Chapter I

INTRODUCTION TO ENERGETIC MATERIALS

Energetic materials, in other words, substances with high amount of stored chemical energy,

constitute one of mankind's early scientific discoveries. Although energetic materials were

developed later than many other early human technologies, combustion and ability to control it is

considered, as one of the defining technological achievements of early humans. Energetic

materials present a wide range of substances ranging from the common fuels such as coal, petrol

etc., to the most powerful explosives. Early energetic materials were used primarily for

entertainment, but their potential in the realm of armed conflicts were soon identified. In modem

era, however, the manifold application of energetic materials reaches well beyond those original

uses in entertainment and warfare. Today, energetic materials find use in mining, construction,

demolition, safety equipments, rocketry, space exploration etc, among various other military and

civilian applications. The search for new materials with desired properties like increased density,

better stability, safety etc., has been one of the challenges faced by the scientific community all

over the world, especially in the space applications. It's possible to tailor make new energetic

species in order to fit into various applications, by changing the explosophoric units (groups like

-N02, -N03, -ON02, -N3, -Cl04 etc.,) within the molecules. In perspective of the new

emerging space applications, energetic materials with improved density and better thermal

stability have been looked up as promising candidates. Through this chapter a detailed survey

has been carried out on the development and uses of various energetic materials. The chapter is

divided in to two parts, where the first part deals with the background and terminology of

energetic materials along with their classification and the next part deals with the review on

synthesis and uses of such materials with special emphasis on nitrogen rich heterocyclics.

2

1.1 Back Ground and History of Energetic Materials

The history of energetic materials is well chronicled in literature. The earliest energetic

material developed was Black powder, which was commonly called as gun powder, in about the

middle of the thirteenth century [ 1,2,3]. The first testing of the black powder occurred in 220 BC,

as a result of an accident, when the Chinese alchemists accidently made black powder while

separating gold from silver through a low temperature reaction. They added charcoal to a hot

mixture of KN03 and sulfur, a process they believed to separate gold from its ore, which resulted

in a tremendous explosion. During the early thirteenth century Roger Bacon experimented with

KN03 and prepared black powder. In 1320 Berthold Schwartz used Bacon's writings to prepare

black powder and studied its properties. The results indicated the composition to consist of

KN03, Sulfur and Charcoal, where all the ingredients are powdered and mixed thoroughly. The

composition varied from time to time until the more or less standardized KNO]iC/S (75/15/10 by

weight) was adopted. Properties of black powder depend considerably on the charcoal used. The

density was usually 1. 7 glee, but depends on the compression during the pressing process. Black

powder ignited readily at about 300°C for the normal product and about 340°C for the sulfur less

composition. Later in 1425 with the development of Coming process (granulating), the ballistic

performance of black powder increased, resulting in its use as gun powder. By the 15th and 16111

centuries the usage and application of black powder was worldwide. Black powder found both

civilian and military applications[4]. It was commonly used as a blasting aid in mines and for

demolition purposes. But soon during the early 19th century the limitations of black powder as a

blasting explosive became apparent.

3

Even though Glauber introduced ammonium nitrate (AN) in 1654, it was not until the

beginning of 19th century when AN was considered as a replacement for KN03 in black powder,

by Grindel and Robin(5). Its explosive property was reported by Reise and Millon in 1849, when

it was found that mixtures of AN and charcoal explodes when heated. There was a tremendous

increase in the development of energetic materials during the World war time, when America

and Russia introduced several research projects to develop powerful ammunitions.

The quest for a better energetic component got underway when the Italian Professor

Ascanio Sobrero prepared nitroglycerine in 1846(6,7). As he became aware of the explosive

property of liquid nitroglycerine, he discontinued his work. Later Immanuel Nobel devised a

process to manufacture nitroglycerine and erected a small manufacturing plant with his son,

Alfred in 1863.

The Nobel family suffered so many mishaps due to the high sensitivity of nitroglycerine. Two of

his manufacturing plants got blown off and lost his brother in a major explosion. To reduce the

sensitivity Alfred mixed an absorbent clay, kieselguhr with nitroglycerine, which came to be

known as 'guhr dynamite'. Nitroglycerine had the advantage of having both the fuel and oxidizer

part in the same molecule, giving intimate contact of the two entities. Due to its unpredictability

and sensitive nature, nitroglycerine was seldom promoted as a common energetic material.

During this period much interest was concentrated on nitrating relatively common materials like

silk, wool, resins, wood, cotton etc. In 1833 Braconnot nitrated starch, and later Pelouze nitrated

wool, cotton etc, but didn't realize that he made nitrocellulose. In 1846 Schonbein along with

Bottger independently developed nitrocellulose and soon it became one among the major

developments in this field. Nitrocellulose was not used for military and commercial purposes

until 1868, when Brown came up with a solution that dry nitrocellulose can be detonated safely

4

using a mercuric fulminate detonator and for wet nitrocellulose, dry nitrocellulose can be used as

a booster charge to make it initiate. Thus the storage safety of nitrocellulose in wet conditions

was proved, making the pathway to a greater usage of nitrocellulose in both civilian and military

purposes. After the development of nitrocellulose as an explosive, in 1875 Alfred Nobel

discovered that a gel was obtained on mixing nitrocellulose with nitroglycerine, which was

developed to produce blasting gelatin, dynamite and later in 1888, ballisite, the first smokeless

powder. It was a mixture of nitrocellulose, nitroglycerine, benzene and camphor. In 1889, a rival

product was prepared by British Government under the name Cordite, and it remained the major

propellant of British forces until 1930's. With the better energetic properties exhibited by nitro

compounds, a greater interest was focused on the preparation of nitro derivatives of organic

compounds.

1.1.1 Military Explosives(8-14)

In 1885 Turpin found that Picric acid could be used as an effective replacement for black

powder, and it replaced black powder in all British ammunitions by 1888. Picric acid was found

mentioned in the literature as early as 1742, and it was used as a fast dye in the second half of

19th century. In 1830 Welter explored the possibility of using picric acid as an explosive. During

the late 1870' s pi crate salts were prepared and found to be useful as propellants. Eventually

picric acids and salts thereof was accepted world wide as the basic explosives for military uses.

Tetryl, another explosive was also being developed the same time as picric acid. Tetryl

was first prepared by Mertens in 1877, and its structure was established by Romburgh in 1883. It

was frequently used as base charge in blasting caps.

Trinitrotoluene, one among the standard explosives of all times, was first prepared by

Wilbrand in 1863. Beilstein and Kuhlberh in 1870 studied the detailed preparation of TNT and in

5

1880 Hepp prepared the pure isomer, which was characterized for its structure by Claus and

Becker in 1883. Around 1902 Germans and British started experimenting with TNT in place of

picric acid. By around 1914 TNT became the standard explosive for all armies during World

War I. Use of a mixture of TNT and AN, called amatol, became widespread to relieve the

shortage of TNT for filling up munitions, caused by the limited availability of toluene from coal

tar.

After the World War I, major research programmes were inaugurated to develop new and

more powerful explosives. From these programmes came ROX and PETN. PETN was one

among the common explosives used during the World War II. PETN was first prepared in 1894

by nitration of pentaerythritol. But PETN was mostly substituted by other explosives like ROX,

because the former was more sensitive to impact and has less chemical stability. RDX was first

prepared by Henning in 1889 for medicinal uses. Herz succeeded in preparing RDX through

direct nitration of hexamine, although in low yields, and suggested its use as an explosive in

1920. In 1940 Meissner developed a continuous process to prepare RDX(Type B) in

comparitatively good yields. Bachman developed a manufacturing process to develop RDX

from hexamine which gave the greatest yield, and later the process was modified and taken up by

the Tennessee-Eastman company in USA during the World War II, to prepare the purest form of

RDX(Type A). Mostly through the World War II RDX was used as in explosive compositions

with TNT, as an effort to increase the explosive power. Research and development continued

throughout World War II to develop new explosives and explosive formulations.

Cyclotetramethylenetetranitramine commonly called HMX or 'Octogen' was developed by the

end of the World War II. The higher blasting power of HMX was utilized to increase the

demolishing power of TNT, based formulations (Octol, 75% HMX and 25% TNT).

6

During the late 1950's polymer bonded explosives, PBX came into existence, which

greatly reduced the sensitivity of explosive crystals by embedding these crystals in to a

polymeric matrix. HMX based PBX was developed during the late 1960's and early 1970's using

Teflon as the binder for projectile and lunar seismic experiments. The polymeric binder was

chosen based on the application rendered by these PBX.

Heat-resistant explosive compounds like Hexanitrostilbene(HNS),

Triaminotrinitrobenzene(TATB)(l 5, 16), Nitrotriazolone(NTO)(l 7,18,19), etc were developed

during the following decades. Shipp prepared HNS in 1966 and TA TB in 1978 by Adkins.

Preparation of NTO was first reported in 1905 through the nitration of triazolone, but it wasn't

until 1987 when Lee, Chapman and Coburn reported the explosive properties of NTO. NTO is

now widely being used in explosive formulations, PBX, airbag inflators etc. Strained ring or

caged nitro compounds and nitro cubanes termed as High Energy Density Materials(20, 21)

developed recently showed much improved density and thermal stability. 2,4,6,8, 10, 12-

hexanitrohexazaisowurzitane or HNIW, commonly known as CL-20(22,23) one among such

recent developments, was developed by Amie Neilsen in 1987. Boyer and coworkers reported

the synthesis of TEX(24), another caged nitro compound, in 1990, through a two step process

using formamidine and glyoxal. The higher density (1.99 glee) can be attributed to its three

dimensional cage structure. Nitro cubanes like Octanitrocubanes and Heptanitrocubanes,

successfully synthesized in 1997 and 2000 by Eaton and coworkers, proved to be one of the most

powerful explosives with predicted detonation velocities greater than 10,000 mis.

1.1.2 Current field of interest:

Throughout the last decade there has been consistent development in the field of

environmentally compatible green energetic materials(25) for defence and space applications.

7

The safety hazards caused by the metalized explosives and environmental pollution generated by

the usage of carbon rich explosives have imparted the need for safer explosives. The focus of the

present scientific community has been to develop High-energy Density Materials (HEDMs),

using High Nitrogen content molecules. Heterocyclic nitrogen rich molecules like Imidazoles,

Triazoles, Tetrazoles, Triazines, Tetrazines (26-30)etc are extensively used as the effective

precursors for the preparation of such HEDMs, owing to its greater density, when compared to

its carbon analogues.

1.2. Terminology and Classification of energetic materials

Having explained the history of energetic materials, it's quite useful to describe

the terms used above for the sake of clarity. The broad spectrum of energetic materials can be

subdivided in to different groups, namely Explosives, Propellants and Pyrotechnics. The

following flow chart in figure I. I gives a general awareness of the primary classification of

energetic materials on the basis of their characteristic combustion behaviour. It shows the

breakdown of energetic materials into various classes designed for specific purposes associated

with different types of military applications and civilian uses.

8

Liquid N itramine/Seminitramine

....--_,_-....--------'--,..-----1 Composite

Single Based Double Based

Propellants

Explosives 1-------i Pyrotechnics ---1

High Explosives

ignition illumination heat generation delay composition noise generation smoke generation gas generation

military

Primary Explosives Secondary Explosives 1------i

Booster Explosives civil

Fig.I. I Primary classification of energetic materials

1.2.1 High Explosives

ITetall acce.lerating

eneral purpose r blast nder water

ITck blastingeismic xplosive welding emolition

Explosives are chemical compositions, which when initiated by a suitable stimulus,

disassociate almost instantaneously into other more stable products. This reaction is known as

high-order or low-order detonations. Detonations are exothermic, self sustaining chemical

reactions which propagates in such rapidity that the rate of advance of the reaction zone in to the

unreacted material exceeds the velocity of sound. The rate is always > 2000 mis. The detonation

generates a shock wave that acts on its surroundings with great brisance, or shattering power,

before the pressure of the exerted hot gases can take effect. Based on the order of detonation as

well as on the ease of initiation, explosives can be categorized into Primary and Secondary.

9

1.2.1.1 Primary explosives, also called initiating explosives, are a class of highly

sensitive explosives that can be initiated by the weakest of stimuli and can explode whether they

are confined or unconfined. They undergo a very rapid transition from burning to detonation and

have the ability to transmit the detonation to less sensitive materials. The heat and shock of these

primary explosives can vary but it's comparable to that of secondary explosives. The detonation

velocities are in the range of 3000-5500 mis. Typical primary explosives which are widely used

are Lead azide, Lead styphnate, Lead mononitroresorcinate(LMNR), potassium

dinitrobenzofuroxan(KDNBF), Barium styphnate etc.

1.2.1.2 Secondary explosives differ from the primary in that they cannot be detonated

readily by mild stimuli and are generally more powerful than the primary explosives. They can

only be initiated by the shock produced by the primary explosive and once its goes off the result

will be devastating. The molecular structure breaks down on explosion leaving, momentarily, a

disorganized mass of atoms, which recombine to give predominantly gaseous products, evolving

a considerable amount of heat. The shock wave generated move to the surroundings at very high

velocity exceeding 5500 mis. The common secondary explosives used are TNT, Tetryl,

Nitroglycerine, Picric acid, PETN, RDX, HMX, TEX, CL-20 etc.

Table 1.1 and 1.2 briefs the structure and properties of the commonly used primary and

secondary explosives (3).

10

Table I. I Structure and properties of common Primary explosives

Primary Explosives

Name

Lead Azide

Silver Azide

Mercuric

fulminate

Lead

styphnate

Structure

N=N===N

Pb/ +

'--....... + -

N==N===N

+

Ag--N===N====N

o..--N--

C--Hg--c--

N--..o

0

�'*� I# �

Q Pb 2' ,H20

-

Density

@20 ° C

(gcc-1)

(a form)-

4.17

(� form)-

4.93

5.1

4.42

3.06

Ignition Heat of

temperature Formation

(OC) (kJkg-1)

327-360 + 1420

273 -

170 + 941

267-268 -1826

11

Tetrazene

hydrate

__..- N /Nn2

11 rN

�/N

, N

..__�� N

11-NH2

', ,0,, /� 'w·· I ',,H

1.7 140

Table I.2 Structure and properties of common Secondary explosives

Secondary explosives

Name Structure Density

@20° C

(gcc· 1)

Nitroglycerine H

1.59 I

H-C-O-N02

IH-C-O-N02

IH-C-O-N02

H

TNT 1.65

(Trinitro toluene) CH3

02N

� N02

#

N02

+ 1005

Ignition Heat of

temperat Formation

ure (kJkg" 1)

(o C)

200 -1633

300 -261.5

12

Tetryl

(2,4,6

trinitropheny l

methylnitramine)

PETN

(Pentaerythritol

tetranitrate)

HNS

(Hexa

nitrostilbene)

RDX

( Cyclotrimethy lene

trinitramine)

H3C'-,,, /

"'u2

N

02N �

N02

#

N02

02 NO-CH2 H2C-ON02 '\./ /c""-

02NO-CH2 H2C-ON02

�����:P-� N02 02N

02N"-.. � / N02

N N

lN) N02

1.73 185 117.7

1.76 202 -1683

1.74 325 + 128.l

1.82 260 + 318

13

HMX (a. form)- 280 + 252.8

(Cyclo N02 1.87

�N

tetramethy lene 02N-N l (� form)-

tetranitramine) \

._/-N02

1.96

N02 (y form)-

1.82

(o form)-

1.78

1.2.2 Propellants

Propellants differ from explosives in that the process by which they liberate energy is through

Deflagration. It is a surface phenomenon, where the reaction front moves parallel to the burning

surface with relatively high rate, (ie < 2000 mis) and is propagated through heat transfer(3 l).

Deflagration of a propellant builds up high pressures, through the liberation of large volume of

gases at high temperature, without the generation of shock waves. This allows work to be

performed by the pressurized gases, without much catastrophic effects. On initiation of

propellants, local, finite hotspots are developed, either through friction or compression of voids,

which in tum produces heat and volatile intermediates, resulting in highly exothermic reactions

in gas phase. This process generates more than enough energy to initiate the decomposition of

the newly created surfaces, thus making the reaction self-sustaining.

The rate of deflagration will increase with increase in confinement. The linear burning

rate of propellants, r in mm/s at a given pressure P, can be represented as (32);

r = aP" , where a is the burning rate co efficient and n is the burning rate index.

14

As the deflagration is a surface phenomenon, the mass of propellant, m consumed in unit

time, m· will be a function of the surface area of the propellant. The relation can be presented as;

m· = r Ap , where r is the burn rate, p is the density and A the surface area of

burning surface of the propellant.

Propellants can be further classified into different groups based on their chemical constitution

and application as, Single base, Double base, Nitramine/seminitramine, Composite Propellants

and Liquid propellants(33-37).

1.2.2.1 Single base propellants

As the name suggests, these propellants are having only one component, capable of

generating high pressure gases on deflagration. These are mainly used as gun propellants and are

seldom used in rocketry. Examples include nitrocellulose. Reinforced nitrocellulose, under the

name 'cordite' is used in guns and ammunitions as propellants. Since it lacks a binder, the

mechanical properties are poor and hence cannot be made into grains to be used as rocket

propellants.

1.2.2.2 Double base propellants

These contain two energetic ingredients viz., nitrocellulose and nitroglycerine, in which

nitroglycerine is the plasticizer. Other nitrate esters like glycol dinitrate, diethylene glycol

dinitrate, metriol trinitrate etc., were also found to be effective plasticizers for nitrocellulose.

Modified double base propellants were effectively used in rocketry until the early 1970's, which

was later replaced by the composite propellants.

1.2.2.3 Composite propellants

The energy level of the double base propellants were increased by the addition of

crystalline oxidizers like Ammonium perchlorate, Ammonium nitrate etc into the polymeric

15

matrix. These propellants were found to be useful as solid rocket propellants. Such composite

propellants used binders like polysulfides, polybutadiene-acrylic acid, polybutadiene­

acrylonitrile-acrylic acid, polyurethane, and carboxy or hydroxyl terminated polybutadienes.

Nitrate and perchlorate salts were used as effective oxidizers. During the late 1980,s significant

amount of metal fuel, viz., Aluminium was used to increase the burn rate of such soild rocket

propellants.

1.2.2.4 Liquid Propellants

Liquid propellants forms the main source of thrust for modern rockets, especially

multistage rockets used for space exploration. Common liquid propellants are from the

Hydrazine family, viz., Hydrazine, Monomethylhydrazine(MMH), Unsymmetrical

dimethylhydrazine(UDMH) etc. Most of the fuels of hydrazine family are hypergolic with

normal oxidizers at room temperature and pressure, but the safe combustion is difficult. Hence

most rockets use hydrazine alone or a mixture of 50% hydrazine with other fuels like MMH or

UDMH. New generation rockets use bi-propellants, with hydrazine and UDMH, where

hydrazine acts as an additive to enhance the ballistic performance.

1.2.3 Pyrotechnics

Pyrotechnics-'the art of making Fire"( 4), is one of the oldest terms used in connection with

energetics. During the early period pyrotechnics were used for entertainment. Those were

chemicals used to produce heat, motion and beautiful optical and acoustic effects. It is believed

to have originated in India and soon Chinese started manufacture of pyrotechnic mixtures and

used it for entertainment. Slowly during the 15th century, pyrotechnic mixtures found their space

in battle fields. These mixtures, mainly black powder, were employed for firing cannons and

16

other big artilleries. To date the field of pyrotechnics has developed and diversified, that these

mixtures are extensively used for civilian, military and space applications.

An elaborate definition is as follows,

"Fireworks, pyrotechnics, or artificial fireworks as they were formally called are contrivances

that by the agency of fire produce audible, visible, mechanical and thermal effects useful for

industrial or military purposes or for entertainment".

A typical Pyrotechnic mixture consists of uniform blend of an Oxidiser and a Fuel, in a binder.

The choice of the oxidiser and the fuel depends greatly on the application for which it's used. For

example, metal fuels and inorganic oxidisers are used for generating hot flares, whereas organic

fuels and inorganic oxidisers are used for generating hot gases for pressure actuated applications.

Pyrotechnic mixtures are used as Illuminating flares, signaling flares, colored and white smoke

generators, tracers, incendiary delays, fuses, photo-flash compounds, gas generators and igniter

compositions. A pyrotechnic process, deflagration, differs from ordinary combustion by not

requiring ambient air. The highly exothermic reactions are based on simple redox reactions

among the fuel and oxidizers. The level of performance of these mixtures depends on factors

like, degree of confinement (loading density), Particle size of ingredients, sensitivity/ignitability,

moisture etc. Tables 1.3 & 1.4 details the use and properties of commonly used pyrotechnic

compositions and smoke generators.

17

Table 1.3 Use and Properties of few pyrotechnic compositions

Pyrotechnic Pyrotechnic Heat of Gas volume Ignition

effect composition reaction Cm3 / g Temp.

Cal /g oC

Heat / Gas Titanium/Potassium 1650 250 630

perchlorate

Delay Zirconium/Barium 500 12 485

chromate/Potassium

perchlorate

Light Magnesium/Sodium 1500 74 640

nitrate/binder

Smoke Zinc/potassium 620 62 475

perchlorate/

hexachlorobenzene

Photo flash Barium 2150 15 700

nitrate/potassium

perchlorate

Motion Charcoal/sulphur/ 650 450 375

potassium nitrate

18

Table I.4 Different pyrotechnic compositions for smoke generation

Substance Colour

(Wt%) Yellow Red Green Blue

KCL03 25 28 28

KN03 25

S8 16

Wheat flour 15 15 15

Sudan yellow 59 10

Rhodamine 24

Para Red 36

Methylene blue 17 17

Indigo Pure 30 40

1.2.3.1 Igniter and Initiatory compositions

Pyrotechnic compositions used in igniters and initiatory are flame producers, which are

often electrically ignited. Typical igniters generate high temperature with only small amount of

gas or no gas. These are the first firing elements used for ignition of Solid boosters, gas

generators and liquid-powered motors or to activate other temperature actuated systems. Igniters

used for solid-propellant rockets are almost exclusively of the pyrotechnic type. Table presented

below lists down few such compositions and their features.

19

Table 1.5 Properties and applications of various initiatory and igniter compositions

Pyrotechnic composition Effect Application Tign. �HR P max

oc cal g· 1 ksc

CaSii/KCl03/Pb(SCN)2 Heat Squib 275-285 925 NIA

/Pb2[Fe(CN)6]

Bl KClO)i Heat Squib 325-375 1250 NIA

Pb(SCN)z

Bl KN03/Pb(SCN)2 Heat Squib 450-475 1660 NIA

Ti/CuO Heat Initiators 843-860 740 7.0

Zr/ KCl04 Heat Initiators 510-525 1340 NIA

Al/NH4Cl04 Heat & Gas Igniters 328-356 2440 60.0

Al/KCl04 Heat& Gas Igniters 573-607 2340 38.0

B/KN03 Heat& Gas Igniters 520-549 1600 29.0

Zr/KN03 Heat Igniters 730-810 1000 19.0

Al/Fe304 Heat Igniters NIA 1711 NIA

Ti/ KCl04 Heat& Gas Igniters 630 1650 39.0

20

1.2.3.2 Gas generators

Gas generators are pyrotechnic mixtures where the decomposition results in the

liberation of large volume of gases at high pressures. The components (ie the fuel and oxidizers)

are chosen in such a way that the combustion of the mixture leads to the generation of gases

without much un-bumed particles. These are used for sudden pressurization or inflation such as

seat belt tensioners, safety air bags in vehicles, fire fighting systems etc. Conventional system

used a pre stored compressed air, which is selectively released to inflate the air bag. The large

consumption of space and safety issues promoted the use of pyrotechnic gas generators. For a

long time air bags used NaN3 fuelled pyrotechnic gas generators with inorganic-oxidizers like

perchlorate salts and nitrate salts of alkali metals as well as organic-oxidizers like guanidine

nitrate(GN), nitrocellulose(NC) etc,.

1.3 Performance Scale for Propellants and Explosives

Performance of energetic materials cannot be expressed in a single characteristic. The

performance of a fuel is directly related to the amount of heat released, whereas the performance

of explosives have less direct relationship with the amount of energy released. The performance

of these material are dependent on the detonation velocity, the packing density, the gas liberated,

heat of explosion, oxygen balance etc. For explosives both brisance and strength are used in

describing performance, where as for propellants, specific impulse is viewed as a key measure of

performance.

21

1.3.l Brisance of Explosives

Brisance of an explosive can be considered as its shattering power or the

destructive fragmentation effect on its near vicinity. In other words it is the ability of the

detonation products to do destructive work on its surroundings. The measure of brisance is the

amount of detonation products motion at initial stages of expansion. The relevant parameter is

the loading density and the velocity of detonation, which in turn depends on the density of the

material. Experimentally the amount of brisance for explosives can be determined using different

methods of measuring impulses, by ballistic pendulum, by crusher compression on brisance

meter or impulse meter(Kast brisance meter), by the velocity of a charge shell or by the Oree of

deformation of materials which are near to the explosive charge.

L. T. Eremenko and D. A. Ncsterenko(38) in their work correlated the relationship of

brisance on the atomic- molecular structures of explosives and proposed a method for calculating

detonation pressure using the relative impulse values, which are either experimentally or

theoretically calculated.

1.3.2 Detonation Velocity and Pressure

The supersonic combustion waves, the detonation waves were first

observed experimentally by Chapman and Jouguet in the late 1800's. They hypothesized the

steady-state detonation condition is reached once there is detonation and the detonation

parameters, velocity of wave front and the pressure of gases remains constant for the process.

This state was later named as the CJ state and the velocities and pressures were termed as CJ

velocity and <;J pressure respectively.

Detonation velocity (VOD) or detonation rate can be easily described as the rate of

propagation of a detonation in an explosive. When the explosive is in its theoretical maximum

22

density and having the critical diameter, the detonation velocity is characteristic of each

explosive and is not influenced by external factors. Experimental values of VOD and pressure of

a few EM's are listed in table 1.6. Under ideal conditions, detonation can be seen as a shock

wave progressing through the explosive. The shock wave front compresses and heats the

explosive initiating the exothermic chemical reaction, which gets completed in microseconds.

The energy thus liberated feeds the shock front and drives it forward. At the same time the

gaseous products behind this shock wave are expanding, a rarefaction moves forward towards

the shock front. The shock front, chemical reaction and the leading edge of the rarefaction are all

in equilibrium; moving forward at the same speed, which can be called as the detonation velocity

and the pressure created by the combustion products corresponds to the detonation pressure.

Table 1.6 Experimental values of VOD and Detonation pressure of classical pnmary and

secondary explosives

Explosive Classification Density Velocity of Detonation

(glee) Detonation Pressure

(mis) (GPa)

Lead azide 4.60 5300 33.6

Silver azide 5.10 - -

Lead Styphnate Primary 3.08 5200@2.9g/cc -

Mercuric fulminate 4.42 4250@3.0g/cc -

Tetrazene 1.70 - -

Nitro glycerine 1.59 7600 20.7

Trinitro toluene 1.65 6900 20.5

23

Tetryl 1.73 7570 25.8

PEIN 1.76 8400 32.3

HNS Secondary 1.74 7000 -

RDX 1.81 8750 36.2

HMX 1.90 9100 40.9

CL-20 2.04 9400

1.3 .3 Specific Impulse

Specific impulse is a means of characterizing and evaluating the properties of a

propellant and is viewed as a key measure of propellant performance. Specific impulse can be

interpreted as the thrust derived per unit weight flow of propellant. It can be considered as the

theoretical maximum of the mechanical work that can be derived from the adiabatic

decomposition of a unit mass of propellant into decomposition products to an environment of

specified pressure, which is usually the near total vacuum of outer space. It's the enthalpy release

converted into the kinetic energy of the exhaust jet. The simplified relation can be given by

equation

Isp = Tc 112 N 112

Where,

Tc is the combustion chamber temperature and N is the number of moles of gaseous products

produced. The chamber temperature depends upon the heat release, given by the equation,

Tc= -6H/C

p

Where,

24

�H is the heat of reaction, and Cp

is the specific heat capacity. The heat of the reaction �H can

be generated using the equation,

�H = �Hproducts- �Hreactants

In the simplest of definitions, Specific impulse can be defined as the thrust achieved per unit

weight of propellant and is given by the equation,

lsp = F.t/W

Where,

Isp is the specific impulse, F is the thrust in N, t the time in seconds and w the weight of

the propellant in grams.

The specific impulse of the propellants depends directly on Oxygen balance and Heat of

reaction. Although the density of the propellant should be as high as possible to store maximum

energy per volume, it is far less a critical factor in the performance of propellants, having a small

effect on Isp. One of the primary ways to improve the Isp

of propellants is by increasing enthalpy

release and increasing the average molecular weight of the exhaust gases to attain more working

fluid. In order to achieve this, the oxygen balance should be greater (best option is to have

positive oxygen balance).

1.4 Properties crucial for the High Performance of Explosives and Propellants

1.4.1 Loading Density:

Loading density can be defined as the ratio between the weight of the

explosive and the explosion volume, ie the space in which the explosive is detonated. Loading

density is one of the key parameters defining the performance of propellant powders and

explosives.

25

The brisance of an explosive or the specific impulse of propellants depends directly on

density (loading density in case of explosives). Velocity of Detonation (VOD), one of the

defining parameters about the strength of an explosive depends directly on density. ie " The

wave velocity is proportional to the density of the explosive material'. For most explosives, the

relationship between detonation velocity and density of the explosive is closer to linear over

reasonable ranges of density, ie,

D= j + kpo,

Where,

j and k are constants particular to each explosive.

For heterogeneous explosives the velocity of detonation increases and then decreases as the

compaction density increases. But for the case of homogenous explosives, velocity of detonation

increases as density increases. (figure 1.2.)

7000

6500

6000

5500

5000

4500

4000

.3500

.3000

0.8 0.95 1.05 1.1 1.2

Effect of density on detonation ve loclty of hlsh exploslves 11, nltroauanldlne

,.__ _____________________________ .. _____________________ ___.

Fig 1.2 Effect of density on Detonation velocity (NGu)

26

1.4.2 Oxygen balance

Oxygen Balance is a method of quantifying how well an energetic species provides its

own oxidant. It's the parameter that tells us whether the oxygen atoms present in the molecule

are sufficient or insufficient for the complete oxidation of the fuel elements like carbon and

hydrogen. There are various ways of defining the oxygen balance, ie we can balance it for CO or

for CO2 (39). OB can also be represented in terms of weight percent oxygen in the explosive or

in terms of oxidant per 100 grams explosive (OB100) (40). For an organic explosive of general

formula CxHyNwOz, the OB values can be calculated using the following equations.

OB % = (1600/MW) (z-x-y/2) (1) (balanced for CO)

OB%= (1600/MW) (z-2x-y/2) (2) (balanced for CO2)

1.4.3 Heat of Reaction

The energy liberated when a fuel undergoes oxidation or an explosive detonates is called

Heat of reaction. Oxidation reactions produce heat because the internal energy of the reactant

molecules is higher than the internal energy of the product molecules. This difference between

the internal energies of the reactants and products is called heat of reaction

1.5 Theoretical Prediction of detonation parameters: Detonation Velocity and Detonation

Pressure

The importance of predicting the performance parameters of explosive compounds was

identified to be vital, since it can greatly reduce the laborious and expensive task of synthesizing

them, if found not suitable for the application. As per the present scenario, explosive compounds

27

are designed using molecular engineering approaches and predicted the performance parameters

using different computer codes, so that the poor candidates can be eliminated even before

synthesizing them. This can greatly reduce the safety issues dealing with the instability of the

proposed candidate. There are a number of reliable computer codes programmed on the basis of

different Equations of State, as well as theoretical relations to predict the detonation parameters,

detonation velocity, pressure, temperature of explosion, the heat of explosion etc. The principal

methods to determine the detonation parameters include the Becker-Kistiakowsky-Wilson

(BKW), the Lennard-Jones-Devonshire (LJD) and the Jacobs-Cowperthwaite-Zwisler (JCZ)

equations of state. Different computational software including CHEETAH, TIGER, EXPLOS,

and LOTUSES etc can be used to predict the explosive properties of a variety of energetic

materials. Certain computer codes like the NASA-CEC-71 and REAL predicts the ballistic

performance parameters of energetic materials, employing the virial equations that are applicable

at high temperature and pressure.

Many researchers have come up with different empirical relationships derived from EOS

to determine the detonation velocity and pressure of explosives. The dependence of density on

the detonation parameters of explosives is shown in the equations coined by Kamlet and

Jacobs( 41-44 ), which invokes the thermo-chemical properties of an idealized detonation reaction

and predicts the detonation velocity and detonation pressure. (or Chapman Jouget pressure, Pc-j).

The idealized reaction assumes that all of the available oxygen is used to convert carbon to

carbon dioxide and no carbon monoxide is formed. Considering the case of a CHNO explosive,

the reaction can be depicted as,

CcHhNnOo = (n/2) N2+ (h/2) H20+ (o/2 - h/4) CO2 + (c - o/2 + h/4) C.

For such a reaction, the empirical relationships are,

28

D = 1.0l<l>112(1+1.3po),

Pc-j = 1.56pa2 <l>,

Where,

<l> = NM 112Q 112 and N =no. of moles of gaseous detonation products per g of molecule, M

= average molecular weight of detonation product gas and Q = chemical energy of detonation.

The term Q is the heat of reaction of the compound and can be determined from the heat of

formation of the compound using the relation,

Q= �H0 r (reaction products) - �H0 r ( compound)/ Molecular weight

Stine in 1990 derived a relatively accurate method for estimating the detonation

parameters using the atomic composition, density and heat of formation of the explosive.

Accordingly, for a CHNO explosive represented as CaHbNcOct, where a, b, c and d are the atomic

fractions of the respective number of atoms, the relation for VOD was reported as per the

equation,

D = 3.69 + (-13.85a + 3.95b + 37.74c + 68.1 ld + 0.6917 �Hr) (p/M)

Rothstein and coworkers( 45,46) derived an empirical relationship between the detonation

velocity and the composition of a CHNO type explosive at its maximum theoretical density.

They predicted the detonation velocity using a factor 'F', which solely depends on the chemical

composition of the explosive.

From the 'F' factor, the detonation velocity at theoretical maximum (TMD), D', can be

determined using the relation,

F = 0.55 D' + 0.26

29

or

D' = (F - 0.26)/0.55 (km/s)

where, F is given by the relation

F= {100* [ n(O)+ n(N) - n(H)/2n(O) + A/3 - n(B)/1.75 - n(D)/4 - n(E)/5 ]/Molecular weight}­

G

where,

G = 0.4 for liquid explosives, for solid explosives G = O; A = 1 if the compound is aromatic,

otherwise A = 0 and where for one mole of the composition, n(O) = number of oxygen atoms,

n(N) = number of nitrogen atoms, n (H) = number of hydrogen atoms, n(B) = number of oxygen

atoms in excess of those already available to form CO, and H,O, n(C) = number of oxygen atoms

doubly bonded directly to carbon as in carbonyl> C = 0, n(D) = number of oxygen atoms singly

bonded directly to carbon as in a> C- 0 - R linkage where R can equal - H, - NH4, - C, etc. and

n(E) = number of nitrato groups existing either in a nitrate ester configuration or as a nitric acid

salt such as hydrazine mononitrate.

Finally the detonation pressure can be correlated to the detonation velocity using the relation,

Pc.J = 93.3D' - 456 (kbar)

Recently Mohammad Hossein Keshavarz(47-52) derived empherical relationships to

calculate the detonation as well as the thermochemical parameters of explosives. A relation

connecting the detonation velocities of various CHNO explosives at their nominal densities with

the elemental composition and some structural parameters was also derived. The final form of

the optimized correlation for an explosive of general formula CaHbNcOd can be given as:

30

D (km/s) = 7.678 - 0.1977a - 0.1105b + 0.2940c +0.0742d- 0.6347nNR - 0. 7354nmN

where a, b, c and d represents the stoichiometric coefficients, nNR is the number of -N N- or

NH4 + in explosive and nmN is the number of nitro groups (-N02) attached to carbon in

nitrocompounds in which a=l.

1.6 Future Developments: New Class ofHEDMs - Green Explosives

In the past decades fascinating and novel molecular structures have been synthesized and

a number of them are being used in rocket propulsion and ordnance systems. The present status

of energetic materials has been achieved through rigorous experimentation and testing for novel

materials with enhanced performance. As previously indicated, the new class of High Energy

Density Materials has attracted the attention of scientists all over the world, working in the

energetic field. The need for an environmentally suitable explosive has paved the way to look

into nitrogen rich compounds as potential candidates, as the combustion of these materials

liberates nitrogen as the major component. Energetic materials should possess the following

properties in order to be classified as the HEDMs,

1. High Density (preferably above 1.6)

2. High thermal stability (Tm> 150)

3. No heavy metals and halogens

4. Nitrogen rich (preferably above 60%)

5. Not too toxic (should avoid inorganic azides)

6. High detonation parameters(VOD and Pc.J)

7. Less sensitive to Impact and Friction

31

Triazoles, tetrazoles, tetrazines, triazines etc are the nitrogen rich molecules that have

been the centre of attraction for the targeted HEDMs. The high energy releases of the compounds

are due to the presence of adjacent nitrogen atoms poised to form nitrogen (N N). Such

transformations are followed by enormous energy release due to the wide difference in the

average bond energies of N-N (160 kJ/mol) and N=N (418 kJ/mol) compared to that of N N

(954 kJ/mol). As a natural consequence they liberate greater amount of gas per gram of the

explosive, making them suitable to be used as the cool gas generators or environment friendly

explosives. Through the next part of the chapter synthetic procedures for different HEDMs will

be discussed.

Part II

1. 7 An overview on the chemistry and synthesis and properties of Energetic derivatives of

Triazoles

1. 7 .1 Triazole chemistry

In five membered ring systems the presence of three nitrogen atoms defines an interesting

class of compounds, the triazoles. These may be of two types, the 1,2,3 triazoles or vicinal

triazoles(commonly represented as the u-triazoles) and the 1,2,4-triazoles or s-triazoles. Bladin

was the first to coin the name triazole for the carbon-nitrogen ring and described the derivatives

as early as 1885. (52,54) The nomenclature and method of numbering has been the same and has

not changed over the years. The original method is as per depicted in the formula (55,56)

32

Scheme I.1 s-triazole nomenclature

1.7.1.1 1,2,4 Triazoles

1,2,4 triazoles and their derivatives can be considered as cyclic hydrazidines with

hydrogen atom on either the hydrazide nitrogen or on amide nitrogen .. These triazoles exists in

two stable tautomeric forms, lH-1,2,4-triazole and 4H-l ,2,4-triazole. Out of these two the lH

tautomer predominates and much of the chemistry of 1,2,4 triazoles is a reflection of this

structure(57,58). 1,2,4-triazoles represents the more stable triazole ring when compared with the

1,2,3 triazole, owing to the absence of any triazene bonding with in the ring

Scheme 1.2 Tautomeric forms of s-triazoles

1. 7. l .2Structure and Properties

In order to satisfactorily represent the structure of 1,2,4- triazole many factors like

polarity, aromaticity, amphoteric nature and substitution nature of the nucleus were taken into

account. On comparison with other five membered heterocyclics, azoles in general have higher

33

melting point and dipole moment.(59) The boiling point of 1,2,4-triazole was unusually high in

comparison with those of furan and pyrrole even though there is only a slight difference in

molecular weights. Substitution in the nucleus was found to decrease the boiling and melting

points.

The unsubstituted triazole nucleus is readily soluble in polar solvents, whereas the solubility

tends to decrease as the nitrogen atom in the nucleus is substituted. By analyzing the infrared

spectrum KT Potts(60) explained the existence of associated triazole nucleus which is stabilized

by resonance. This can be interpreted as an intermolecular association in which the hydrogen

atom of the imino group protonates an unsaturated nitrogen of an adjacent molecule, the two

charges being stabilized by resonance( 61 ). Infrared spectrum showed the presence of two bands

characteristic of ammonium type and immonium type band, which was absent in gas-phase

spectrum as well as in the spectrum of substituted triazole nucleus(62).

1,2,4- triazole nucleus presents a highly stable nucleus owing to the presence of aromatic

sextet formed by contributions of one pi electron each from atom joined by double bonds and of

two electrons from a nitrogen atom. The resonance stabilized aromatic structures of the 1,2,4

triazole can be represented from the structures as below

Scheme 1.3 Resonance stabilized structures of s-triazoles

34

The six electrons required for the aromatic sextet, as per the Huckel's (4n+2) rule, are

made up of four from the double bonds and two from the pyrrole type nitrogen atom. The

structure can be represented as an overall negative charge on the ring, balanced by a

corresponding positive charge on the hydrogen atom. Dipole moment(63,64) data indicate that

there is a appreciable contribution from the resonance form that has the negative charge more

closely associated with the nitrogen atom in the 4-position.

1.420 N\

( /

N 1.383 N

1.383

Pi-electron densities of

neutral molecule

1.093 N\

/ N 1.136 '-N/

1.136

Pi-electron densities of anion

Scheme I.4 Electron densities of s-triazole and its anion

Excellent evidence that a pair of electrons from a nitrogen atom of 1,2,4-triazole enters into

the molecular orbital is obtained from the study of the infrared spectra of triazole nucleus and its

substituted derivatives. 3-aminotriazole behaves chemically like a typical aromatic amine and its

infrared spectrum shows characteristic strong NH deformation frequency of a primary amine at

1642 cm·'. In N-acetyl triazole this absorption occurs at 1765 cm·', in a higher frequency than

the normal open chain, saturated ketone at 1725-1705 cm· 1, whereas that of an N,N' substituted

amide occurs at 1650 cm· 1• This indicates that there is no contribution from the dipolar form of

an amide. The ultraviolet absorption spectra of several 1,2,4-triazoles have been examined and

found to obey Beet's lambert's law in concentrations not exceeding 2*10"4 M.

35

Very little attention has been paid to the physical properties of 1,2,4-triazole and its

derivatives. In a study of the heat of combustion of compounds containing high percentage of

nitrogen, several triazole derivatives were studied, and are tabulated below.

Table 1.7 Heat of combustion of some energetic triazoles(65)

SI.no. Compound Heat of combustion

kcal/mo I

1 3-amino-1,2,4-triazole -343.10

2 3-amino-1,2,4-triazolium nitrate -318.01

3 3-amino-5-methyl-1,2,4-triazolium nitrate -466.68

4 5-methyl-3-nitrimino-1,2,4-triazole -465.67

5 3-nitrimino-1,2,4-triazole -317.45

1.7.lJReactivity: Nucleophilic substitution reactions ofTriazole ring

1,2,4-triazolate anion is sufficiently nucleophilic to induce substitution on aromatic

electrophilic positions. Nucleophilic substitution of the fluorine atom in 2-flourobenzonitrile

by 1,2,4-triazole gave a 10:1 mixture of 2-[l ,2,4]-triazol-1-yl benzonitrile and the

corresponding 4-isomer in 66% yield.

A similar reaction of 1,2,4-triazole in the presence of a weak base like Cs2C03 , with 2-cyano-

3-flouropyridine gave 3-[l,2,4]-triazol-1-yl-pyridine-2-carbonitrile as the sole product in a

yield of92%

Reaction of 2,5-dichlorobenzonitrile with 1,2,4-triazole gave 5-chloro-2-[1,2,4]triazol-1-

ylbenzonitrile as the sole product in quantitative yield.

36

N-arylation of Benzotriazole nucleus with 2-chloro-3-nitropyridine in DMSO gave the sole

arylated product by using Na2C03 as the base(66).

1. 7 .1.4 Common methods of preparation of 1,2,4-triazoles

Many excellent methods are available for the synthesis and conversion of oxygen

and sulfur containing triazoles to the parent triazoles. The earlier reported methods were all

distinguished by their simplicity and the low yields. These methods have now be replaced by

later modifications or by more efficient reactions. One among the most efficient routes

describes the refluxing of 1,3,5-triazines with hydrazine hydrochloride in ethanol (67-69).

The table given below shows the methods available for the synthesis of 1,2,4-triazoles.

Table 1.8 General methods for Triazole ring synthesis

Reactants Conditions Yield Reference

Hydrazine Distillation at 280 0 c 30 70

hydrochloride and

formamide

Diformylhydrazide Autoclave at 200 uc, 24 hr 70-80 70

and excess

ammoma

1,3 ,5 triazine and Refluxing for 8 hrs, in ethanol Quantitative 67-69

hydrazine

hydrochloride

Urazole and Heating to 180-200 uc - 71

phosphorous

pentasulfide

37

3-methyl-1,2,4- Final decarboxylation at 120 °c - 72,73,74

triazole with

excess of KMn04

1. 7 .2 Synthesis of Energetic derivatives of triazoles

1.7.2.l ANTA

3-amino-5-nitro-1,2,4-triazole(ANT A) is one among the mostly studied energetic

compounds derived from the triazole nucleus. ANT A was first synthesized by Pevzner

et al(75), starting from the commercially available 5-amino-1,2,4-triazole by converting

the latter into the acetamido derivative using acetic anhydride and later nitrating it with

an acetic acid/nitric acid mixture, followed by hydrolysis by hydrochloric acid. This

three step synthetic route however gave variable and poor yields, with the max limit of

20%.

1hr 2hr

I 0% HCI, reflux

5hr

Scheme I.5 Pevzner method for synthesis of ANT A

38

A second route was reported by the same group in 1982(76), which involves the

oxidation of 1-acyl-3,5-diamino-1,2,4-triazole with hydrogen peroxide in the presence

of sodium tungstate.

N02 N-i

�N H2N N/

ANTA

Scheme I.6 Synthesis of ANTA from H202/NaW04

In an alternative route reported by Stinecipher(77), ANT A was obtained through a

two step process from 3,5-diamino-1,2,4-triazole. The first step involves the

diazotization of the latter to prepare 3,5-dinitro triazole and in the next step the dinitro

derivative is partially reduced using hydrazine hydrate at elevated temperatures.

N--(

NH2

,){ \N H2N N/

H

NaN02, H2S04

60 °C

1hr

N--(

N02

02N/)N

+

NH4

N---(N02

,){ \N02N N/

H

NHz-NH2.HzO

80 °C, 1.5hr

Scheme I. 7 Stinecipher method for synthesis of ANT A

NH3

N--(

N02

,){ \NH2N N/

H

ANTA

39

Table I. 9 Properties of ANT A

Property Ref.

Colour Light Yellow

Molecular Weight 1 29

Melting Point(°C) 238 77, 78, 79,80

Decomposition Temperature(°C) 2 43

Density(gcc- 1) 1.819

Enthalpy of formation(kJmor 1) +61.1

1.7.2.2 ADNT

Ammonium salt of 3,5-dinitro-1,2,4-triazole represents one of a favourable

precursor for the development of new energetic compounds. Simpson et al(81)

presented a synthetic route which describes ADNT synthesis from 3,5-diamno-1,2,4-

triazole by diazotization followed by displacement of diazo with nitro group.

NaN02

, 8iSO 460 °C

1 hr

ADNT

Scheme 1.8 Synthesis of ADNT

NH3

40

Table I. IO Properties of ADNT

Property

Colur

Molecular Weight

Melting Point

Decomposition Temperature(°C)

1.7.2.3 DNAT

Ref.

Light Yellow

176

168-170 81

225

The requirement for nitrogen rich energetic compounds with increased thermal

stability and insensitivity, paved the way for the development of new triazolyl compounds.

One among such compound is 5,5'-dinitro-3,3'azo-IH-1,2,4-triazole (DNAT).(82) Hiskey

etal synthesized DNAT by oxidative coupling of ANTA.

KMn04

, Cone. HCI

50 °C H

N-N

c?,N--(\Jl_NH2

Scheme 1.9 Synthetic route towards DNAT

Minor

+

Major

DNAT

41

Table 1.11 Properties of DNAT

Property

Colour

Molecular Weight

Decomposition Temperature(°C)

Density(gcc · 1)

Enthalpy of formation(kJmor 1)

1.7.2.4 NTO

Ref.

Yellow

254

172 82

1.88

+407.4

NTO represents one of the chemically and thermally stable explosives derived from

the triazole nucleus. It is also known as oxynitrotriazole(ONTA) in german literature. It

has performance properties comparable that of RDX, while its sensitivity is close to the

insensitive 1,3,5-triamino-2,4,6-trinitro benzene(TATB). NTO was first reported in 1905

by the nitration of 1,2,4-triazolone.(83) NTO has been used as a potential explosive since

1980's. It was used in systems where insensitivity and stability is of more concern than

performance, as in low vulnerability ammunitions. (LOYA) Alain Becuwe and A. Delcos

showed the synthesis of NTO by nitration of triazolone formed by condensing

semicarbazide hydrochloride and formic acid. (84)

42

HCOOH )( 100 oc HN NH

�J

NTO

70% HN03

or, Cone. �S0/HN03

Scheme 1.10 Schematic representation of synthetic route towards NTO

Table 1.12 Properties ofNTO

Property

Colour

Molecular Weight

Decomposition Temperature(°C)

Density(gcc· 1)

Enthalpy of formation(kJmol" 1)

1.7.2.5 ATZ

Ref.

White solid

130.1

273 83,84

1.91

-117.22

3-azido-1,2,4-triazole represents one of the stable azido derivatives of triazole

ring system. G. C Denault and co-workers(85) reported the synthesis of 3-azido triazole

from 4-amino-3-hydrazino-s-triazole hydrochloride by diazotization using sodium nitrite.

43

Synthesis of other derivatives of 3-azido triazoles, ie 3-azido-5-amino triazole etc, were

also reported in literature

8iS04

, NaN02

aq. eth -5 °C

NaN3

40 °C 1hr

Scheme I. I I Azido triazole syntheses from amino triazole

Table I. I 3 Properties of A TZ

Property

ATZ

Color Colorless -off-white

Melting Point 121-123

Decomposition temperature (°C) 190

Enthalpy of formation(kJmol"1) 458

1.7.2.6 NTRZ (Nitrimino triazole)

Ref.

85

Nitrimino triazoles, (previously reported as nitramino triazole) are known from

1950 as reported by Henry(86). Nitrimino triazoles and their derivatives are powerful

energetic compounds with VOD comparable to that of RDX(87). Literature provides the

evidence on the synthesis of new family of energetic compounds based on nitimino

triazolate anions. Even though these compounds show improved thermal stability, they

are highly sensitive for impact, especially the parent nitrimino triazoles.

Henry reported the synthesis of 3-methyl-5-nitramino triazole from acetamido-3-

nitroguanidine employing anhydrous sodium carbonate. Throughout the literature

44

different methods are presented dealing with nitration of amino azoles , the most

commonly used ones being, i) Nitric acid in Acetic anhydride(88), ii) alky nitrite in

alcohol(89), iii) nitronium salt (N02BF4) in acetonitrile(90-92), iv) nitric acid in

sulphuric acid(93,94). Katritsky(95) et al described the synthesis of 4-nitrimino-1,2,4-

triazole from 4-amino-1,2,4-triazole from sulphuric acid, nitric acid mixture, at elevated

temperature.

90% fiiS04

, 70% HN03

O °C, 30 mins

�. RT 2hr

Scheme I.12 Synthetic route towards NITRZ

NITRZ

Pevzner et al described the synthesis of 3(5)-nitrimino-1,2,4-triazole, from 3(5)-

arnino-1,2,4-triazole. The exact structure and properties of the nitrimino triazole was

unequivocally described by Astachov et al (96).

Table I.14 Properties of NITRZ

Property Ref.

Colour white

Decompostion temperature(uC) 207 96

Enthalpy of formation (kJmo1"1) -

45

1.8 Conclusions

The different synthetic methods of preparation of various energetic derivatives of

triazoles are reviewed and their properties and applications are brought into lime light.

The synthesis of nitro, azido, dinitro, amino as well as nitrimino derivatives of the parent

triazole nucleus is described.

1.9 Scope of work

Nitro aromatics were the classical high-energy materials, which found much use

mainly in military applications. Much of the interest in the early part of 19th century was

nitration of common materials like wool, cotton, resins etc to produce nitro compounds

which can be used as powerful explosives. The brisance power of such nitro aromatics

was quite high and few had good thermal stability ( eg. TA TB, HNS) and most of them

were highly insensitive (eg. TNT, ONT, TNB etc). These nitroaromatics were considered

as first generation explosives. Later during the world wars, research and development on

new explosives paved the way for the development of powerful nitramine explosives like

ROX, HMX etc. These explosives showed good thermal stability and better brisance

power than the nitro aromatics, but were more sensitive than the latter. The increased

level of toxicity and health hazards caused by these energetic materials, initiated the

development of new generation explosives, 3 rd and 4th generation also referred to as

'Green explosives'. The skeleton of these 'green explosives' were made of nitrogen rich

heterocyclics, mostly triazole or tetrazole ring systems. These energetic molecules have

high nitrogen content, better thermal stability, insensitive and are highly denser.

46

Through this work an effort is made to create new energetic molecules by linking

the nitro aromatic moiety with nitrogen rich heterocyclics and thus studying its

properties. By incorporating nitrogen rich heterocyclics on nitro aromatics, a new class of

energetic compounds can be derived, which may exhibit combined properties of both

class of energetic molecules. Most of the energy obtained from these compounds results

partly through oxidation of the carbon backbone, as traditionally found for carbon rich

explosives, and partly from higher positive heats of formation of heterocyclics. In order

to develop such a class of energetic species dinitrobenzene was chosen as the nitro

aromatic and different energetic derivatives of 1,2,4-triazole were used as the nitrogen

rich heterocyclic part. The aim was to develop novel energetic molecules with good

thermal stability as well as improved density and insensitivity. Different methods and

experimental procedures were used to predict as well as to experimentally establish the

performance of these molecules as energetic species. Thermal and kinetic studies were

used to find out the phenomenological and decomposition kinetics of these compounds.

Computational as well as performance simulations were utilized for predicting the

performance of the selected compounds in various systems. Based on the experimental

results, the possible applications of these compounds were also described through the

work. Thus the work explains in detail the synthesis, characterization and performance

evaluation of a few numbers of energetic compounds of triazolyl ring systems.

47

1.10 References

1. Urbanski T, Chemistry and Technology of Explosives, vols. 1-4, pergamon press, 1964,

1965, 1967, 1984.

2. Roth J, Capener E. L, Encyclopedia of Explosives and Related Items, vol. 8, US Army

Armament Research and Development Command, New Jersey, 1978.

3. Rudolf Meyer, Josef Kohler, Axel Homburg, Explosives, 5th Edition, Wiley-VCH erlag

Gmbh, Germany, 2002.

4. John A. Conkling, Chemistry of Pyrotechnics, Basic Principles and Theory, Marcel

Dekker Inc., New York, 1985.

5. Akhavan, J, The Chemistry of Explosives, 2nd edition, Royal Society of Chemistry,

Cambridge, 2004.

6. Bailey, A, Murray, S. G, Explosives propellants and Pyrotechnics, Brassey's(UK),

Maxwell Pergamon, London, 1989.

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