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Reaction tuning of selectively deposited nano-thermite inks for thrust and heat depositionRaghav RamachandranPurdue University

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Recommended CitationRamachandran, Raghav, "Reaction tuning of selectively deposited nano-thermite inks for thrust and heat deposition" (2016). OpenAccess Theses. 888.https://docs.lib.purdue.edu/open_access_theses/888

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PURDUE UNIVERSITY

GRADUATE SCHOOL

Thesis/Dissertation Acceptance

This is to certify that the thesis/dissertation prepared

By

Entitled

For the degree of

Is approved by the final examining committee:

To the best of my knowledge and as understood by the student in the Thesis/Dissertation

Agreement, Publication Delay, and Certification Disclaimer (Graduate School Form 32),

this thesis/dissertation adheres to the provisions of Purdue University’s “Policy of

Integrity in Research” and the use of copyright material.

Approved by Major Professor(s):

Approved by:

Head of the Departmental Graduate Program Date

Raghav Ramachandran

Reaction Tuning of Selectively Deposited Nano-thermite Inks for Thrust and Heat Deposition

Master of Science in Mechanical Engineering

Steven F. Son

Chair

Ibrahim E. Gunduz

Co-chair

Jeffrey F. Rhoads

Co-chair

Steven F. Son

Jay P. Gore 12/1/2016

REACTION TUNING OF SELECTIVELY DEPOSITED NANO-THERMITE INKS

FOR THRUST AND HEAT DEPOSITION

A Thesis

Submitted to the Faculty

of

Purdue University

by

Raghav Ramachandran

In Partial Fulfillment of the

Requirements for the Degree

of

Master of Science in Mechanical Engineering

December 2016

Purdue University

West Lafayette, Indiana

ii

ACKNOWLEDGEMENTS

I want to express my sincere gratitude to Professor Steven Son for allowing me to join his

research group and giving me the opportunity to work on several cutting edge projects as

well as providing me with his support and guidance. I would also like to thank Professor

Emre Gunduz for his guidance with my experimental setups and helpful discussions.

Many thanks to Vasant Vuppuluri, Trevor Fleck, Gabriel Diez, and Eric Westphal, who

at some point or other have aided me in safely handling all of the nano-thermite batches

and helped with my experiments. A special thanks goes to Professor Jeffrey Rhoads for

his leadership on this project, as well as for serving on my committee. I would also like

to acknowledge Dr. Cathie Condron for funding this project through the Defense Threat

Reduction Agency grant no. HDTRA 1-15-1-0010.

iii

TABLE OF CONTENTS

Page

LIST OF TABLES ............................................................................................................. iv

LIST OF FIGURES .............................................................................................................v

ABSTRACT ....................................................................................................................... vi

CHAPTER 1. INTRODUCTION ........................................................................................1

1.1 Nano-aluminum and Nano-thermite combustion....................................................1

1.2 Integration of Nano-Energetics with MEMS ..........................................................5

1.3 Motivation ...............................................................................................................9

CHAPTER 2. EXPERIMENTAL METHODS .................................................................10

2.1 Sample Preparation ...............................................................................................10

2.2 Thrust Measurements ............................................................................................12

2.3 Thermal Measurements .........................................................................................13

CHAPTER 3. RESULTS AND DISCUSSION .................................................................15

3.1 Thermochemical Equilibrium Calculations ..........................................................15

3.2 Thrust Measurement Results.................................................................................16

3.3 Thermal Measurement Results .............................................................................21

CHAPTER 4. CONCLUSIONS AND FUTURE WORK .................................................25

LIST OF REFERENCES ...................................................................................................27

APPENDICES

APPENDIX A: NANO-THERMITE MIXING PROCEDURE ........................................31

APPENDIX B: SAMPLE MIXING SHEET .....................................................................34

iv

LIST OF TABLES

Table Page

1.1 Nano-thermite performance summary. .......................................................................7

v

LIST OF FIGURES

Figure Page

1.1 Schematic of a semi-conductor bridge. ....................................................................5

1.2 Combustion chamber without and with converging-diverging nozzle. ...................6

1.3 Micro-machined thruster. .........................................................................................7

1.4 Schematic of microelectronic thruster incorporating nano-thermite. ......................8

2.1 Fixture and syringe for resonant mixing of nano-thermites. ..................................10

2.2 Image of deposited nano-thermite which has been dried. ......................................11

2.3 Thrust measurement schematic. .............................................................................12

2.4 Thermal measurement schematic. ..........................................................................13

2.5 Region of interest used to average temperature of wafer. ....................................14

3.1 Predicted performance of Al-CuO. ........................................................................16

3.2 Predicted performance of Al-Bi2O3. ......................................................................16

3.3 Summary of thrust performance of Al-CuO. .........................................................19

3.4 Summary of thrust performance of Al-Bi2O3. .......................................................19

3.5 Typical force traces for Al-CuO. ...........................................................................20

3.6 Typical force traces for Al-Bi2O3. .........................................................................20

3.7 Schlieren imaging of stoichiometric Al-CuO. .......................................................20

3.8 Schlieren imaging of Al-CuO at very fuel-rich conditions (=8). .........................21

3.9 Schlieren imaging of stoichiometric Al-Bi2O3. .....................................................21

3.10 Schlieren imaging of Al-Bi2O3 at very fuel-rich conditions (=10). .....................21

3.11 A typical nAl temperature trace measured on the back side of a wafer. ...............23

3.12 Typical Al-CuO temperature traces. ......................................................................23

3.13 Typical Al-Bi2O3 temperature traces. ....................................................................23

3.14 Summary of heat deposition with Al-CuO samples. .............................................24

3.15 Summary of heat deposition with Al-Bi2O3 samples. ............................................24

vi

ABSTRACT

Ramachandran, Raghav. M.S.M.E., Purdue University, December 2016. Reaction Tuning

of Selective Nano-thermite Inks for Thrust and Heat Deposition. Major Professor: Steven

F. Son, School of Mechanical Engineering.

Currently, there is very little systematic work that quantifies the performance of

energetic materials in terms of thrust or heat deposition applied to electronic circuits. A

better understanding of the interactions between nano-scale energetic materials and

electronic systems, as a function of stoichiometry is needed for enhanced defeat. Both of

these needs are addressed in this research. Formulations of Al-CuO and Al-Bi2O3 nano-

thermites were prepared at different equivalence ratios and selectively deposited onto

silicon substrates and the thrust and heat deposition of these materials was quantified.

Both nano-thermite systems produced maximum thrust near stoichiometric ratios, and

more fuel-rich mixtures led to significant decreases in thrust. In contrast, fuel-rich

formulations of both nano-thermite systems resulted in increased heat deposition, due to

less gas production, which removed hot products from substrate surfaces, and lower heat

deposition was observed near stoichiometric ratios. The heat transfer from the Al-CuO

system increased more than the Al-Bi2O3 system at fuel rich conditions. The developed

methodology can be used to characterize and optimize other materials for possible

integration into electronic devices.

1

CHAPTER 1: INTRODUCTION

1.1 Nano-aluminum and Nano-thermite Combustion

In the field of energetic materials, monomolecular explosives are widely used. As

the fuel and oxidizer are already combined in a single molecule, breaking molecular

bonds results in a release of energy over a short timescale. The integration of most of

these materials in small geometric configurations is difficult, as their volumetric heats of

combustion are too low to successfully allow for propagation at the micro-scale [1]. In

addition, these materials may be difficult to ignite and quench immediately after ignition

within these geometries [2]. Metal fuels have been shown to have higher volumetric

combustion enthalpies and energy densities, which makes them an ideal candidate for use

in micro-scale energetics. Although boron has the highest known volumetric heat of

combustion when ignited in air, boron particle ignition delays are too large to be utilized

in energetic formulations [3].

Aluminum is a preferred fuel in blast explosives and propellants due to its wide

availability. Ignition characteristics of aluminum particles are highly dependent on

particle size. Particles with diameters greater than 100 m only ignite once the aluminum

oxide shell melts. However, nano-scale aluminum (less 100 nm in diameter) exhibits

rapid combustion at ignition temperatures closer to the melting point of aluminum [3].

As the particle diameter decreases, the specific surface area of the aluminum particles

increases. This allows for a higher reactivity at the particle surface, increasing the overall

reactivity [1]. All aluminum particles feature a 2-5 nm alumina shell on their surface.

For micron-scale particles, the effect of the oxide shell on particle mass can be neglected.

With nano-scale aluminum, the active aluminum content in nano-scale aluminum

particles needs to be accounted for since the alumina shell is considered to be inert [4].

2

A thermite reaction is an oxidation-reduction reaction consisting of a metal and a

metal-oxide behaving as a fuel and oxidizer, respectively. Traditionally, these reactions

were used in the formation of structural metals through their high temperature reactions,

as in the welding of railroad tracks [5]. Fischer and Grueblich have tabulated the

theoretical maximum density, adiabatic flame temperature, gas production, and heat of

combustion for a variety of thermite reactions [6]. Aluminum is a commonly used fuel in

these reactions. Thermites mixed with micron-scale constituents react through the

diffusion of particles. The mixing of nano-scale aluminum and nano-scale metal oxides

(<100 nm) results in the formation of so-called nano-thermites. The high interfacial

contact between the nanoparticles and the small diffusion scales enhance the chemical

kinetics between the fuel and oxidizer, allowing for high energy releases and rapid

pressurization upon ignition [7]. Nano-thermites have also been referred to as metastable

intermolecular composites, as the mixture stays in an inert form prior to ignition [8].

However, special care is needed when handling nano-thermites, as these materials are

very sensitive to impact, spark, and friction [2].

While there are advantages to other approaches of synthesizing nano-thermite

formulations, including self-assembly and ball-milling, physical mixing is the most

commonly used method to process large batches of nano-thermite [8]. As nano-thermites

are highly sensitive to electrostatic discharge, wet processing of the materials is highly

recommended. Pusynzki et al. demonstrated the ability to process nano-thermites in

water, if the nano-aluminum was well coated with hydrophilic materials like ammonium

dihydrogen phosphate or hydrophobic materials like oleic acid. While coating nano-

aluminum increased the resistivity of converting to aluminum oxide, full oxidation

occurred within 15 hours of mixing [9]. Otherwise, it is preferred that nano-scale

powders are typically mixed in an aprotic solvent to prevent the oxidation of nano-

aluminum [10].

Ultrasonic mixing has been a popular physical mixing technique for nano-

thermites [8]. This method is widely used to prepare powders, but it may pose some

problems for the development of inks for deposition. One major drawback is the larger

3

amounts of solvent needed to mix a suspension without inducing a combustion reaction

of the nanoparticles. With larger amounts of solvent in suspension, the phase separation

of particles occurs over time. In contrast, resonant mixing uses a low resonant frequency

to agitate a bulk mixture, which allows for the mixing of highly viscous suspensions [11].

Near stoichiometric conditions, the propagation of nano-thermites is driven by

advection. Rapid pressurization upon combustion allows for the thermal transport of

molten material to unreacted zones, leading to fast reactions. Experimental results also

confirm that maximum propagation speeds correlate with peak pressurization and low

densities [12]. Zachariah and Egan also contend that conduction and radiation are

negligible in experimental conditions used in previous propagation experiments, based on

physical properties of the “burn tubes” used in such experiments [13]. In contrast,

conductive heat transfer dominates the combustion of very fuel rich nano-thermite

systems [14] or nano-thermites prepared at high packing densities [12]. The inclusion of

binders has been another technique to tune the reactivity of nano-thermite mixtures. The

addition of fluoropolymer binders has reduced the peak pressurizations and increased the

ignition sensitivities of nano-thermites. Such binders are well utilized because of the

aluminum-fluorine reactions that occur [15].

Dutro et. al stated that there are three different types of propagations depending

on the equivalence ratio: steady propagation driven by advection, an unsteady

acceleration wave, and a steady slow propagation driven by conduction [14]. Similarly, a

wide range of propagation behaviors were seen through varying the addition of alumina

in nano-thermites as a diluent. Here, three distinct propagation regimes were observed: a

constant velocity regime, a constant acceleration regime, and an unsteady spiraling

combustion wave [16].

Several theories have been proposed regarding the chemical kinetics and reaction

mechanisms that control the combustion of nano-scale thermites. As the alumina layer

on nano-aluminum acts as a barrier to reaction, the ignition of nanothermites either

occurs through diffusion in the alumina layer or by breaking the oxide layer [13].

Therefore, the ignition of nano-thermites highly depends on heating rates. Nano-

4

thermites heated at “slower” heating rates exhibit reactions dominated by diffusion

processes, similar to micron-scale thermites. The melt-dispersion reaction mechanism is

proposed at higher heating rates (greater than 1E6 K/s). Here, the aluminum core melts

and leads to the fracture of the alumina shell, allowing for the dispersion of molten

aluminum [17].

Another theory postulates that gaseous oxygen may drive the ignition of nano-

thermites. Metal oxides decompose into a stable phase, releasing gaseous oxygen at high

temperatures. Rapid heating experiments were conducted to monitor temperatures at

which O2 is released. Only certain nano-thermite systems had similar ignition and O2

release temperatures, suggesting that other reaction mechanisms dominate nano-thermite

combustion [18].

Recent literature proposes that the nano-thermite reactions are strongly driven by

condensed phase reaction mechanisms [19-20]. Specifically, the sintering of aggregated

particles into bigger structures was observed with a variety of nano-thermite systems.

Similar to the melt dispersion mechanism, molten aluminum disperses and interacts with

the oxidizer, increasing the interfacial area and speeding reaction rates [19]. High-speed

imaging with nano-second temporal resolution showed that there were large interfaces

between the fuel and oxidizer, resulting in changes in particle morphology after ignition.

[20].

The two nano-thermite systems relevant to the experiments described herein are

aluminum-copper oxide (Al-CuO) and aluminum-bismuth trioxide (Al-Bi2O3). Both

nano-thermites prepared at stoichiometric conditions are known to be gas generators and

have been investigated in a variety of experiments. Al-CuO is expected to produce more

heat and less gas generation compared to Al-Bi2O3 [6]. Al-Bi2O3 is considered to be one

of the most reactive nano-thermite systems. Bismuth boils at temperatures beneath the

reaction temperature of Al-Bi2O3, and the evaporation of bismuth coincides with peak

pressurization. The evaporation of bismuth allows for more gas release, allowing for

more advective transport of reacted particles and therefore increasing the propagation

speeds [21].

5

1.2 Integration of Nano-Energetics with MEMS

The integration of energetic materials in electronic systems has been considered

for the past four decades. Anti-tamper devices integrated with aluminized pyrotechnic

formulations were developed in order to address critical needs for self-destruction within

electronics. One specific example cited describes the use of aluminum and tungsten

oxide thin films [22-23].

Likewise, semi-conductor bridge (SCB) technology was developed as an alternate

method for the initiation and ignition of explosive powders. A schematic can be seen in

Fig 1.1. SCBs have demonstrated operability with lower energy inputs and smaller

volumes of traditional bridge-wires on electro-explosive devices [24]. Experimental

work has been conducted to assess the possibilities of integrating nano-thermites with

SCB initiators [25].

Figure 1.1: Schematic of a semi-conductor bridge [24].

The possibilities for integrating energetic materials in microelectronic systems has

widely increased with the advancements in nanotechnology, resulting in the emergence of

the on-chip micro-energetics field [26-27]. Recent safe-and-arm devices utilize

miniaturized electromechanical components to arm and disarm pyrotechnic initiators in

munitions [28].

Another potential application is in the field of power generation. While nano-

energetic materials could potentially be used in microscale devices for energy harvesting

6

[26], nano-scale metal fuels could also be used in large scale applications. One example

postulates that if nano-aluminum is burned in suspension with air as the oxidizer, and the

metal oxide products could then be captured and reconverted into metal fuels using phase

separation technology [29].

The utilization of nano-energetics in micro-actuation and micro-propulsion

applications has been well documented. The combustion of energetic materials provides

the largest actuation pressures at the microscale in comparison to other types of actuation

[26-27]. An early example uses a thin film propellant heated with resistive wire onto a

silicon substrate [30]. Other examples integrate deposited energetic material onto

fabricated Joule-heating initiators [31].

There are many examples of energetic materials used in micro-propulsion

applications [27]. Nano-thermite formulations have also been considered in micro-

propulsion devices has been considered [21, 32-35]. In one early set of experiments, Al-

CuO powder was prepared ultrasonically and pressed into pellets of different densities

within combustion chambers as seen in Fig 1.2. Thrust measurements were measured

using a high impedance force sensor [32].

Figure 1.2: Combustion chamber without and with converging-diverging nozzle [32].

Results verified that high density thermites had lower thrusts than low density

thermites, as their propagation was driven by conduction over advection. Converging-

divergences had no effects on thrust for low density thermites, but the effect of

confinement increased thrust with high density thermites [32].

7

More recent experiments from the same research group described thrust

measurements performed with the experimental setup seen in Fig. 1.3. Both Al-CuO and

Al-Bi2O3 were prepared ultrasonically and pressed into low density thermites within a

chamber [32-33]. Initial experiments were done to validate that the optimal

stoichiometry of Al-Bi2O3 for maximum thrust were at slightly fuel rich conditions, even

with the inclusion of nitrocellulose as a binder [33].

Figure 1.3: Micro-machined thruster [33].

Table 1.1: Nano-thermite performance summary [34].

Nanothermite Average

thrust, N

Duration,

ms

Impulse,

mN*s

Isp, s Isv,

mN*s*mm-3

CuO/Al 4.6 ± 0.4 5.1 ± 0.5 23.0 ± 0.6 20.2 ± 1.3 0.5 ± 0.0

CuO/Al/NC

(2.5%)

2.7 ± 0.4 10.4 ± 0.4 28.3 ± 4.2 24.2 ± 3.8 0.6 ± 0.1

CuO/Al/NC

(5.0%)

0.8 ± 0.2 23.1 ± 3.6

19.5 ± 6.9 15.7 ± 4.3 0.4 ± 0.2

Bi2O3/Al 46.1 ± 2.6 1.7 ± 0.0 78.6 ± 4.8 41.4 ± 1.2 1.7 ± 0.1

Bi2O3/Al/NC

(2.5%)

22.4 ± 2.8 4.9 ± 0.4 108.8 ± 4.3 54.2 ± 1.7 2.3 ± 0.1

Bi2O3/Al/NC

(5.0%)

7.8 ± 1.6 13.7 ± 2.0 105.7 ± 3.9 59.4 ± 2.9 2.2 ± 0.1

8

The effects of incorporating binder for both nano-thermite systems at quantities

up to 5 weight percentage binder were also examined. The inclusions of binders into

nano-thermite mixtures were shown to decrease thrust and increase impulse due to their

longer reaction timescales. The results from these experiments can be seen in Table 1.1

[34].

Another paper discusses integrating nano-thermite directly into a fabricated

silicon device. Puchades et al. describes all the steps taken to fabricate the device. After

applying a silicon nitride layer through chemical vapor deposition and etching a resistive

aluminum element as the micro-heater, a chamber is created through a wet etch.

Although many details of the Al-Bi2O3 nano-thermite preparation process are unstated, it

is known that the thermite was prepared in hexane and deposited into the chamber using

an automated dispenser. Few force measurements were made by measuring the

thermite’s change in mass on a scale. Maximum forces of 600 mN were recorded for a

10 mg sample [21].

Figure 1.4: Schematic of microelectronic thruster incorporating nano-thermite [21].

More recently, impulse measurements on Al-CuO thermites were done using a

torsional pendulum and a displacement sensor. Here, loose powder was prepared with

nitrocellulose (NC) binder at different quantities either through ultrasonic mixing or

electrospray. Deep reaction ion etching methods were used to create a chamber within a

9

silicon substrate with a patterned ignitor. The loose nano-thermites are placed within the

chambers and ignited using Joule heating. Experimental results from the paper indicated

that average thrust and specific impulse values increase with the addition of binder,

although the measured thrusts were far below the other experiments. Estimated specific

impulse values were 10-30 seconds for 1.5 mg samples [35].

1.3 Motivation

Although the integration of energetic materials with electronics systems has been

studied and demonstrated for more than a decade, there are no standard or uniformly

accepted characterization experiments to quantify the integration of energetic materials in

electronic systems. In fact, energetic materials are often chosen based on availability or

convenience. Some may assume stoichiometric mixtures of energetic composites would

be suitable for all applications; however, this may not be true. Tailoring and optimizing

performance requires an understanding of how a deposited energetic material interacts

with a system, the specifics of which can vary widely. Consequently, a simplified

representative system would be valuable for studying the performance of deposited

energetic materials and to quantify their performance using a few simple tests and useful

metrics. The objective of this study is to develop experiments, where nano-energetics

can be quantitatively compared for thrust or heat deposition applied to a substrate using

force transducers and infrared imaging. A second objective is to explore the effects of

mixture stoichiometries on Al-CuO and Al-Bi2O3 nano-thermite systems.

10

CHAPTER 2: EXPERIMENTAL METHODS

2.1 Sample Preparation

Nano-thermite inks are formulated in 500 mg batches following a procedure

described in detail by Nellums et al. [11]. Briefly, nano-aluminum (80 nm, 27.1 m2/g,

Novacentrix) was mixed either with nano-copper oxide (d<50 nm, 29 m2/g, Sigma-

Aldrich) or nano-bismuth oxide (38 nm, 18 m2/g, Nano-Arc) using a resonant mixing

process. Contents are placed and sealed in a 10 mL syringe, the syringe is placed within

a Teflon holder, and the materials are then mixed for a total of 16 minutes.

Figure 2.1: Fixture and syringe for resonant mixing of nano-thermites.

Nellums et al. demonstrated optimal mixing at a 30% volumetric solids loading;

however, the nano-bismuth oxide used in those studies had a lower specific surface area

than the materials used in this paper, therefore the solids loading was adjusted [11]. The

increase in specific surface area and decrease in particle diameter allow for intimate

mixing to be achieved at lower volumetric solids loadings.

11

A polar solvent is advantageous for the mixing of nano-thermites since a

suspension can be maintained longer in polar solvents [10]. Here, all nano-thermite

formulations were prepared at an 8% volumetric solids loading in N-N-

dimethylformamide (DMF), which a polar solvent.

Prior to mixing and deposition, 40 mg nylon washers (3.2 mm inner diameter and

0.8 mm thickness) were epoxied onto 13 mm x 13 mm silicon substrates (obtained from

Ultrasil Corporation) in order to maintain the deposited material diameter constant

between samples. Preliminary experiments showed that variations in deposit diameter

had a large effect on the dynamics of reaction propagation, which negatively affected the

reproducibility. Extra care was taken to ensure that any residual epoxy is carefully

removed within the inner diameter of the washers prior to the deposition of nano-thermite

inks.

After physical mixing, nano-thermite inks were immediately transferred from the

syringe into a 1.5 mL polypropylene vial. Materials were then directly pipetted in within

the washers to simulate a printed energetic ink with a SciLogex 2-20 microliter pipette.

The four equivalence ratios tested in the following experiments were chosen based on

predicted equilibrium calculations made on CHEETAH, an equilibrium code [10].

Stoichiometric (=1) and near-stoichiometric (=1.5) conditions for both nano-thermites

were considered to determine the point of maximum thrust.

Figure 2.2: Image of deposited nano-thermite which has been dried.

12

The next conditions were fuel rich conditions with predicted very low amounts of

gas generation (=4 for Al-CuO and =5 for Al-Bi2O3). The last conditions used were

very fuel rich conditions with essentially no predicted gas generation. The equivalence

ratios were double that of the previous tests for consistency (=8 for Al-CuO and =10

for Al-Bi2O3).

Depending on the equivalence ratio of the mix, the appropriate volumes of nano-

thermite inks needed to be pipetted within the nylon washers in order to produce about 4

to 5 mg of dry nano-thermite. Immediately after deposition, the deposited nano-thermites

were set to dry under exposure to an infrared lamp for 45 minutes. An image of a dried

deposited nano-thermite is shown in Fig. 2.2.

2.2 Thrust Measurements

A detailed schematic of thrust measurements can be seen in Fig. 2.3. A Kistler

9215 piezoelectric force transducer was used to measure the thrust of the dried nano-

thermite. The sensor was calibrated by the manufacturer to measure between a 0.01-200

N and has a reported 25 kHz natural frequency. Silicon substrates with the deposited

droplets were placed onto a small 6.35 mm diameter cylindrical aluminum mount

attached to the sensor. Single leads of electric wire connected to a PKB50 20kV high

voltage generator ignited the nano-thermite samples via spark discharge. This was used to

minimize the heating of the substrate from the ignition source.

Figure 2.3: Thrust measurement schematic.

13

A Kistler Type 5010 Signal Conditioner and a DPO 4034 Oscilloscope was used

to collect the force signal. The light emitted from the reaction was detected by a Thor

Labs DET10 photodiode, which triggered the oscilloscope. A transistor-transistor logic

(TTL) pulse was sent from the oscilloscope to a high-speed Phantom v7.3 Camera to

capture high-speed videos of the ignition event. In order to better observe the plume

from the nano-thermite combustion, schlieren imaging using an LED light source and

two concave mirrors was used (Edmund Optics 71-013). The aperture of the camera lens

(Nikon AF Nikkor 28-105mm 1:3.5-4.5D) was used as the knife-edge. All videos were

recorded at 88888 frames/second (fps). It should be noted that all wafers used in this

experiment were 500 m in thickness to avoid any wafer fracture post ignition.

2.3 Thermal Measurements

A FLIR ASC655 camera was used to image the back-side of a silicon wafer using

a 25.4 mm diameter Thor-Labs gold plated mirror mounted at a 45° angle, which was

96% reflective in the infrared spectrum, as illustrated in the schematic in Fig. 2.4.

Figure 2.4: Thermal measurement schematic.

All of the substrates in this study were 300 µm in thickness to produce a good

signal range. The back side of the wafer was coated with two coats of black Rust-Oleum

High Heat Spray Paint in order to create a near blackbody reference and to prevent

infrared transmission through the silicon wafer for thermal imaging. All of the samples

14

were ignited using the same spark ignition method described earlier, and all of the

subsequent videos were recorded at 200 fps.

Through video playback on FLIR’s Examin-IR software, a region-of-interest was

created to spatially average the temperature of the wafer’s back side over time, as seen in

Fig. 2.5 The maximum temperature rise, ΔT, was used to determine the heat deposition

within the substrates using equation 2.1:

Q = mCPΔT

The specific heat of silicon used was 0.712 J/gK [36], and the mass of the wafer was

about 117.4 +/- 0.2 mg, based on an average of 24 measurements.

Figure 2.5: Region of interest used to average temperature of wafer.

15

CHAPTER 3: RESULTS AND DISCUSSION

3.1 Thermochemical Equilibrium Calculations

Equilibrium calculations were performed using CHEETAH 7.0, a thermochemical

equilibrium code developed by Lawrence Livermore National Laboratory with a

modified JCZS product library [10]. The fuel-to-oxidizer ratios were varied over the

experimental ranges in order to estimate adiabatic flame temperatures and combustion

products at a constant atmospheric pressure. CHEETAH calculations predict the

maximum adiabatic flame temperatures near stoichiometric conditions for both nano-

thermite systems, as seen in Figs. 3.1 and 3.2.

The Al-CuO system exhibits an adiabatic flame temperature through a wide range

of equivalence ratios as compared to Al-Bi2O3 because of the formation of liquid copper

throughout. Liquid alumina may help to contribute to the high predicted peak

temperature in the Al-Bi2O3 system, but the amounts formed are much lower than that

produced in the Al-CuO system at stoichiometric conditions.

Two peaks in gas generation are observed in the Al-CuO system. The first peak

corresponds to gaseous copper and the second peak results from the formation of Al2O

and Al2O2 gases, that are produced as the mixture becomes more fuel-rich and there is not

enough oxygen to fully oxidize aluminum to Al2O3. Aluminum oxide gases are produced

until approximately =4.5, where the product formation is primarily liquid copper or

solid alumina. In contrast, gaseous bismuth dominates the gas generation in the Al-Bi2O3

system, with the maximum amount being produced at fuel rich conditions.

16

Figure 3.1: Predicted performance of Al-CuO.

Figure 3.2: Predicted performance of Al-Bi2O3.

3.2 Thrust Measurement Results

A second order 1 MHz Butterworth low pass filter was applied to the force

transducer traces to remove the effects of mechanical resonance and high frequency

electrical noise from the capacitor discharge unit. Impulse values were computed using

the trapezoidal function in MATLAB to integrate the force trace over time. Specific

impulse was calculated using equation 3.1,

1 1.5 4 8

1500

2000

2500

3000

1000

Adia

bati

c F

lam

e T

em

pera

ture

(K

)

1.25

2.5

3.75

5

Tota

l G

as

Pro

ducts

(m

ol/

kg)

Adiabatic Flame Temperature

Gas Products

1 1.5 5 101000

1500

2000

2500

3000

Ad

iab

ati

c F

lam

e T

em

pera

ture

(K

)

1.25

2.5

3.75

5

To

tal

Gas

Pro

du

cts

(m

ol/

kg

)

Adiabatic Flame Temperature

Gas Products

17

𝐼sp =∫ F dt

mg, (3.1)

where F is force, m is initial mass of the sample and g is the gravitational constant. Thrust

measurements were conducted on a set of four samples at each equivalence ratio tested.

Peak thrusts divided by sample mass and specific impulses of the samples are plotted in

the bar graphs in Figs. 3.3 and 3.4.

Typical force traces of the Al-CuO samples are shown in Fig. 3.5. As seen in Fig

3.3., thrust generation decreases by more than a factor of two between =1 to =1.5, and

between =1.5 to =4. These observations are similar to what was observed by Sanders et

al., where the peak pressures and combustion velocities occurred near =1.1 [10]. A

shock wave followed by hot reaction products can be visibly seen at stoichiometric

conditions in Fig 3.7. as the reaction propagates very quickly. For more fuel-rich cases,

the reaction is slower with less gaseous products and no shock wave, but the hot reaction

products are still observed. Comparing the experimental results to CHEETAH

calculations suggests that the shock wave formation and the majority of thrust generation

may be largely driven by the production of copper gas since the observed shock

formation corresponds to high predicted gas production. As the mixture becomes more

fuel-rich, there is more variability in the results, likely due to lower flammability and

slower burning rate.

Samples from the very fuel rich Al-CuO (=8) set demonstrated more sample-to-

sample variation. Certain samples from this set exhibited combustion behavior, which

appeared to be driven by a quick burn, possibly a subsonic convection reactive wave. In

Fig. 3.8, an unsteady combustion front causes the reacted or unreacted powders to be

thrown in different directions off the substrate. Other samples ignited within the washers,

but did not emit the necessary amount of light to trigger the photodiode, resulting in no

recorded force signal and indicating a possible slow deflagrative wave. An alternative

triggering mechanism can be used to obtain force signals for such samples, as mentioned

later. Previous literature has reported unsteady combustion behavior and variable reactive

performance at fuel-rich conditions for nano-thermite systems [14].

18

Typical force traces of the Al-Bi2O3 samples can be seen in Fig. 3.6. The

maximum peak thrusts and specific impulses for Al-Bi2O3 are observed at slightly fuel

rich conditions (=1.5). Correlating the observed experimental results to isobaric

CHEETAH calculations, the observed shock waves and thrust generation appears to also

be driven by the rapid formation of gaseous bismuth. Sanders et al. showed that the peak

propagation velocities and pressures were at =1.3 or this system [10]. Compared to

stoichiometric conditions, the propagation velocities at =1.6 were higher, but the

pressures recorded were lower [10]. Samples mixed at an equivalence ratio of =5

generated much weaker shock waves upon ignition and propagated slower than their

stoichiometric counterparts. Based on experimental observations, as seen in Fig. 3.10,

the combustion of the samples prepared at =10 is relatively slow, and solid powders are

released from the substrate over a longer period of time. Attempts were also made to test

dried deposited nano-aluminum (no oxidizer) for thrust measurements, but these samples

failed to ignite with a spark. With the low amounts of material tested in this experiments,

it is likely that the agglomeration and higher densities of these deposited materials

inhibited ignition. Comparable amounts of nano-aluminum powder were instead ignited

for a thrust measurement. Similar to select tests with the very fuel rich Al-CuO, the nAl

ignited within the washers without releasing any products from the substrate. Since the

samples did not generate a recorded force signal using the photodiode, an alternative

triggering mechanism was needed. The signal conditioner was set to the minimum

amplification settings, and the signal output was used to trigger a force measurement.

This verified that the ignition of nAl powders generated negligible amounts of thrust.

19

Figure 3.3: Summary of thrust performance of Al-CuO.

Figure 3.4: Summary of thrust performance of Al-Bi2O3.

20

Figure 3.5: Typical force traces for Al-CuO.

Figure 3.6: Typical force traces for Al-Bi2O3.

Figure 3.7: Schlieren imaging of stoichiometric Al-CuO.

0 100 200 300

0

15

30

45

Time (s)

Thru

st (

N)

=1

=1.5

=4

=8

0 100 200 300

0

100

200

300

Time (s)

Thru

st (

N)

=1

=1.5

=5

=10

21

Figure 3.8: Schlieren imaging of Al-CuO at very fuel-rich conditions (=8).

Figure 3.9: Schlieren imaging of stoichiometric Al-Bi2O3.

Figure 3.10: Schlieren imaging of Al-Bi2O3 at very fuel-rich conditions (=10).

3.3 Thermal Measurement Results

All thermal heating measurements were conducted on a set of four samples at

each equivalence ratio considered. The Al-Bi2O3 samples prepared at =1 and =1.5 were

not tested in this experiment, as the energy generated from these samples fractured the

substrates used. Because deposited nano-aluminum inks were not ignitable with the

spark generator, 4 mg of dry deposited nAl powders were instead used to determine the

22

heating effects. As the nAl powder did not gasify, no products of combustion left the

washer after ignition. Therefore, a greater amount of energy could be effectively

transferred conductively for heating the substrate. All of the substrates showed heating at

low heating rates with spatially uniform temperature distributions throughout. A typical

temperature trace measured on the backside of a silicon wafer can be seen in Fig. 3.11.

Figs 3.12 and 3.14 and summarize the heating results of several Al-CuO samples.

While fuel-rich samples effectively deposit more heat into substrates, there is larger

variability between tests. Nano-thermites at fuel rich conditions can display more

stochastic combustion behavior between samples that are presumably propagating within

a subsonic convection regime, as observed in other work for fuel-rich nano-thermites.

One of the four samples from the =8 set produced very slow heating and a slow cool-off

on the backside, similar to the behavior observed in the nAl powder ignition.

The other samples from the =8 set generated similar amounts of heat within the

substrates as compared to the =4 samples, although there was a significant decrease in

the heating rate, as seen in Fig 3.12. The variability in the heating performance of the

fuel-rich Al-CuO nano-thermites can also be attributed to the significant decrease in their

sensitivity to electro-static discharge [37]. The reduction in electrostatic sensitivity could

increase ignition delays, changing the overall ignition time of each sample prepared at

such conditions.

Fuel rich Al-Bi2O3 formulations were observed to be less effective at heating the

silicon substrates, but do exhibit quick heating profiles (see Figs. 3.13 and 3.15),

performing similarly to stoichiometric and near-stoichiometric Al-CuO. Despite a

significant reduction in gas generation, there is less condensed phase product formation

necessary for larger amounts of energy transfer within the substrate. As indicated by the

schlieren imaging, ignition of the samples prepared at = 10 result in solid products

being thrown away from the substrate over time. It is also possible that some of these

samples exhibit incomplete combustion, with the partially combusted products

propagating away from the substrate.

23

Figure 3.11: A typical nAl temperature trace measured on the back side of a wafer.

Figure 3.12: Typical Al-CuO temperature traces.

Figure 3.13: Typical Al-Bi2O3 temperature traces.

24

Figure 3.14: Summary of heat deposition with Al-CuO samples.

Figure 3.15: Summary of heat deposition with Al-Bi2O3 samples.

25

CHAPTER 4: CONCLUSIONS AND FUTURE WORK

This work describes experiments used to quantify the thrust and heating in

electronic circuit analogues integrated with nano-thermite based energetic materials.

Nano-thermites processed through resonant mixing methods were deposited into washers

attached to silicon substrates. Tests were conducted on the dried nano-thermites (4 to 5

mg) to determine thrust generation using a force transducer and heat deposition into

silicon substrates using infrared thermometry. At near stoichiometric conditions, both Al-

CuO and Al-Bi2O3 generated maximum thrusts and shock-wave formation was visible in

schlieren imaging. The tradeoff between thrust and heat deposition was clearly visible by

altering stoichiometry, as increased heat deposition was observed with fuel-rich nano-

thermite formulations, and the reduction in gas generation resulted in more hot products

remaining on the substrates post ignition. The results seen through experimentation

match thermochemical equilibrium calculations predicted by CHEETAH 7.0.

More stochastic combustion behavior was observed with fuel-rich Al-CuO

formulations, resulting in different conductive heating performances between samples.

As nano-thermite formulations failed to spark ignite beyond a certain threshold, nano-

aluminum powder ignition was tested to determine heat deposition. With very few hot

reaction products leaving the substrates, most of the combustion energy was effectively

transferred into conductively heating the silicon substrates.

The low quantity of materials tested in these experiments could potentially

represent a simplified version of existing micro-electronic devices integrated with

energetic material. While this paper focused solely on nano-thermite formulations, these

experiments could be used to quantify other nano-thermite formulations as well as other

classes of energetic materials that propagate well at the microscale.

26

There is a wide number of combinations possible for mixing other nano-thermite

systems. Utilizing other nano-scale metal oxides with lower densities may be useful

forcreating nano-thermite inks with a lower likelihood of phase separation. The addition

of the appropriate dispersants may also help to increase the shelf life of nano-thermite

inks and allow for the possibilities of inkjet printing.

Future work could also investigate the effects of incorporating various binders on

thrust and heat deposition. In addition to particular fluoropolymers and nitrocellulose

that have been used in the past with nano-thermites, the addition of a conductive polymer

into a nano-thermite mixture should be investigated for its potential use in a conductive

bridgewire application. Different nanoaluminum-fluorocarbon mixtures would also be

worth considering for these experiments. Other materials that could be tested for micro-

energetic applications include intermetallic and high nitrogen materials.

28

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27

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APPENDICES

31

APPENDIX A: NANO-THERMITE MIXING PROCEDURE

Procedure For:

Preparation of 500 milligrams of Nanothermite through

Resodyn mixing. Deposition of Nanothermite Ink

Droplets.

Date Created/Created By: 02/04/2015 Raghav Ramachandran

Location: ZL4 Rooms 110 and 116.

Notes:

Potential hazards and Mitigation:

● Unintentional ignition of reacted material – Use of PPE (appropriate gloves,

cotton clothing, eye protection, hearing protection, ESD strap), Keep away from

open flame. Minimize amounts in storage.

● Unintentional exposure to organic solvents – Work in fume hood. Wear

appropriate goggles and gloves.

● Unintentional exposure to nanomaterials – Work in fume hood. Wear appropriate

goggles and gloves.

● Unforeseen circumstances/accidents – two people present and aware during

experimentation.

Section 1 – PPE to be used during operation ☐ 1. Fume hood

☐ 2. Cotton/organic fiber clothing/lab coat (no nylon/synthetic fiber)

☐ 3. Nitrile gloves

☐ 4. Leather gloves

☐ 5. Face shield

☐ 6. Hearing protection

☐ 7. Safety glasses

☐ 8. Long pants

☐ 9. Close-toed shoes

☐ 10. Waste container.

Section 2 – Chemicals/Materials/Equipment required ☐ 1. Nano-Aluminum Powder.

32

☐ 2. Nanopowder Metal Oxides : Bismuth oxide, copper oxide, or molybdenum

oxide.

☐ 3. Surfactant: PVP for example.Solvent : DMF, Hexanes, IPA, Acetone, Ethanol, or

Methanol. Note that alcohols can react slowly with nano-aluminum.

☐ 4. 325 Mesh Sieve.

☐ 5. Sieve Shaker.

☐ 6. Metal Brush.

☐ 7. Water

☐ 8. 10 mL BD Syringe

☐ 9. PTFE Holder for BD Syringe

☐ 10. AIRTECH Flashbreaker 1 Tape

☐ 11. Dispersing Needle

☐ 12. Scale

☐ 13. Resodyn Acoustic Mixer

Section 3 – Preparation of Nanothermite Ink through Resodyn mixing. ☐ 1. Make sure that a water-filled container for waste disposal is present near the

fume hood. Refer to Section 5.1 for more details.

☐ 2. Ensure that all nanopowders are separately sieved in the fume hood using a 325

mesh sieve before any sort of mixing. Place sieve on pan on vibrator/sieve shaker. If

necessary, use an appropriate metal brush to sieve nanoparticles.

☐ 3. Always ensure that the brush and sieve are clean when a new material is being

sieved.

☐ 4. Cover the BD Syringe tip with a small piece of the AIRTECH tape.

☐ 5. Remove plunger from the syringe.

☐ 6. Measure the appropriate amount of surfactant and deposit into the BD Syringe.

☐ 7. Measure the appropriate amount of nanoaluminum and deposit into the BD

Syringe. Note that nanoaluminum needs to be removed from the glovebox in room 103.

☐ 8. Measure the appropriate amount of metal oxide powder and deposit into the BD

Syringe.

☐ 9. Nearly invert syringe (tip up) and tap with spatula to clear syringe tip. Add the

appropriate amount of solvent using the variable volume pipette. Make sure solvent does

not reach tip.

☐ 10. Remove tape from syringe tip and vent (depress plunger) until 1mL of volume

remains. Reseal Tip.

☐ 11. Carefully insert syringe into PTFE holder. Cut the plunger with a razor blade

such that the syringe is flush with the bottom of the PTFE holder.

☐ 12. Place sample in Resodyn in a remote room. Be sure that sample is properly

clamped in place with the lock nut tightened, and that the mixing container bottom is

seated inside the raised outer shoulder.

☐ 13. Put on leather gloves, hearing protection, face shield, and lab coat. Place sample

in Resodyn in room 116.

☐ 14. Verify that no personnel are present in the mixing room and close all mixing

room doors.

☐ 15. Connect to COM port 2 on RAMWARE. Verify connection and operation of

remote monitor station before beginning to mix.

33

☐ 16. Start Resodyn mixing at 0% intensity and quickly ramp up to 80% and run for

480 seconds. Invert sample in Resodyn and repeat steps 11-13. This must be done in

60 seconds. ☐ 17. Follow the procedures outlined in Section 4.

Section 4 – Transportation of Material ☐ 1. Carefully remove BD Syringe from the PTFE Holder. Pull the plunger back a

little bit before the full syringe is removed.

☐ 2. Carefully remove tape from syringe tip.

☐ 3. Transfer materials from syringe into the ~2.5mL plastic vials.

☐ 4. Place syringe and tape in waste container.

☐ 5. Carefully position materials in an appropriate ammo box and store in fume hood.

☐ 6. Place a sign on the box informing other students that only Dr. Son, Gunduz, and

Rhoads are authorized to transport the material.

Section 5 – Storage, Handling, and Disposal of Nanothermite Inks ☐ 1. Fill a container (usually a Menards green pail is used) with water.

☐ 2. Handling: After the nanothermite ink dries, it becomes very ESD sensitive. When

handling this material, the following PPE must be used: face shield, hearing protection,

leather gloves, ESD strap, and lab coat.

☐ 3. Disposal: Place BD syringe, weighing papers in the waste, and any kim wipes

with nanothermite ink residue into waste container.

Section 6 – Emergency Procedures ☐ 1. Unintentional ignition of material resulting in bodily injury: If minor injury is

sustained from the incident (e.g., minor burns), treat with appropriate first aid and contact

a supervisor. If major injury is sustained, immediately call 911 and then contact a

supervisor.

☐ 2. Unintentional ignition of material resulting in uncontrolled fire: Ensure that fire

is extinguished using appropriate fire extinguisher and then contact a supervisor. If fire is

sufficiently uncontrolled, immediately pull a fire alarm, evacuate the building, call 911,

and then contact a supervisor. If a fire is sufficiently controlled (e.g., unintentional

ignition of paper on a burn plate), let the fire self-extinguish, and then contact a

supervisor.

☐ 3. Unforeseen injury: If minor injury is sustained from the incident (e.g., cuts,

minor burns, etc.), treat with appropriate first aid and immediately contact a supervisor. If

major injury is sustained, call 911 and then contact a supervisor.

34

APPENDIX B: SAMPLE MIXING SHEET

Reaction Equation

Al + 3/2 CuO ----> 1/2 Al2O3 + 3/2 Cu

Enter total amount of nanothermite (g). 0.50

Enter mass percentage of surfactant used (%). 0.00

Total amount of Nanothermite desired to be mixed (g). 0.50

Enter the Percentage of Active Aluminum in nAl (%) 82.00

Enter Phi (Equivalence Ratio) 4.00

Amount Surfactant 0.00

MW of Aluminum (g/mol) 26.98

MW of Aluminum Oxide (g/mol) 101.96

MW of Copper Oxide (g/mol) 79.55

Mass Fraction Aluminum in nAl 0.82

Mass Fraction Aluminum Oxide in nAl 0.18

Mole Fraction Aluminum in nAl 0.95

Mole Fraction Aluminum Oxide in nAl 0.05

Moles of Alumina (Product in Reaction) 0.53

Moles of Copper Oxide 1.42

Mass Fraction nAl (stoichiometric conditions) 0.22

Mass Fraction Copper Oxide (stoichiometric conditions) 0.78

35

Amount of Novacentrix nAl needed (stoichiometric conditions) 0.11

Amount of Copper Oxide needed (stoichiometric conditions) 0.39

Fuel to Oxidizer Ratio (Stoichiometric Conditions and mass based) 0.28

Fuel to Oxidizer Ratio (Actual) 1.10

Amount of Novacentrix nAl needed (g) 0.26

Amount of Copper Oxide needed (g) 0.24

Enter the volumetric solids loading. 10.30

Density of Aluminum (g/mL) 2.70

Density of Aluminum Oxide (g/mL) 3.95

Density of Copper Oxide (g/mL) 6.31

Density of Surfactant (g/mL) 1.20

Volume of Al-CuO Nanothermite (mL) 0.13

Volume of Nanothermite and Surfactant (mL) 0.13

Volume of Solvent needed (mL) 1.13

Total Volume 1.26