonium nitrate by using the Kissinger method

and reported that the activation energy of AN was 102.6kJ/mol. This value is very close

to the activation energy value of 98.8kJ/mol reported in the present work. It may be noted

that the kinetic calculations in our case are based on TG data whereas, the heat flow data

has been used to calculate the activation energy in the above mentioned work.

The decomposition of ammonium nitrate takes place in the following manner

NH4NO3 (l) → N2O(g)+2H2O(g) (3.2)

The nitrous oxide produced in this manner further decomposes to release nitrogen and

oxygen gases as per the following reaction.

N2O (g) → N2(g)+ ½ O2(g) (3.3)

54

The overall reaction of the decomposition of ammonium nitrate may be represented in the

following manner.

NH4NO3 (l) → N2(g)+ 2H2O(g) + ½ O2(g) (3.4)

The equation (3.4) shows that one mole of ammonium nitrate i.e. 80 g releases half mole

of the oxygen gas i.e.16 g. Therefore, the amount of oxygen liberated per gram of the

ammonium nitrate is 0.2 grams. The oxygen balance of ammonium nitrate is therefore

nearly 20 percent. The ammonium nitrate therefore has a positive oxygen balance

although it is lower than that of the ammonium perchlorate.

The thermal analysis of ammonium nitrate shows two phase transformations. One of the

phase changes takes place near 56oC and ammonium nitrate is considered to be thermally

unstable due to this phase change. The ammonium nitrate melts at 170oC. The

decomposition of ammonium nitrate takes place in single stage and results in almost

complete mass loss. The decomposition peak temperature of AN under these

experimental conditions is nearly 283oC.

The decomposition products of ammonium nitrate are benign to the atmosphere as they

are chlorine free. This is not the case for AP where the decomposition products contain

HCl which is further converted to chlorine gas and becomes harmful for the ozone layer.

Therefore, despite the inherent drawback such as phase changes and low burning rate AN

based propellants are gaining popularity and the research work is now being focused to

shift the first phase change of AN to a higher temperature and to increase the burn rate of

AN using some suitable modifiers[25].

3.5 Thermal and kinetic behaviour of Potassium Nitrate

The simultaneous thermal analysis of potassium nitrate has been carried to investigate its

thermal behaviour and decomposition. The TG/DTA curve showing the thermal

decomposition of potassium nitrate is shown in the Figure 3.5.

55

Figure 3.5 TG/DTA curve of potassium nitrate

Potassium nitrate has been heated up to 1100ºC and the thermal events have been

observed. Two prominent and very sharp endothermic peaks are visible in the Figure

below a temperature of 400oC. The first endothermic peak appears near a temperature of

130ºC. This peak represents the solid state phase transformation in the potassium nitrate

due to which its crystal structure changes from rhombic to trigonal. The next endothermic

peak is very sharp and it indicates the melting of the potassium nitrate. The peak appears

at a temperature of 334ºC. A series of overlapping endothermic peaks mark the

decomposition of the potassium nitrate. The Potassium nitrate begins to decompose near

590ºC and the decomposition takes place over a wide range of temperature.

The mass loss curve of the potassium nitrate is also presented in the Figure 3.5. The TG

curve remains almost stable from room to nearly 600ºC. Beyond this temperature, the

potassium nitrate begins to decompose and the TG curve shows mass loss. The rate of

decomposition is slow initially in the temperature range of 600 to 750ºC. The TG curve

shows a mass loss of about 30 percent in this temperature region. The thermal

56

decomposition takes place rapidly from 750ºC onwards and shows a major mass loss.

The potassium nitrate decomposes at a higher temperature as compared to the other

oxidizers investigated in this work.

Figure 3.6 Graph for calculation of activation energy of Potassium Nitrate

The experimental data obtained from TG analysis of potassium nitrate has been processed

for the calculation of the activation energy. The activation energy of the potassium nitrate

by using the Horowitz and Metzger method is found to be 278.2 kJ/mol. The value of

activation energy is the highest amongst the oxidizers investigated in this work. The

corresponding value of the enthalpy of activation in this case is 271.5kJ/mol. The

relevant kinetic graph plot is shown in Figure 3.6.

Stern et al. have reported that the decomposition of potassium nitrate takes place in the

following manner [26].

2 KNO3 → 2KNO2+ O2 (3.5)

The equation (3.5) shows that the potassium nitrate decomposes to potassium nitrite with

liberation of some oxygen. The potassium nitrite formed in this manner further

57

decomposes to release more oxygen along with the production of potassium oxide as

shown in the following equation.

2KNO2 → K2O+N2+3/2 O2 (3.6)

From equations (3.5) and (3.6), the overall reaction for the decomposition of ammonium

nitrate has been described as:

2 KNO3 → K2O+N2+5/2 O2 (3.7)

The equation (3.7) states that the decomposition of two moles of potassium nitrate (202

g) produces two and a half moles of free oxygen (80 g).The amount of oxygen released

per gram of this oxidizer is 0.396 g. The oxygen balance of this oxidizer is nearly 40

percent which is very high.

The investigation of thermal behaviour potassium nitrate shows that the oxidizer changes

its crystal structure near 132oC due to a solid state phase transformation. This phase

change however does not affect the oxidizer during storage because it is sufficiently

higher than the maximum ambient temperature of 70oC. The potassium nitrate melts near

334oC under these experimental conditions. The decomposition temperature of this

oxidizer is quite high and also the decomposition takes place over a wide range of

temperature. The activation energy of this oxidizer is very high and in the present work, it

was found to be 278.2 kJ/mol. The corresponding enthalpy of activation is 271.5 kJ/mol.

3.6 Thermal and kinetic behaviour of Potassium Permanganate

The TG/DTA curve of the thermal decomposition of potassium permanganate is shown in

the Figure 3.7. Thermal behavior of the potassium permanganate has been studied up to a

temperature of 550ºC. The heat flow curve of this oxidizer shows two different peaks.

The first peak is exothermic in nature and it appears at a temperature of 284ºC. The peak

begins near a temperature of 240ºC close to the melting point of potassium

permanganate. The peak is also associated with a mass loss and represents the

decomposition of potassium permanganate. The next peak is endothermic in nature and

represents a high temperature decomposition step of potassium permanganate near 511ºC.

58

The mass loss curve of potassium permanganate is also presented in Figure 3.7. The

potassium permanganate decomposes in two mass loss steps. The TG curve shows no

change in the sample mass till a temperature of 240ºC and beyond this temperature the

decomposition of potassium permanganate takes place with a mass loss of nearly18

percent. The mass loss curve becomes stable again till the second step of the mass loss

starts near 450ºC. Only about 2 percent more mass is lost in this stage.

Herbstein et al., have reported that the decomposition of potassium permanganate takes

place in the following manner [27].

10 KMnO4→2.65 K2MnO4+[2.35 K2O·7.35·MnO2.05]+6O2 (3.8)

The K2MnO4 produced in this manner is further decomposed and liberates more oxygen

as shown in equation (3.9).

10 K2MnO4→ 5·7 K3MnO4+ 0·5(2·9 K2O,8·6 MnO2·1)+ 3·40 O2 (3.9)

Figure 3.7 TG/DTA curve of potassium permanganate

59

It may be calculated from equation (3.8) and (3.9) that the decomposition of ten moles of

potassium permanganate i.e.1580 g produces 6.901 moles of oxygen i.e.

220.83g.Therefore, of oxygen liberated per gram of this oxidizer is 0.14 grams. The

oxygen balance of this oxidizer is nearly 14 percent.

3.7 Thermal and kinetic behaviour of Barium Nitrate

The DTA curve of barium nitrate has been presented in Figure 3.8. The heat flow curve

does not show any thermal event till a temperature of 592oC. The first peak is

endothermic in nature and represents the melting of barium nitrate near 592oC. Thermal

decomposition of the barium nitrate does not show a single peak and it rather appears as a

series of overlapping endothermic peaks. The major peak representing the decomposition

of barium nitrate takes place near 690oC. The barium nitrate does not undergo any solid

state phase changes as opposed to ammonium perchlorate, ammonium nitrate and

potassium nitrate. It is therefore considered to be thermally stable till its melting and

subsequent decomposition.

Figure 3.8 TG/DTA curve of barium nitrate showing the heat flow and mass loss

60

The TG curve of the barium nitrate is also shown in Figure 3.8. There is no mass loss

initially and the TG curve remains stable till melting of the oxidizer. The thermal

decomposition of barium nitrate is associated with some mass loss in the temperature

range of 580oC to 700

oC. The TG curve shows nearly 40 percent mass loss in the

temperature range mentioned above. The thermal decomposition of barium nitrate

completes near 700oC.

Figure 3.9 represents the kinetic plot of barium nitrate. The activation energy of the

barium nitrate by using the TG data has been calculated to be nearly 211kJ/mol. The

corresponding value of the enthalpy of activation has been found to be 205.5kJ/mol.

Figure 3.9 The kinetic plot of barium nitrate representing the Horowitz Method

The decomposition reaction of barium nitrate has been previously reported in literature

and it is presented below [15,28].

Ba(NO3)2 → BaO + 2NO+ O2 (3.10)

61

The equation above shows that one mole of barium nitrate (261 g) produces 3/2 moles of

oxygen (48 g). Therefore, the oxygen liberated by on gram of barium nitrate as per the

above equation is nearly 0.18 g.

It is important to mention here that some of the previous studies on barium nitrate have

reported that high temperature decomposition mechanism of barium nitrate results in the

production of nitrogen gas and the decomposition is described below [29].

Ba(NO3)2 → BaO + N2+ O2 (3.11)

The high temperature decomposition of barium nitrate liberates 5/2 moles (80 g) of

oxygen per mole of barium nitrate (261 g). The oxygen balance of barium nitrate in this

case is nearly 30 percent.

The investigation of barium nitrate shows that it is thermally very stable and does not

undergo any phase changes. There is no thermal event prior to its melting. The

decomposition peak temperature under these experimental conditions was 690oC. The

decomposition of barium nitrate is associated with nearly 40 percent mass loss. The

activation energy has been found to be nearly 211kJ/mol and the enthalpy of activation

was 205kJ/mol.

3.8 Comparative Analysis – Discussion

The data obtained from the experimental work has been summarized and presented in

Table 3.1. The results depict that ammonium perchlorate decomposes near 309oC. The

comparison with other oxidizers shows that this temperature is higher than the

decomposition temperature of ammonium nitrate and potassium permanganate but it is

significantly lower than the decomposition temperature of potassium nitrate and barium

nitrate. The high temperature decomposition temperature of AP is 388oC and it is also

lower than the decomposition temperatures of potassium nitrate and barium nitrate.

Ammonium perchlorate undergoes a solid state phase transformation near 242oC.

62

Table 3.1 Comparative data of the oxidizers

The activation energy of the high temperature decomposition stage of ammonium

perchlorate is nearly 155kJ/mol. The activation energy of AP is higher than that of

ammonium nitrate but lower than the activation energy of potassium nitrate and barium

nitrate. The activation energy of AP is neither too high nor too low. It is therefore

considered to be adequately sensitive as well as safe. The oxygen balance of AP is nearly

34 percent and it is higher than the oxygen balance of the ammonium nitrate, potassium

permanganate and barium nitrate.

The decomposition peak temperature of ammonium nitrate is 283oC and it is the lowest

amongst all the five investigated oxidizers. Due to low decomposition temperature, it is

relatively easy to decompose this oxidizer. Ammonium nitrate shows two phase changes

S.No Oxidizer Decomposition

peak temperature

1st Phase

change

temperature

Activation

Energy

(kJ/mol)

Oxygen

released per

gram

1

Ammonium

Perchlorate

309oC

242oC

155.3

0.34 grams

2 Ammonium

Nitrate

283oC 56

oC 98.8 0.20 grams

3 Potassium Nitrate 625oC 130

oC 278.2 0.40 grams

4 Potassium

Permanganate

284oC No phase

change

------ 0.14 grams

5 Barium Nitrate 690oC No phase

change

211.2 0.30 grams

63

prior to its melting and decomposition. The first phase change takes place near 56oC and

it is very undesirable. The temperature cycling through this phase change temperature

may cause volume changes in the composition based on this oxidizer. The volume change

may lead to development of cracks and ultimately bursting of the rocket motor containing

ammonium nitrate based propellant. The second phase change of ammonium nitrate

taking place near 130oC is considered to be safe. The phase changes of potassium nitrate

and ammonium perchlorate take place at 130oC and 242

oC respectively and they are safe.

The activation energy of ammonium nitrate is nearly 98.8kJ/mol and it is the lowest

amongst all the five oxidizers. Ammonium nitrate requires less energy to decompose and

the decomposition takes place at a lower temperature. The activation energy of potassium

nitrate is nearly three times higher than the activation energy of ammonium nitrate. The

oxygen balance of ammonium nitrate is nearly 20 percent. It is higher than the oxygen

balance of potassium permanganate but lower than the oxygen balance of ammonium

perchlorate, barium nitrate and potassium nitrate.

The decomposition peak temperature of potassium permanganate is nearly 284oC and it is

lower than the decomposition temperatures of ammonium perchlorate, potassium nitrate

and barium nitrate. The oxidizer can be easily decomposed due to its low decomposition

temperature as compared to the other oxidizer. The main advantage of the potassium

permanganate as an oxidizer is that it is thermally very stable and does not undergo any

solid state phase change unlike the other oxidizers such as ammonium nitrate, potassium

nitrate and even ammonium perchlorate.

The thermal decomposition of the potassium nitrate begins near 590ºC and the

decomposition is represented by overlapping endothermic peaks over a wide range of

temperature. The first endothermic peak appears near 625oC. The decomposition

temperature of potassium nitrate is very high as compared to ammonium perchlorate,

potassium permanganate and ammonium perchlorate. The oxidizer exhibits a solid state

phase transformation near 130oC. This phase change however takes place at a

temperature higher than the maximum possible ambient temperature and therefore does

not influence its thermal stability. The activation energy of potassium nitrate is nearly

278kJ/mol and it the highest amongst all the investigated oxidizers. The activation energy

64

and the decomposition temperature of potassium nitrate are both very high. It means that

the potassium nitrate is difficult to decompose and requires a lot of energy for its

decomposition but at the same time it is considered to be safe. The oxidizer has a very

high oxygen balance of nearly 40 percent which is highest amongst the five oxidizers.

Barium nitrate also decomposes at a very high temperature and the main decomposition

peak appears near 690oC. This value is higher than the peak temperatures of the other

oxidizers investigated in this work. The decomposition of barium nitrate is endothermic

in nature. Barium nitrate is thermally very stable and it does not show any phase change.

The activation energy of barium nitrate calculated in the present work is 212kJ/mol. This

value is higher than the activation energies of ammonium nitrate, potassium

permanganate and ammonium perchlorate. The oxidizer is difficult to decompose due to

high decomposition temperature and high activation energy. The oxygen balance of

barium nitrate is nearly 30 percent and it is much higher than the oxygen balance of

potassium permanganate and ammonium nitrate.

3.9 Conclusions

Thermal and kinetic behaviour of five different oxidizers including ammonium

perchlorate, ammonium nitrate, potassium nitrate, barium nitrate and potassium

permanganate has been investigated under exactly similar reaction conditions. The

comparison of phase transformations, thermal decomposition, activation energy and

oxidizing ability of the above mentioned oxidizers has been carried out to identify the

pros and cons of each individual oxidizer.

The thermal and kinetic investigation of ammonium perchlorate showed that the oxidizer

exhibits a solid state phase transformation near 242oC. AP decomposition takes place in

two stages i.e. low temperature decomposition stage and a high temperature

decomposition stage. Both the decomposition stages are exothermic in nature. The

activation energy of the high temperature decomposition stage of AP is nearly155 kJ/mol.

The oxygen balance of the AP is nearly 34 percent and considered to be fairly good.

Ammonium nitrate undergoes two phase transformations. The first one appears at a

temperature of 56oC and considered to be a concern for the thermal stability. The oxidizer

65

decomposes in a single stage and the decomposition peak temperature of ammonium

nitrate is 283oC.The decomposition of ammonium nitrate is endothermic in nature. The

activation energy of ammonium nitrate is nearly 99kJ/mol. The oxygen balance of

ammonium nitrate is approximately 20 percent. The thermal and kinetic behaviour of

potassium nitrate showed that the oxidizer undergoes a solid state phase transformation

near 130oC. The thermal decomposition of potassium nitrate is endothermic in nature and

takes place over a wide range of temperature. The activation energy of the oxidizer is

very high and it is 278kJ/mol. The oxygen balance of potassium nitrate is very good i.e.

40 percent.

The potassium permanganate is thermally stable and does not undergo any phase

transformation. The thermal decomposition temperature of the oxidizer is 284oC. The

decomposition of potassium permanganate is exothermic in nature. The oxidizer has

relatively low oxygen balance of nearly 14 percent. The thermal and kinetic behaviour of

barium nitrate revealed that it is thermally stable and does not undergo any phase change.

The decomposition peak temperature of this oxidizer is very high i.e. 690oC. The

decomposition of barium nitrate is endothermic in nature. The activation energy of

barium nitrate is very high and it nearly 211kJ/mol. The oxygen balance of barium nitrate

is good and it is close to 30 percent.

References

[1] J.A. Conkling, C. Mocella, Chemistry of Pyrotechnics: Basic Principles and

Theory, CRC Press, 1985.




[4] I. Tuukkanen, E.L. Charsley, S. Goodall, P.G. Laye, J.J. Rooney, T.T. Griffiths,

H. Lemmetyinen, Thermochimica acta 443 (2006) 116.


[6] V.P. Sinditskii, V.Y. Egorshev, A.I. Levshenkov, V.V. Serushkin, Propellants,

Explosives, Pyrotechnics 30 (2005) 269.

[7] V. Boldyrev, Thermochimica Acta 443 (2006) 1.

66

[8] O. Biblarz, G.P. Sutton, Rocket Propulsion Elements, Wiley, 2011.

[9] E. Santacesaria, S. Carra, Reaction Kinetics and Catalysis Letters 5 (1976) 317.

[10] M. Rajić, M. Sućeska, Journal of thermal analysis and calorimetry 63 (2000) 375.

[11] M. Zou, X. Jiang, L. Lu, X. Wang, Journal of hazardous materials 225 (2012)

124.

[12] R. Gunawan, D. Zhang, Journal of hazardous materials 165 (2009) 751.

[13] M.K. Abhay, P.D. Devendra, Research Journal of Chemistry and Environment 14

(2010) 94.

[14] M.E. Brown, K.C. Sole, M.W. Beck, Thermochimica acta 89 (1985) 27.

[15] A. Eslami, S. Hosseini, S. Pourmortazavi, Fuel 87 (2008) 3339.



[17] M.E. Brown, Introduction to thermal analysis: techniques and applications,

Springer, 2001.




119.


(2007) 27.



[22] M. Shamsipur, S.M. Pourmortazavi, S.S. Hajimirsadeghi, Combustion Science

and Technology 183 (2011) 575.

[23] A. Said, R. Al-Qasmi, Thermochimica acta 275 (1996) 83.

[24] S. Vyazovkin, C.A. Wight, Chemistry of materials 11 (1999) 3386.

[25] C. Oommen, S. Jain, Journal of hazardous materials 67 (1999) 253.

[26] K.H. Stern, E. Weise, High Temperature Properties and Decomposition of

Inorganic Salts. Part 2. Carbonates. DTIC Document, 1969.

[27] F. Herbstein, M. Kapon, A. Weissman, Journal of thermal analysis 41 (1994) 303.

67

[28] O.Q. González, J.L. Burell, E. Martínez, C. Barrera, R. Acosta, N. Ruiz, Revista

Cubana de Química 16 (2010).


1111.

68

Chapter 4

Modification of barium nitrate and

ammonium perchlorate

4.1 Synthesis of micro porous barium nitrate with improved ignition

reliability

The work presented in this chapter reports the synthesis and characterization of a micro

porous barium nitrate that may be used effectively as an effective pyrotechnic oxidizer.

The pyrotechnic compositions commonly comprise some solid fuel and an oxidizer. The

oxidizers are important ingredient of pyrotechnic/propellant compositions and provide

oxygen for the ignition of the fuel. The pyrotechnic compositions are used to obtain

certain special effects that include heat, light sound and smoke. These effects are not only

used in the fireworks but in certain important military applications [1,2].

4.2 Introduction

Quite a lot of compounds are being used as oxidizers in various pyrotechnic/ propellant

compositions. Some of these oxidizers include potassium chlorate, potassium perchlorate,

strontium nitrate, barium nitrate, ammonium nitrate and ammonium perchlorate to name

a few [3-7]. Potassium chlorate had been one of the most widely used oxidizer in the field

of pyrotechnics for quite some time. It has advantage of ease and reliability of ignition

and it is relatively cost effective as compared to some other oxidizers [8,9]. However,

potassium chlorate suffers from some serious safety concerns and the pyrotechnic

compositions containing this oxidizer are considered to be unsafe. The use of this

oxidizer has caused some accidents in the past due to which its use in pyrotechnics has

been restricted and even banned in several parts of the world. Compared to potassium

chlorate, the barium nitrate is considered to be relatively much safer. But there are some

69

intrinsic draw backs in the combustion behaviour of barium nitrate [10]. The pyrotechnic

compositions based on barium nitrate are difficult to ignite and they can easily flame out

even after ignition. These factors pose some serious questions regarding the reliability of

compositions that contain this oxidizer. The oxygen content, reliability and safety are

some of the important factors that are considered while assessing the suitability of any

oxidizer concerting its use in the pyrotechnic/propellant compositions [11]. The oxidizing

ability of the barium nitrate is fairly well and makes it suitable for its use as an oxidant

for pyrotechnics. While, barium nitrate is quite safe as a pyrotechnic oxidizer, its ignition

reliability needs improvement. The ignition reliability of barium nitrate can be increased

by different methods. One way is to use some thermal decomposition catalysts that will

sensitize the oxidant and improve its reliability and ease of ignition [12-14]. However, in

the present work, inorganic vesicants have been used to modify the performance of the

barium nitrate by producing micro pores which render the oxidant as more sensitive and

easy to ignite [15,16]. The vesicants that have been used for the modification of barium

nitrate are sodium bicarbonate, potassium carbonate and ammonium perchlorate. The

micro porous barium nitrate modified by this method has been characterized using X-ray

diffraction (XRD) and scanning electron microscope (SEM). Two pyrotechnic mixtures

were formulated by using pure and modified barium nitrate as oxidant and the aluminum

powder as a fuel. The ignition behaviour of pyrotechnic mixture containing the vesicant

modified barium nitrate has not been reported earlier to the best of our knowledge.


Analytical grade barium nitrate by Scharlau (Spain), potassium carbonate by Merck

(Germany), sodium bi-carbonate by Sigma Aldrich (Germany) have been used in the

present work to prepare the modified barium nitrate. The defence grade ammonium

perchlorate was purchased from National Development Complex (Pakistan). The

thermally saturated solution of barium nitrate was prepared and approximately five per

cent inorganic vesicant was added to this solution in each case. The above mentioned

solution was heated and the contents were evaporated by careful heating and allowed to

crystallize. The crystals formed in this manner were then heated in the furnace to

70

decompose the vesicant. The final product was finely ground and sieved through 100

mesh sieve. The characterization of the modified barium nitrate and pure barium nitrate

was carried out by different analytical techniques for comparison. The XRD instrument

by STOE Germany has been used for the structural analysis and the sample scan range

was from 10o

to 70o. Scanning electron microscope JSM- 6490 has been used in the

present work and the micrographs were taken at various magnifications. The thermal

behaviour and ignition of the pyrotechnic compositions containing pure and modified

barium nitrate was carried using the Diamond TG/DTA instrument of Perkin Elmer. The

experiments were performed at a heating rate of 10oC /min for the pyrotechnic mixtures.

The nitrogen gas has been used as an inert medium and its flow rate was maintained at

100ml/min. The crucibles made up of alumina pans were used to hold the sample and the

reference.

4.4. Results and Discussion

4.4.1 SEM Analysis

The SEM micrographs of the pure barium nitrate are shown in Figure 4.1 at two different

magnifications. It is clear from the Figure that pure barium nitrate is solid in shape and

its surface feature is quite smooth and plain. There are no pores or cracks in the

composition. The SEM micrographs of the vesicant modified barium nitrate are shown in

4.2. Three different vesicants have been used to modify the barium nitrate and create

pores in it. These vesicants include potassium carbonate, ammonium perchlorate and

sodium bi-carbonate. It is evident from Figure 4.2 that the morphology and surface

feature of barium nitrate modified with each vesicant is different in all the three cases.

The modified barium nitrate in each case is porous and fluffy with relatively uneven

surface. The surface is quite different in appearance as compared to the pure barium

nitrate. It can be seen from Fig. 4.2 (a, b, c) that barium nitrate modified with different

vesicants is porous and size of the pores and the number of pores is different in all the

cases. Figure 4,2 (a) shows that barium nitrate modified with the potassium carbonate as

a vesicant has the maximum number of pores, however, the pore size is comparatively

smaller than other vesicants.

71

Figure 4.1 SEM micrographs of pure barium nitrate

The surface feature is relatively rough and uneven as opposed to the smooth and even

surface of pure barium nitrate. The surface of barium nitrate modified with sodium

bicarbonate is shown in Figure 4.2 (b) and it also shows some distinct pores whereas the

surface feature has been least effected in this case due to the modification. The surface is

not as smooth as the pure barium nitrate but better than the one modified with potassium

carbonate. Barium nitrate modified with ammonium perchlorate is shown in Figure 4.2

(c). It has relatively less number of pores as compared to other versions of vesicant

72

modified barium nitrate. The size of the pores is larger as compared to other vesicants in

this case.

Figure 4.2(a) SEM micrographs of barium nitrate modified

with potassium carbonate

Figure 4.2(b) SEM micrographs of barium nitrate modified

with sodium bicarbonate

73

The measurement of the pore size from the SEM images shows that all the pores are in

the range of the micrometers and modified barium nitrate is regarded as a micro porous

structure. The porous material in general is more reactive as compared to the solid

material due to its larger exposed area, so the modified barium nitrate is likely to be more

reactive. Moreover, physical properties of the barium nitrate modified with different

vesicants are expected to be different.

Figure 4.2(c) SEM micrographs of barium nitrate modified

with ammonium perchlorate

4.4.2 Bulk density

Bulk density of pure barium nitrate and the modified versions has been calculated using

graduated cylinder method. The bulk density of all the four versions of barium nitrate has

been presented in Table 4.1. The data show that the bulk density of the barium nitrate

decreases after the modification in all the three cases, however to a different extent.

Numerous factors can influence the bulk density of any material. One of the most

important factors is the entrapped air and the interstitial air. The bulk density is found to

74

decrease in all the modified versions of barium nitrate. It is obvious that the development

of pores in the barium nitrate is the main reasons for the decrease in the bulk density.

Table 4.1 Bulk density of pure and modified barium nitrate

Barium nitrate modified with potassium carbonate has the lowest bulk density and the

percentage decrease in this case is nearly 12 %. The bulk density of barium nitrate

modified with sodium bi carbonate and ammonium perchlorate decreased by

approximately 10 % and 4 % respectively.

4.4.3 XRD Analysis

XRD analysis of pure barium nitrate and the modified samples has been carried out to see

different type of structural changes that may have resulted due to the modification. The

XRD data show that the crystal structure of the barium nitrate before and after the

modification was cubic and there was no change in any of the modified samples. The

XRD spectra of pure and the modified barium nitrate have been shown in Fig. 4.3 and 4.4

respectively (Miller indices have been marked). A total of fifteen distinct peaks were

observed in the XRD spectra of pure barium nitrate out of which five main diffraction

peaks were observed at 2θ position of 18.96o, 21.93

o, 36.6

o, 38.5

o and 59.15

o. It is

observed from Fig. 4.4 (a) that the total number of diffraction peaks decreased in barium

Inorganic

Vesicant

Mass of the

sample (g)

Volume of the

sample (cm3)

Bulk density

(g/cm3)

Percent decrease

in bulk density

Pure barium

nitrate

9.947 5 1.98 Nil

Potassium

carbonate

8.715 5 1.74 12.12

Ammonium

perchlorate

9.561 5 1.91 3.66

Sodium bi-

carbonate

8.951 5 1.79 9.59

75

Figure 4.3 XRD spectra of pure barium nitrate

Figure 4.4 (a) XRD spectra of barium nitrate modified with ammonium perchlorate

76

nitrate modified with the ammonium perchlorate, however, the peak position of the main

diffraction peaks did not change. The intensity of main diffraction peaks decreased

significantly which meant that the barium nitrate after modification with ammonium

perchlorate became less crystalline. The average crystallite size of barium nitrate also

decreased after modification with ammonium perchlorate (Table 4.2).

Table 4.2 Lattice parameters of pure and modified barium nitrate

Figure 4.4 (b) XRD spectra of barium nitrate modified with potassium carbonate

Composition Cell Parameters Volume of the

Cell 106 pm

3

Crystallite

size nm a (nm) b (nm) c (nm)

Pure barium nitrate

81.184

81.184

81.184

535.07

79

Potassium carbonate

modified

81.260 81.260 81.260 536.58 53.6

Ammonium

perchlorate modified

81.190

81.190

81.190

535.19

73.2

sodium bi-carbonate

modified

81.100

81.100

81.100

533.41 35.3

77

For the barium nitrate modified with the potassium carbonate, the number of peaks and

the peak positions did not change as shown in Figure 4.4(b). However, in this case the

intensity of some of the main diffraction peaks increased significantly and the modified

barium nitrate became more crystalline as opposed to the one modified with ammonium

perchlorate. The average crystallite size, however, decreased in this case as well. The

XRD spectra of barium nitrate modified with sodium bicarbonate is shown in Figure 4.4

(c). It is seen from the spectra that the relative intensity of the main diffraction peaks

decreased greatly which meant that the modification decreased the crystallinity of the

barium nitrate as in case of ammonium perchlorate.

Figure 4.4 (c) XRD spectra of barium nitrate modified with sodium bi carbonate

The crystal structure of the modified versions of barium nitrate remained the same in all

the three cases showing that the modification did not affect the geometry of the crystal.

The crystallite size has reduced greatly in all the modified versions of barium nitrate,

therefore, it is expected that the modification has increased the reactivity of the barium

nitrate. The information regarding different cell parameters as well as the values of

78

crystallite size have been presented in Table 4.2. The data shows that no significant

variation has occurred in the value of the cell parameters due to the modification of

barium nitrate. The parameters a, b and c have not changed much and the volume of the

cell also remains the same.

The reduction in the crystallite size of the grain exposes more surface area and therefore

makes it more reactive. The most significant variation has been seen for barium nitrate

modified with sodium bi carbonate where the average crystallite size has reduced from 79

nm to 35.3 nm.

4.4.4 Thermal Analysis of binary pyrotechnic mixtures

Thermal analysis is the most widely used technique to investigate the thermal

decomposition and ignition of pyrotechnic mixtures [17-19]. The DTA curve of

pyrotechnic mixture containing micro sized aluminum powder and pure barium nitrate is

shown in Fig. 4.5.

Figure 4.5 Heat flow curves of aluminum with pure

and modified barium

79

The heat flow curve of the pyrotechnic mixture made with modified barium nitrate is also

presented in Fig. 4.5. The DTA curve of the pyrotechnic mixture consisting of aluminum

powder and pure barium nitrate shows an endothermic peak near 583oC close to the

melting point of the barium nitrate showing that the barium nitrate in the pyrotechnic

mixture melts near this temperature. Just after melting, the mixture decomposes

exothermically near a temperature of 635oC. This temperature is regarded as the ignition

temperature of the pyrotechnic composition based on pure barium nitrate. On the other

hand, it is clearly seen from the DTA curves that pyrotechnic composition formulated

with modified barium nitrate shows only a single exothermic peak and ignites at a

temperature of 606oC. Since this temperature is much lower than the ignition

temperature of pure barium nitrate based composition so it may be concluded that the

ignition behaviour of modified barium nitrate has improved.

4. 5 Conclusions

The micro porous barium nitrate has been synthesized using three different inorganic

vesicants to improve its ignition reliability as a pyrotechnic oxidant. The SEM results

reveal that the surface features and morphology of the barium nitrate has changed after

the modification. The results indicate the presence of micro sized pores in the modified

versions of barium nitrate and it has a porous structure, whereas, pure barium nitrate has

a very solid and smooth surface. It was found that the barium nitrate modified with

potassium carbonate has the highest number of pores. The exposed area of vesicant

modified barium nitrate, therefore, increases due to the production of micro pores and

improves the reactivity of the pyrotechnic mixture. The bulk density of barium nitrate has

been found to reduce in all the cases after the modification. This reduction in the bulk

density is attributed to the production of pores which result in lowering of the bulk

density. The crystal structure and the cell parameters of the barium nitrate did not change

after the modification. The crystallite size of the barium nitrate reduced after

modification with vesicants. The pyrotechnic composition based on aluminum powder

and modified barium nitrate ignited at a lower temperature as compared to the one

80

formulated with pure barium nitrate showing that the reactivity of modified barium

nitrate has increased as a pyrotechnic oxidant. The results obtained in the present work

encourage the use of vesicant modified barium nitrate as a safe and reliable oxidant for

the pyrotechnic applications.

4.6 Improvement in the thermal and kinetic behaviour of

ammonium perchlorate using nano magnesium oxide

catalyst

The work presented in this section of the chapter is focused on the improvement of the

thermal cum kinetic behaviour of ammonium perchlorate. The thermal behaviour of

ammonium perchlorate is known to be influenced by the addition of various catalysts.

The magnesium oxide nano particles have been used as thermal decomposition catalyst to

increase the reactivity of the ammonium perchlorate.

4.7 Introduction

Ammonium perchlorate finds extensive use as an inorganic oxidizer in composite solid

propellants. It is one of the most frequently researched oxidizer due to its high utility in

various propellant and pyrotechnic compositions [20,21]. A remarkably large quantity of

this oxidizer is also used in space shuttles [22]. More than million pounds of ammonium

perchlorate is consumed in space shuttles in a single launch. The nature of the thermal

decomposition of the ammonium perchlorate plays an important role in the combustion

behavior of the propellants. The thermal decomposition of ammonium perchlorate is

known to exhibit significantly high sensitivity to different kinds of additives. The

additives are assumed to influence the thermal decomposition and the reactivity of AP

based compositions [23]. Effect of nano metals and nano sized transition metal oxides on

the thermal decomposition of ammonium perchlorate has therefore been an area of

interest for the researchers concerned with energetic materials [24-31].

81

The effect of addition of MgO nano particles on the thermal as well as the kinetic

behavior of ammonium perchlorate has been investigated in the present work. The nano

sized metal oxides are considered to be more efficient as a catalyst due to relatively

greater exposed area as compared to the micro sized particles [32]. The MgO

nanoparticles having an average size of 20 to 30 nm have been used as a catalyst in this

work to increase the reactivity of the AP. The nano particles were characterized by using

Scanning Electron Microscope (SEM) and X-ray diffraction. The decomposition kinetics

of AP and different energetic composition containing AP strongly depends upon the

morphology and size of the nano additives. The catalytic effect of nano MgO on the

thermal decomposition of AP has been reported previously by Guorong Duan et al. [33].

The earlier work reports the effect of different percentages of nano magnesium oxide on

the thermal decomposition of AP and shows that two decomposition stages completely

merge with each other by using 4 percent MgO as a catalyst. At lower percentages of

MgO, the decomposition stages do not completely merge into each other. The present

work is an attempt to increase the reactivity of ammonium perchlorate and focuses on

calculating the change in the kinetic parameters of the catalyzed and non-catalyzed AP.

The kinetic parameters of MgO catalyzed AP are not reported in the earlier mentioned

work.

Thermal analysis is an effective tool for the study of energetic materials and their

ingredients. It is widely used to obtain thermal and kinetic data to gain an insight into the

decomposition reactions [34-36]. The kinetic data gives useful information regarding the

thermal decomposition and reactivity of any material. Simultaneous thermal analysis has

been used to accomplish the experimental work presented in this part. The kinetic

parameters of pure and catalyzed ammonium perchlorate have been calculated by using

non isothermal approach based on Kissinger method.

The focus in this work has been to determine the kinetic parameters such as activation

energy, frequency factor, reaction rate constant as well as the enthalpy of activation. This

work attempts to elucidate the decomposition mechanism of the pure AP and the one

catalyzed with a small amount of nano MgO on the basis of data obtained during the

82

analytical experiments. Comparative analysis of the pure and the catalyzed ammonium

perchlorate has been carried out to monitor the changes in thermal and kinetic

parameters. The experimental results indicate that nano MgO has made a strong catalytic

effect on the thermal decomposition of ammonium perchlorate and reduced the two

distinct stages of decomposition of ammonium perchlorate to only one stage. Moreover,

the reactivity of AP has also increased.


The defence grade ammonium perchlorate and MgO nano powder have been used to

carry out this part of the work. The MgO and AP and were mixed in the weight ratio of 4

percent and 96 percent respectively. Thorough mixing of the ingredients was carried out

and the samples were pre heated for six hours before the conduct of experiments to make

sure that there was no moisture in the sample. MgO nano particles were characterized

using Scanning Electron Microscope (SEM) and X-ray diffraction. The XRD instrument

by STOE Germany is used for the analysis. The sample was scanned from 10o to 70

o.

JSM- 6490 scanning electron microscope was used to get the micrographs of the

nanoparticles. The methods of thermogravimetery (TG) and differential thermal analysis

(DTA) have been used to study the decomposition process of pure and catalyzed

ammonium perchlorate. Diamond TG/DTA instrument by Perkin Elmer has been used for

simultaneous thermal analysis of the sample. The instrument performs simultaneous TG

and DTA measurements on one and the same sample. The aluminum crucibles were

used to hold the samples for the analysis. The nitrogen gas was used for producing inert

atmosphere during the conduct of all the experiments. To investigate the effect of heating

rate on decomposition peak temperature of ammonium perchlorate catalyzed with nano

MgO, the experiments were conducted at four different heating rates i.e. 2oC/min,

6oC/min, 10

oC/min and 20

oC/min. The samples were heated up to 500

oC till the

completion of high temperature decomposition reaction using multiple heating rates. The

peak temperature data obtained from heat flow curves has been used to calculate the

kinetic parameters of pure and catalyzed ammonium perchlorate using Kissinger and

83

Ozawa methods. The methods have been described in detail in chapter 2, therefore only

the final equations are presented here.

4.8.1 Kinetic Methods

The details of the kinetic methods are given in Chapter 2 and are not discussed here. The

final form of the Kissinger equation is given below[37] :

(

)

(4.1)

In the equation (1), is the peak temperature, is the heating rate, “A” represents

frequency factor, “R” is the gas constant, “T” represents sample temperature, “n”

represents reaction order and “E” represents activation energy in kJ/mol . When

activation energy “E” is known, the frequency factor is obtained in the following manner

[38,39]:

[ (

)]

(4.2)

And finally, the reaction rate constant can be found by using the famous Arrhenius

equation.

k = A (

) (4.3)

The representative equation of the Ozawa Method is given below[40]:

(

( )) (

) (4.4)

84

In the above equation (4.4), “Ea” is the activation energy in kJ/mol, “β” is the heating rate

and “Tp” is the peak temperature corresponding to a specific heating rate and “R” is the

universal gas constant.

4.9 Results and Discussion

4.9.1 Analysis of Nano particles

The SEM image of the nano sized MgO is shown in Figure 4.6. The nano particles show

good regularity in terms of their morphology and they are found to be very well

dispersed. The particles are elliptical in shape and not perfect spheres. The average

diameter of the elongated side of particles measured from SEM images is between 20 to

30 nm. The measurements show that the average diameter of the shorter side of these

particles is between 15 to 20 nm. The MgO powder, therefore, can be truly regarded as

nano sized and considered to be very suitable for the catalytic activity.

Figure 4.6 SEM micrograph of nano sized MgO powder used

for catalyzing AP

85

The XRD pattern of nano sized MgO powder is shown in Figure 4.7. Five main

diffraction peaks can be seen in the spectra and these peaks are located at 37.92o, 42.56

o,

62.19 o

, 73.26 o

, 78.42 o

. These peaks typically represent MgO and the peak data is in fair

agreement with the reference pattern No 01-074-1225. The Miller Indices have been

assigned to all the five peaks as shown in the Figure. The peaks are in general broad,

showing that the particle size is fairly small. The crystallite size of the MgO particles has

been calculated using Scherer formula for three different faces and the average crystallite

of the sample was found close to 9nm.

Figure 4.7 XRD spectra of the nano sized MgO powder

4.9.2 Analysis of Pure AP

Heat flow curve of the pure ammonium perchlorate shows three major peaks representing

three distinct events (Figure 4.8). First endothermic peak of ammonium perchlorate

appears near a temperature of 241 ºC and corresponds to solid state phase transformation

during which the crystal structure of ammonium perchlorate changes from orthorhombic

to cubic [41]. Next two peaks appearing in the curve are both exothermic in nature. The

first exothermic peak appears at 310.7 ºC and it marks the low temperature

86

decomposition stage of ammonium perchlorate. The second exothermic peak appears at

384.1 ºC and marks the high temperature decomposition stage of ammonium perchlorate.

Figure 4.8 also presents the weight loss curve of ammonium perchlorate. The weight loss

curve shows two events as opposed to three events exhibited in the heat flow curve. It is

due to the fact that the first endothermic peak corresponding to the phase change is not

associated with any weight loss.

Figure 4.8 TG/DTA curve of pure ammonium perchlorate

It is also confirmed from TG curve that decomposition of pure ammonium perchlorate is

a two stage process. The first weight loss stage is between 280 ºC and 320 ºC resulting in

more than 20 percent weight loss of the sample. The second weight loss stage starts at

340 ºC and ends close to 400 ºC causing complete decomposition of ammonium

perchlorate. Ammonium perchlorate decomposes to form various products. The

decomposition equation of ammonium perchlorate has been reported as follows [21].

2NH4ClO4 → N2 + 3H2O+2HCl+2.5O2 (4.5)

87

The effect of heating rate on the heat flow curve of AP has been shown in Figure 4.9. The

Figure shows that the first endothermic peak shows only a slight variation due to the

Figure 4.9 Effect of heating rates on the thermal decomposition of

pure ammonium perchlorate

heating rates; however, there is a noticeable change in low temperature as well as high

temperature decomposition peak temperature. These peaks shift to higher temperatures

with increase in the heating rates. The onset and peak temperatures for low temperature

and high temperature decomposition stages change significantly when the heating rates

are increased from 2oC/min to 20

oC/min (Table 4.3).

Table 4.3 Effect of heating rate on decomposition peaks

Heating Rate

(oC/min)

Low Temperature Stage High Temperature Stage

Onset Temp (oC) Peak Temp (

oC) Onset Temp (

oC) Peak Temp (

oC)

2

258.7

276.3

326.4

357.9

6 267.8 299.1 338.6 374.2

10 283.9 310.7 342.2 384.1

20 299.5 332.9 357.6 412.1

88

The data presented in Table 4.3 has been used to calculate the kinetic parameters of pure

AP. Representative graph for calculation of the kinetic parameters of low stage

decomposition of AP based on Kissinger method is shown in Figure 4.10(a) while the

graph for calculation of the kinetic parameters of the high temperature decomposition

stage has been shown in Figure 4.10 (b).

Figure 4.10 (a) Kissinger Plot for Low stage decomposition of pure AP

The data points of the plot of ln β/Tp2

against the reciprocal peak temperature ( 1/Tp) fit

into a straight line and the slope of this line gives the value of the activation energy. The

activation energy of AP by Kissinger method has been found to be 103.7kJ/mol and

138.1kJ/mol for low temperature and high temperature decomposition stages

respectively. The data points obtained from the plot of ln β/Tp2

aginst (1/Tp) for the high

temperature decomposition stage also fit into a straight line and the activation energy is

determined from the slope of this line. The corresponding values of frequency factor are

1.17x 107

sec-1

and 6.13x 10

8 sec

-1 for low and high temperature decomposition stages

89

respectively by Kissinger Method. The rate of reaction constant for low temperature stage

is 6.1 x 10-3

sec-1

and for the high temperature stage is 6.41 x 10-3

sec-1

.

Figure.10 (b) Kissinger plot for high stage decomposition of pure AP

The values of activation energy, frequency factor, reaction rate constant and enthalpy of

activation have been presented in Table 4.4 for both the stages separately.

Table 4.4 Summary of Kinetic data of non-catalyzed ammonium perchlorate

Kinetic

Method

Ea (kJ/mol) A* (sec-1

) k(sec-1

) ∆H#

(kJ/mol)

Low High Low High Low High Low High

Kissinger

103.7

138.1

1.17x 107

6.13x 108

6.1 x 10-3

6.41 x 10-3

99.1

132.8

Ozawa 107.6 141.8 2.65x 107 1.01 x10

9 6.2 x 10

-3 6.85 x 10

-3 102.7 136.3

* Values of A, k and ∆H#

have been reported for peak temperature corresponding to heating rate

of 10oC/min temperature

90

The representative graph of Ozawa method for calculation of the kinetic parameters of

low temperature decomposition of AP has been shown in Figure 10 (c).

Figure 4.10 (c) The Ozawa plot for low temperature decomposition

stage of pure AP

The data points obtained from the plot of log β against (1000/Tp) fit into a straight line

and the activation energy is determined from the slope of this line. Similarly, the Ozawa

plot for high temperature decomposition stage of ammonium perchlorate has been

presented in Figure 4.10 (d). The results of the kinetic evaluation of pure AP by Ozawa

method indicate that the activation energy of AP is 107.6 kJ/mol for low temperature

decomposition stage and it is 141.8kJ/mol for high temperature decomposition stage.

These values of the activation energy are slightly different from the ones calculated by

the Kissinger method. The corresponding values of frequency factor, rate of reaction

constant and enthalpy of activation have been presented in Table 4.4 for both the stages

separately for comparison.

91

Figure 4.10 (d) The Ozawa plot for high temperature decomposition

stage of pure AP

Figure 4.10(d) Ozawa plot for high temperature decomposition stage of ammonium

perchlorate

4.9.3 Analysis of Catalyzed AP

The heat flow curve of AP catalyzed with 4 percent MgO nano particles is shown in

Figure 4.11. This curve is quite different from the one for pure AP. A total of two peaks

can be seen from the Figure as opposed to three peaks in case of pure AP. First

endothermic peak which represents the phase change of AP remained unchanged and

appeared almost at the same temperature showing that the catalyst did not interfere much

with the phase transformation of AP. The second peak which is exothermic in nature

represents the rapid decomposition of AP due to addition of MgO. This peak appears at a

temperature of 364oC and shows that the two stages of AP decomposition have now

reduced to one. The decomposition of the catalyzed AP takes place in a single stage at a

temperature which is higher than the low temperature decomposition stage and lower

than the high temperature decomposition stage. The TG curve of the catalyzed AP also

92

confirms this result and a sharp decline in the weight of the sample can be seen in a single

step as opposed to two distinct weight loss steps for pure AP.

Figure 4.11 TG/DTA Curve of AP + 4 Percent MgO

The effect of multiple heating rates on the thermal decomposition of the MgO catalyzed

AP is shown in Figure 4.12.

Figure 4.12 Effect of heating rates on thermal decomposition of AP + 4 Percent MgO

93

The curve shows that the first endothermic peak related to the phase transformation

remains almost unaffected. On the other hand, the decomposition peak shifts to higher

temperatures with increasing heating rates. The decomposition peak temperature

increases from 335oC to 381

oC when the heating rate is increased from 2

oC/min to

20oC/min. The Figure also confirms that two stages of AP decomposition have been

reduced to a single stage in all the cases. This shows that the nano magnesium oxide has a

strong catalytic effect on the thermal decomposition of AP and it is likely to alter the

associated kinetic parameters as well. Peak temperature data at different heating rates

has been used to determine the kinetic parameters of decomposition reaction of catalyzed

AP. The kinetic parameters of AP catalyzed by MgO were estimated by using Kissinger

and Ozawa method for comparison with the kinetic parameters of pure ammonium

perchlorate. Kinetic results obtained by using both the methods confirm that the presence

of magnesium oxide as a catalyst has significantly altered the kinetic parameters and

increased the reactivity of AP.

Figure 4.13(a) Kissinger Plot for decomposition of catalyzed AP

94

Representative graph of Kissinger method for AP+ 4% MgO is shown in fig 4.13(a). The

plot of ln β/Tp2

aginst the reciprocal peak temperature (1/Tp) gives a nearly straight line

and slope of the line gives the activation energy value of 159.1 kJ /mole for the catalyzed

AP. The representative graph of Ozawa method for calculation of the kinetic parameters

of the catalyzed version of AP has been shown in Figure 4.13 (b).

The Figure shows that the data points obtained from the plot of log β against (1000/Tp)

are nearly linear and fit into a straight line and the activation energy is determined from

the slope of this line. The activation energy of the catalyzed AP calculated with Ozawa

method is 161.2kJ/mol and this value is quite close to the one obtained by Kissinger

method. The values of rate of reaction constant, frequency factor, activation energy and

enthalpy of activation for decomposition of the catalyzed version of AP are presented in

Table 4.5 for the sake of comparison.

Figure 4.13(b).Ozawa Plot for the decomposition of catalyzed AP

95

The data shows that the frequency factor has increased tremendously after the addition of

MgO, therefor; the reaction rate constant has also increased. The frequency factor has

been found to be 6.13 x 108

s-1

and 8.7 x1010

s-1

for the pure and catalyzed AP

respectively by the Kissinger method. The rate constant has also increased in the

catalyzed version of AP. The rate constant for the pure and catalyzed AP is 6.41 x 10-3

and 7.85 x 10-3

s-1

respectively as calculated by the Kissinger. This is nearly twenty three

percent increase in the value of the rate constant. The kinetic analysis by the Ozawa

method also showed increase in the values of both the frequency factor and the reaction

rate constant. The frequency factor by Ozawa method was found to be 1.01 x 109 s

-1 for

the pure AP and 1.3 x 1011

s-1

for the catalyzed AP. The rate constant was found to be

6.85 x 10-3

s-1

and 8.1 x 10-3

s-1

respectively for pure and catalyzed AP.

∆

H# is the enthalpy of activation

* Values of A, k and ∆H# have been reported for peak temperature corresponding to

heating rate of 10oC/min temperature.

4.9.4 Comparison of thermal and Kinetic Parameters

There is a noticeable difference in the thermal behavior and kinetic parameters of pure

and catalyzed ammonium perchlorate. The heat flow curve of pure AP shows one

endothermic and two exothermic peaks. Whereas, the addition of nano particles of MgO

reduces two exothermic peaks of AP decomposition to one peak showing that both the

stages have merged and the decomposition takes place in single stage. Moreover, the

DTA results indicate that the decomposition peak of the catalyzed AP is quite sharp as

Table 4.5 Summary of Kinetic data of catalyzed ammonium perchlorate

Kinetic Method Ea (kJ/mol) A* (sec-1

) k(sec-1

) ∆H#(kJ/mol)

Kissinger 159.1 8.7x1010

7.85x10-3

153.8

Ozawa 161.2 1.3x 1011

8.1x10-3

155.9

96

compared to the decomposition peak of pure ammonium perchlorate. The decomposition

of the catalyzed AP takes place at a temperature that is higher than the peak temperature

for the low temperature stage but lower than the peak temperature of the high temperature

decomposition stage. The mass loss curve presented in Figure 4.8 shows two distinct

mass loss steps in case of pure AP. However, mass loss curve of the catalyzed AP

presented in Figure 4.11 clearly shows that the mass is lost in a single step confirming

that the two stage decomposition has been reduced to a single stage. The decomposition

of AP in single stage instead of two different stages is likely to make it more suitable as

an oxidizer for energetic formulations. The propellants and pyrotechnic compositions

mainly comprise fuel and the oxidizer besides some other additives. The decomposition

of the oxidizer plays the most important role of providing the oxygen for the combustion

of fuel. The decomposition of the oxidizer in a single stage is therefore advantageous for

getting the optimal results. The comparison of results obtained by thermal analysis

shows that the addition of MgO does not have any noticeable impact on the endothermic

peak related to the phase transformation of AP. The decomposition peak of the catalyzed

AP is quite sharp as compared to the DTA peak for pure ammonium perchlorate.

The effect of multiple heating rates on the thermal decomposition of pure and catalyzed

AP has been presented in Figures 4.9 and 4.12 respectively. The results indicate that in

both the cases the decomposition peak temperatures of the AP shift to higher values when

the heating rates are increased. This kind of trend is very common in the field of thermal

analysis. The peak decomposition temperature of the low temperature stage of pure AP

increases from 276.3oC to 332.9

oC when the heating rates are increased from 2

oC/min to

20oC/min. Whereas, the peak decomposition temperature of the high temperature stage of

pure AP increases from 357.9oC to 412.1

oC when the heating rates are increased from

2oC/min to 20

oC/min.

Data presented in Table 4.4 and 4.5 shows that the activation energy of catalyzed AP is

higher than pure AP. The activation energy value increases from 138.1kJ/mol to

159.1kJ/mol for the high temperature decomposition stage of ammonium perchlorate as

calculated by Kissinger method. Similarly, the activation energy values of AP increase

97

from 14.8kJ/mol to 161.2kJ/mol for the high temperature decomposition stage of

ammonium perchlorate as calculated by the Ozawa method. Increase in activation energy

apparently means that the AP after addition of catalyst has become difficult to

decompose. It is generally observed that the addition of catalyst lowers the activation

energy; however, this effect may differ from case to case. Some researchers have

reported contrary to this observation based on the experimental results where the

activation energy of ammonium perchlorate has increased due to the addition of the

catalyst [32,42]. The rate of reaction, however, has increased despite increase in the

activation energy in the present work due to the addition of catalyst. This is considered to

be due to the reason that the rate of reaction is directly proportional to the frequency

factor (A). In our case frequency factor has increased significantly, therefore, the overall

rate of reaction has also increased.

The kinetic calculation by Kissinger method show that the value of frequency factor

increased from 6.13 x 108

s-1

to 8.7 x1010

s-1

due to the addition of MgO as a catalyst for

high temperature decomposition stage of AP. This is nearly two orders of magnitude

increase in the value of the frequency factor. This huge increase in the frequency factor

increases the overall reactivity of catalyzed AP despite increase in the activation energy.

The rate of reaction constant of catalyzed AP is 7.85 x 10-3

s-1

as compared to 6.41 x 10-3

s-1

for Pure AP showing that addition of nano particles of MgO has increased the overall

reactivity. These results indicate that the reaction rate constant has increased by nearly 23

percent by the addition of 4 percent MgO. Moreover, the catalyzed AP gives higher value

of the enthalpy of activation. The kinetic results obtained by the Ozawa method show

that the frequency factor increased from 1.01 x 109 s

-1 for the pure AP to 1.3 x 10

11 s

-1 for

the catalyzed AP. This is also two orders of magnitude change in the value of the

frequency factor. The reaction rate constant of the catalyzed AP calculted by Ozawa

method was also found increased to 8.1 x 10-3

s-1

from 6.85 x 10-3

s-1

for the pure AP.

4.10 Conclusions

Catalytic effect of MgO nano particles on the thermal decomposition and kinetic

parameters of ammonium perchlorate has been investigated by using simultaneous

98

thermal analysis including DTA and TG. MgO nano particles were characterized using

SEM and XRD. The particles were found to be elliptical in shape and showed regularity

in their shape. The average size of the nano particles measured from SEM images was

found to be between 20-30 nm. The experimental results confirm that the MgO nano

particles had a strong catalytic effect on the thermal decomposition of ammonium

perchlorate as well as on the associated kinetic parameters. The following important

changes were observed in the thermal and kinetic behaviour of the AP due to addition of

the nano catalyst. The DTA peaks reduced from three to two in the catalyzed version of

AP. The main decomposition peak in case of catalyzed AP was found to be sharp as

compared to the one for pure AP. The two distinct decomposition stages of the pure AP

decomposition merged with each other and reduced to a single stage. The TG curve also

showed single step decomposition. The thermal decomposition of catalyzed AP took

place at a temperature significantly lower than the peak temperature for the high

temperature decomposition of the pure AP but higher than the peak temperature of the

low temperature decomposition. Activation energy of the catalyzed AP was increased by

21kJ/mol from 138.1kJ/mol for pure AP to 159.1kJ/mol for the catalyzed version of AP.

The frequency factor A increased by two orders of magnitude due to the addition of MgO

catalyst. The increase in frequency factor caused nearly twenty three percent increases in

the rate of reaction constant despite increase in the value of activation energy. The

enthalpy of activation has also increased by approximately 16 percent in the catalyzed

version of AP.

References


[2] J.P. Agrawal, High energy materials: propellants, explosives and pyrotechnics,

John Wiley & Sons, 2010.

[3] P. Simoes, L. Pedroso, A. Portugal, J. Campos, Thermochimica acta 319 (1998)

55.

[4] S. Vyazovkin, C.A. Wight, Chemistry of materials 11 (1999) 3386.

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calorimetry 84 (2006) 557.

[10] M. Yao, L. Chen, J. Yu, J. Peng, Procedia Engineering 45 (2012) 567.


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(2004) 34.


[14] A.A. Vargeese, K. Muralidharan, Applied Catalysis A: General 447 (2012) 171.



[16] Z.-h. MEI, G.-y. ZENG, H. QIAN, C.-x. Lü, Chinese Journal of Energetic

Materials 1 (2011) 008.



[18] I. Tuukkanen, S. Brown, E.L. Charsley, S. Goodall, P.G. Laye, J.J. Rooney, T.T.

Griffiths, H. Lemmetyinen, Thermochimica acta 426 (2005) 115.


[20] P.W.M. Jacobs, H. Whitehead, Chemical Reviews 69 (1969) 551.



[22] G.P. Sutton, O. Biblarz, Rocket propulsion elements, John Wiley & Sons, 2010.

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[25] V. Boldyrev, Thermochimica acta 443 (2006) 1.

[26] D. Majda, A. Korobov, U. Filek, B. Sulikowski, P. Midgley, D. Vowles, J.

Klinowski, Chemical Physics Letters 454 (2008) 233.

[27] R. Gonçalves, J. Rocco, K. Iha, Thermal decomposition kinetics of aged solid

propellant based on ammonium perchlorate–AP/HTPB binder. 44th

AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Hartford, CT, USA, 21–

23 July, 2008.

[28] L. Bircumshaw, B. Newman, Proceedings of the Royal Society of London. Series

A. Mathematical and Physical Sciences 227 (1955) 228.


[30] M. Zou, X. Jiang, L. Lu, X. Wang, Journal of hazardous materials (2012).

[31] D. Majda, A. Korobov, U. Filek, B. Sulikowski, P. Midgley, D.A. Nicol, J.

Klinowski, Chemical Physics Letters 504 (2011) 185.




(2007) 27.

[34] S. Vyazovkin, Analytical chemistry 74 (2002) 2749.

[35] A. Eslami, S.G. Hosseini, M. Bazrgary, Journal of thermal analysis and


[36] L.V. Kakumanu, N. Yadav, S. Karmakar, (2014).


[38] S. Vyazovkin, C.A. Wight, Thermochimica acta 340 (1999) 53.

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374 (2001) 159.

[40] B. Janković, M. Marinović-Cincović, M.D. Dramićanin, (2013).

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[42] F. Solymosi, T. Bansagi, Kinet. Catal 4 (1963) 73.

101

Chapter 5

Thermal and kinetic evaluation of some

magnesium and aluminum fuelled


5.1 Introduction

The work presented in this chapter reports the experimental results concerning the

thermal decomposition, ignition and kinetic parameters of three different pyrotechnic

compositions containing either aluminum or magnesium as a metallic fuel. The

pyrotechnic compositions are generally made up of some kind of fuel and one or more

oxidizers. The pyrotechnic compositions are used for both military and civilian

applications and the reactions produce special effects such as heat, light, sound and

coloration [1-4]. Oxidizers release the oxygen when they are thermally decomposed and

the liberated oxygen then reacts with the fuel releasing heat in the process. Nature of both

the fuel and the oxidizer has an effect on the ignition behavior and the reliability of the

pyrotechnic composition [5,6]. The reaction between fuel and the oxidizer in pyrotechnic

composition can be termed as a redox reaction. These reactions can be very effectively

investigated by the use of thermal analysis techniques such as differential thermal

analysis (DTA) and thermogravimetery (TG). Thermal analysis is a safe and effective

tool to monitor the ignition temperatures and the reaction kinetics of the energetic

material due its ability to analyze a small quantity of sample. Different types of energetic

materials including pyrotechnic mixtures are frequently investigated using this technique

[7-13]. The ability of DTA and TG to analyze a very small amount of sample i.e. few

milligrams makes it a very safe and suitable choice for investigation of the energetic

materials including explosives, propellants and pyrotechnics. The safety aspects are given

a special attention while carrying out any kind of analysis of the energetic compositions

102

to avoid the damage to the analytical instrument as well as the operator performing the

analysis.

It is important to mention here that the choice of an oxidizer for use in a certain specific

composition plays an important role on the performance of the pyrotechnic mixture. The

ignition and combustion behaviour of pyrotechnics depends on different factors such as

the oxygen balance, thermal stability and the decomposition temperature of the oxidizer.

High oxygen balance means that the less amount of oxidizer would be required for the

complete combustion of the fuel in any energetic composition. The thermal stability of

the oxidizer is necessary for safe storage and reliable operation of the pyrotechnic

composition formulated with it. The decomposition temperature of the oxidizer bears an

impact on the final ignition of the pyrotechnic mixture. Three different oxidizers have

been used in the present work to formulate the pyrotechnic mixtures containing either

aluminum or magnesium as a fuel. These oxidizers include barium nitrate, potassium

permanganate and ammonium perchlorate. All these oxidizers are used frequently in

different kinds of pyrotechnics [14-20]. Aluminum and magnesium are perhaps the most

widely used metallic fuel in the pyrotechnic compositions which produce a very high

light output. Such compositions are used in making different type of flares as well as in

tracer applications [21,22].

The theory and mechanisms involved in the decomposition reaction of propellants and

explosives are generally well defined, however, there is lack of relevant theory and

experimental data pertaining to the decomposition kinetics of the pyrotechnics [1]. The

aim of the present work is to carry out thermal cum kinetic analysis of three different

pyrotechnic compositions and their ingredients to elucidate the reaction process and

assess the safety and reactivity of these mixtures. The thermal behaviour of the individual

oxidizers and fuels has been discussed before the thermo kinetic behaviour of the actual

mixtures is discussed to have a better idea regarding the mechanism of thermal

decomposition.

The investigated pyrotechnic mixtures include Mg + NH4ClO4, Mg + KMnO4 and Al +

Ba(NO3)2. Thermal and kinetic analysis of Mg + KMnO4 has been reported earlier as well

but the critical ignition temperature has not been reported for this composition [21].

103

Moreover, the thermal cum kinetic investigation and the critical ignition data of

pyrotechnic mixtures based on Al + Ba(NO3)2 and Mg + NH4ClO4 is being reported in

detail in the present work. The kinetic parameters have been estimated using non

isothermal data based on the Kissinger method. The focus in the present work was to

assess the thermal stability and reactivity of the above mentioned compositions so that

their reliability during storage and for operational use could be estimated on the basis of

the thermal, kinetic data obtained in a series of experiments.

5.2 Experimental conditions

The information regarding the chemicals used in the present work has already been

discussed in detail in chapter 2 and therefore only the conditions under which the

experiments were performed have been discussed here. The particle size analysis of the

fuels was carried out to find out their mean size. The average particle size of the

aluminum powder was 20 µm and the average particle size of the magnesium powder

was 24 µm. Laser scattering particle size distribution analyzer LA-920 by HORIBA has

been used for the analysis. The XRD instrument by STOE Germany has been used for

analysis of the end product of the oxidation. All the three oxidizers were sieved through a

300 mesh sieve before mixing with fuel as a pyrotechnic mixture. The fuel and the

oxidizers were mixed in their respective stoichiometric mass ratios. Thermo analytical

instrument by Shimadzu has been used to carry the present research work. TG/DTA

curves of all the ingredients and the mixtures were obtained by using a heating rate of

10°C min-1

. The air atmosphere was used for the analysis of fuels whereas nitrogen gas

was used for the analysis of oxidizers. Alumina pans were used to hold the sample. The

effect of heating rate on the heat flow curve of the pyrotechnic mixtures was analyzed at

four different heating rates i.e. 10°C min-1

, 20°C min-1

, 30°C min-1

and 40°C min-1

. The

sample mass was kept close to 3mg for all the three pyrotechnic mixtures. Nitrogen gas

was used for providing the inert atmosphere and the flow rate of nitrogen was maintained

at 50 ml min-1

.

104


The particle size distribution of both aluminum and magnesium is shown in Figure 5.1

and 5.2 respectively.

Figure 5.1 Particle size distribution of aluminum powder (Davg= 20µm)

Figure 2 Particle size distribution of magnesium powder (Davg= 24µm)

105

The particle size analysis of both the fuels i.e. magnesium and aluminum powder has

been carried out to estimate their average particle size by using the particle size

distribution analyzer. The results indicate that the average particle size of the aluminum

powder was nearly 20 µm whereas; the average particle size of the magnesium powder

was nearly 24 µm.

5.3.1 Thermal analysis of the pyrotechnic ingredients

Although thermal decomposition data of the pyrotechnic ingredients used in this research

work is available in the literature; however, to elucidate the mechanism of decomposition

of pyrotechnic mixtures thoroughly, thermal behaviour of all the ingredients was carried

out under identical experimental conditions.

TG curve of pure magnesium powder is shown in Figure 5.3. The curve shows that the

mass of the sample increases significantly in the temperature range of 400oC to 900

oC

due to oxidation of magnesium to magnesium oxide in the presence of air.

Figure 5.3 TG curve of magnesium powder showing the mass gain due to oxidation

106

The TG curve initially shows a slight mass loss in the temperature range of 200oC to

350oC due to decomposition of magnesium hydroxide and evaporation of the residual

water. The mass gain is quite slow in the temperature range of 350oC to 550

oC. The main

oxidation reaction takes place near 600oC due to ignition of magnesium fuel with

atmospheric oxygen and resulting in a large mass gain. The mass gain slows down after

650oC and oxidation completes near 900

oC. The total mass gain of magnesium powder

was approximately 67 percent which is very close to the theoretical value showing that all

the magnesium has oxidized to MgO as per the following equation.

Mg + ½ O2 → MgO (5.1)

XRD analysis of the residue was also carried out to see the end product of oxidation of

magnesium. The XRD spectra presented in Figure 5.4 shows five distinct diffraction

peaks.

Figure 5.4 XRD spectra of the residue obtained after oxidation of magnesium powder

showing typical MgO peaks

107

The analysis of the data shows that these are the characteristic peaks representing MgO

and match with the reference pattern No 01-074-1225. Miller Indices have been marked

on the peaks. XRD result also confirms the complete oxidation of Mg.

The TG curve of aluminum powder has been shown in Figure 5.5. TG curve shows a

two-step mass gain of aluminum powder to aluminum oxide in the presence of air. The

TG curve shows a very small decrease in the mass of the sample due to removal of water

from the aluminum hydroxide in the temperature range of 200oC to 300

oC. The first stage

of oxidation takes place between temperature ranges of 400oC and 700

oC and results in a

mass gain of about 20%. The mass gain is slow till 800oC and after this temperature the

main oxidation reaction starts with a rapid increase in mass of the sample showing nearly

50% more mass gain up to 1000oC and continues to increase. Complete oxidation of the

sample results in the production of aluminum oxide.

2Al + 3/2O2 → Al2O3 (5.2)

Figure 5.5 TG curve of aluminum powder showing the mass gain due to

oxidation in air

TG/DTA curve of pure barium nitrate is shown in the Figure 5.6. The curve shows that

barium nitrate melts near 592oC showing an endothermic peak. Decomposition of

barium nitrate takes place in a series of overlapping endothermic peaks. The TG curve

108

shows that the decompositions of barium nitrate are associated with 40 percent of mass

loss in the temperature range of 590oC to 700

oC. It decomposes close to 690

oC releasing

oxygen. The decomposition reaction of barium nitrate has been previously reported in

literature and presented below[5,23]:

Ba(NO3)2 → BaO + 2NO+ O2 (5.3)

Figure 5. 6 TG/DTA curve of barium nitrate showing the heat flow and mass loss

curves

The thermal behaviour of ammonium perchlorate is described briefly here as it was

discussed in detail in Chapter 3. First endothermic peak of ammonium perchlorate

appears at 241ºC and it corresponds to a solid phase transformation of ammonium

perchlorate during which its crystal structure changes from orthorhombic to cubic [24-

26]. The next peak is exothermic in nature and appears at 310.7ºC (Table 1). This peak

represents the low temperature decomposition of ammonium perchlorate. The last

exothermic peak appears close to 384.1ºC and depicts the high temperature

decomposition of ammonium perchlorate.

109

Table 5.1 Summary of experimental thermal data

# The temperatures have been mentioned at maximum/minimum heat flux. All of these are

the actual experimental results.

T*

It is the temperature range during which there is a change in the mass of the sample

(increase or decrease).

TG curve confirms the decomposition of ammonium perchlorate in two distinct stages.

The decomposition of ammonium perchlorate yields the following products as reported

by J. A. Conkling and C. Mocella in their book “Chemistry of pyrotechnics: basic

principles and theory” [2].

2NH4ClO4 → N2+ 3H2O+2HCl+2.5O2 (5.4)

The data obtained from the DTA curve of potassium permanganate indicates two distinct

peaks (represented in chapter 3). First peak is exothermic in nature that starts with the

melting of the sample and appears at a temperature of 284oC. This peak represents the

decomposition of potassium permanganate and is associated with a mass loss. Second

peak appearing near 511ºC is endothermic in nature and represents high temperature

decomposition stage of potassium permanganate. The TG curve also shows a slight mass

loss corresponding to this peak.

S.No Composition Mass Ratio

(Percentage)

Fusion Temp#

(oC)

Decomposition

Temp# (

oC)

T*(

oC)

1 Ba(NO3)2 100 592 690 580-700

2 NH4ClO4 100 ------ 310 280-410

3 KMnO4 100 ------ 284 240-290

4 Al + Ba(NO3)2 17/83 569 601 550-620

5 Mg + NH4ClO4 34/66 ----- 338 330-380

6 Mg + KMnO4 18/82 ------ 292 260-320

110

5.3.2 Thermal analysis of the pyrotechnic compositions

The heat flow curve of Al + Ba(NO3)2 pyrotechnic composition is shown in Figure 5.7.

The curve depicts that mixture melts close to 569oC. This temperature is lower than the

melting point of both the fuel and the oxidizer. This is thought be due to mechanism of

solid state reaction during which the more reactive ingredient of the mixture (fuel in this

case) migrates to the oxidation interface [21]. Just after melting, the pyrotechnic mixture

decomposes exothermically at a temperature of 601oC and ignition of the fuel and

oxidizer takes place. TG curve remains stable until melting of the mixture and then it

shows a mass loss of approximately 20 percent. Such a behavior of the TG curve means

that the pyrotechnic mixture is highly stable till its melting temperature. The oxidation

reaction between fuel and oxidant is shown below.

2 Al + Ba(NO3)2 → BaO + 2 NO+ Al2O3 (5.5)

Figure 5.7 TG/DTA curve of Al + Ba(NO3)2 pyrotechnic composition showing the

mass loss and heat flow curves

111

The heat flow curve of Mg + NH4ClO4 pyrotechnic mixture shows an endothermic peak

at 254oC (Figure 5.8). This peak is due to the phase change in ammonium perchlorate and

takes place at a slightly higher temperature than pure ammonium perchlorate. This

pyrotechnic mixture does not melt prior to decomposition. There is a sharp exothermic

peak at 338oC which represents the decomposition of this pyrotechnic mixture without

melting. TG curve does not show any mass loss until the fuel and oxidizer mixture

ignites. The decomposition is associated with nearly 17 percent mass loss in the

temperature range of 330oC to 380

oC. It is postulated that the oxygen released due to the

decomposition of ammonium perchlorate reacts with the magnesium fuel and ignites the

pyrotechnic mixture. The oxidation reaction between the fuel and oxidizer is represented

below.

5 Mg + 2 NH4ClO4 → 5 MgO + 3 H2O+ 2 HCl + N2 (5.6)

Figure 5.8 TG/DTA curve of Mg + NH4ClO4 pyrotechnic composition showing the


112

The heat flow curve of Mg + KMnO4 has been shown in Figure 5.9. A single exothermic

peak of the mixture can be seen near a temperature of 292oC and there is no thermal

event prior to this peak. The exothermic peak represents the decomposition of the

pyrotechnic mixture. This pyrotechnic mixture does not show any kind of phase

transformation or melting before the decomposition. The reaction between the two has

been reported as follows [21].

Mg +2KMnO4 → MgO + K2O+ 2MnO2 +O2 (5.7)

Potassium permanganate decomposes to release oxygen and oxidizes the metal fuel to

form MgO. There is no variation in the TG curve till the melting of the oxidizer near

240oC. The TG curve shows approximately 11 percent mass loss in the temperature

range of 260oC - 320

oC.

Figure 5.9 TG/DTA curve of Mg + KMnO4 pyrotechnic composition showing the


113

5.3.3 Variation in peak temperatures at multiple heating rates

The effect of heating rates on the heat flow curves has been used to calculate the kinetic

parameters of the pyrolants. Standard ASTM method was used for determination of the

kinetic parameters based on the multiple heating rate experiments and employing

Kissinger method. The DTA experiments were carried out as per the guidelines given by

the international committee of thermal analysis and calorimetry (ICTAC)[27]. The effect

of heating rates is similar in all the three pyrotechnic mixtures in the sense that the

decomposition peaks shift to higher temperatures with increase in the heating rate as seen

in Figures 5.10 (a), 5.10 (b) and 5.10 (c). The effect of heating rates on the decomposition

peaks of mixture containing Al and Ba(NO3)2 is shown in Figure 5.10 (a) and the results

are presented in Table 5.2.

Table 5.2 Effect of heating rate on the decomposition peak temperature of the

pyrotechnic mixtures

S.NO Composition Heating rate

(oC min

-1)

Peak Temp

(oC)

1

Al + Ba(NO3)2

10

600.6

20 624.9

30 641.6

40 655.7

2 Mg + NH4ClO4 10 338.1

20 346.4

30 352.5

40 358.1

3 Mg + KMnO4 10 292.5

20 302.9

30 310.8

40 319.5

114

The decomposition temperature of the mixture increases from 601oC to 656

oC when the

heating rate increases from 10oC/min to 40

oC/min. The effect of heating rate for the

mixture containing Mg and NH4ClO4 is shown in Figure 10(b). The data presented in

Table 5.2 shows that the peak temperatures increase from 338oC to 358

oC when the

heating rate increases from 10oC/min to 40

oC/min. Similarly the effect of heating rate on

the thermal decomposition of the mixture containing Mg and KMnO4 is presented in

Figure 10 (c).

Figure 5.10(a)Effect of heating rate on thermal decomposition of mixture containing Al

and Ba(NO3)2

The data presented in Table 5.2 shows that the peak temperatures increase from 292oC to

319oC when the heating rate increases from 10

oC/min to 40

oC/min. Ideally speaking, the

heating rate should not have an effect on the melting peak of the mixture; however,

Figure 10 (a) shows that the melting peaks also shifted to higher temperatures with

increasing heating rate. This is thought to be due to non-uniform heating of the sample

caused by the thermal lag phenomena. The sample is exposed for less time to the specific

heating range at higher heating rates. Similar effect of increase in the melting peak

temperature due to increasing of the heating rate for pyrotechnics and other mixtures has

been reported by other researchers as well [5,14]. The shift in the peak temperatures

115

caused by the heating rate has been used to calculate the kinetic parameters using

Kissinger method.

Figure 5.10(b) Effect of heating rate on thermal decomposition of mixture

containing Mg and NH4ClO4

Figure 5.10(c) Effect of heating rate on thermal decomposition of mixture

containing Mg and KMnO4

116

5.3.4 Kinetic evaluation of the mixtures

Kinetics is an important aspect in the study of decomposition reactions. Evaluation of

thermal and kinetic stability of energetic materials is mandatory to access their reactivity

and thermal stability during processing, handling, storage and operation. Various

methods have been reported in literature for calculation of the kinetic parameters. In this

case, kinetic parameters of the pyrotechnic compositions have been determined by using

non isothermal data based on the Kissinger method [28]. The detailed description of the

Kissinger method for evaluation of the kinetic parameters from heat flow data has been

presented in chapter 2.

The heat flow data obtained by multiple heating rates experiments was used to construct

the Kissinger plot of (

) versus

. The data points are nearly linear and fit into a

straight line. The slope of the graph based on the above mentioned relationship is used to

calculate the value of activation energy. The plots of (

) against the reciprocal peak

temperature based on Kissinger method are shown in Figure 5.11(a),5.11(b) and 5.11(c)

for compositions comprising Al + Ba(NO3)2 , Mg + NH4ClO4 and Mg + KMnO4. The

kinetic and thermodynamic parameters of the pyrotechnic mixtures have been presented

in Table 5.3.

Table 5.3 Kinetic parameters of pyrotechnic mixtures

*Tb is the critical ignition temperature.

Composition E

(kJmol-1

)

Frequency

factor A (s-1

)

Reaction

Constant k(s-1

)

(kJmol-1

)

*Tb (oC)

Al + Ba(NO3)2

155.9 + 4 %

8.58x106

4.0 x 10-3

148.6

599.2

Mg + NH4ClO4 214.3 + 7 % 2.38x1016

11.5 x 10-3

209.2 338.4

Mg + KMnO4 134.6 +10 % 2.28x1010

8.4 x 10-3

129.9 289.3

117

Important thermo dynamic parameters known as enthalpy of activation have been

calculated using the relationship shown in equation (5.8) [29].

(5.8)

Figure 5.11 (a) Kissinger plot for determination of activation energy of pyrotechnic

composition containing Al + Ba(NO3)2

Kinetic analysis of the mixture containing Al and Ba(NO3)2 shows that the activation

energy of this mixture is 156kJ/mol. The frequency factor for this mixture is nearly

8.58x106s

-1. The reaction rate constant is nearly 4.0 x 10

-3s

-1. The Kinetic analysis of the

mixture containing Mg and NH4ClO4 shows that the activation energy of this mixture is

214kJ/mol. This mixture has the highest value of the activation energy amongst the three

mixtures. The frequency factor for this mixture is nearly 2.38x1016

s-1

it is quite high. It is

due to such a high value of frequency factor that the reaction rate constant of this mixture

118

Figure 5.11 (b) Kissinger plot for determination of activation energy of pyrotechnic

composition containing Mg + NH4ClO4

Figure 5.11 (c) Kissinger plot for determination of activation energy of pyrotechnic

composition containing Mg + KMnO4

119

is the highest and it is nearly 11.5 x 10-3

s-1

. The rate constant of this mixture is the

highest. Kinetic analysis of the mixture containing Mg and KMnO4 shows that the

activation energy of this mixture is close to 135kJ/mol. The mixture has the lowest value

of the activation energy amongst the three compositions. The frequency factor for this

mixture is nearly 2.28x1010

s-1

. The reaction rate constant is nearly 8.4 x 10-3

s-1

. The

reaction constant in this case is higher than the mixture containing Al and Ba(NO3)2 but

lower than the mixture containing Mg and NH4ClO4.

5.3.5 Estimation of critical ignition temperature

It is the described as the minimum temperature up to which the energetic materials can be

heated without the risk of runaway reaction [30,31]. Critical ignition temperature is often

estimated by researchers working on energetic materials using thermo kinetic parameters

obtained from heat flow data and employing the equations (5.9) and (5.10). This

approach is now being commonly used for quite some time to estimate the critical

ignition temperature on the basis of DSC/DTA data [32,33].

√

(5.9)

(5.10)

Where is the critical ignition temperature , E is the activation energy, R is the gas

constant, is the onset temperature corresponding to → 0 approximated using

regression coefficients b and c.

Critical ignition temperature Tb is an important parameter to determine the safety and

stability of energetic materials for their safe handling during the storage and operation.

The pyrotechnic compositions are considered to be safe for storage, handling and

operation if this temperature is sufficiently higher than the maximum possible ambient

temperature. The maximum possible ambient temperatures during storage are generally

120

up to 70oC. The values of have been calculated by using equation (5.10) from the

temperature data at multiple heating rates and using the linear regression formula.

The values of obtained in this case are 585.2oC, 332.3

oC and 284.2

oC respectively for

the pyrotechnic mixtures containing Al / Ba(NO3)2, Mg /NH4ClO4 and Mg /KMnO4

respectively. The values of critical ignition temperatures calculated using the above

mentioned values of are 599.2oC, 338.4

oC and 289.3

oC respectively for mixtures

based on Al / Ba(NO3)2, Mg /NH4ClO4 and Mg /KMnO4. The data is presented in Table

5.3.

5.3.6 Comparative analysis of the kinetic data and ignition temperatures

Thermal decomposition, ignition and kinetic analysis of three pyrotechnic mixtures has

been carried out in this work. The main focus in the present work was to determine the

thermal stability and relative reactivity of the three different pyrotechnic mixtures

containing fuel and oxidizers. The kinetic analysis of two pyrotechnic compositions

containing Mg/NH4ClO4 and Al / Ba (NO3) has not been reported earlier to the best of

our knowledge. The kinetic analysis of the third composition containing Mg and KMnO4

however has been reported earlier as well by S.M. Pourmortazavi et al [21].

The data presented in Table 3 shows that pyrotechnic mixture based on Mg and KMnO4

has the minimum value of the activation energy and enthalpy of activation as compared

to the other two investigated mixtures. The critical ignition temperature of this

composition has been found to be found to be 289.3oC which is the lowest amongst the

three compositions depicting that it is the least stable composition out of above

mentioned compositions. However, even this value of critical ignition temperature is

fairly higher than the normally encountered storage and operational temperature and

therefore it is considered to be thermally stable for handling, storage and operation. The

values of activation energy and enthalpy of activation calculated in our work are 134.6

kJmol-1

and 129.9 kJmol-1

respectively that are very close to the reported values of

135KJmol-1 and 130 kJmol-1

by S.M. Pourmortazavi et al. However, the critical ignition

temperature in our case is 289.3oC which is not reported in the earlier work.

121

The comparison of data presented in Table 5.3 for the three compositions investigated in

this work shows that the activation energy of the mixture containing Mg and KMnO4 has

the lowest value amongst these three mixtures. It means that this composition is relatively

easy to ignite and requires less amount of energy for its ignition. The activation energy of

the mixture containing Al and Ba(NO3)2 is nearly 156 kJ/mol and it is higher than the

activation energy of the former mixture. The activation energy of the mixture containing

Mg and NH4ClO4 is the highest amongst the three mixtures i.e.214kJ/mol. This mixture is

therefore difficult to ignite as compared to the others.

It is obligatory to mention here that mixture containing the ammonium perchlorate as the

oxidizer of magnesium requires more activation energy and relatively difficult to ignite as

compared to the one containing potassium permanganate as the oxidizer. It is therefore

evident that the replacement of the ammonium perchlorate with potassium per magnate as

the oxidant of the magnesium fuel lowers the ignition temperature as well as the

activation energy.

The comparison of the data shows that the frequency factor of the composition containing

aluminum fuel and barium nitrate as oxidant is the lowest. The frequency factor for this

composition is nearly 8.58x106s

-1. The frequency factor of composition containing

ammonium perchlorate is the highest and its value is nearly 2.38x1016

s-1

. The frequency

factor of the composition containing potassium permanganate is lower than the one

containing ammonium perchlorate but higher than the one containing barium nitrate. Its

value was found to be nearly 2.28x1010

s-1

.

It would not be out of place to mention here that the frequency factor plays a vital role in

determining the rate constant. It is perhaps equally important from kinetics point of view

as the activation energy. The activation energy alone can only predict the ease or

difficulty of ignition of any investigated composition but it does not give any information

regarding the rate at which that reaction proceeds unless the frequency factor is known.

An interesting comparison between the two compositions based on Mg + NH4ClO4 and

Mg + KMnO4 will give a fair idea regarding the importance of the frequency factor. The

122

activation energy of the composition congaing ammonium perchlorate is nearly 58

percent higher than the mixture containing potassium permanganate. However, despite

such a high value of activation energy the rate constant of pyrotechnic mixture containing

NH4ClO4 is nearly 37 percent higher than mixture containing KMnO4. The main reason

for this is that the high value of the frequency factor that increases the overall reactivity.

Similarly, the reaction rate constant of the mixture containing barium nitrate is the lowest

despite relatively lower activation energy as compared to the mixture containing

ammonium perchlorate.

The comparison of the reaction rate data shows that composition based on Al + Ba(NO3)2

is the least reactive amongst the three with rate constant value of 4.0 x 10-3

s-1

. The

composition based on Mg /KMnO4 is more reactive with the rate constant value of 8.4 x

10-3

s-1

and the composition based on Mg + NH4ClO4 is the most reactive of the three

with rate constant value of 11.5 x 10-3

s-1

. Relative reactivity of these mixtures has been

found to obey the following order: Mg + NH4ClO4 > Mg + KMnO4 > Al + Ba(NO3)2 on

the basis of above mentioned discussion and the data presented in Table 5.3. The

enthalpy of activation of the three mixtures is nearly149 kJ/mol, 209 kJ/mol and 129.9

kJ/mol for compositions containing Al + Ba(NO3)2, Mg + NH4ClO4 and Mg + KMnO4

respectively.

The thermal stability of the mixtures can be assessed by the value of the critical ignition

temperatures. The pyrotechnic composition based on Al + Ba(NO3)2 has the highest value

of critical ignition temperature i.e. 599.2oC and can be regarded as the most stable

compositions amongst the three. The rate of reaction constant for this composition is the

lowest and therefore it is considered to be as the least reactive composition out of the

above mentioned compositions. The pyrotechnic composition based on Mg + KMnO4

has the lowest value of critical ignition temperature i.e. 289.3oC and can be regarded as

the least stable compositions amongst the three. It is reemphasized here that even this

value of critical ignition temperature is sufficiently higher than the ambient temperatures

and therefore the composition is thermally stable even though it is less stable in

comparison to the other two compositions.

123

The pyrotechnic composition based on Mg + NH4ClO4 has the critical ignition

temperature of 338.4oC and can be regarded as a stable pyrotechnic composition.

Moreover, the rate of reaction constant for this composition is the highest and therefore it

is considered to be as the most reactive composition out of the above mentioned

compositions.

Thermal and kinetic analysis of pyrotechnic compositions provides meaningful

information regarding different mechanism involved in the decomposition process.

Moreover, the kinetic data along with the critical ignition temperature is helpful in

determining the safety and stability of the pyrotechnic compositions during storage,

handling and operation. The kinetic evaluation of different fuel/oxidant mixtures can help

resolve the safety issues associated with the pyrotechnics to avoid accidental initiation.

5.4 Conclusions

The evaluation of thermal stability and reactivity of any pyrotechnic composition is

mandatory to ensure its safety and reliability during storage, handling, transportation and

operation. Experimental investigation of the thermal and kinetic behaviour of three

pyrotechnic compositions based on aluminum and magnesium fuel has been carried out

to elucidate the decomposition process and to evaluate the stability and reactivity. The

experimental results and the kinetic data of the pyrotechnic mixture containing Al /

Ba(NO3)2 depict that the mixture melts close to a temperature of 569oC and decomposes

exothermically at 601oC showing the ignition of the fuel. There is no thermal event prior

to melting which means that the pyrotechnic mixture is highly stable till its melting

temperature. The mixture is least reactive amongst the three investigated mixtures and it

has the lowest value of the reaction rate constant. The mixture is most stable thermally

due to highest value of its critical ignition temperature i.e. nearly 599oC.The mixture has

a moderate value of the activation energy which is nearly 156kJ/mol.

The experimental results and the kinetic data of the pyrotechnic mixture containing Mg/

NH4ClO4 revealed that the heat flow curve of pyrotechnic mixture shows an endothermic

peak at 254oC representing the phase change of AP. The mixture does not melt and

124

decomposes exothermically near 338oC showing the ignition of the magnesium fuel. The

mixture has the highest value of activation energy amongst the three compositions and

therefore it is relatively difficult to ignite. Once ignited the mixture is very reactive and

has the highest value of the reaction rate constant. The mixture is thermally very stable

and has a fairly high value of the critical ignition temperature.

The experimental results and the kinetic data of the pyrotechnic mixture containing

Mg/KMnO4 depict that there is no thermal event such as phase change or melting in this

mixture prior to its decomposition. The mixture decomposes exothermically at 292oC

depicting the ignition of the magnesium fuel. It is the least stable pyrotechnic

composition amongst the three and has the lowest value of the critical ignition

temperature. The mixture is very easy to ignite as it has the lowest value of activation

energy. The mixture is fairly reactive and its rate constant is higher than the mixture

containing barium nitrate but lower than the one containing ammonium perchlorate.

The upshot of the above discussion is that all the three compositions investigated in the

present work are thermally stable and the relative reactivity of these pyrotechnic mixtures

decreases in the following order:

Mg + NH4ClO4 > Mg + KMnO4 > Al + Ba(NO3)2

References





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Momenian, Journal of hazardous materials 162 (2009) 1141.

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[17] M.W. Beck, M.E. Brown, Thermochimica acta 65 (1983) 197.

[18] J.J. Granier, M.L. Pantoya, Combustion and Flame 138 (2004) 373.

[19] Y. Wang, J. Zhu, X. Yang, L. Lu, X. Wang, Thermochimica Acta 437 (2005) 106.

[20] E.C. Koch, D. Clement, Propellants, Explosives, Pyrotechnics 32 (2007) 205.


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Zecheru, Journal of Thermal Analysis and Calorimetry 115 (2014) 1407.

[23] O.Q. González, J.L. Burell, E. Martínez, C. Barrera, R. Acosta, N. Ruiz, Revista

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127

Chapter 6

Thermal decomposition of the magnesium

fueled pyrotechnic compositions and the

effect of accelerated aging

6.1 Introduction

The work presented in this chapter reports the experimental results concerning the

thermal decomposition and kinetic parameters of two military pyrotechnic compositions

containing magnesium fuel. The focus in the present work was to qualitatively identify

the effect of the accelerated aging by storing the compositions at high temperature and

humidity levels. The investigated pyrotechnic compositions viz. SR-524 and SR-562

are used in various military applications. The SR-524 pyrotechnic composition contains

magnesium fuel and sodium nitrate is the oxidizer. The SR-562 pyrotechnic composition

contains magnesium, sodium nitrate and calcium oxalate.

The pyrotechnic mixtures are typically made up of finely divided metallic or nonmetallic

fuel and inorganic oxidizers. These compositions are used to produce some special

effects like light, heat, sound and colours in various military and civilian applications [1-

4]. The fuel reacts with the oxygen that is released when the oxidizer is thermally

decomposed and heat is released in this process. The ignition behaviour of the

pyrotechnics depends on nature of the fuel as well as the oxidizer [5,6]. The performance

of the pyrotechnic mixtures is affected by many factors including humidity, temperature,

impact and thermal shock [7]. The investigation of the aging behaviour of the

pyrotechnics is important to identify different physical and chemical changes that may

have a detrimental effect on the ignition behaviour of the pyrotechnics. The changes

caused by the aging process may lead to the poor performance of the pyrotechnic

composition and it may not produce the desired output. In certain cases, the composition

128

may even fail to ignite [8]. On the other hand, some of the changes may lead to the

accidental initiation of these compositions and many accidents have been reported in the

past during the processing, handling and storage [9].

Extensive exposure of the energetic materials to high levels of humidity and elevated

temperatures is detrimental and likely to deteriorate their performance. The stability of

different materials such as pyrotechnics, propellants and polymers is known to have been

seriously affected by high humidity. The effect of high temperatures and high humidity

levels on aging process of the pyrotechnics and propellants has been investigated by

many researchers [10-15].

Guo et al. investigated the effect of moisture content on the thermal behaviour and

stability of propellants [16]. The authors reported that moisture content in the form of

different high humidity levels played an important role in changing the reaction

mechanism of the propellant and also reported that low humidity levels did not affect the

reaction. Similarly I.M Tuukkanen et al. investigated the effect of aging on magnesium-

strontium nitrate pyrotechnic composition and reported that the decomposition

temperature of the aged composition lowered by 17oC and the reaction product of the

aging were strontium nitrite and magnesium hydroxide [17]. The authors also reported

that the overall reactivity of the pyrotechnic mixture increased after aging in contrast to

the generally expected behaviour of decreasing reactivity due to aging.

Liqiong Wang et al. reported the effect of temperature and humidity on the thermal

behaviour of pyrotechnic compositions containing the strontium nitrate. The authors

showed that the critical temperature of thermal explosion decreased as the humidity

levels were increased and that both the high humidity and high temperature have a strong

influence on the thermal behaviour and kinetics of the investigated pyrotechnic mixtures

[7].

In this case we have investigated the effect of aging on two military pyrotechnic

compositions. The pyrotechnic composition SR-524 is used as a tracer. It contains

magnesium as a fuel and sodium nitrate is used as an oxidizer. The pyrotechnic

129

composition SR-562 is used as a flare. It is based on the magnesium fuel and contains

calcium oxalate and sodium nitrate as the oxidizers. The magnesium-sodium nitrate

system was previously studied by W.P.C et al [18]. The authors reported the kinetic

parameters calculated from the mass loss data obtained by TG and predicted the time

after which the reaction is expected to reach some amount of conversion at a certain

temperature on the basis of the experimental results. The authors have carried out the

thermo gravimetric analysis of the pyrotechnic composition and evaluated the kinetic

parameters from the weight loss data.

The work pertaining to the aging of these military pyrotechnic compositions viz. SR524

and SR 562 has not be reported earlier to the best of our knowledge of the published

literature. The work presented in this chapter is focused on the investigation of thermal,

kinetic and morphological changes that take place as a result of aging on the basis of

comparison of the results obtained for aged and fresh compositions. The kinetic

parameters in our case have been calculated by using the heat flow data obtained from the

DTA experiments. The accelerated aging of the pyrotechnic composition was carried out

for 30 days at a temperature of 70oC and relative humidity of 70 percent. The thermal

behaviour was investigated using TG/DTA and the kinetic parameters were calculated

using Kissinger method.

Thermal analysis is a very suitable technique for the analysis of energetic materials

especially due to the safety reasons. A very small amount of sample typically in the range

a few milligrams are required for the analysis. The thermal decomposition and kinetic

parameters of energetic materials including explosives, pyrotechnics and propellants are

often investigated by using different thermal analysis technique [19-24]. Different

methods of thermal analysis have proved to be a good tool to follow the aging process of

the energetic materials including pyrotechnics. Thermal data can be further utilized for

determination of the kinetic parameters and evaluate the safety of the pyrotechnic

materials before and after the aging process.

The reaction product of the aging is identified by comparing the XRD spectra of the fresh

and aged compositions. The scanning electron microscope is used to observe the surface

130

features and morphology to identify the changes in the physical condition of the

pyrotechnic mixture in the bulk form.

6.2 Thermal decomposition kinetics and effect of accelerated aging on

SR-524 pyrotechnic composition

SR-524 military pyrotechnic composition contains 58 percent magnesium as a fuel, 38

percent sodium nitrate as an oxidizer and 4 percent boiled linseed oil. The accelerated

aging of pyrotechnic composition was carried out in a humidity controlled oven for 30

days at 70oC and 70 percent relative humidity. Unlike propellants, there are no specific

aging criteria for the pyrotechnics. The military compositions are conventionally tested

up to 70oC to cater for the effect of maximum possible ambient temperatures. Moreover,

various aging studies on pyrotechnics and propellants have been previously carried out in

a highly humid atmosphere (generally more than 60 percent relative humidity) to get

meaningful data after the aging process.


Defence grade pyrotechnic composition SR-524 was purchased from Pakistan Ordnance

Factories and used for experiments without any modification. The physical and chemical

changes before and after aging were monitored using scanning electron microscope

(SEM) and X-ray diffraction technique. The XRD instrument with Cu kα radiations by

STOE, Germany has been used for carrying out the structural analysis and

characterization of the reaction products of aging. The scan range during the analysis was

from 20o to 80

o. Physical changes resulting from the aging process were monitored by

using scanning electron microscope JSM- 6490 LA, JEOL, Japan. Thermal analysis of

aged and fresh pyrotechnic composition was carried out using the Diamond TG/DTA

instrument by Perkin Elmer. The alumina crucibles were used to hold the samples for

analysis. The pyrotechnic samples were heated from room temperature to 850oC to

monitor the thermal events and decomposition process. All the experiments were

conducted in inert nitrogen atmosphere and the flow rate of the gas was maintained at 50

ml min-1

. Experiments were performed with four different heating rates to calculate the

131

kinetic parameters of the fresh and aged samples. The heating rates of 10oC min

-1, 20

oC

min-1

, 30oC min

-1 and 40

oC min

-1 were used. To cater for the effect of higher heating

rates and keeping in view the safety considerations, the sample in all experiments was

kept close to 1mg. Arrhenius kinetic parameters were estimated by using Kissinger

method from the heat flow data. The method requires the heat flow data at multiple

heating rates for kinetic calculations. The details of the kinetic method have been

described in Chapter 2 in detail.

6.4 Results and discussion

6.4.1 Thermal analysis of the fresh and aged composition

The heat flow curve of fresh and aged SR-524 pyrotechnic composition has been

presented in Figure 6.1.

Figure 6.1 The heat flow curves of fresh and aged SR-524


132

The curve for the fresh composition shows a small endothermic peak near 315oC which is

due to the melting of the sodium nitrate present in the pyrotechnic mixture. The second

curve which is highly exothermic in nature appears at a temperature of 538oC. This peak

represents the ignition of magnesium fuel due to reaction with oxygen released as a result

of oxidizer decomposition. The endothermic peak at 315oC is not clearly visible because

of the small heat flow value and it has been masked due to very high heat flow value of

the ignition peak. The curve for aged composition also shows one endothermic and one

exothermic peak similar to the fresh composition. The small endothermic peak has not

been affected much; however, the exothermic peak representing the ignition of fuel has

been shifted to a lower temperature due to the aging process. The exothermic peak for

aged composition appears at 524oC which is slightly lower than the fresh composition.

The effect of heating rate on the heat flow curve of the fresh pyrotechnic composition is

shown in Figure 6.2(a).

Figure 6.2 (a) Effect of multiple heating rates on the heat flow curve of

Fresh SR 524 pyrotechnic composition

133

It is seen that the small endothermic peak corresponding to the melting of sodium nitrate

does not shift with increasing heating rates. This peak has been masked because the

ignition peak has a very large heat flow value as compared to the endothermic peak. The

exothermic peak related to the ignition of the magnesium fuel shifts to higher

temperatures when the heating rates are increased. The similar effect of shift in peak

temperatures due to increasing heating rates has been reported by many other researchers

for different type of energetic materials including pyrotechnic compositions. The peak

temperatures for the fresh composition were 527.2oC, 538.7

oC, 547

oC and 555.9

oC

corresponding to the heating rates of 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC min

-1

respectively (Table 6.1).

*The peak temperatures corresponding to different heating rates have been reported at the

value of maximum heat flux

The effect of heating rate on the heat flow curve of aged pyrotechnic composition is

shown in Figure 6.2(b). The curve shows that the peak tempearture inreases from 498oC

Table 6.1 Kinetic parameters of SR-524 pyrotechnic composition calculated by

Kissinger method.

Heating Rate

(oC min

-1)

Peak Temperature*

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

K

(sec-1

)

∆H#

(kJ mol-1

)

10 527.2 3.85 x 1014

8.00 x 10-3

20 538.7 256+9% 4.34 x 1014

15.57 x 10-3

249

30 547.0 4.35 x 1014

22.85 x 10-3

40 555.9 3.79 x 1014

29.82 x 10-3

134

Figure 6.2(b) Effect of multiple heating rates on the heat flow curve of

Aged SR 524 pyrotechnic composition

*The peak temperatures corresponding to different heating rates have been reported at the

value of maximum heat flux

Table 6.2 Kinetic parameters of aged SR 524 pyrotechnic composition caculated by

Kissinger method.

Heating Rate

(oC min

-1)

Peak Temperature*

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

k

(sec-1

)

∆H#

(kJ mol-1

)

10 497.9 1.06 x 105 3.71 x 10

-3

20 524.5 110 +6% 1.12 x 105 6.93 x 10

-3 103

30 541.4 1.14 x 105 9.98 x 10

-3

40 558.6 1.04 x 105 12.76 10

-3

135

to 558oC when heating rate increses from 10

oC min

-1to 40

oC min

-1(Table 6.2). It is also

seen from Figure 2(b) that the shape of the decomposition peak of the aged compositions

at different heating rates is not very regular and smooth as compared to the fresh

composition. This is considered to be due to the physical changes caused in the

pyrotechnic compositions during the aging process and will be discussed on the basis of

SEM and XRD results.

The mass loss curve of fresh and aged composition has been presented in Figure 6.3. The

mass loss curve of fresh composition remains stable till 300oC. There is a gradual mass

loss above 300oC till 500

oC due to slow decomposition of the boiled linseed oil present in

the pyrotechnic composition. There is a rapid increase in the mass of the sample above

500oC due to oxidation of the magnesium fuel by the oxygen released due to the thermal

decomposition of the oxidizer.

Figure 6.3 The mass loss curve of fresh and aged SR 524


The curve stabilizes again near 600oC. The mass loss curve of the aged composition

shows a slight mass loss till 300oC due to removal of physically adsorbed water.

136

Moreover, it shows mass loss of nearly 6 percent between temperature range of 300oC

and 350oC due to removal of chemically bonded water due to thermal decomposition of

magnesium hydroxide. This change in the mass loss curve shows that aging of the

pyrotechnic composition in a highly humid atmosphere has resulted in the production of

magnesium hydroxide. The presence of magnesium hydroxide has also been detected in

the XRD spectra of the aged composition. Rest of the events in the mass loss curve are

similar to the ones discussed for the fresh composition.

6.4.2 Kinetic Analysis of fresh and aged compositions

The kinetic analysis of the fresh and aged pyrotechnic composition SR-524 has been

carried out by using the Kissinger method. The graphs for determination of the kinetic

parameters of fresh and aged compositions are shown in Figure 6.4(a) and Figure 6.4(b)

respectively.

Figure 6.4(a) The Kissinger graph for determination of kinetic parameters of

fresh SR 524 pyrotechnic composition

137

They are based on the heat flow data obtained at multiple heating rates. The experiments

were performed as per the recommendations of the international committee of thermal

analysis and calorimetry (ICTAC) [25].

The results of the kinetic analysis are presented in Table 6.1 and 6.2 for fresh and aged

compositions respectively. The activation energy of the fresh composition was found to

be 256 kJ mol-1

and the corresponding value of enthalpy of activation was 249 kJ mol-1

.

The frequency factors showed slight variation with respect to peak temperatures and its

average value was found to be 4.08 x1014

sec-1

. The rate constant for the fresh

composition was nearly 8x10-3

sec-1

corresponding to heating rate of 10oC min

-1.

Figure 6.4(b) The Kissinger graph for determination of kinetic parameters of

aged SR 524 pyrotechnic composition

The kinetic analysis of aged composition shows that there is a substantial variation in the

kinetic parameters after the aging of pyrotechnic composition at high temperatures and

138

high humidity levels. The activation energy has decreased prominently by 146 kJ mol-1

from 256 kJ mol-1

for the fresh composition to 110kJ mol-1

for aged composition (Table 2).

The enthalpy of activation also decreased in the aged composition and its value came out

to be 103 kJ mol-1

. There is a huge decrease in the value of frequency factor due to large

decrease in the activation energy. The average value of the frequency factor for the aged

composition is 1.09 x 105. The rate constant also decreased for the aged composition and

its value was found to be 3.71 x 10-3

at a heating rate of 10oC min

-1.

The kinetic analysis shows that there is a decrease in the rate constant or in other words

there is a decline in the reactivity of the aged pyrotechnic composition despite the fact

that the activation energy has decreased tremendously. Ideally speaking, the decrease in

the activation energy should increase the rate of reaction. This however is not that simple

because the Arrhenius kinetic parameters are interlinked with each other and the rate

constant is determined by a complex interplay between the values of the temperature,

activation energy and the frequency factor. For instance, the increase in the temperature

increases the rate constant due to increase the value of the negative exponential term in

the rate equation i.e. (

). On the other hand, the increase in temperature lowers the

frequency factors due to the decrease in the value of positive exponential term i.e.

(

) and also decrease in the value of

. Thus increase in temperature lowers

the rate constant by lowering the frequency factor. We have just discussed the effect of

temperature here, although, rate constant shows a similar kind of dependence on

activation energy and frequency factor as well. Therefore, the experimental results may

vary from case to case basis and the experimental results in our case agree fairly with the

theoretical interpretation.

6.4.3 SEM Analysis of the fresh and the aged compositions

The surface feature of the fresh pyrotechnic composition SR-524 has been presented in

Figure 6.5 (a). It is seen that the surface of the fresh composition is very smooth and solid

without any pores and cracks. The magnesium particles can also be seen in the

micrograph. The micrographs were captured for the fresh and aged pyrotechnic

139

compositions in the bulk form to monitor the changes caused due to aging at high

temperatures and high humidity levels.

Figure 6.5 (a) SEM micrographs of the fresh SR-524 composition showing

smooth and solid surfaces

The SEM micrographs for the aged composition are presented in Figure 6.5 (b) for

comparison with the fresh composition. The most prominent change visible in the

micrographs is the presence of micro sized cracks. The development of the cracks is

certainly due to the aging of the pyrotechnic composition. During the thermal analysis of

the aged composition it was observed that the heat flow peaks are not very smooth and

regular. The development of the cracks seems to be the reason for the peak irregularity in

the aged composition. Moreover, the development of cracks is one of the main reasons

for such a huge decrease in the activation energy of the aged composition. The cracked

and porous compositions are generally easy to ignite as compared to the solid ones. The

aged composition therefore is easy to ignite due to the presence of cracks as compared to

the fresh composition due to relatively very low value of activation energy. The

140

Figure 6.5(b) SEM micrographs of aged SR-524 composition showing micro cracks

development of cracks however is a safety concern and the compositions can be

dangerous during storage as well as during the operational use.

6.4.4 XRD Analysis

The XRD pattern of the fresh pyrotechnic composition is presented in Figure 6.6 (a).

Nearly fifteen main diffraction peaks can be seen from the spectra. The peak data

matches precisely with the reference pattern number 00-004-0770 for magnesium powder

and reference pattern number 01-079-2056 for Sodium nitrate. Six peaks out of the

fifteen are characteristic peaks of sodium nitrate whereas eight peaks represent

magnesium. One of the peaks appearing at angular position of 47.819o could be assigned

to both magnesium and sodium nitrate. The peaks have been labeled accordingly in the

Figure. The crystal structure of the magnesium is hexagonal in this case and the crystal

structure of the sodium nitrate is rhombohedral.

XRD pattern of the aged pyrotechnic composition is presented in Figure 6.6 (b) The

figure shows some additional diffraction peaks representing magnesium hydroxide which

141

means that some of the magnesium present in the pyrotechnic composition has reacted

with the water vapors to form magnesium hydroxide as the reaction product of aging. The

Figure 6.6 XRD spectra of fresh and aged SR 524 pyrotechnic composition

TG curve also showed the presence of the magnesium hydroxide. These peaks were

absent in the XRD pattern of the fresh pyrotechnic composition. This clearly indicates

that high humidity levels strongly effect the chemical composition of the pyrotechnics.

Rest of the diffraction peaks are very much similar to the ones for the fresh composition.

The diffraction peak data of the aged composition matches with the reference pattern

number 00-004-0770, 01-079-2056 and 00-044-1482 for magnesium, sodium nitrate and

magnesium hydroxide respectively. The crystal structure and the cell parameters of the

magnesium and sodium nitrate did not change due to the aging. The crystal structure of

the magnesium hydroxide produced as the reaction product of the aging is hexagonal.

The mass loss curve also showed the presence of magnesium hydroxide in the aged

142

version. There was some additional mass loss in the temperature range of 300oC-350

oC

for the aged composition which could be due to the thermal decomposition of the

magnesium hydroxide formed due to the aging process.

6.4.5 Comparison of fresh and aged compositions

The comparison of the experimental results obtained for the fresh and the aged

compositions is obligatory to establish the overall effect of aging on the SR-524

pyrotechnic composition. The comparison of the DTA curve shows that the aged

composition ignites at lower temperature as compared to the fresh composition. The TG

results also confirm the lowering of the ignition temperature after aging. The aged

composition showed the ignition peak close to a temperature 524oC which is 14

oC lower

than the corresponding peak of the fresh composition. The DTA results indicate that the

ignition peaks shift to higher temperatures with increasing heating rates and the trend is

similar for both the fresh and aged compositions. The comparison of the kinetic

parameters showed prominent changes in the aged composition. The kinetic analysis

revealed that the activation energy of the aged composition decreased greatly by nearly

57 percent. The frequency factor also decreased noticeably after the aging. The aged

composition was found to be less reactive as compared to the fresh composition. There

was a noticeable decrease of nearly 53 percent in the rate of reaction constant of the aged

composition. The decrease in the activation energy meant that the composition after

aging became easier to ignite, however, the reaction proceeded at a lower rate due to

decrease in the overall rate constant.

The comparison of the SEM graphs of the fresh and aged pyrotechnic compositions

reveals that the surface of the aged composition showed some micro cracks as opposed to

a very solid and smooth surface of the fresh composition. The micro cracks could be one

of the reasons for such a large decrease in value of the activation energy. The comparison

of the XRD pattern of the fresh and aged composition showed some additional peaks for

the aged composition that were identified as the characteristic peaks of the magnesium

hydroxide. Some of the magnesium reacted with the water vapours at high temperatures

143

to form magnesium hydroxide as the reaction product of aging. The comparison of results

obtained by thermal analysis, kinetics, XRD and SEM clearly indicates that the exposure

of pyrotechnic compositions to high humidity and high temperatures for longer duration

has significantly altered the overall behaviour of the composition.

6.4.6 Evaluation of the pyrotechnic thermal stability

The critical ignition temperature is considered to be an important parameter pertaining to

the thermal stability and safety of energetic materials such as explosives, propellants and

pyrotechnics. It is the minimum temperature beyond which there is a chance of thermal

runaway reaction for the energetic materials [26,27]. It can be effectively used as a

thermal safety standard for the propellants, explosives and pyrotechnics. The energetic

composition is assumed to be safe for handling and storage if the critical ignition

temperature is adequately higher than the ambient temperature at which the composition

is to be stored. The critical ignition temperature can be easily estimated by using the

thermal and kinetic parameters and employing the under-mentioned formulae [28]. This

approach is common in the scientific community working on pyrotechnics for more than

a decade.

√

(6.1)

(6.2)

Where is the critical ignition temperature , E is the activation energy, R is the gas

constant, is the onset temperature corresponding to → 0 approximated using

regression coefficients b and c.

144

For the fresh and aged pyrotechnic composition, the values obtained by using equation

(6.2) are 518.6oC and 480.8

oC respectively. The above mentioned values of have been

used to calculate the critical ignition temperatures by using equation (6.1). The critical

ignition temperature of the aged composition was found to be 499.6oC which is lower

than the value of 527.6oC for the fresh composition. It would not be out of place to clarify

here that although the critical ignition temperature has decreased by 28oC due to aging,

however, the composition is still thermally safe due to the fact that the critical ignition

temperature of the aged composition is exceptionally higher than the ambient range of

temperatures. The SEM results, however, suggest that the development of micro cracks is

critical for its safety and the composition may be unpredictable during its storage,

ignition and functioning.

6.5 Conclusions

The experimental results confirmed that high temperatures and high humidity levels had a

pronounced effect on the thermal and kinetic behaviour of the pyrotechnic composition

based on the magnesium and sodium nitrate. Thermal analysis showed that the

decomposition peak temperature of the aged composition lowered by 14oC. The kinetic

parameters of the pyrotechnic mixture changed considerably after the aging. The

activation energy of aged composition decreased nearly by 57 percent and the frequency

factor also decreased noticeably. The aging process made the composition easy to ignite

by lowering the activation energy, however, the overall reactivity of the composition

reduced due to decrease in the rate constant. The rate constant decreased due to a large

decrease in the value of the frequency factor. XRD results suggest that the high humidity

levels resulted in the production of magnesium hydroxide as the reaction product of the

aging process. The combined effect of high temperature and high humidity resulted in the

development of micro sized cracks in the bulk composition and made it relatively unsafe

for storage and ignition as compared to the fresh composition. It is therefore concluded

that high humidity and elevated temperatures have a strong influence on the thermal

behaviour and kinetic parameters of pyrotechnic composition investigated in this work.

145

6.6 Accelerated aging of SR-562 pyrotechnic composition and

investigation of its thermo kinetic parameters

SR-562 pyrotechnic composition contains 50 percent magnesium powder, 35 percent

sodium nitrate, 11 percent calcium oxalate and 4 percent lithographic varnish. The

composition is used as a flare for high heat and light output applications. The accelerated

aging of SR-562 pyrotechnic composition has been carried out in a humidity controlled

oven at 70oC and 70 percent relative humidity for 30 days. As discussed earlier, unlike

propellants, there are no specific aging criteria for the pyrotechnics. The military

compositions are conventionally tested up to 70oC to cater for the effect of maximum

possible ambient temperatures. Moreover, various aging studies on pyrotechnics and

propellants have been previously carried out in a highly humid atmosphere (generally

more than 60 percent relative humidity) to get meaningful data after the aging process.

The composition after the aging was investigated in the present work for its thermal

behaviour, kinetics and morphology. For the sake of comparison, the fresh composition

was also investigated to quantify the changes in the aged composition.


Defence grade pyrotechnic composition SR-562 was procured from Pakistan ordnance

factories. The composition was aged at a temperature of 70oC and 70 percent RH for a

period of 30 days. Scanning electron microscopy and X-ray diffraction were used to

observe changes induced by the aging process. The reaction products after the aging have

been identified by using XRD instrument by STOE; Germany. Scanning electron

microscope JSM- 6490 LA by JEOL, Japan has been used. Diamond TG/DTA instrument

of Perkin Elmer has been used to carry out the thermal analysis. The experiments were

carried out in the temperature range of 30oC to 850

oC to elucidate the mechanism of

thermal decomposition. Nitrogen gas with a flow rate of 50 ml min-1

has been used to

provide inert atmosphere during the conduct of experiments. Kinetic parameters of the

fresh and aged compositions were calculated by Kissinger method from the heat flow

146

data. For the kinetic evaluation, the experiments were carried out at multiple heating rates

of 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC min

-1. A very small amount of sample

nearly 1 mg has been used in all the experiments to cater for the safety aspects. Alumina

pans were used to hold the sample in all the experiments.

6.8 Results and discussion

6.8.1 Thermal Analysis of the fresh and aged compositions

Thermal analysis of the fresh SR-562 pyrotechnic composition has been carried out from

room temperature to 850oC to identify and analyze different thermal events including the

ignition of the pyrotechnic mixture. The DTA curves of the fresh and the aged SR-562

pyrotechnic compositions at a heating rate of 20oC min

-1have been shown in Figure 6.7.

The DTA curve of the fresh composition exhibits two distinct peaks; one of which is

endothermic and the other is exothermic in nature.

Figure 6.7 Comparison of the DTA curve of fresh and aged

SR-562 pyrotechnic composition

147

The first peak is very small and endothermic in nature that appears at a temperature of

311oC. This peak represents the melting of sodium nitrate which is present in this

composition. This peak is not clearly visible as it has been masked due to relatively a

very large heat flow output of the next peak. The second peak in the Figure is very

prominent and highly exothermic in nature. This peak appears at a temperature of

595.4oC and signifies ignition of the pyrotechnic mixture. The ignition of the magnesium

fuel takes place due to availability of sufficient oxygen produced as a result of the

thermal decomposition of the two oxidizers. The DTA curve of the aged pyrotechnic

composition is also shown in Figure 6.7 for comparison with the fresh composition. The

aged composition also exhibits two distinct peaks like the fresh composition. The first

peak endothermic peak did not change but the location of the second peak has been

changed on the temperature axis. For the aged composition, the exothermic peak related

to ignition of the pyrotechnic mixture appears at a temperature of 646.8oC. This

temperature is nearly 51oC higher than the ignition temperature of the fresh composition.

Figure 6.8 represents the effect of multiple heating rates on the DTA curve of the fresh

SR-562 pyrotechnic composition.

Figure 6.8 Effect of heating rate on the DTA curve of fresh SR-562 pyrotechnic composition

148

The Figure shows that the position of the first endothermic peak has not changed much

with the increasing heating rates. The peak is very small and has been masked due to very

large heat flow output of the ignition peak. The ignition peak has been shifted to higher

temperatures at higher heating rates. The ignition peaks appear at temperatures of

580.5oC, 595.4

oC, 605.7

oC and 615.2

oC respectively for heating rates of 10

oC min-1,

20oC min-1, 30

oC min-1 and 40

oC min-1 respectively and the data is presented in Table

6.3. Many researchers working in the field of energetic materials have reported the

similar trend of increase in the peak temperatures due the increase in the heating rates.

Figure 6.9 shows the effect of multiple of heating rates on the DTA curve of the aged SR-

562 pyrotechnic composition. The effect of heating rate is very much similar to the one

seen for the fresh composition. Ignition peaks of the aged composition also shifted to the

Figure 6.9 Effect of heating rate on the DTA curve of aged SR-562

pyrotechnic composition

149

higher temperatures with the increase in the heating rates. The peak temperatures for the

aged composition are 626.1oC, 646.8

oC, 662.9

oC and 674.7

oC respectively for the heating

rates of heating rates of 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC min

-1. The ignition

peak data of the aged sample at different heating rates has been shown in Table 6.4. It

can be seen from the data presented in Table 6.3 and 6.4 that for all the heating rates,

ignition temperature of the aged composition is higher than the corresponding ignition

temperature of the fresh composition.

*The peak temperatures corresponding to different heating rates have been

reported at the value of maximum heat flux

Table.6.3 Kinetic parameters of fresh SR-562 composition calculated from the

heat flow data.

Heating Rate

(oC min

-1)

Peak Temp.*

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

K

(sec-1

)

∆H#

(kJ mol-1

)

10 580.5 3.00 x 1012

6.59 x 10-3

20 595.4 239.5+6% 3.25 x 1012

12.73 x 10-3

232.3

30 605.7 3.23 x 1012

18.65 x 10-3

40 615.2 2.97 x 1012

24.34 x 10-3

Table.6.4 Kinetic parameters of aged SR-562 composition calculated from the

heat flow data.

Heating Rate

(oC min

-1)

Peak Temp.*

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

K

(sec-1

)

∆H#

(kJ mol-1

)

10 626.1 3.06 x 108 4.61 x 10

-3

20 646.8 186.2+5% 3.33 x 108 8.82 x 10

-3 178.8

30 662.9 3.17 x 108 12.78 x 10

-3

40 674.7 3.07 x 108 16.62 x10

-3

150

6.8.2 Kinetic Analysis of the fresh and aged composition

Kissinger method has been used to calculate the kinetic parameters in this work. The

relevant graphs for the fresh and aged pyrotechnic mixtures have been given in Figure

6.10(a) and Figure 6.10(b) respectively. The heat flow data from the DTA experiments at

four different heating rates has been used to construct the Kissinger kinetic plot. Multiple

heating rate experiments were performed in accordance with the guidelines provided by

the international committee of thermal analysis and calorimetry (ICTAC) [25]. The

kinetic parameters of both versions of the sample were calculated by following the

Standard ASTM method.

Figure 6.10 (a) Kissinger graph for calculation of Kinetic parameters of

fresh SR-562 pyrotechnic composition

Kinetic evaluation of both the fresh and the aged pyrotechnic compositions has been

carried out to compare and contrast the results. The kinetic parameters of both the

samples have been included in Table 6.3 and 6.4 respectively for the fresh and aged

compositions. Energy of activation for the fresh SR-562 pyrotechnic composition came

out to be 239.5 kJ mol-1

; whereas, the enthalpy of activation in this case was found to be

232.3 kJ mol-1

. The pre-exponential factor changed slightly in each case due to change in

151

the values of peak temperatures and heating rates. The average value of the pre-

exponential factor came out to be 3.11 x1012

sec-1

. The rate of reaction constant of the

fresh pyrotechnic mixture was found to be 6.59x10-3

sec-1

at a heating rate of 10oC min

-1.

The details are presented in Table 6.3.

Figure 6.10 (b) Kissinger graph for calculation of Kinetic parameters of

aged SR-562 pyrotechnic composition

It was revealed after the kinetic evaluation of the aged sample that the aging of

pyrotechnic composition at high temperatures and humidity changed both the thermal and

kinetic parameters significantly. The activation energy of the aged composition was

found to be 186.2 kJ mol-1

. This value is notably lower than the activation energy of the

fresh sample. The activation energy of the pyrotechnic mixture after aging has lowered

by 22 percent. The value of the enthalpy of activation of the aged sample was also found

to be lowered and in this case came out to be also decreased in the aged composition and

its value came out to be 178.8 kJ mol-1

. The decrease in the activation energy has greatly

decreased the value of the pre-exponential factor of the aged composition. For the aged

composition, the average value was found to be 3.15 x 108. Also, the reaction rate

constant in this case was found to be 4.61 x 10-3

for the heating rate of 10oC min

-1 which

is lower than the one for the fresh sample.

152

The comparison of the thermal and kinetic data show the ignition temperature has

increased for the aged composition due to which the rate constant should also have

increased but in this case the rate constant has rather decreased. It is clear from the

Arrhenius rate equation that the rate constant is dependent on three different factors i.e.

frequency factor, activation energy and the temperature. One of the reasons for the

decrease in the rate constant of the aged composition in this particular case is the

temperature dependence of the frequency factor „A‟ that has reduced significantly due to

increase in the value of the peak temperature and lowering of the activation energy. The

decrease in the frequency factor is responsible for the decrease in the reactivity of the

aged composition. Therefore, the experimental results in this work are consistent with the

theoretical justification.

It is interesting to note that the aged composition on one hand has become easy to ignite

as it requires less amount of activation energy for the start of reaction and on the other

hand it has become less reactive because the reaction proceeds slowly as compared to the

fresh composition.

6.8.3 SEM Analysis of fresh and aged compositions

SEM analysis of both the compositions was carried out to identify any physical changes

caused by the aging process. The surface of the fresh composition SR-562 was

examined by SEM and the relevant micrograph is presented in Figure 6.11(a). The Figure

shows that surface of the composition quite solid and even and there are no cracks at all.

The micrographs for both the fresh and aged compositions were recorded in the bulk

form at same level of magnification to see the differences clearly and for better

comparison of the results. Figure 6.11(b) presents the SEM micrograph of the aged SR-

562 pyrotechnic composition. it can be seen from the Figure that the surface features of

the aged composition are different as compared to the fresh composition. The micro sized

cracks are visible at the surface of the aged composition. These cracks have been

developed due to the aging of the composition at high temperatures and humidity levels.

In the kinetic analysis of the aged composition, the activation energy was found to be

lowered. The development of these micro cracks could be the reason for decrease in the

activation energy because the solid compositions are in general more difficult to ignite

153

whereas the cracked or porous compositions are relatively easy to ignite. It is however a

point of concern because the cracked energetic compositions are not considered to be safe

for handling, storage and operation and therefore considered to be less reliable.

Figure 6.11(a) SEM micrographs of fresh SR 562 pyrotechnic composition

showing solid and even surface

Figure 6.11(b) SEM micrographs of aged SR 562 pyrotechnic composition at

showing micro sized cracks

154

6.8.4 XRD Analysis of fresh and aged compositions

Figure 6.12 (a) shows the XRD pattern of fresh SR-562 pyrotechnic composition. There

are about twelve diffraction peaks which are visible in the spectra. Some of the peaks for

different components of the mixture have overlapped and therefore represent the presence

of more than one component. The peak data of the fresh composition agrees with

reference pattern 00-004-0770 that indicates the presence of magnesium powder,

reference pattern 01-085-1467 which indicates the presence of sodium nitrate and

reference pattern 00-035-0914 for calcium oxalate. Ten peaks in the spectra confirm the

presence of magnesium and four peaks indicate the presence of sodium nitrate, three

peaks in the spectra are characteristic of calcium oxalate. Each individual peak has been

labeled accordingly to show the presence of different components of the mixture.

Magnesium, sodium nitrate and the calcium oxalate particles present in this sample have

hexagonal, rhombohedral and orthorhombic crystal structures respectively.

Figure 6.12(a) XRD spectra of fresh SR-562 pyrotechnic composition showing the

presence of magnesium, sodium nitrate and calcium oxalate.

155

Figure 6.12(b) XRD spectra of aged SR-562 pyrotechnic composition showing the

presence of magnesium hydroxide as the reaction product of aging.

Figure 6.12(b) represents the XRD spectra of the aged SR-562pyrotechnic composition.

It is clearly seen from the Figure that few additional diffraction peaks appear in the

spectra. The analysis of these peaks reveals that they are representative peaks of

magnesium hydroxide. The presence of magnesium hydroxide in the spectra indicates

that due to high temperatures and elevated humidity levels some part of the magnesium

powder present in the aged mixture reacts with the water vapors and results in the

production of magnesium hydroxide. The magnesium hydroxide peaks were not at all

present in Figure 6.12(a) representing XRD spectra of the fresh composition and

remaining peaks are similar for both the compositions. These results confirm that

simultaneous presence of high humidity levels and high temperatures has seriously

affected the chemical composition of the aged composition SR-562. The XRD pattern in

this case is consistent with the reference patterns number 00-004-0770, 01-085-1467, 00-

035-0914 and 00-044-1482 for magnesium, sodium nitrate, calcium oxalate and

156

magnesium hydroxide respectively. The cell parameters and crystal structures of all the

components of the pyrotechnic mixture remained the same. The magnesium hydroxide

produced as a result of aging in this case has hexagonal crystal structure.

6.8.5 Comparison of fresh and aged compositions

The comparative analysis of the experimental results obtained for the fresh and the aged

compositions is briefly described here to establish the overall effect of aging on the SR-

562 pyrotechnic composition. The comparison of the DTA results showed that the aged

composition ignited at higher temperature as compared to the fresh composition. The

ignition peak of the aged composition appeared close to a temperature 646.8oC which is

51oC higher than the corresponding peak of the fresh composition. The DTA results

indicate that the ignition peaks shift to higher temperatures with increasing heating rates

and the trend is similar for both the fresh and aged compositions. The comparison of the

kinetic parameters showed prominent changes in the aged composition. The kinetic

analysis revealed that the activation energy of the aged composition decreased by nearly

22 percent. The frequency factor also decreased noticeably after the aging. The aged

composition was found to be less reactive as compared to the fresh composition. There

was a noticeable decrease of nearly 30 percent in the rate of reaction constant of the aged

composition. The decrease in the activation energy meant that the composition after

aging became easier to ignite, however, the reaction proceeded at a lower rate due to

decrease in the rate constant.

The comparison of the SEM graphs of the fresh and aged pyrotechnic compositions

revealed that the surface of the aged composition showed some micro cracks as opposed

to a very solid and smooth surface of the fresh composition. The micro cracks could be

one of the reasons for the decrease in value of the activation energy. The comparison of

the XRD pattern of the fresh and aged composition showed some additional peaks for the

aged composition that were identified as the characteristic peaks of the magnesium

hydroxide. Some of the magnesium reacted with the water vapours at high temperatures

to form magnesium hydroxide as the reaction product of aging. Similar reaction of

magnesium with the water vapours was observed for SR-524 composition as well.

157

The comparison of results obtained by thermal analysis, kinetics, XRD and SEM clearly

indicates that the exposure of pyrotechnic composition SR-562 to high humidity and high

temperatures for longer duration has significantly altered the overall behaviour of the

composition.

6.8.6 Estimation of thermal stability of the pyrotechnics

There is an important parameter known as the critical ignition temperature that can be

used to predict the thermal stability of the pyrotechnic, explosives and propellant. For the

energetic materials, it is defined as the minimum temperature which if exceeded may lead

to a thermal runaway reaction. This parameter can be used to predict the safety of the

pyrotechnic compositions during storage and handling. For the pyrotechnic compositions

to be safe for storage, the critical ignition temperature should be sufficiently higher than

the maximum possible ambient temperatures. Once thermal and kinetic parameters of a

composition are known, it is easy to estimate the critical ignition temperature by using

the equation (6.1) described above [29]. This method of estimating the critical ignition

temperature is now very common since last one decade and it is being effectively used by

researchers working in the field of energetic material including pyrotechnics.

The value for the fresh SR-562 pyrotechnic composition has been found to be

570.6oC from equation (6.2) and for the aged composition it has been found to be

612.1oC. The critical ignition temperatures for both the compositions have been

calculated from equation (6.1) by using the values mentioned above. The critical

ignition temperature for the fresh composition came out to be 582.4oC and it is lower than

the critical ignition temperature of the aged composition which is 629.8oC. Both these

temperatures are sufficiently higher than the maximum possible ambient temperatures

and therefore the fresh and the aged composition are thermally stable.

6.9 Conclusions

The results indicate that aging of the SR-562 pyrotechnic composition at extreme

conditions of temperature and humidity has changed the thermal behaviour, kinetic

parameters, chemical composition and the surface features to a large extent. The

comparison of thermal data for a heating rate of 20oC min

-1 showed that the ignition

158

temperature of the aged composition increased by nearly 51oC. The aging process also

changed the kinetic parameters of the SR-562 pyrotechnic composition due to the

changes in the thermal behaviour. The kinetic analysis showed nearly 22 percent decrease

in the value of activation energy of the aged composition as compared to the fresh

composition. The pre exponential factor was found to be lowered as well in the aged

sample. The rate constant was decreased by nearly 30 percent after the aging despite

decrease in the value of activation energy due to the fact that the pre exponential factor

decreased considerably. This means that the aged composition became easy to ignite due

to lowering of activation energy but its reactivity decreased due to lowering of the

reaction rate constant. XRD results of the aged composition depicted that magnesium

hydroxide has been formed due to reaction of some of the magnesium with water

vapours. SEM micrographs showed the presence of micro sized cracks on the surface of

the pyrotechnic composition and therefore the aged composition is likely to be unsafe

during storage and operational use.

References





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(1997) 263.

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and calorimetry 85 (2006) 151.

[13] M.D. Judge, Propellants, Explosives, Pyrotechnics 28 (2003) 114.

[14] Y. Li, W. Song, C. Xie, D. Zeng, A. Wang, M. Hu, Materials chemistry and

physics 97 (2006) 127.

[15] Ö. Hocaoğlu, F. Pekel, S. Özkar, Journal of applied polymer science 79 (2001)

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Lemmetyinen, Thermochimica acta 417 (2004) 223.

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Griffiths, H. Lemmetyinen, Thermochimica acta 426 (2005) 115.




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160

Chapter 7

Thermal decomposition kinetics of nano zinc

oxide and nano magnesium oxide catalyzed

composite solid propellant

7.1 Introduction

Propellant compositions based on ammonium perchlorate (AP) and hydroxyl-terminated

polybutadiene (HTPB) are frequently used in the field of composite solid propellants [1-

3]. Ammonium perchlorate is used as an oxidizer for the combustion of hydroxy-

terminated polybutadiene (HTPB) binder which also acts as a fuel. Although some new

oxidizers such as ammonium dinitramide (ADN) and hydrazinium nitroformate (HNF)

have been inducted as a replacement of ammonium perchlorate (AP), however, AP is still

preferred in variety of propellant and pyrotechnic formulations due to its high density,

good oxygen balance and excellent compatibility with other ingredients. The impact of

different additives especially transition metal oxides (TMOs) on thermal decomposition

and kinetic behaviour of AP and propellant formulations containing AP has always

attracted attention of the scientific community working in the field of energetic materials

[1,4,5]. This chapter presents the effect of a small percentage of zinc oxide and

magnesium oxide nano particles on the thermal decomposition and the kinetic behaviour

of HTPB/AP composite solid propellant. Variation in the thermal decomposition pattern

of AP/HTPB composite consequently influences the combustion process.

Transition metal oxides (TMOs) are often used to catalyze the thermal decomposition of

the AP/HTPB based composite solid propellant and to improve their performance by

enhancing the burn rates [6-8]. They are commonly referred as the burn rate modifiers.

161

Different factors such as concentration of TMOs, their particle size and the exposed

surface area influence the overall effect of these modifiers on the burn rate of propellant

[9]. The nano sized additions are now preferred over the conventional micro sized

additions [10]. The smaller size of catalyst particles increases the exposed surface area

and enhances the catalytic activity considerably.

A lot of research has been carried out to see the effect of micro/nano sized transition

metal oxides on the thermal decomposition and kinetics of AP and composite solid

propellants. Patil et al. investigated the effect of nano ferric oxide on the thermal

decomposition of the HTPB/AP based composite solid propellant and reported a higher

heat release of 40 percent in the catalyzed version [6]. The authors, however, reported a

significant increase in the activation energy of the catalyzed propellant contrary to a

general trend of decrease in activation energy. Similarly, Duan et al. have recently

investigated the effect of different percentages of nano MgO on the thermal

decomposition of AP and reported that it has a strong catalytic effect on the

decomposition of ammonium perchlorate [7]. The work presented in chapter 4 also

described the effect of addition of nano magnesium oxide catalyst on the thermal

decomposition kinetics of ammonium perchlorate. The results clearly indicated the

effectiveness of the catalyst and significant changes in thermal as well as the kinetic

behaviour were observed. Sun et al. have reported the effect of ZnO twin cones on the

thermal decomposition of ammonium perchlorate and observed that these cones have a

strong catalytic effect on AP and significantly lower its decomposition temperature [11].

The authors, however, have not computed the kinetic parameters in their work. Tang et

al. has very recently reported the catalytic effect of ZnO nano crystals on the thermal

decomposition of AP. They reported that both the decomposition temperature and the

activation energy were lowered by the addition of ZnO [12].

In the present work, we have reported the effect of nano sized zinc oxide and magnesium

oxide on the thermal decomposition and kinetic behaviour of composite solid propellant

containing HTPB/AP. The nano particles of zinc oxide and magnesium oxide were

characterized using SEM and XRD techniques before mixing with the propellant

composition. Moreover, comparative analysis of the thermal and kinetic data has been

162

carried out for catalyzed and un-catalyzed composite solid propellant. Thermal analysis

of the propellant composition was carried out using differential scanning calorimetry

(DSC) and Thermogravimetery (TG). The Arrhenius kinetic parameters were estimated

by using four different kinetic methods based on non-isothermal data obtained from DSC

and TG.

Thermal analysis is an effective analytical technique for the investigation of different

materials including energetic materials such as explosives, propellants and pyrotechnics

[13-15]. The technique is essentially very safe due to the requirement of an extremely

small amount of the sample for the analysis. Differential scanning calorimetry (DSC) is

often used to investigate the thermal decomposition and the kinetic parameters of highly

energetic materials including various types of propellants [16,17].


Defence grade composite solid propellant has been used to accomplish the present work.

The composite solid propellant containing 80 percent AP, 15 percent HTPB and 5 percent

additives was synthesized by National Development Complex on our request as per the

above mentioned specifications using a slurry cast technique. One of the compositions

was prepared without the addition of nano catalysts and will be called as composition „A‟

in the entire chapter. The other composition was prepared by adding 2% zinc oxide nano

particles and will be called as composition „B‟. The third composition was prepared by

adding 2% nano magnesium oxide and will be called as composition “C” in this chapter.

The bimodal particle distribution containing AP particles of two different sizes was used.

The size of the smaller AP particles was in the range of 121-131µm and that of the larger

AP particles was between 334-362µm. The ratio of the two different sized particles was

50/50 by mass of the total AP present in the composite propellant. Zinc oxide nano

particles were synthesized via Sol-Gel method originally reported by Omivar et al.

(Omivar, 2012) while magnesium oxide was purchased off the shelf. The nano particles

of ZnO and MgO were characterized using scanning electron microscope (SEM) and X-

ray diffraction techniques. The XRD instrument by STOE, Germany with Cu kα

radiations was used for structural analysis. The angular scan range was kept between 20o

to 80o. Scanning electron microscope JSM- 6490 LA, JEOL, Japan has been used to

163

acquire the micrographs of ZnO and MgO nanoparticles. Thermal analysis of the

catalyzed and un-catalyzed versions of composite solid propellant was carried out by

using differential scanning calorimetry (DSC). The DSC823e instrument by Mettler

Toledo has been used in this work for thermal analysis. The crucibles made up of

aluminum were used to carry out the analysis of both the samples. The propellant

samples were heated from room temperature to 550oC to analyze the complete

decomposition process. Inert atmosphere was maintained during the conduct of

experiments by using nitrogen gas at a flow rate of 50 ml min-1

. The experiments were

conducted at a set four different heating rates for composition A, B and C to calculate the

kinetic parameters. The heating rates were 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC

min-1

. For safety considerations, and to mitigate the effect of higher heating rates the

mass of the sample was kept very small i.e. between 1.5 to 2 mg in all experiments. Heat

flow data obtained from the DSC experiments was then used for determination of the

Arrhenius kinetic parameters for both the propellant compositions using Kissinger

method. The Diamond TG instrument by Perkin Elmer has been used for thermo

gravimetric analysis of composition A, B and C.

7.3 Kinetics of thermal decomposition

Kinetic parameters give important information regarding the decomposition process.

Kinetic parameters of the two versions of the propellant were determined using four

different kinetic methods including Kissinger method, Flynn–Wall–Ozawa method,

Friedman method and Kissinger-Akahira-Sunose method. Brief description of the kinetic

methods is presented below.

7.3.1 Kissinger Method

The details of the Kissinger method are given in Chapter 2 and the final equation is

described below [18]:

(

)

(7.1)

164

In the above equation, is the temperature of decomposition peak, is the heating rate,

A is frequency factor, R is the gas constant, T is the sample temperature, n is the order

of reaction and E is the energy of activation in kJmol-1

. The frequency factor is calculated

by the following equation:

[ (

)]

(7.2)

The rate of reaction constant k can be determined when both E and A are known by the

famous Arrhenius equation.

k = A (

) (7.3)

The kinetic parameters of composition A, B and C calculated by this method are

presented in Table 7.1 and 7.3 and 7.5 respectively.

7.3.2 Friedman Method

Friedman method is one of the most widely used isoconversional method. The final

equation of the Friedman method is given below [19]:

(

) [ ]

(7.4)

Where β is the heating rate (oCmin

-1), α is the degree of conversion, A is the pre

exponential factor, is the activation energy in kJmol-1

, T is the sample temperature and

R is the universal constant.

7.3.3 Flynn–Wall–Ozawa Method

The final equation of the FWO method is given below [20]:

(

) (

) (7.5)

In equation (7.5), is the temperature corresponding to a certain degree of conversion

α, represents the heating rate, A is frequency factor, R is the universal constant and

165

is the energy of activation in kJmol-1

corresponding to a certain degree of conversion α.

7.3.4 Kissinger-Akahira-Sunose Method

The final equation of the KAS method is given below [21]:

(

) (

)

(7.6)

In the equation (7.6), is the temperature corresponding to a certain degree of

conversion α, represents the heating rate, A is frequency factor, R is the universal

constant and is the energy of activation in kJmol-1

corresponding to a certain degree

of conversion α .


7.4.1 Characterization of nano particles of ZnO

The morphology and particle size of the catalyst has a strong effect on the catalytic

mechanism; therefore, a brief description of these features is presented here. Figure 7.1

Figure 7.1 XRD pattern of nano sized zinc oxide used as

actalyst (Miller indices marked)

166

presents the XRD pattern of the ZnO nano particles. Nine diffraction peaks are present in

the XRD pattern and are located at 2 theta position of 31.78o, 34.41

o, 36.24

o, 47.52

o,

56.58o, 62.81

o, 66.38

o, 67.89 and 69.01

o. The peak data agrees with reference pattern No

01-076-0704 showing that these are characteristic peaks of zinc oxide. Corresponding

Miller indices have been marked on the diffraction peaks. The XRD spectra shows the

crystalline nature of the nano particles and crystal structure of the zinc oxide in this case

is hexagonal with cell parameters as a = 0.3253 nm, b = 0.3253 nm and c = 0.5213 nm.

Scherer formula has been used for calculation of the crystallite size of zinc oxide using

XRD data and the average crystallite size came out to be nearly 35 nm.

Figure 7.2 shows the SEM micrograph of ZnO nano particles. The results show that the

particles are well dispersed. The particle morphology is seen to be cylindrical and rod

type in this case. The measurements from SEM image show that the average diameter of

the zinc oxide nano particles is nearly between 80-90nm. The particles are therefore

Figure 7.2 SEM micrograph of zinc oxide nano particles

used as a catalyst

167

expected to cause good catalytic activity in the thermal behaviour of composite solid

propellant due to small size and large exposed surface area.

7.4.2 Characterization of nano particles of MgO

Particle size and the morphology of catalyst play an important role in its catalytic

behaviour and therefore, a summary of these characteristics has been presented for nano

particles of magnesium oxide. Figure 7.3 shows the SEM micrograph of the MgO nano

particles. The particles are seen to be well dispersed and exhibit regularity in their

morphology. The particles are not perfectly spherical rather they seem to be elliptical in

shape. The measurements from the SEM micrograph show that average diameter of the

magnesium oxide particles is between 20 to 30 nm (measured for the elongated side). On

the other hand, the average diameter was found to be nearly 15 to 20 nm for the shorter

side. The size of MgO particles is therefore well within the range of nano meters and

considered to be very suitable for its use as a catalyst.

Figure 7.3 SEM micrograph of magnesium oxide nano particles

MgO nano particles were also characterized by using X-ray diffraction. The XRD spectra

of the MgO nano particles have been presented in Figure 7.4.

168

Figure 7.4 XRD spectra of the MgO nano particles

There are five main diffraction peaks in the spectra which are located at a peak position

of 37.9o, 42.6

o, 62.2

o, 73.3

o, 78.4

o. The diffraction data agrees well with the reference

pattern No 01-074-1225 and the above mentioned peaks represent magnesium oxide

(MgO). The Miller Indices corresponding to all the have been marked in the Figure. The

diffraction peaks are not very sharp showing that the particles are not very crystalline and

have a small particle size. Scherer formula has been used to calculate the crystallite size

of the nano particles and the average crystallite size has been found to be nearly 9nm.

7.4.3 Analysis of Propellant composition A

(a) Thermal analysis

The representative DSC curves for composite solid propellant composition A are shown

in Figure 7.5 at four different heating rates i.e. 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and

40oC min

-1.

169

Figure 7.5 Effect of heating rate on the DSC curve of composition A

representing un-catalyzed CSP

The DSC curves shows two distinct peaks one of which is endothermic and the other is

exothermic in nature. The endothermic peak appears near a temperature of 250oC and

represents an important event of solid state phase transformation of AP present in the

propellant. The crystal structure of AP changes from orthorhombic to cubic as a result of

this phase transformation. This peak did not shift much by the increasing heating rates.

The exothermic peak represents the decomposition of the composite solid propellant and

appears at different temperatures as a consequence of different heating rates. The peak

temperatures are nearly 398.8oC, 414.5

oC, 427.1

oC, 437.9

oC corresponding to heating

rates of 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC min

-1 respectively. The analysis of

composition A was necessary to quantify the effect of the catalyst under similar

experimental conditions and subsequent comparison of results obtained for composition

B.

In our earlier work on thermal cum kinetic comparison of various oxidizers, it was

observed that the AP decomposition exhibited two exothermic peaks corresponding to a

170

low temperature and a high temperature decomposition stage contrary to a single stage

complete decomposition of the composite solid propellant in this case[22]. This is

thought to be due to the strong binding of the AP particles in the polymeric matrix of

HTPB which also acts as a fuel and the complete decomposition of the AP takes place in

a single step.

The mass loss curves of propellant composition A was obtained at multiple heating rates

and the effect of heating rate on the mass loss pattern is shown in Figure 7.6 for

compositions A. The results show that the mass loss step shifts to higher temperatures

when the heating rates are increased. The extent of conversion (α) can be easily

determined from the mass loss data by using the following formula.

(7.7)

Figure 7.6 Mass loss pattern of propellant composition A at multiple heating rates

171

In the above equation, and represent the initial and the final mass of the sample

and represents the sample mass any given time t or at any given temperature T. The

isoconversional kinetic analysis of both the compositions has been carried out at six

different values of conversion degree (α). The overall kinetic analysis of composition A

and B has been carried out by four different methods.

(b) Kinetic analysis by using DSC data (composition A)

The heat flow data obtained from DSC experiments at multiple heating rates has been

used to calculate the kinetic parameters of the composite propellant by the Kissinger

method. The representative graph of the Kissinger method used for determination of the

kinetic parameters is shown in Figure 7.7. The Figure represents the plot of reciprocal

peak temperature against ln β/Tp2. The activation energy in this case was found to be

Figure 7.7 Graph representing the Kissinger method for determination of kinetic

parameters of composition A from DSC data.

172

127kJmol-1

from the heat flow data. This value is in accordance with the different values

reported in the literature. Rocco et al examined the thermal degradation of HTPB/AP

based propellant using DSC at five different heating rates [23]. They reported the

activation energy of the composite propellant as 126.2 kJmol-1

and134.5 kJmol-1

by

Kissinger and Ozawa method respectively. Both these values are in fair agreement with

the activation energy of 127.1 kJ determined in our case. The values of frequency factor,

rate constant and enthalpy of activation have been presented in Table 7.1 for propellant

composition A. The data show that the values of pre exponential factors vary with the

change in the peak temperature due to increase in the heating rate. These values,

however, are quite close to each other and the average value of the frequency factor is

4.6x107s

-1. The rate constant increases with increase in the peak temperature.

(c) Kinetic analysis by using TG data (composition A)

The TG data obtained at multiple heating rates has also been used to evaluate the kinetic

parameters of the composite propellant. The reaction mechanism of the thermal

decomposition of composite solid propellants varies with temperature and therefore the

Table 7.1 Kinetic parameters of composite solid propellant calculated from

DSC data by Kissinger Method (Composition A)

Heating Rate

(oC min

-1)

Peak Temp.

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

k

(sec-1

)

∆H#

(kJ mol-1

)

10 398.8 4.46 x 107 5.66 x 10

-3

20 414.5 127+7% 4.94 x 107 1.07 x 10

-2 121

30 427.1 4.79 x 107 1.56 x 10

-2

40 437.9 4.44 x 107 2.01 x 10

-2

173

activation energy also changes as a function of degree of conversion of the sample. It

would not be out of place to mention here that the decomposition of composite propellant

is a complex and multistep process. Therefore, three different isoconversional methods

based on the TG data have also been used to elucidate the reaction mechanism of the

composite solid propellant. The activation energy of composition A was estimated for α =

0.2, α = 0.3, α = 0.4, α = 0.5, α = 0.6 and α = 0.7. The activation energy was found to be

121.9kJmol-1

, 133kJmol-1

and 139.9kJmol-1

for KAS, Friedman and FWO method

Table 7.2 Activation energy data corresponding to different degrees of conversion for

propellant composition A (calculated from TG data)

respectively (α=0.4). The activation energy data corresponding to other conversion

degrees is presented in Table 7.2. The representative graphs for estimation of activation

energy of composition A by all the three isoconversional methods are given in Figure

7.8(a), 7.8(b) and 7.8(c) for (α=0.4).

It has been found during the kinetic analysis that the activation energy of composite solid

propellant shows some variation with respect to the extent of conversion (α) as the

reaction proceeds. The trend for such a variation has been shown in Figure 9 which

presents the “E” versus “α” curve for composition A by three different isoconversional

methods mentioned above.

Activation Energy (kJmol-1

)

Method

Friedman 126.5 126.9

133

127.8

124

125.9

FWO 133

133.5

139.9

134.5

130.5

132.4

KAS 115.8

116

121.9

116.6

112.7

114.5

174

Figure 7.8 (a) Graph representing the Flynn–Wall–Ozawa (FWO) method for

determination of kinetic parameters of composite solid propellant from

TG data

Figure 7.8(b) Graph representing the Friedman method for determination of kinetic

parameters of composite solid propellant from TG data

175

Figure 7.8 (c) Graph representing the Kissinger-Akahira-Sunose (KAS) method for

determination of kinetic parameters of composite solid propellant from

TG data

Figure 7.9 Plot showing the activation energies versus extent of conversion for

composition A by three different methods (calculated from TG data)

176

7.4.4 Analysis of Propellant composition B


The DSC curves representing the thermal decomposition of the composition B are shown

Figure 7.10 at four different heating rates of 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC

min-1

. The curves show two main peaks similar to the composition A as discussed earlier.

However, a very small exothermic peak is also seen near the main decomposition peak

which may be attributed to partial decomposition of composite solid propellant. The

major peaks for thermal decomposition of composition B appear at 346.9oC, 362.4

oC,

374.1oC and 384.1

oC corresponding to heating rates of 10

oC min

-1, 20

oC min

-1, 30

oC min

-

1 and 40

oC min

-1 respectively. Although, conventionally used catalysts with particle size

in the range of micrometers also have the capability to catalyze the decomposition of

composite propellant, however, the results with the nano sized catalyst are generally

much better due to smaller particle size and greater exposed surface area.

Figure 7.10 Effect of heating rate on the heat flow curve of composition B

containing 2% nano zinc oxide as a catalyst

177

The pronounced effect in this case is the lowering of the decomposition temperature due

to the addition of the zinc oxide nano particles. The peak temperature lowers by

approximately 52oC at a heating rate of 10

oC min

-1. Similar trend is seen for the other

heating rates.

The mass loss curve obtained from thermogravimetery (TG) confirms the lowering of

decomposition temperature in composition B. The comparison of the mass loss curve for

composition A and B has been presented in Figure 7.11. The effect of heating rate on the

propellant composition B is shown in Figure 7.12.

Standard ASTM method has been followed for the calculation of the kinetic parameters

of composition A and B using DSC data based on Kissinger method. The experiments

were performed as per the recommendations of the international committee of thermal

analysis and calorimetry(ICTAC) [24].

Figure 7.11 Mass loss curve of propellant composition A and B

178

Figure 7.12 Mass loss pattern of propellant composition B at multiple heating rates

(b) Kinetic analysis by using DSC data (composition B)


used to calculate the kinetic parameters of the propellant composition B as well for

comparison with composition A. The Kissinger graph for propellant composition B has

been presented in Figure 7.13. The effect of heating rate on the DSC curves and the

details of the kinetic data for composition B have been presented in Table 7.3.

T

h

Table 7.3 Kinetic parameters of composite solid propellant catalyzed with nano zinc

oxide calculated from DSC data by Kissinger Method (Composition B)

Heating Rate

(oC min

-1)

Peak Temp.

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

k

(sec-1

)

∆H#

(kJ mol-1

)

10 346.9 3.67 x 107 6.05 x 10

-3

20 362.4 116+6% 4.03 x 107 1.15 x 10

-2 111

30 374.1 3.92 x 107 1.66 x 10

-2

40 384.1 3.65 x 107 2.15 x 10

-2

179

e activation energy of composition B containing 2 percent catalyst has been found to be

116.1kJ mol-1

from the DSC data. This value is lower than the activation energy of

127.1kJ mol-1

for un-catalyzed propellant composition A. The decrease in the

decomposition temperature and activation energy of the composition B shows that a

significant catalytic activity has taken place. Moreover, the rate constant of the catalyzed

propellant composition B has also increased showing that it has become more reactive.

Figure 7.13 Graph representing the Kissinger method for determination of kinetic

parameters of composition B from DSC data.

Although, the mechanism of effect of the TMOs on the decomposition of composite

propellants is not very well understood, the fact that these TMOs significantly alter the

pattern of AP decomposition and modify the performance of composite propellants is

well established. The effects of lowering of the activation energy, decrease in the

decomposition temperature and increase in the rate constant are consistent with the

general trend reported in the literature on the effect of addition of TMOs on the thermal

cum kinetic behaviour of AP and propellants based on HTPB/AP. For instance, Patil et al

180

reported the effect of nano sized copper chromite and copper oxide on the thermal

decomposition of AP and they found decease in the peak temperature as well as the

activation energy in both the cases [8]. Moreover, Duan et al has very recently

investigated the thermal decomposition of AP catalyzed with different percentages of

nano magnesium oxide [7]. The authors reported that the decomposition peak

temperature is significantly lowered by the addition of MgO nano particles.

In case of the catalyzed thermal decomposition of composite solid propellant, the

frequency factor of composition B has decreased due to the decrease in the activation

energy. The average value of the frequency factor for composition B is 3.8 x 107s

-1 which

is lower than the average value of 4.6 x 107s

-1 for composition A. The rate constant for

composition B increases despite decrease in the frequency factor due to lowering of the

activation energy, which facilitates the rate of reaction. For both the propellant

compositions, the rate constant increases with increase in the peak temperatures as a

consequence of increasing heating rates. The enthalpy of activation also decreases due to

the decrease in the activation energy for composition B.

(c) Kinetic analysis by using TG data (composition B)


parameters of the composite propellant by three different methods. The isoconversional

kinetic analysis of the composition B also confirms that the activation energy of the

composite solid propellant has lowered by the addition of catalyst. The activation energy

of composition B has been determined for α = 0.2, α = 0.3, α = 0.4, α = 0.5, α = 0.6 and α

= 0.7 for comparison with composition A under identical conditions.

The activation energy for composition B was found to be 107.2 kJmol-1

, 117.5 kJmol-1

and 123.5 kJmol-1

for KAS, Friedman and FWO method respectively (α=0.4) by using

TG data. The activation energy values corresponding to different conversion degrees are

presented in Table 7.4. The representative graphs of all the three isoconversional

181

methods for estimation of activation energy of composition B are given in Figure

7.14(a),7.14(b) and 7.14(c) for (α=0.4).

Table 7.4 Activation energy data corresponding to degree of conversion for

propellant composition B ( calculated from TG data).

Figure 7.14(a) Graph representing the Friedman method for determination of kinetic


data

Activation Energy (kJmol-1

)

Method

Friedman 125.9

118.2

117.5

121.6

125.4

127.2

FWO 132.5

124.3

123.5

127.9

131.9

133.8

KAS 116.1

108.1

107.2

111.2

114.8

116.5

182

Figure 7.14(b) Graph representing the FWO method for determination of kinetic

parameters of nano zinc oxide catalyzed composite propellant from

TG data

Figure 7.14 (c) Graph representing the KAS method for determination of kinetic

parameters of zinc oxide catalyzed composite propellant from TG

data

183

The isoconversional kinetic analysis shows that the activation energy of composite solid

propellant composition B varies with the extent of conversion (α) as the reaction

proceeds. The trend of activation energy variation in this case is shown in Figure 7.15

which represents the “E” versus “α” curve for composition B by three different

isoconversional methods.

Figure 7.15 Plot showing the activation energies versus extent of conversion for

composition B by three different methods (calculated from TG data)

The exact mechanism by which TMOs influence the decomposition kinetics of composite

solid propellants is yet to be clearly understood. Ebrahim et al proposed a mechanism for

the thermal decomposition of ammonium perchlorate in the presence of p type

semiconductors such as CuO as a catalyst on the basis of electron transfer to ammonium

ion from perchlorate ion [25].

This electron transfer process is considered to be the rate controlling step. The „p‟ type

semiconductors (ZnO in this case) have a large number of holes on their surface due to

which, they accelerate the electron transfer process.

(7.8)

184

is the oxygen atom that is abstracted from oxide and represents positive

hole in valence band of the oxide.

The comparison of the results for propellant compositions A and B shows that the

addition of nano zinc oxide catalyst has significantly altered the thermal and kinetic

behaviour of the composite propellant. It has shown good catalytic activity by lowering

the decomposition temperature and increased the reactivity of the composite propellant.

The results of thermal cum kinetic analysis for composition A and B are presented in

detail in Table 7.1, 7.2, 7.3 and 7.4.

7.4.5 Analysis of Propellant composition C


Thermal analysis of the composite propellant catalyzed with magnesium oxide nano

particle has also been carried out in the temperature range of 50oC-550

oC. The DSC

curves for catalyzed composite propellant at multiple heating rates are presented in

Figure 7.16.

Figure 7.16 The DSC curves for MgO catalyzed composite propellant at

multiple heating rates

185

The curve for the catalyzed propellant also shows two main peaks similar to non-

catalyzed version of the propellant. First peak related to phase transformation of AP did

not change much and appeared nearly at the same temperature close to 250oC. The

second peak however has changed a lot in terms of its position and shape. The peak

appears at a temperature of 377.1oC at a heating rate of 10

oC min

-1. This means that the

decomposition temperature of the catalyzed composite propellant has lowered by

approximately 22oC due to addition of MgO nano particles. Moreover, the decomposition

peaks in the catalyzed propellant are relatively sharp as compared to the ones for non-

catalyzed propellant. The decomposition peaks in this case also shift to higher

temperatures when the heating rates are increased. The peak temperatures for the

catalyzed version of propellant are 377.1oC, 391.5

oC, 400.6

oC and 407.9

oC respectively

for heating rates of 10oC min

-1, 20

oC min

-1, 30

oC min

-1 and 40

oC min

-1.

The TG curves for the catalyzed propellant are shown in Figure 7.17 at different heating

rates.

Figure 7.17 The TG curves for the MgO catalyzed propellant at multiple heating rates

186

In this case, there is no significant change in the mass of the sample till 300oC but beyond

this temperature there is a rapid mass loss due to decomposition of the composite

propellant. The TG curve also confirms lowering of the decomposition temperature in the

catalyzed version of propellant. The effect of heating rate is similar to the one seen for

non-catalyzed sample of composite propellant.

(b) Kinetic analysis by using DSC data (composition C )


used to calculate the kinetic parameters of the propellant composition C. The multiple

heating rate experiments were performed under exactly the same set of experimental

conditions to investigate the effect of the catalyst on the kinetic behaviour of composite

propellant. The Kissinger method for catalyzed version of the propellant has been

depicted in Figure 7.18.

Figure 7.18 The Kissinger graph for catalyzed version of the

propellant (Composition C)

187

The activation energy calculated from the slope of the Kissinger graph came out to be

155.4kJ mol-1

for the catalyzed propellant. This value is higher than the activation energy

of 127.1kJmol-1

for the non-catalyzed version of the propellant calculated by the

Kissinger method. The average value of the frequency factor or the pre exponential factor

in this case has been found to be 2.27X1010

and it is nearly three orders of magnitudes

higher than the value obtained for non-catalyzed propellant (Table 7.5). The rate constant

in this case is 7.37 x 10-3

which is nearly 30 percent higher than that of the non- catalyzed

propellant.

Generally speaking, the addition of catalyst should have lowered the value of activation

energy but in our case the experimental results and the kinetic evaluation shows that it

has rather increased. During the literature survey, it was found that the activation energy

of AP may increase or decrease by the addition of catalyst and the results differ from case

to case. Some researchers have reported on the basis of actual experimental results that

the activation energy of ammonium perchlorate was found to increase after the catalytic

activity [6,9,26]. It is important to note here that the rate constant has increased by 30

percent despite the increase in the activation energy. The main reason for this is that the

rate constant has a very strong dependence on the frequency factor A which has increased

by three orders of magnitude and, therefore, the overall rate constant has increased. The

rate constant further increases with the increase in the peak temperatures.

Table 7.5 Kinetic data obtained by Kissinger Method for MgO catalyzed

composite propellant from DSC data (Composition C)

Heating Rate

(oC min

-1)

Peak Temp.

(oC)

Ea

(kJ mol-1

)

A

(sec-1

)

k

(sec-1

)

∆H#

(kJ mol-1

)

10 377.1 2.25 x 1010

7.4 x 10-3

20 391.5 155+ 2% 2.31 x 1010

14.1 x 10-3

150

30 400.6 2.30 x 1010

20.5 x 10-3

40 407.9 2.23 x 1010

26.8 x 10-3

188

The comparative analysis of the two versions of the propellant by Kissinger method

clearly indicates that the kinetic behaviour of the composite propellant has significantly

changed by the addition of nano magnesium oxide as a catalyst. The activation energy of

the catalyzed propellant has increased by 22 percent and the frequency factor has

increased tremendously by 3 orders of magnitude. It is worth mentioning here that the

overall reactivity of the composite propellant has increased due to addition of the catalyst

because the rate constant has increased by nearly 30 percent.

(c)Kinetic analysis by using TG data (composition C )


parameters of the composite propellant by three different methods. The isoconversional

analysis of the non-catalyzed propellant showed that the activation energy value changes

as the degree of conversion increases. Therefore, it was obligatory to carry out the

isoconversional kinetic analysis of the catalyzed propellant as well to elucidate the

changes caused by the catalyst. Three different isoconversional methods have been used

for the analysis which includes Friedman method, Flynn–Wall–Ozawa method and

Kissinger-Akahira-Sunose method. With each kinetic method, activation energy has been

calculated at six different values of conversion (α) i.e. α = 0.2, α = 0.3, α = 0.4, α = 0.5, α

= 0.6 and α = 0.7.

The detailed analysis of the catalyzed propellant showed that the activation energy has

increased by the addition of the catalyst throughout the decomposition process. The

increase in activation energy was confirmed by all the three isoconversional methods. For

instance the activation energy of the non-catalyzed propellant at 30 percent conversion (α

= 0.3) was found to be 116 kJ/mol, 126.9 kJ/mol and 133.5 kJ/mol for KAS, Friedman

and FWO method respectively. Whereas, for the same degree of conversion (α = 0.3), the

activation energy values of the catalyzed propellant were found to be 142.6 kJ/mol, 153.2

kJ/mol and 161.2 kJ/mol respectively KAS, Friedman and FWO method. Similar kind of

increase in activation energy can be seen for the other degrees of conversion from Table

7.6. The representative graphs of all the three kinetic evaluation method are presented in

Figure 7.19(a), 7.19(b) and 7.19(c).

189

Table 7.6 Activation energy data corresponding to different degrees of conversion for

MgO catalyzed composite propellant calculated from TG data

Figure 7.19(a) Representative graph of Friedman method for kinetic evaluation

of the propellant composition C

We had noted for the non-catalyzed propellant that the activation energy depends on the

degree of conversion and shows the variation. Similarly, the catalyzed propellant also

Activation Energy *(Eα)

Method E0.2 E0.3 E0.4 E0.5 E0.6 E0.7

KAS 118.8

142.6

152.5

160.4

157.5

164.2

Friedman 132.2

153.2

163.3

171.3

168.5

175.3

FWO 139.1

161.2

171.8

180.1

177.2

184.4

*(Eα) represents the activation energy corresponding to a specific degree of

conversion (α)

190

Figure 7.19(b) Representative graph of Flynn–Wall–Ozawa method for kinetic

evaluation of the propellant composition C

Figure 7.19(c) Representative graph of Kissinger-Akahira-Sunose method for kinetic

evaluation of the propellant composition C

shows variation in the activation energy values for different conversions. The “E” versus

“α” curve of the catalyzed version of the composite propellant has been shown in Figure

191

7.20 which indicates the activation energy trend against the degree of conversion (α) by

three isoconversional methods.

Figure 7.20 The “E” versus “α” curve of the composite propellant composition C

It is evident from the thermal and kinetic analysis presented above that the magnesium

oxide nano particles play an important role in catalyzing the thermal decomposition of

the composite propellants. Similarly some other transition metal oxides (TMOs) are also

reported in literature for catalyzing the decomposition of the AP and AP containing

composite propellants.

7.5 Estimation of critical ignition temperature

Critical ignition temperature (Tb) is an important parameter related to explosives,

propellants and pyrotechnic. This parameter can be used to predict the safety of energetic

materials during storage, processing and transportation. The propellant charge may be

considered safe if its Tb is adequately higher than ambient range of temperatures

encountered during its storage, transportation and use. Critical temperature of thermal

192

ignition (Tb) can be estimated from the obtained thermal and kinetic parameters and

employing the under mentioned formulae [27,28]. The method was proposed by Tonglai

et al. and it is very common now [29].

√

(7.9)

Where is the critical temperature of thermal ignition, “E” represents activation

energy, “R” is the universal gas constant. The “ ” is the onset temperature

corresponding to → 0 and can be estimated using polynomial regression coefficients

using equation (11).

(7.10)

The values of obtained by using equation (7.10) are 381oC and 329

oC respectively for

the propellant composition A and B respectively. The values of critical temperatures

calculated from equation (7.9) and using the above mentioned values of are 391oC

and 337oC respectively for composition A and B respectively. Although, the critical

temperature of composition B has been lowered by 51oC, however, it is still sufficiently

higher than the commonly encountered range of temperatures during storage and

handling.

Similarly value of for propellant composition C obtained by using equation (7.10) is

369oC. The value of critical ignition temperature obtained by using equation (7.9) is

376oC. This value is lower than the one for composition A but it is still higher than the

commonly encountered range of temperatures during storage and handling.

7.6 Conclusions

Three versions of composite propellant have been investigated in this chapter and the

important results are described below for composition of composition A, B and C. The

propellant composition “A” showed an endothermic peak near 250oC representing the

solid state phase transformation of AP present in the composite propellant. The propellant

composition “A” decomposed thermally at a temperature of 398.8oC at a heating rate of

10oCmin

-1. The activation energy of the non-catalyzed propellant was found to be

193

127kJmol-1

by Kissinger method. The frequency factor was found to be 4.46 x 107 per

second and the rate of reaction constant was 5.66 x 10-3

per second. The activation energy

data of the composition “A” showed variation with respect to the degree of conversion α.

The thermal and kinetic evaluation of composite propellant composition “B” showed that

the first endothermic peak of the composition “B” did not change much after the addition

of the ZnO catalyst. The propellant composition “B” decomposed thermally at a

temperature of 346.9oC which is nearly 52

oC lower than the peak temperature of the

composition “A”. The activation energy of ZnO catalyzed propellant was fond to be

116kJmol-1

which is slightly lower than the activation energy of composition “A”. The

frequency factor of the composition “B” decreased to 3.67 x 107 per second and the rate

of reaction constant increased to 6.05 x 10-3

per second. The activation energy data of the

composition “B” also showed some variation with respect to the degree of conversion α,

however, the trend was different as compared to composition “A”.

The thermal analysis of propellant composition “C” showed that the first endothermic

peak of the composition “C” did not change much after the addition of the MgO catalyst.

The propellant composition “C” decomposed thermally at a temperature of 377.1oC

which is nearly 22oC lower than the peak temperature of the composition “A”. The

activation energy of MgO catalyzed propellant was fond to be 155kJmol-1

which is higher

than the activation energy of composition “A”. The frequency factor of the composition

“C” was found increased by three orders of magnitude to 2.25 x 1010

per second and the

rate of reaction constant increased by nearly 30 percent. The activation energy data of

the composition “C” also showed variations with respect to the degree of conversion α,

however, the trend was different as compared to composition “A” and “B”.

References

[1] G. Singh, I.P.S. Kapoor, S. Dubey, P.F. Siril, Propellants, Explosives,

Pyrotechnics 34 (2009).

[2] K. Kreitz, E. Petersen, D. Reid, S. Seal, Combustion Science and Technology 184

(2012) 750.

194

[3] K. Ishitha, P. Ramakrishna, Combustion and Flame (2014).

[4] S. Ramamurthy, P. Shrotri, Journal of Energetic Materials 14 (1996) 97.

[5] S. Shioya, M. Kohga, T. Naya, Combustion and Flame 161 (2014) 620.




(2007) 27.



[9] K. Kishore, V.R.P. Verneker, M.R. Sunitha, Journal of Applied Chemistry and

Biotechnology 27 (1977) 415.


[11] X. Sun, X. Qiu, L. Li, G. Li, Inorganic chemistry 47 (2008) 4146.

[12] G. Tang, S. Tian, Z. Zhou, Y. Wen, A. Pang, Y. Zhang, D. Zeng, H. Li, B. Shan,

C. Xie, The Journal of Physical Chemistry C 118 (2014) 11833.

[13] M. Kohga, K. Okamoto, Combustion and Flame 158 (2011) 573.

[14] M. Shamsipur, S.M. Pourmortazavi, S.S. Hajimirsadeghi, Combustion Science

and Technology 183 (2011) 575.

[15] M. Shamsipur, S.M. Pourmortazavi, M. Roushani, A.A. Miran Beigi, Combustion

Science and Technology 185 (2013) 122.

[16] J. Lima, A. Frutuoso, K. Iha, M. Ionashiro, J. Matos, M. Suárez-Iha, J. Therm.

Anal. Calorim 75 (2004) 551.

[17] Y. Zhu, H. Huang, H. Ren, Q. Jiao, Journal of Energetic Materials 32 (2014) 16.


[19] M. Venkatesh, P. Ravi, S.P. Tewari, The Journal of Physical Chemistry A 117

(2013) 10162.

[20] T. Vlase, G. Vlase, N. Birta, N. Doca, Journal of thermal analysis and calorimetry

88 (2007) 631.

[21] B. Janković, Chemical Engineering Journal 139 (2008) 128.

[22] A.Q. Malik, Caspian Journal of Applied Sciences Research 2 (2013).

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[23] J. Rocco, J. Lima, A. Frutuoso, K. Iha, M. Ionashiro, J. Matos, M. Suárez-Iha,




[25] E. Alizadeh-Gheshlaghi, B. Shaabani, A. Khodayari, Y. Azizian-Kalandaragh, R.

Rahimi, Powder technology 217 (2012) 330.

[26] B. Dubey, N. Singh, J. Srivastava, A. Ojha, Indian J Chem 40 (2001) 841.



1111.

[29] Z. Tonglai, H. Rongzu, X. Yi, L. Fuping, Thermochimica Acta 244 (1994) 171.

196

Chapter 8

General Conclusions

Energetic materials viz. propellants and pyrotechnics being very sensitive in nature

require some special and safe techniques for their thermal and kinetic studies. Thermal

analytical techniques are essentially very safe for the investigation of energetic materials

due to the fact that a very small quantity of sample in the range of few milligrams is

required for the analysis. The ignition of such a minute quantity of the energetic materials

does not damage the instrument or its surroundings. Thermal techniques including DTA,

DSC and TG were mostly used for the accomplishment of the present work along with

some other techniques which include X-ray diffraction technique (XRD), scanning

electron microscope (SEM) and particle size analyzer. The set of above mentioned

thermal techniques provided important information such as decomposition, ignition, solid

state phase transformations, melting etc. Accordingly, the obtained thermal data was

used to calculate the kinetic parameters vis-à-vis reactivity to elucidate the reaction

mechanism of the pyrotechnics and propellants. The scanning electron microscopy and x-

ray diffraction provided valuable information regarding crystal structure, phase

composition and morphology.

Thermal cum kinetic behaviour of five different oxidizers including ammonium

perchlorate, ammonium nitrate, potassium nitrate, barium nitrate and potassium

permanganate was investigated under similar reaction conditions. On the basis of

comparative analysis in terms of solid state transformations, decomposition temperatures,

activation energy and oxygen balance data of the five oxidizers, it was found that

ammonium perchlorate has an edge over the remaining oxidizers and that is why it is

used in wide variety of propellant and pyrotechnic composition.

The micro porous barium nitrate was synthesized to improve its ignition reliability by

using three different vesicants. The results indicate the presence of micro sized pores in

197

three modified versions of barium nitrate. The exposed area of vesicant modified barium

nitrate increased due to the production of micro pores and improved the reactivity of the

barium nitrate. The bulk density of barium nitrate was found to reduce in all the cases

after the modification. The reduction in the bulk density is attributed to the production of

micro pores. The crystal structure and the cell parameters of the barium nitrate did not

change after the modification. The crystallite size of the barium nitrate reduced after

modification with vesicants. The pyrotechnic composition containing aluminum powder

and micro porous barium nitrate ignited at a lower temperature as compared to the one

formulated with pure barium nitrate.

The MgO nano particles were used to catalyze the thermal decomposition of ammonium

perchlorate. The DTA peaks reduced from three to two in the catalyzed version of AP.

The two distinct decomposition stages of the pure AP decomposition merged with each

other and reduced to a single stage. The thermal decomposition of catalyzed AP took

place at a temperature significantly lower than the peak temperature of the high

temperature decomposition stage but higher than the peak temperature of the low

temperature decomposition stage. The frequency factor increased and caused nearly

twenty three percent increases in the rate of reaction constant despite increase in the value

of activation energy.

Experimental investigation of the thermal and kinetic behaviour of three pyrotechnic

compositions containing aluminum or magnesium fuel was carried out to elucidate the

decomposition process and to evaluate their stability and reactivity. The pyrotechnic

mixture containing Al / Ba(NO3)2 is the least reactive amongst the three investigated

mixtures as it has the lowest value of the reaction rate constant. The mixture is most

stable thermally due to the highest value of its critical ignition temperature i.e. nearly

599oC. Activation energy of the mixture containing Mg/ NH4ClO4 is the highest amongst

the three compositions and therefore it is relatively difficult to ignite. However, once

ignited the mixture is very reactive and has the highest value of the reaction rate constant.

The composition is thermally very stable and has a fairly high value of the critical

ignition temperature. The pyrotechnic mixture containing Mg/KMnO4 is very easy to

ignite as it has the lowest value of activation energy and it is fairly reactive. The rate

198

constant is higher than the mixture containing barium nitrate but lower than the one

containing ammonium perchlorate. All the three investigated compositions were found to

be thermally stable and the relative reactivity of these pyrotechnic mixtures decreased in

the following order:

Mg + NH4ClO4 > Mg + KMnO4 > Al + Ba(NO3)2

The effect of aging i.e. high temperatures and elevated humidity levels on two very

commonly used military pyrotechnics was studied. SR-524 pyrotechnic composition

containing the magnesium and sodium nitrate was investigated before and after the aging

and the decomposition temperature of the aged composition was found to be lowered by

14oC. The kinetic parameters of the pyrotechnic mixture also changed considerably after

the aging. The activation energy of aged composition decreased nearly by 57 percent and

the frequency factor also decreased noticeably. The aging process made the composition

easy to ignite by lowering the activation energy, however, the overall reactivity of the

composition reduced due to decrease in the rate constant. High humidity levels resulted in

the production of magnesium hydroxide as the reaction product of the aging process.

Similarly, ignition temperature of the aged SR-562 pyrotechnic composition was found to

increase by nearly 51oC. There was nearly 22 percent decrease in the value of activation

energy of the aged composition as compared to the fresh composition. The reaction rate

constant was decreased by nearly 30 percent after the aging despite decrease in the value

of activation energy due to the fact that the pre exponential factor decreased considerably.

The aged composition became easy to ignite due to lowering of activation energy but its

reactivity decreased due to lowering of the reaction rate constant. The aged composition

showed the presence of magnesium hydroxide formed due to reaction of some of the

magnesium with water vapours. It is therefore concluded that high humidity and elevated

temperatures have a strong influence on the thermal behaviour, kinetic parameters and

morphology of pyrotechnic compositions containing the magnesium fuel.

Three different versions of the composite propellant were evaluated for their thermal

behaviour and kinetics. The magnesium oxide and zinc oxide nano particles were used as

catalysts. The propellant composition “A” decomposed thermally at a temperature of

199

398.8oC. The activation energy of the propellant was found to be 127kJmol

-1 by the

Kissinger method. The frequency factor was found to be 4.46 x 107 per second and the

rate of reaction constant was 5.66 x 10-3

per second. The activation energy data of the

composition “A” showed variations with respect to the degree of conversion α.

ZnO catalyzed propellant composition “B” decomposed thermally at a temperature of

346.9oC which is nearly 52

oC lower than the peak temperature of the composition “A”.

The activation energy of this composition was fond to be lower than the activation energy

of composition “A”. The frequency factor of the composition “B” decreased to 3.67 x 107

per second and the rate of reaction constant increased to 6.05 x 10-3

per second. The

activation energy data of the composition “B” also showed some variation with respect to

the degree of conversion α, however, the trend was different as compared to composition

“A”.

MgO catalyzed propellant composition “C” decomposed thermally at a temperature of

377.1oC which is nearly 22

oC lower than the peak temperature of the composition “A”.

The activation energy of MgO catalyzed propellant was fond to be 155kJmol-1

which is

higher than the activation energy of composition “A”. The frequency factor of the

composition “C” was found to be increased by three orders of magnitude to 2.25 x 1010

per second and the rate of reaction constant increased by nearly 30 percent. The

activation energy data of the composition “C” also showed variations with respect to the

degree of conversion α, however, the trend was different as compared to both the

composition “A” and “B”.

It was found during the literature review that there is a lack of theoretical understanding

and experimental work concerning the reaction kinetics of the pyrotechnics. The earlier

reported work presents some individual studies concerning thermal behaviour, kinetics,

aging etc. of the pyrotechnic compositions and their ingredients. The present work was

aimed at providing a systematic insight into the thermal behaviour, kinetics, aging and

morphological aspects of pyrotechnics/propellants. The comparative analysis of five

oxidizers was carried out to identify their strengths and weaknesses and to get an insight

200

into the role of these oxidizers when they are subsequently used in various energetic

compositions. Two selected oxidizers i.e. ammonium perchlorate and barium nitrate were

then modified to increase their reactivity by using a nano catalyst and inorganic vesicants

respectively. Three pyrotechnic mixtures were synthesized by adding aluminum and

magnesium fuel to oxidizers and their ignition, kinetics and stability was studied.

Temperature and humidity are the most important factors that influence the shelf life and

ignition behaviour of the pyrotechnics. Therefore, the effect of high temperatures and

high humidity was studied for two commonly used military pyrotechnic compositions.

The thermal and kinetic behaviour of the composite solid propellant was investigated in

detail and the reactivity of the composite propellant was enhanced by using nano sized

magnesium oxide and zinc oxide.

In conclusion, the studies pertaining to the thermal behaviour, kinetics, thermal stability,

aging and the effect of different additives provide valuable information to the user for

assessing the stability and reactivity of pyrotechnic/propellant compositions during

processing, handling, storage, transportation and usage. These studies are also beneficial

for improving the behaviour of existing compositions as well as for the formulation of the

new energetic compositions for specific requirements.

Future Recommendations

1. Thermal decomposition, ignition behaviour and kinetic evaluation of Boron and

Tin fueled pyrotechnic compositions may be carried out to assess their thermal stability

and relative reactivity by using different oxidizers.

2. The catalytic decomposition of composite solid propellants to improve their

thermal behaviour and reactivity by using some nano sized transition metal oxides, that

are not reported earlier, could be an interesting piece of research.

thermal, kinetic and morphological studies of available

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