Melissa Mileham- Stability and Degradation Processes of Energetic Materials

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FLORIDA STATE UNIVERSITY

COLLEGE OF ARTS AND SCIENCES

STABILITY AND DEGRADATION PROCESSES OF

ENERGETIC MATERIALS

By

Melissa Mileham

A Dissertation submitted to theDepartment of Chemistry and Biochemistry

in partial fulfillment of therequirements for the degree

Doctor of Philosophy

Degree Awarded:Summer Semester, 2008



The members of the Committee approve the dissertation of Melissa Milehamdefended on July 1, 2008.

______________________________ Albert E. StiegmanProfessor Directing Dissertation

______________________________ Vincent Salters

Outside Committee Member

______________________________ Kenneth GoldsbyCommittee Member

______________________________ John DorseyCommittee Member

Approved:

_______________________________________________________ Joseph Schlenoff, Chair, Department of Chemistry and Biochemistry

The Office of Graduate Studies has verified and approved the above named committee

ii



ACKNOWLEDGMENTS

I would like to thank Dr. Albert E. Stiegman for his invaluable guidance and

support throughout my graduate career. Thank you for answering all of my questions and

not laughing (too hard) at all of my mistakes. My experiences in the lab as well as the

people I have met have truly been priceless.

I would also like to thank Dr. Lambertus J. van de Burgt for sharing even just a

fraction of your knowledge about lasers with me. I really appreciate all of your help and

guidance over the last couple of years.

Dr. Michael P. Kramer, thank you for not only providing funding for my research,

but always having an active role in it as well. Thank you for giving me this opportunity,

and I can honestly say that there was never a dull moment. Surprising yes, but never dull.Also, many thanks to all of the staff members, especially the machine and glass

shop, without whom some of my work would not have even been possible.

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TABLE OF CONTENTS

Acknowledgments……..…………………………………………………………………iii

List of Tables ……………………………………………………………………………..v

List of Figures ……………………………………………………………………………viList of Abbreviations ………………………………………………………………….....ix

Abstract …………………………………………………………………………………...x

INTRODUCTION…..…………………………………………………………………….1

GENERAL EXPERIMENTAL …………………………………………………………..6

1. SURFACE STABILITY AND DEGRADATION STUDIES OF TNT ON METAL

OXIDES……………………………………………………………….…………………13

2. SURFACE STABILITY AND DEGRADATION STUDIES OF PETN ON METAL

OXIDES………………………………………………………………………………….28

3. LASER INITIATION PROCESSES IN THERMITE ENERGETIC MATERIALS

STUDIED BY A LASER DESORPTION IONIZATION (LDI) TECHNIQUE………..41

4. PHOTO-THERMAL INIATION PROCESSES OF ORGANIC-INORGANIC

HYBRID METASTABLE INTERSTITIAL COMPOSITE (MIC) MATERIALS……..53

SUMMARY……………………………………………………………………………...64

REFERENCES…………………………………………………………………………..66

BIOGRAPHICAL SKETCH…………………………………………………………….70

iv



LIST OF TABLES

Table 1. TNT coverage levels on metal oxides surveyed………………………………..17

Table 2. Surface coverage of PETN on each metal oxide……………………………….32

Table 3. Effects of PETN on heat and energy given off by MIC composites…………...54

v



LIST OF FIGURES

Figure 1. Molecular structures of common high energy materials………………………..2

Figure 2. Schematic of an isocratic pumping system in HPLC…………………………...7

Figure 3. FT-IR spectrometer schematic………………………………………………….9

Figure 4. Principle of the MALDI process………………………………………………10

Figure 5. Scheme of a time-of-flight mass spectrometer………………………………...11

Figure 6. The four-level pumping system of the Nd:YAG laser………………………...12

Figure 7. Percent decomposition over time of TNT deposited on various metal oxidesubstrates…………………………………………………………………………………17

Figure 8. Differential scanning calorimetry trace of pure TNT………………………….19

Figure 9. DSC of a) 1.4 and b) 4 monolayers of TNT deposited on MnO2……………...20

Figure 10. DSC of a) 1 monolayer and b) 4 monolayers of TNT deposited onCuO………………………………………………………………………………………20

Figure 11. DSC traces of TNT deposited on KBr, LiF, and SiO2………………………..21

Figure 12. HPLC chromatograms of the products of 1.4 monolayer of TNT deposited onMnO

2(a) initially and at 50 °C for (b) 16 (c) 24 and (d) 42 days……………………….23

Figure 13. HPLC chromatograms of the products of 1 monolayer of TNT deposited onCuO (a) initially and at 50 ˚C for (b) 31 (c) 39 and (d) 58 days…………………………23

Figure 14. (a) Gas chromatogram showing two principle components at 11.83 and 11.9minutes whose mass spectra identify them as (b) trinitrobenzene and (c) unreactedTNT………………………………………………………………………………………25

Figure 15. Plot of the decomposition data of TNT on MnO2 over time fit to equation2…......................................................................................................................................26

Figure 16. The evolution of a brown gas from PETN on the surface of MoO3 after beingstored at 50 °C……………………………………………………………………………33

Figure 17. Differential Scanning Calorimetry (DSC) of pure PETN with a vented sample pan (note exothermic processes are above the baseline)………………………………...34

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Figure 18. DSC of (a) <2, (b) 2, and (c) 6 monolayer coverage of PETN deposited onnano-scale MoO3................................................................................................................36

Figure 19. DSC of (a) <2, (b) 2, and (c) 6 monolayer coverage of PETN deposited onmicron-scale MoO3………………………………………………………………………37

Figure 20. DSC traces of PETN deposited on (a) SiO2 and (b) KBr…………………….37

Figure 21. Percent PETN remaining on MoO3 vs. time…………………………………38

Figure 22. FT-IR spectroscopy of PETN on the surface of MoO3 at 100 ˚C after a) 1, b) 5,c) 24, d) 27.5, and e) 46 hours (♦=CO2, ▲=N2O, =N2O4, =NO2)…………………39

Figure 23. SEM images of (a) 50 nm, (b) 100 nm, and (c) micron-scale aluminum particles…………………………………………………………………………………..43

Figure 24. LDI-TOF mass spectra of 50 nm aluminum at a laser energy density of (a)1.74 and (b) 1.90 J/cm2…………………………………………………………………..45

Figure 25. Aluminum ions formed at a laser energy of 1.90 J/cm2 for each aluminum particle size………………………………………………………………………………47

Figure 26. Major aluminum ions formed ([Al]+ + [Al2]+ + [Al2O]+) for each particle size

at increasing energy densities……………………………………………………………48

Figure 27. LDI-TOF mass spectra of iron(III) oxide at a laser density of (a) 1.58 and (b)1.74 J/cm2………………………………………………………………………………..49

Figure 28. LDI-TOF mass spectra of thermite mixtures at a laser energy density of (a)1.42, (b) 1.58, (c) 1.74, and (d) 1.90 J/cm2………………………………………………51

Figure 29. Relative amounts of pure iron cluster species formed during laser desorptionof the Fe2O3 control and thermite samples at an energy density of 2.054 J/cm2………...51

Figure 30. Mixed Al/Fe oxide species formed at an energy density of 2.054 J/cm 2 for 50and 100 nm and micron aluminum thermite mixtures…………………………………...52

Figure 31. Photo-thermal initiation of Fe2O3/100 nm Al samples with increasing amountsof PETN at 1064 nm (arrow indicates the position of the laser plume emission)……….57

Figure 32. Initiation time (ms) as a function of PETN coverage in mg at 1064 nm……..57

Figure 33. Deflagration duration time (ms) as a function of the amount of PETN at 1064nm………………………………………………………………………………………..58

vii



Figure 34. Initiation time of Al/Fe2O3 MIC materials with various concentrations of PETN initiated with a single pulse of 532 nm radiation…………………………………61

Figure 35. Energy density (J/cm2) required in order to initiate the thermite/PETN mixtureat 1064 nm………………………………………………………………………………..62

viii



LIST OF ABBREVIATIONS

TNT 2,4,6-trinitrotoluene

TNB trinitrobenzene

RDX hexahydro-1,3,5-trinitro-1,3,5-triazine (Royal DemolitioneXplosive)

HMX octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (High Molecular weight rdX)

PETN pentaerythritol tetranitrate

MIC metastable interstitial composite

LDI/TOF MS laser desorption ionization time-of-fight mass spectrometry

BET Brunauer-Emmett-Teller (mathematical formulation for surfacearea anaylsis)

YAG yttrium aluminum garnet (common garnet crystal used in lasers)

FE-SEM field emission scanning electron microscopy

DSC differential scanning calorimetry

TGA thermogravimetric analysis

HPLC high-performance liquid chromatography

GC-MS gas chromatography – mass spectrometry

FID flame ionization detector

FT-IR fourier-transform infrared

ix



ABSTRACT

The use of binary inorganic solid-state fuel/oxidant redox processes typified by

the classic aluminum/iron(III) oxide thermite reaction in combination with traditional

energetic materials such as 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) has been of

interest in order to produce higher output explosives. This dissertation focuses on the

stability and degradation processes that occur with the combination of binary inorganic

fuel/oxidant systems with high energy materials. First is a discussion on the stability of

2,4,6-trinitrotoluene (TNT) as well as pentaerythritol tetranitrate (PETN) deposited onto

the surface of metal oxides through a wet impregnation technique, which is followed by a

discussion of the study of the initiation processes of the aluminum/iron(III) oxide

thermite reaction using laser induced desorption-ionization time-of-flight mass

spectrometry. Finally, the photo-thermal initiation of an aluminum/iron(III) oxide

thermite with PETN deposited on the surface of the iron(III) oxide in increasing

increments was studied using a single pulse of a Nd:YAG laser at differing wavelengths

in order to understand the effects of the presence of PETN on the time to initiation, as

well as the deflagration duration of the thermite reaction.

x



INTRODUCTION

Stability and Degradation Processes of TNT and PETN on Metal Oxides

Recently there has been a new approach to the fabrication of high output

explosives and pyrotechnics in which binary inorganic solid-state reactive materials are

used in combination with traditional organic high energy explosives such as 2,4,6-

trinitrotoluene (TNT); hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX); octahydro-1,3,5,7-

tetranitro-1,3,5,7-tetrazocine (HMX); and pentaerythritol tetranitrate (PETN) as shown in

figure 1. The binary inorganic materials are typified by the classic thermite reaction

(equation 1), where a stoichiometric amount of a metal (fuel) and a metal oxide (oxidant)

are mixed and a highly exothermic redox reaction occurs.

Fe2O3 + 2Al Al2O3 + 2Fe ΔH = 3.97 kJ/g (1)

These binary inorganic materials have the advantage of having a high energy

density, in fact several times higher than that of a traditional high energy material.

However, the rate of energy release is much slower than that of conventional explosives

making it difficult to exploit these materials for explosive purposes. Therefore, in order to

overcome this problem, new composite materials have been developed in which the

binary inorganic systems are combined with conventional explosives such as TNT, PETN,RDX, and HMX. Thus the energy release in these combined systems is driven by the

conventional explosive, resulting in a rapid release of energy in a controlled fashion.

Previous studies have indicated that the combination of these hybrid organic/inorganic

materials provides great promise in the exploitation of the high energy density provided

by the binary inorganic mixtures. TNT, for example, has been combined with various

oxidant phases and fuels such as MnO2 and Al as well as Al-Zr and Mg-Al alloys for use

as high energy density explosives for high penetration applications and as components for

boosters and primers.1-3 PETN, on the other hand, has been studied with binary inorganic

materials where Al, Al alloys, and Zr have been used as fuels and Pb 3O4, Fe2O3, and

MnO3 have been used as the oxidant for their use as boosters and primers as well as their

use in a laser initiated system.2, 4

1



Figure 1. Molecular structures of common high energy materials

There is one problem that exists in the incorporation of conventional organic high

energy explosives with fuel/oxidant binary inorganic mixtures, however, in that there is a

possibility that an inherent incompatibility between the inorganic and organic phases may

occur. This incompatibility arises from the deleterious surface chemistry between the

organic material and the metal or metal oxide phase in the composite, which may arise

from an interfacial chemical process that may occur either as a reduction at the surface of

the metal or an oxidation at the surface of the oxide. The magnitude of these processes

may be affected by the degree of contact that occurs between the organic materials and

the metal or metal oxide surfaces, as well as the temperature, humidity, and other ambient

conditions of the storage and handling of these systems.

The chemical stability of both TNT and PETN when deposited onto the surface of metal oxides such as MnO2, CuO, WO3, MoO3, Bi2O3, SnO2, and Fe2O3 has been

investigated and is reported hereafter. In order to imitate storage conditions of such

materials, the samples were placed in an oven set at 50 °C, periodically removing

aliquots to be analyzed chromatographically in order to determine if a decomposition of

the energetic materials had occurred at the surface of the oxide. In a case where

2



degradation of the energetic material occurred, subsequent studies were completed to

understand the degradation process as well as any products that are produced at the

surface of the oxide.

Laser Initiation Processes of Thermite Energetic Materials Using Laser Desorption

Ionization (LDI)

The traditional thermite reaction (equation 1) in which stoichiometric amounts of

aluminum and iron oxide react to form aluminum oxide and iron metal has become a

great interest in the production of high energy materials due to their high energy density.

However, these materials have not been broadly useful due to slow mass-transport

processes of the reaction, which leads to a slower energy release and subsequently alower obtained power. It has recently been determined though that the energetic

properties of these binary inorganic materials may be enhanced by having at least one of

the components in the mixture of nanometer size, usually the fuel. The nano-scale fuel

causes a more intimate contact between the fuel and the oxidizer, which enhances the

mass-transport and therefore leads to an increase in power in these materials known as

metastable interstitial composites (MIC).5, 6

The thermite reaction is highly exothermic and thermal initiation of the reaction

occurs at high temperatures once the melting point of one of the components, typically

the metal has been achieved.7 This initiation begins the reactive processes which liberate

heat causing the reaction to accelerate. Ignition of the reaction occurs once the reaction

becomes self-sustaining and propagates. The incident laser energy put into a sample for

laser initiation causes high localized heating to occur, which then leads to ignition of the

bulk sample. The time to ignition is defined as the time at which the energy released by

the reaction becomes greater than or equal to the energy put into the composite by the

laser. Both the laser excitation event and the subsequent ignition and propagation are

extremely high temperature events, which generate both liquid and gas phase (plasma)

species. The chemical reactions occurring within and between these phases as well as

with the solid material all contribute to the net combustion process. This complexity and

the fact that the reaction occurs so quickly has made it difficult to observe the species

3



formed during the reaction other than the final products and therefore thoroughly

understand the reaction. Recently, however, this has been addressed by Dlott et al. by

using time-resolved spectroscopy, where a reaction between aluminum and nitrocellulose

was initiated by flash-heating the mixture with a 100 ps laser pulse in the near infrared.8, 9

Short-pulse photo-thermal initiation allows for the use of time-resolved spectroscopic

techniques in order to monitor reaction dynamics, interpret some of the intermediate

species of the reaction, and to observe specific structural changes in the reactive

components. Time-resolved studies of the combustion of Al/MoO3 MIC composites have

been studied and have identified neutral species such as AlO that form in the process.10

Previous studies of standard binary fuel/oxidant systems have involved

thermochemical measurements such as temperature profiles, burn rates, and non-

isothermal calorimetry (DSC, DTA, and TGA).11-13 Recent studies of MIC materials haveused laser-induced photo-thermal initiation of binary inorganic systems in order to

determine properties such as ignition time and burn rate as well as study the effect of

particle size on combustion properties. Pantoya et al. studied the combustion velocities

and ignition times of an Al/MoO3 thermite system with varying aluminum particle

diameter by observing the reaction over time using high-speed cameras.14, 15 A similar

study on the ignition process of a magnesium and barium oxide pyrotechnic mixture were

completed by Östmark et al. where the mixture was initiated using a CO2

laser and the

subsequent ignition process was observed using high-speed photography which was

synchronized with the laser pulse.16 Each study found that the combustion properties of

binary inorganic mixtures is dependant upon the particle size of the fuel.

The use of photo-thermal laser initiation coupled with time-of-flight mass

spectrometry (LDI/TOF MS) in order to directly observe the ionic species formed in the

plasma phase of both conventional Al/Fe2O3 thermite and the corresponding MIC

materials is discussed. Though this does not show the reaction to completion, the species

formed in the plasma allows for the study of the reactive processes of various aluminum

sizes in the reaction. This may be the first use of this technique to characterize reactive

species in the initiation phase of binary inorganic reactions.

4



Photo-thermal Initiation Processes of Hybrid Organic/Inorganic Metastable Interstitial

Composite (MIC) Materials

Two approaches used separately or in tandem have been devised in order to

overcome the slow energy release found in binary inorganic materials compared to that of

traditional energetic materials. One is the development of metastable interstitial

composite (MIC) materials in which one of the components (typically the fuel) is of

nanoscale dimensions. Improvements in the rate of energy release are generally attributed

to better mixing of the components and a more intimate fuel/oxidant contact. 5,6 Another

approach, applied to conventional (i.e. micron scale) thermite-type compositions as well

at to MIC materials, is to mix traditional organic high-energy materials such as 2,4,6-

trinitrotoluene (TNT); hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX); and pentaerythritoltetranitrate (PETN) into the inorganic system to form an organic/inorganic composite. In

these systems the energy release of the binary fuel/oxide system is driven by the

conventional explosive, thereby releasing energy much more rapidly and in a controlled

fashion. Studies to date have indicated that this approach affords great promise in

exploiting the high energy density provided by binary inorganic energetic compositions.1-

4

It was determined from thermal analysis studies that depositions of at least 6

monolayers resulted in thermal properties for the decomposition of the organic species

that were the same as the bulk. This suggests that even in small amounts incorporation of

the organic phase may result in observable changes in the energy release properties of the

composite, which are amenable to laboratory study. We report here a study of the energy

release dynamics of the Al/Fe2O3 MIC materials with depositions of 16-127.9 mg of

PETN per gram of thermite. The study is carried out using short pulse (ns) photo-thermal

initiation, with the dynamics of the process studied by time-resolved spectroscopic

techniques.

5



GENERAL EXPERIMENTAL

Liquid Chromatography

Liquid Chromatography (LC) is an analytical technique where the components of

a mixture are separated in solution at room temperature. It is one of the most commonly

used separation techniques due to its versatility in that there are two interactive phases

(stationary and mobile phases) and this interaction may be altered in order to change the

selectivity of the system. Each component in a mixture should interact differently with

each of the two phases causing a separation to occur. The time it takes each component to

travel through the column to the detector is the retention time and is what allows for the

characterization of the species. High Performance Liquid Chromatography (HPLC)describes a liquid chromatography technique where the mobile phase is mechanically

pumped through a column containing the stationary phase; therefore, the instrument

consists of an injector, pump, column, and detector (figure 2). Reversed-phase

chromatography is the most widely used technique and is used to separate neutral

molecules by their degree of hydrophobicity. The typical stationary phase has an organic

functional group such as –CH3, -C4H9, -C8H17 and –C18H37 chemically attached to silica.

The functional group affects the retention time, which increases exponentially with chain

length, as well as the column selectivity and efficiency. The typical mobile phases used in

reversed-phase chromatography include a polar solvent, usually water, which is mixed

with a slightly less polar solvent such as methanol or acetonitrile. The polarity of the

solvent is inversely proportional to its eluting strength and the mixture used as the mobile

phase is chosen to give the desired separation.17, 18

6



Figure 2. Schematic of an isocratic pumping system in HPLC

Gas Chromatography - Mass Spectroscopy

Gas chromatography is a method used for separating and analyzing mixtures of

volatile compounds. Similar to liquid chromatography, gas chromatography takes

advantage of varying interactions occurring between each component and the two phases

(stationary and mobile). The main difference, however, is that gas chromatography uses a

gas as a mobile phase, and therefore in order to use this technique all components must

be volatile and thermally stable making it somewhat more limited than liquid

chromatography. Due to the fact that there is little interaction between the molecules ingas phase, the mobile phase primarily acts as a way to move the components through the

system and thus the distribution equilibria of the components is determined by their vapor

pressure and sorption by the stationary phase. Since vapor pressure plays a key role in the

separation, the column must be heated to a temperature high enough to provide an

appropriate analysis time. The most commonly used carrier gases are helium and

nitrogen; however, argon, hydrogen, and carbon dioxide may also be used. The carrier

gas is determined by its compatibility with the detector of the system, which in this study

a flame ionization detector (FID) was used due to its ability to detect organic molecules.

The column can either be a packed column or a capillary open tubular column, each with

its advantages and several options for stationary phases which will depend on the types of

solutes being separated. The gas chromatograph can also be coupled to a mass

spectrometer, which ionizes the components and subsequently separates and detects them

7



based on their mass-to-charge (m/z) ratio. This feature allows the confirmation of the

identity of each analyte in a mixture as well as any unknowns.19, 20

Differential Scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) is a thermal analysis technique that

measures the heat flow associated with thermally driven endothermic or exothermic

transitions in a material. The DSC is capable of giving thermodynamic information such

as phase changes, melting, crystallization, product stability, and oxidative stability. Two

identical pans, one reference and one containing the sample, are placed on platforms that

are connected to individual furnaces. In a general experiment, the two pans are

simultaneously heated at a specific rate, typically not more than 20 ˚C/min, measuring thedifference in heat flow between the sample pan and the reference pan. The result is a plot

of heat flow versus temperature on which the features correspond either to an absorption

or release of energy in the sample upon heating due to either phase transitions or a

decomposition of the material.21

Gas Physisorption

Gas adsorption/desorption techniques are used in order to find characteristics of

porous materials such as surface area, pore size, and pore volume. Nitrogen gas is used at

incrementally higher pressures to dose a sample at liquid nitrogen temperatures. This

known dosing pressure is compared to a measured actual pressure, the difference between

which yields the amount of gas adsorbed by the sample. An isotherm plot is given by

plotting the measured volumes of adsorbed gas versus the relative pressure at which these

adsorptions took place. There are six classifications of physisorption isotherms, each of

which indicate the different adsorbent-adsorbate interactions. The first few points of the

isotherm are used to calculate the surface area, typically using a mathematical equation

developed by Brunauer, Emmett, and Teller (BET), while other equations are used to

calculate the pore size and volume.22

8



Fourier-Transform Infrared Spectroscopy (FT-IR)

Fourier-transform infrared (FT-IR) spectroscopy is a technique used for observing

the vibrations of the atoms in a molecule. It has become a very versatile technique, in that

almost any type of sample can be used, e.g. liquids, powders, films, gases, and surfaces.

The spectrum is obtained by using an interferometer, where radiation from the source

passes through before hitting the sample, in which interference of radiation between two

beams produces the transmitted beam to the sample at 90° from the input beam. The

fraction of incident radiation that is absorbed by the sample at a particular energy is then

determined and is converted to an absorbance spectrum using a fourier transformation

process. A schematic of the instrument is shown in figure 3.23

Figure 3. FT-IR spectrometer schematic

Laser Desorption Ionization Time-of-Flight Mass Spectrometry (LDI-TOF MS)

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry

(MALDI-TOF MS) is a technique that is most often used for, but not limited to, the

analysis of biomolecules because significant decomposition of the fragile molecules does

not occur and large mass ranges can be detected. MALDI-TOF mass spectrometry allows

species to be converted from the solid phase to gas-phase ions with little fragmentation

due to the simultaneous vaporization and ionization processes. Since these processesoccur in a single step, it is referred to as a desorption/ionization technique. This single-

step process is accomplished because the samples are placed under a vacuum of at least

10-6 torr, which allows the sample to sublime directly into the ion source with little or no

heating.24

9



The sample is usually mixed in a large excess of a matrix compound, which is

typically a weak acid that absorbs light readily at the wavelength of the laser. The sample

is then deposited onto the solid surface of the target, which is made out of a conducting

metal, most often stainless steel. The mixtures used in this case act as a matrix

themselves, and thus a typical matrix is not needed. Therefore, small holes were drilled

into the sample plate into which the mixture was tightly packed in order to ensure that the

sample would stay in place, especially under a high vacuum and the process itself is

referred to as laser desporption ionization (LDI) mass spectrometry. Once the proper

vacuum is reached, the sample may be hit with a brief laser pulse. The irradiated spot is

rapidly heated by the laser energy and becomes vibrationally excited, which results in a

portion of the deposited sample’s surface to be released from the irradiated spot, allowing

the matrix to carry a portion of the analyte into the vapor phase with little or no heating asillustrated by figure 4.24, 25

Since the LDI is equipped with a time-of-flight mass spectrometer each of the

accelerated ions from the source has a different velocity, which is based off of the mass

and charge of the ion. The accelerated ions then move from the source into the drift

region of the analyzer where the different velocities separate the ions in order to keep

them from hitting the detector at the same time. The now mass separated ions hit the

detector region, where they are counted, giving rise to a specific signal for each m/z value

proportional to the number of ions present, producing a mass spectrum.(figure 5). 24

Figure 4. Principle of the MALDI process (Wilkins et al., 2006).

10



Figure 5. Scheme of a time-of-flight mass spectrometer (Wilkins et al., 2006).

Nd:YAG Laser

The Nd:YAG laser uses a neodynium yttrium-aluminum-garnet (Y3Al5O12)

crystal, a common lasing medium for solid-state lasers, where a small amount (~1 %) of

yttrium ions are replaced by neodynium ions in the crystal structure. A flashlamp is used

to excite the neodynium electrons from the ground state and upon relaxation a slow decay

back to the ground state results in a dominant fluorescence around 1064 nm with a slow

decay time (ms), which results in the laser action. The neodynium ion actually has a four-

level pumping system with several different laser transitions that result in fastnonradiative decay to the upper laser level (E2). A simplified version of this system is

shown in figure 6 in which level 4 (E3) represents the combination of all the levels above

the upper laser level in the real atomic system. Level 3 (E2) is the upper laser level and

usually is a long-lived level, while level 2 (E1) is the lower laser level and level 1 (Eo)

represents the ground level The laser may be operated both in a pulsed or continuous

mode. If the laser is pulsed, it is most often operated in the Q-switching mode, which is

an optical switch that opens once the maximum population inversion of the neodynium

ions is acquired, allowing the light wave to run through the cavity, depopulating the

excited laser medium and resulting in a laser pulse. This pulse is less than ten

nanoseconds and gives an output power of 20 mW.26

11



Figure 6. The four-level pumping system of the Nd:YAG laser

12



CHAPTER 1

SURFACE STABILITY AND DEGRADATION STUDIES OF TNT ON METAL

OXIDES

Introduction

An important issue related to the incorporation of such conventional organic high-

energy materials with metal/metal oxide binary compositions is one of inherent

incompatibility between the organic and inorganic phases. This incompatibility arises

from deleterious surface chemistry between the organic and metal or metal oxide phases

in the composite. These deleterious processes arise from interfacial chemical processesincluding, but not limited to, reduction at the metals (fuel) surface and oxidation at the

oxide surface. The magnitude of these processes will likely be exacerbated by the degree

of contact between the organic materials and the metal or metal oxide surfaces and will

be affected by temperature, humidity, and other ambient conditions of storage and

handling.

The chemical stability of 2,4,6-trinitritoluene (TNT) when placed in physical

contact with metal oxide surfaces was investigated. The TNT was deposited at 1-3

monolayers of coverage on the surface of microcrystalline MnO2, CuO, WO3, MoO3,

Bi2O3, SnO2, and Fe2O3 by wet impregnation techniques. The samples were placed in a

50 °C oven and allowed to react over a period of ten months. Periodically, a small portion

of the sample was removed and analyzed chromatographically to determine if products

were being formed at the surface of the oxide. TNT proved to be inert to most oxides;

however, both CuO and MnO2 effected a clean decomposition of the TNT molecule to

trinitrobenzene (TNB).

13



Experimental

Materials

MnO2 (Aldrich, reagent grade, >90% purity, ~10mm particle size), CuO (Aldrich,

nanopowder, ~33 nm), Fe2O3 (Fisher, anhydrous), MoO3 (nano- and micron-scale,

Climax Molybdenum), SnO2 (Keeling & Walker Ltd.), WO3 (Atlantic Equipment

Engineering) and Bi2O3 (Aldrich) were used as received from the manufacturer. 2,4,6-

Trinitrotoluene (TNT) was obtained from Chemservice and sublimed prior to use.

Acetonitrile (Acros, reagent grade) and methanol (EMD Chemicals, HPLC grade) was

also used as received.

Long-term surface reactivity studies

Samples for long-term reactivity studies were prepared using a wet impregnation

technique. Sixty milligrams of TNT, dissolved in acetonitrile, was slurried with one gram

of the metal oxide, after which the acetonitrile was removed under vacuum (10 -3 torr) in

order to leave a dry powder of the oxide with TNT deposited onto its surface. The

approximate coverage area of TNT is 986 m2/g, which is based on the estimated area of a

TNT molecule obtained from treating the molecule as a disk with the outer circumference

defined by the oxygen atoms of the nitro groups (obtained from the crystal structure of

1,3-dinitrotoluene) with a radius extending from the center of the aromatic ring.27

Sealed containers containing the samples were placed in an oven held at 50 °C.

Periodically, a small portion of the powder (~0.10 g) was withdrawn and stirred into one

milliliter of acetonitrile. The solids were allowed to settle and the supernatant liquid

containing the organic species was removed and analyzed by HPLC using a 60:40

H2O/CH3OH solution as the mobile phase. The presence of new compounds formed from

interfacial chemistry was evident in the chromatogram by comparison to the control

samples using a UV detector set to 254 nm. The percent degradation was determined

from the ratio of the integrated peak area of the TNT in the HPLC with the sum of all the

peak areas. For samples that showed only minimal degradation, no attempt was made to

determine the products. For oxides that induced a significant amount of decomposition

the product was characterized by GC-MS.

14



BET surface area analysis

The surface area of each metal oxide used in the survey was found by BET

methods using a Micromeretics ASAP 2020 physisorption surface area and porosity

analyzer. Approximately 0.25 g of each metal oxide was placed in a sample tube and was

degassed under a vacuum of 20 µmHg at 90 °C for sixty minutes in order to remove any

water from the sample. A subsequent heating at 340 °C for 240 minutes was performed in

order to completely degas the material. The analysis was performed at liquid nitrogen

temperatures in order to obtain the surface area for each metal oxide.

HPLC

Liquid chromatography experiments were performed on a Beckman Coulter System Gold HPLC equipped with a 125 Solvent Module, 166 Detector and 508

Autosampler using a Beckman C18 column that is 250 x 4.6 mm. The detector was set at

254 nm in order to detect any organic compounds that may be present. A flow rate of

0.75 mL/min was used for the mobile phase, an isocratic mixture of 60:40 H2O/methanol.

The chromatogram was taken over a period of 30 minutes, allowing all compounds time

to travel through the column. TNT appears at approximately 18 minutes at this flow rate,

assuming only degradation products of the TNT molecule occur, they should elute faster

through the column due to their smaller size.

GC-MS

The GC-MS data were collected on an Agilent 6890+ GC coupled with an HP

5973 MSD (mass selective detector). The GC was equipped with a DB-5 capillary

column which had a length of 30 meters and an i.d. of 0.25 μm using helium as the carrier

gas with a flow rate of 1.5 mL/min. The oven temperature ramp was set as isothermal at

140 ˚C and the injector and detector temperatures were set to 200 and 250 ˚C,

respectively. The detector used was a flame ionization detector (FID), which is used

primarily for organic compounds due to its ability to easily detect hydrocarbons. The

sample was also ionized and analyzed by mass (m/z) in the mass selective detector. The

source of the MSD was set at 100 ˚C. This allows the mass of any of the degradation

15



products of TNT corresponding to elution time through the gas chromatogram to be

determined as well as the products themselves to be identified.

Thermal Analysis

Differential scanning calorimetry (DSC) was performed on a TA Instruments

Q1000 DSC under both O2 and an inert N2 environment. Samples were prepared using

the wet impregnation technique as described above. A two milliliter aliquot of TNT (0.13

M in acetonitrile) was added to one gram of MnO2 or CuO and the solvent was removed

under vacuum. The dry powder containing TNT deposited onto the surface of the metal

oxide was then analyzed using a scan rate of 10 °C/min with a pin prick in the pan in

order to vent the sample pan. When the sample is analyzed in a sealed container, the

decomposition becomes a deflagration event with an abrupt release of energy. Samples of pure TNT and TNT deposited on the surface of inert materials such as silica gel and KBr

as well as LiF were prepared and analyzed under the same conditions in order to

differentiate surface chemistry specific to the metal oxides.

Results and Discussion

Degradation studies

The long-term degradation studies were designed to simulate long-term ambient

storage of multicomponent high-energy materials where TNT would be in intimate

contact with metal oxide surfaces. Relatively low concentrations (~1-3.5 monolayers) of

TNT were deposited onto the oxide surfaces to isolate specific interfacial chemistry and

to allow sensitive detection of reaction products relative to the TNT (i.e. without the

chromatogram being overwhelmed by a large amount of TNT). Table 1 shows the oxides

surveyed in the study and their TNT surface coverage in the prepared samples.

16



Table 1. TNT coverage levels on metal oxides surveyed

Metal Oxide BET Surface Area (m2/g) TNT coverage(equivalent monolayers)

MnO2 40 1.5

CuO 32 1.9WO3 17 3.5

MoO3 (large) 25 2.4

MoO3 (nano) 55 1.1

Bi2O3 26 2.3

SnO2 21 2.8

Fe2O3 33 1.8

A chromatogram of the samples was taken immediately upon preparation to

establish the initial purity. They were then placed in a 50 °C oven under the ambient

atmosphere and allowed to react over the course of approximately eleven months with

samples withdrawn periodically over that time and analyzed by HPLC. The amount of

decomposition of the TNT over time for each oxide surface where a measurable

decomposition is observed is shown in figure 7.

` 2 4

3 2

4 2 5

9 8 5 9

8 1 6 8 1

9 0 2 5

3 3 0 9

M n O 2

C u O

F e 2 O 3

S n O 2

B i 2 O 3

0

10

20

30

40

50

60

70

80

90

100

Percent Decomposition

time (days)

Percent Decomposition of TNT on Metal Oxides

Figure 7. Percent decomposition over time of TNT deposited on various metal oxidesubstrates

17



TNT proved to be inert or slightly reactive on the majority of the oxide surfaces.

In particular, no detectable decomposition was observed for MoO3 and WO3. As

indicated in figure 7, small amounts of decomposition (<10 %) were observed for Bi2O3,

Fe2O3, and SnO2 over the duration of the study. For two of the oxides, CuO and MnO 2,

more extensive decomposition was observed with 21 % of the TNT consumed over CuO

and essentially complete (100 %) consumption observed for MnO2. For the case of CuO,

the decomposition reached its maximum after about one month (figure 7), but did not

react further after that. This result suggests that this reaction is stoichiometric with a

specific surface site, which, once consumed, ceases to react further. Conversely, for

MnO2, the reaction proceeds to completion suggesting either a large amount of surface

sites on the MnO2 or that the reaction is catalytic. An important aspect of TNT reactivity

on both of these surfaces is that the HPLC data indicates that the reaction is quite clean,going primarily to a discrete product of trinitrobenzene as opposed to decomposition into

multiple species with MnO2 being somewhat cleaner than the CuO. Moreover, based on a

comparison of the retention times, the product is the same for both oxides.

Reactivity of TNT on MnO2

Thermal analysis. The thermal analysis of pure TNT using differential scanning

calorimetry (DSC) is shown in figure 8. The thermal scan shows a sharp endotherm at

80 °C, which corresponds to the melting and an exotherm, whose observed peak at this

scan rate (10 °C/min) is at 306 °C, that corresponds to the decomposition of the molecule.

This thermal decomposition process has been studied previously and was found to

initially involve oxidation of the methyl group but, even in the early stages of the reaction,

it is quite complex and ultimately yields telomeric or polymeric materials.28-31 When

considering composites of TNT with inorganic oxides, the effect of the organic-inorganic

interface on these well-known thermal processes is of interest. The DSC plot of TNT

deposited at approximately 1.4 monolayer loadings on MnO2 is shown in figure 9. At

18



Figure 8. Differential scanning calorimetry trace of pure TNT (note that the exothermic processes are above the baseline)

these coverage levels, the melting endotherm is not observed. This result is expected

since no crystalline bulk phase is present. More importantly, however, the thermaldecomposition process occurs at 229 °C, which is significantly lower than the bulk.

When the amount of TNT is increased to >4 monolayers, two exothermic processes are

observed, one whose peak is close to that resolved in the bulk (321 °C) and a low

temperature exothermic process at 278 °C. Our preliminary interpretation of this complex

thermal behavior is that it represents the superposition of interfacial and bulk processes

observable at the higher loadings of TNT though it may reflect a more complex

decomposition process than is initiated at the surface. A similar, but less dramatic,

interfacial effect is observed on CuO with monolayer coverage yielding decomposition

exotherms at 211 °C and 294 °C due to the interfacial effect of the CuO, while excess

loadings showing interfacial and bulk decomposition at 294 and 318 °C respectively

(figure 10).

19



Figure 9. DSC of a) 1.4 and b) 4 monolayers of TNT deposited on MnO2

Figure 10. DSC of a) 1 monolayer and b) 4 monolayers of TNT deposited on CuO

20



It is important to determine whether the interfacial mediation of the

decomposition temperature is specific to those oxides or constituted a general surface

effect. Thermal analysis of monolayers of TNT deposited on inert oxides and salts

indicates that in almost all cases the decomposition temperature is modified (figure 11).

On fumed silica the decomposition temperature is very close to that of the bulk at

310.8 °C indicating that the silica surface is inert. Interestingly, on simple salts the

interfacial effect is pronounced. Specifically, LiF shows a decomposition temperature of

293.0 °C while KBr shows an even stronger surface effect with a decomposition

temperature of 270.9 °C.

These results do not provide a chemical rationale for the interfacial effect, nor do

they provide any specifics on the decomposition pathway(s) on the surface. However, it

is clear from the thermal analysis data that there is a pronounced and surprisingly generalsurface effect between TNT and solid surfaces. This effect might well mitigate energy

release processes or affect long-term stability of heterogeneous composite mixtures.

Figure 11. DSC traces of TNT deposited on KBr, LiF, and SiO2 (note exotherms areabove the line).

21



Kinetics and product analysis. Reaction processes and decomposition pathways of

trinitrotoluene deposited on the surface of microcrystalline MnO2 were monitored by

HPLC. Initial chromatographic data shows only a sharp peak at a retention time (RT) of

18.57 minutes, which is assigned to TNT itself by comparison to a standard solution of

TNT in acetonitrile (figure 12). No other significant products were observed. The

samples were kept at a constant temperature of 50 °C and reanalyzed after 16 days. The

chromatogram shows two peaks, the TNT peak and a new species eluting earlier with a

retention time of 10.26 minutes. After sitting at 50 ˚C for another 8 days, the

chromatogram indicates that this species was still being produced and was present in

approximately equal amounts to the starting material. Also present in the chromatogram

was small amounts of additional species that elute significantly faster. The observed

pattern of more quickly eluting species appearing over time is suggestive of a generaldecomposition of the TNT into lower molecular weight species. Notably, however, these

faster eluting species represent a very small amount of the total dissolved mass and, in

fact, the chromatographic data shows that TNT is actually being converted relatively

cleanly into a single product in a solid-state reaction on the surface of the MnO2. Finally,

chromatograms collected after 42 days show that the new species is now predominant,

suggesting that most of the TNT has been converted. The faster eluting species, while

still present, has not increased significantly in concentration.

Similar observations were made from the surface reaction of TNT with CuO.

Chromatographic data indicates the formation of the same species eluting at 18.57

minutes after 31 days of reacting at 50 °C. While the species that forms is the same, the

extent of reaction is much more limited and, after 39 days, there is little additional

formation of this product. As with MnO2, faster eluting species are also present which,

while still minor, make up a larger fraction of the total dissolved material than they do in

the MnO2 (figure 13).

22



Figure 12. HPLC chromatograms of the products of 1.4 monolayer of TNT deposited on

MnO2 (a) initially and at 50 °C for (b) 16 (c) 24 and (d) 42 days.

Figure 13. HPLC chromatograms of the products of 1 monolayer of TNT deposited onCuO (a) initially and at 50 ˚C for (b) 31 (c) 39 and (d) 58 days.

23



Characterization of the main product of the solid-state reaction was carried out by

GC-MS (figure 14). Injection of the product mixture, after reaction for 31 days on MnO2,

resolved two relatively intense peaks at 11.83 and 11.9 minutes retention time on the GC.

The mass spectrum of the 11.9 minute peak shows the classic fragmentation pattern for

TNT with a weak parent-ion peak at 227.0 m/z and a more intense peak at 210.0 m/z

corresponding to [TNT-OH]+ due to the loss of O from an o-nitro group and a H from the

methyl group with ion bombardment to produce [C7H4(NO2)2(NO)]+.32 The mass

spectrum of the species that elutes at 11.83 minutes shows a strong peak at 213.0 m/z/

and a series of peaks at 167.0, 120.0, and 75.0 m/z. The 213.0 m/z peak can be assigned

as the parent peak of trinitrobenzene (TNB), C6H3(NO2)3, and the progression comes

from sequential loss of NO2 (or NO2 and H) groups. This assignment is confirmed

unambiguously by comparison to the published fragmentation pattern of atrinitrobenzene. The mass spectrum was also collected on some of the minor species

resolved by the gas chromatogram. They have not been fully characterized but, as would

be expected, the slower eluting species appear to be higher molecular weight biphenyl

species and the faster eluting species are lower molecular weight decomposition

fragments. Similar data was collected for the CuO catalysis. The primary product of the

solid-state reaction is TNB, though, as indicated by the HPLC data, conversion to TNB is

not as extensive as it is in the case of the MnO2. The gas chromatogram indicates that

there are generally more species, both slower and faster eluting, than form with MnO2,

suggesting that conversion to TNB is less clean and that there is more net decomposition

of the TNT.

While the mechanistic details of the reaction are not completely known, water has

been observed forming in bulk studies. As such, this suggests that the net reaction is:

24



Figure 14. (a) Gas chromatogram showing two principle components at 11.83 and 11.9

minutes whose mass spectra identify them as (b) trinitrobenzene and (c) unreacted TNT.

A likely pathway is the initial oxidation of the methyl group followed by a rapid

decarbonylation process. Kinetically, the reaction is extremely slow and the plots of the

relative concentration of TNT as a function of time are qualitatively consistent with a

reaction that is pseudo first order in TNT. Further analysis of the data using the

generalized approximate rate equation (equation 2) derived by Wilkinson, where p is the

fraction reacted, t is the time, n is the reaction order, and K is the apparent rate constant is

shown in figure 15.33 Evaluation of the slope (n/2) for the reaction, which is only linear

when p ≤ 0.4, p values greater than 0.4 give a reaction order to the nearest half order.

t

p=

nt

2+

1

K (2)

25



Evaluation of the slope in this instance yields a reaction order of 1.4, consistent with a

first order dependence on TNT and an apparent rate constant of 0.02/day for MnO2. The

half-life for the reaction is 35 days. In the absence of more detailed data about the

reaction products and mechanism, the kinetic analysis cannot be taken too far and only

serves to indicate the relative efficiencies of the reaction. It also should be noted that it is

not known whether the reaction is catalytic, using O2 as the oxidant, or whether it is

stoichiometric with oxygen taken from specific sites on the metal oxide surface.

Figure 15. Plot of the decomposition data of TNT on MnO2 over time fit to equation 2.

The net reaction is of considerable interest from a synthetic standpoint, since there

are, to the best of our knowledge, no previous reports of the direct demethylation of

aromatics. Moreover, there is no cost effective synthesis for TNB even though, from an

energetic standpoint, it is a superior high-energy material. The most direct synthesis

reported to date involves the conversion of phloroglucinol into trioximes withhydroxylamine followed by oxidation to trinitrobenzene with nitric acid.34 As such, a

single-step heterogeneous synthesis may be of commercial utility in producing TNB.

Unfortunately, attempts to scale up as a solution-solid reaction over MnO2 or as a solid-

state reaction involving larger amounts of TNT were not successful. From all the data

obtained so far, the reaction appears to occur as a very slow interfacial reaction.

26



Conclusions

From the standpoint of compatibility, it is clear that the interfacial chemistry

between TNT and inorganic surfaces can be pronounced. As indicated by the thermal

analysis work, this involves at one level, changes in the normal thermal decomposition

pathways of the molecule which may or may not have a net effect on stability or energy

release in bulk composite materials. For redox active metal oxide phases the interfacial

processes can result in a net chemical reaction that will slowly convert the TNT to

another species, and possibly to degradation products. For the specific oxides reported

here, the interfacial reaction converts TNT to TNB relatively cleanly and quantitatively

for MnO2 and with more overall decomposition than for the case of CuO. Clearly, this

result indicates that in composite energetic materials containing these components theTNT will be changing over time, which suggests that the stability and energy release may

also be variable. Moreover, once conversion to TNB is accomplished, further

decomposition may occur and adversely affect properties. Both MnO2 and CuO react

with TNT under relatively mild conditions. For both oxides a major product of this solid-

state reaction is the demethylation of TNT to form TNB. Over CuO this reaction does not

appear to be exclusive as many other species are formed nor does it proceed to

completion.

27



CHAPER 2

SURFACE STABILITY AND DEGRADATION STUDIES OF PETN ON METAL

OXIDES

Introduction

As previously stated the use of binary inorganic solid-state reactive materials in

combination with traditional organic high explosives such as 2,4,6-trinitrotoluene (TNT),

hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-

tetrazocine (HMX) and pentaerythritol tetranitrate (PETN) has recently become a new

approach to the fabrication of high output explosives. The binary inorganic materials arestoichiometric mixtures of a metal (fuel) and a metal oxide (oxidant) that react through a

highly exothermic redox reaction such as the well-known thermite reaction. These binary

inorganic materials have the advantage of having energy densities several times higher

than that of conventional explosives, though the rate of energy release is significantly

lower. In order to take advantage of the energy density of these binary materials, new

composites involving the mixture of conventional explosives with these binary

fuel/oxidant systems have been developed. These new composites allow the energy

release of the system to be driven by the conventional explosive, thus driving the kinetics

of the binary inorganic materials and thereby releasing energy much more rapidly and in

a controlled fashion.

However, it is unknown if an incompatibility between conventional explosives

and metal/metal oxide binary compositions exists. This incompatibility could occur due

to interfacial chemical processes occurring at the surface between the organic and

inorganic phase of the composite. These chemical processes may include a reduction at

the surface of the metal (fuel) and oxidation at the surface of the metal oxide. The

magnitude of these processes will likely be increased by the degree of contact between

the organic materials and the metal or metal oxide surfaces, and may be affected by

temperature and humidity along with other ambient conditions of storage and handling.

28



The chemical stability of pentaerythritol tetranitrate (PETN) when placed in

physical contact with metal oxides is investigated. In this study PETN was placed on the

surfaces of a range of microcrystalline metal oxides including MnO2, CuO, MoO3, WO3,

Bi2O3, SnO2, and Fe2O3 in coverages of 1-3.5 monolayers by a wet impregnation

technique. Samples were then placed in a controlled temperature environment at 50 °C

and checked periodically for the presence of decomposition products using liquid

chromatography. PETN proved to be inert over all of the oxides except MoO3 which

showed the relatively rapid evolution of a brown gas over a period of 48 hours. Analysis

of the evolved gas indicated that it is primarily NO2 along with N2O4, N2O, and CO2.

Experimental

Materials

MnO2 (Aldrich, reagent grade, >90% purity, ~10mm particle size), CuO (Aldrich,

nanopowder, ~33 nm), Fe2O3 (Fisher, anhydrous), MoO3 (nano- and micron-scale,

Climax Molybdenum), SnO2 (Keeling & Walker Ltd.), WO3 (Atlantic Equipment

Engineering) and Bi2O3 (Aldrich) were used as received from the manufacturer.

Pentaerythritol tetranitrate (PETN) was prepared according to the literature and was

stored in acetone (Aldrich, HPLC grade) for safety.35 Acetonitrile (Fisher, HPLC grade)

and toluene (Aldrich, HPLC grade) were used as received.

Long-term surface reactivity studies

A wet impregnation technique was used in preparing the samples for the long-

term reactivity studies. A PETN standard solution (208 μL, ~0.50 M) was diluted with

one milliliter of acetone and subsequently added to one gram of the metal oxide. The

acetone was removed under a vacuum (10-3 torr) leaving a dry powder containing the

metal oxide and the PETN on its surface. The approximate coverage area of the PETN is

1543 m2/g, which is based on the estimated area of a PETN molecule, assuming the

molecule is a disk with the outer circumference defined by the oxygen atoms of the nitro

groups (obtained from the crystal structure of PETN) with the radius extending from the

central carbon atom.36

29



The samples were placed in sealed containers and held at 50 °C in an oven over a

period of several months. Periodically, a small portion of the sample was removed (~0.10

g) and stirred into one milliliter of acetone. The solids were allowed to settle, and the

supernatant was collected containing any organic species and analyzed by HPLC using a

65:35 CH3CN/H2O solution as the mobile phase and toluene as an internal standard.

There was no evidence of new compounds forming in the chromatograph; however, a

comparison between the control sample and each subsequent sample removed showed a

decomposition of the PETN was occurring at the surface of the MoO3. The percent

degradation was determined from the ratio of the integrated peak area of the PETN in the

HPLC to that of the sum of all of the peak areas. For oxides that induced a significant

amount of degradation, the products were analyzed using FT-IR spectroscopy.

BET surface area analysis

The surface area of each metal oxide used in the survey was found by BET

methods using a Micromeretics ASAP 2020 physisorption surface area and porosity

analyzer. Approximately 0.25 g of each metal oxide was placed in a sample tube and was

degassed under a vacuum of 20 µmHg at 90 °C for sixty minutes in order to remove any

water from the sample. A subsequent heating at 340 °C for 240 minutes was performed in

order to completely degas the material. The analysis was performed at liquid nitrogen

temperatures in order to obtain the surface area for each metal oxide.

HPLC

Liquid chromatography experiments were performed on a Beckman Coulter

System Gold HPLC equipped with a 125 Solvent Module, 166 Detector and 508

Autosampler using a Beckman C18 column that is 250 x 4.6 mm. The detector was set at

254 nm in order to detect any organic compounds that may be present. A flow rate of

0.75 mL/min was used for the mobile phase, an isocratic mixture of 65:35

acetonitrile/H2O and toluene as an internal standard. The chromatogram was taken over a

period of 15 minutes, allowing all compounds time to travel through the column. PETN

appears at approximately 3.6 minutes at this flow rate with another peak appearing at 7.5

30



minutes. No other products were seen in the chromatogram due to the fact that the

degradation products appear to be solely gases.

FT-IR

The FT-IR spectra were collected on a Thermo Nicolet Avatar 360 FT-IR in

transmission mode. The sample, prepared as described above, was placed in a quartz tube

and attached to a gas phase IR cell with sodium chloride windows, which was under

vacuum (10-3 torr) allowing only gases evolved from a reaction between MoO3 and

PETN to be seen. A background was taken, and the sample was then opened to the gas-

cell allowing any evolved gas to enter the cell. The sample was heated to 100 °C allowing

a faster evolution of any reaction products and subsequent spectra were taken in the

spectral range of 750 to 4000 cm-1 with a resolution of 4 cm-1 at first every 15 minutesand then at longer intervals until no further product evolution occurred.

Thermal analysis

Differential Scanning Calorimetry (DSC) was performed on a TA Instruments

Q1000 Series DSC under O2 at a ramp rate of 10 °C/min from 40 to 450 °C. Samples

were prepared using the wet impregnation method as described above. An aliquot of 84

µL of a PETN standard solution (~0.50 M) was diluted with a small amount of acetone

and added to 0.25 grams of MoO3 (both nano and micron sizes). The solvent was

removed under a vacuum, and the dry powder containing the PETN on the surface of the

metal oxide was then analyzed. Samples of pure PETN and PETN on the surface of silica

gel and KBr were also analyzed to differentiate surface chemistry specific to the metal

oxides.


Degradation studies

The long-term degradation studies were designed to simulate long-term ambient

storage conditions of multicomponent high-energy materials in direct contact with metal

oxide surfaces. Relatively low concentrations (1-3.5 monolayers) of PETN were

31



deposited onto the oxide surfaces in order to isolate specific interfacial chemistry as well

as to allow the sensitive detection of reaction products. Table 2 shows the oxides

surveyed in the study and their PETN surface coverage in the prepared samples.

Table 2. Surface coverage of PETN on each metal oxide

Metal Oxide BET Surface Area (m2/g) PETN Coverage (equivalent

monolayers)

MnO2 40 2.0

CuO 32 2.5

WO3 17 2.9

MoO3 (large) 25 2.0

MoO3 (nano) 55 0.91

Bi2O3 26 3.1

SnO2 21 3.8

Fe2O3 33 2.4

A chromatogram of the samples was taken immediately following preparation to

establish the initial purity. The samples were then placed in a 50 °C oven under ambient

atmosphere and allowed to react over the course of a couple of months, with aliquots periodically being withdrawn for analysis using HPLC. The amount of decomposition

over time of PETN for most oxide surfaces was found to be very small.

PETN proved to be inert or only slightly reactive on many of the metal oxide

surfaces. The amount of PETN on the surface of Bi2O3, CuO, WO3, Fe2O3, MnO2, and

SnO2 remains fairly constant over the period analyzed (~60 days) with only small

changes in the concentration of PETN, which were within the experimental error of the

measurement. However, PETN did show some decomposition on the surface of MoO3 as

evidenced by a decline in the amount of PETN and the concomitant production of a

brown gas, which can be observed visually in the sample container (figure 16). Increasing

the ambient temperature at which the sample is stored results in a more rapid production

of the gas.

32



Figure 16. The evolution of a brown gas from PETN on the surface of MoO3 after beingstored at 50 °C

Reactivity of PETN with MoO3

Thermal analysis. Thermal analysis of pure pentaerythritol tetranitrate (PETN) using

differential scanning calorimetry (DSC) is shown is figure 17. The thermal scan shows asharp endotherm at 141 °C, which corresponds to the melting and an exotherm, whose

observed peak at this scan rate (10 °C/min) is at 205 °C, corresponds to the

decomposition of the molecule. When the sample is analyzed in a sealed container, the

decomposition becomes a deflagration event with an abrupt release of energy. When

considering composites of PETN with inorganic oxides, the effect of the inorganic-

organic interface on these well-understood thermal processes is of interest. The DSC plot

of PETN at ~2 monolayer coverage on nano-scale MoO3 is shown in figure 18. At

slightly less than 2 monolayer coverage of PETN on MoO3, the decomposition occurs at

a significantly lower temperature of 142 °C, with no melting endotherm being observed.

The lack of a melting endotherm is expected since there is no crystalline bulk phase

present. When the coverage of PETN is increased slightly to approximately 2 monolayers

a melting endotherm begins to be observed at 138 °C. The melting endotherm is

33



superimposed on the middle of the broad decomposition exotherm thereby splitting it into

two apparent peaks at 112 and 144 °C. There is also an exothermic process located at 192

°C, which is lower than for the pure PETN presumably due to interfacial mediation at the

surface of the oxide. An endotherm is also present at 292 °C and 287 °C for the <2 and 2

Figure 17. Differential Scanning Calorimetry (DSC) of pure PETN with a vented sample pan (note exothermic processes are above the baseline)

monolayer samples respectively, which likely represents a desorption of some of the

products from the initial decomposition. A sample with excess PETN on the surface of

MoO3 shows an endotherm at 140.6 °C and two exothermic processes, one whose peak is

close to that resolved in the bulk (193 °C), and a low temperature exothermic process at

164 °C. This may be interpreted as coming from the superposition of the interfacial andthe bulk processes observable at higher loadings of PETN, wherein the melting

endotherm at 140.6 °C, which is more pronounced at the higher loading, now dominates

and obliterates most of the 141 °C interfacial exotherm leaving only a residual spike at

164 °C. The size of the 164 °C exotherm, however, is quite large, therefore a more

complex surface mediated decomposition process that results from the interfacial

34



reactions with the melt cannot be ruled out. An endothermic process just below 300 °C is

not seen in the bulk sample as it is in the monolayer samples due to the fact that it may be

attributed to a decomposition that occurs at the surface, and is therefore solely an

interfacial process.

The DSC data of PETN deposited on nano-scale MoO3 is compared to that of

PETN deposited on the larger micron size MoO3, which has a smaller surface area (figure

19). At a surface coverage of less than two monolayers, an endothermic process at 141 °C

indicating the melt of PETN is observed, as well as an exotherm at 195.4 °C for the

decomposition process. This exothermic process occurs at a lower temperature than that

of the bulk PETN (205 °C) showing that there is some interfacial mediation occurring,

however, the fact that there is a melt present and only one decomposition peak indicates

that there is mostly bulk crystalline phase present. In fact, similar processes are observedfor the samples with a higher loading of PETN, showing that the same interfacial

processes occurring on nano-scale MoO3 do not also occur on the micron scale MoO3. In

short, when the surface area gets small enough, interfacial effects become less significant

and are not observed in the thermal analysis or, alternatively, the large surface area

provided by nano-scale materials will give rise to greater surface degradation effects.

Comparable studies were performed on the remaining metal oxide surfaces, where a

surface mediation effect was only observed at the surfaces of MnO2

and CuO, which

showed the decomposition peak occurring at a lower temperature than that of pure PETN

(149 and 175 °C, respectively).

It is important to determine whether the interfacial mediation of the

decomposition temperature is specific to certain oxides or constituted a general surface

effect. Thermal analysis of monolayers of PETN deposited on inert oxides and salts

indicates that the decomposition temperature is not modified on all surfaces (figure 20).

On fumed silica the decomposition temperature is very similar to that of the bulk PETN

at 201 °C. On simple salts such as KBr, there is also no detectable change in the

decomposition temperature from that of the bulk. Thus the interfacial effect is not a

general effect, but rather a specific interaction that occurs between PETN and the surface

of nano-scale MoO3.

35



Figure 18. DSC of (a) <2, (b) 2, and (c) 6 monolayer coverage of PETN deposited on

nano-scale MoO3

These results do not provide a chemical rationale for the interfacial effect, nor do

they provide any specifics on the decomposition pathway(s) on the surface. It is clear

from the thermal analysis that there is a significant surface reaction with MoO3. This

effect might well mitigate energy release processes or affect long-term stability of

heterogeneous composite mixtures.

36



Figure 19. DSC of (a) <2, (b) 2, and (c) 6 monolayer coverage of PETN deposited onmicron-scale MoO3

Figure 20. DSC traces of PETN deposited on (a) SiO2 and (b) KBr

Product analysis. The reaction processes of pentaerythritol tetranitrate (PETN)

deposited on the surface of molybdenum (VI) oxide were studied using liquid

37



chromatography and fourier-transform infrared spectroscopic techniques. The HPLC

study was conducted over a period of three months wherein small aliquots of the sample

were removed periodically for analysis. The chromatogram shows a modest (1.3 %)

decrease in the amount of PETN present after 49 days (figure 21); however, no additional

peaks in the chromatogram that might represent the decomposition products were

observed, suggesting that the primary decomposition products are likely gaseous species

that diffuse out of the solid. Only a small amount of net decomposition is observed

chromatographically, which suggests that the reaction is localized at the interface and the

catalytic decomposition of the bulk does not occur, at least at the temperatures studied.

95

95.5

96

96.5

97

97.5

98

98.5

99

99.5

100

0 1 3 7 14 21 35 49 70

time (days)

P e r c e n t R e m a i n i n g

Figure 21. Percent PETN remaining on MoO3 after vs. time

The sample for the gas phase FT-IR study was prepared as described previously.

The container holding the sample was connected to a gas phase IR cell that had

previously been evacuated. The sample was set to heat to 100 °C, and spectra were

periodically recorded over a period of two days (figure 22). Peaks associated with the

decomposition of PETN began to appear during the first hour of heating. The products

38



observed by FT-IR were NO2 (1629, 1597 cm-1), N2O4 (1743 and 1270 cm-1), N2O (2236,

2212 cm-1), and CO2 (2359, 2342 and 3727-3600 cm-1).37 The band that occurs at 2918

cm-1 is due to the grease from the stopcocks of the gas cell heating slightly during the

reaction and releasing a small amount of the grease into the cell. The band at 1270 cm-1

has been assigned to N2O4, which is in accordance with the reported values found by

Mélen et al.38 However, this band seems to overlap with other fundamental vibrational

bands of NO2 as well as N2O, which are at 1320 and 1285 cm-1 respectively. A control

sample of the MoO3 without PETN prepared for FT-IR analysis showed that none of the

products found were from the surface of the MoO3 alone.

50

100

150

200

250

300

350

7.50E+021.25E+031.75E+032.25E+032.75E+033.25E+033.75E+03

Wavenumbers (cm-1)

% T

r a n s m i t t a n c e ( a . u . )

a

b

c

d

e

Figure 22. FT-IR spectroscopy of PETN on the surface of MoO3 at 100 ˚C after a) 1, b)

5, c) 24, d) 27.5, and e) 46 hours (♦=CO2, ▲=N2O, =N2O4, =NO2)

39



Conclusions

From the standpoint of compatibility, it is clear that the interfacial chemistry

between PETN and inorganic surfaces can be pronounced. As indicated by the thermal

analysis studies, this involves at one level, changes in the normal thermal decomposition pathways of the molecule which may or may not have a net effect on the stability or

energy release in bulk composite materials. For redox active metal oxide phases the

interfacial processes can result in a net chemical reaction that will slowly convert PETN

into another species, and possibly into degradation products. For the specific oxides

reported here, the surface interactions of PETN with MoO3 produces a gas containing

NO2, N2O, N2O4, and CO2 species. Clearly this result indicates that in composite

energetic materials containing forms of MoO3, the PETN will be changing over time,

which suggests that stability and energy release may also be variable.

40



CHAPTER 3

LASER INITIATION PROCESSES IN THERMITE ENERGETIC MATERIALS

STUDIED BY A LASER DESORPTION IONIZATION (LDI) TECHNIQUE

Introduction

The use of binary solid-state inorganic fuel/oxidant redox processes such as the

classic aluminum/iron oxide thermite reaction in the production of energetic materials is

of great interest due to their high energy densities. It has recently been discovered that the

energetic properties of these materials may be enhanced when at least one of the

components in the mixture is on the nanometer size scale. These materials, which arereferred to as metastable interstitial composites (MIC) have a rapid release of energy that

can be attributed to a more intimate fuel/oxidant contact that occurs in the nano-scale,

which enhances mass transport and provides more power.5,6

Thermal initiation of thermite materials typically involves heating above the

melting point of one of the components, typically the metal. This starts the reaction

processes that liberate heat, causing the reaction to accelerate. Ignition occurs when the

reaction becomes self-sustaining and propagates. For laser initiation, the incident energy

put into the sample by the laser causes high localized heating, which leads to ignition of

the bulk. The time to ignition is defined as the time at which the energy released by the

reaction becomes greater than or equal to the energy put into the composite by the laser.

Both the laser excitation event and the subsequent ignition and propagation of the

mixture are extremely high-temperature events which generate both liquid and gas phase

(plasma) species. Chemical reactions between species occurring within and between

these phases and with the solid material all contribute to the net combustion process. This

complexity coupled with the high temperatures makes these reactions difficult to study on

a microscopic level. This has been addressed recently by Dlott et al. by using time-

resolved spectroscopy and initiating the reaction between aluminum and nitrocellulose by

flash-heating with a 100 ps laser pulse in the near infrared.8,9 Short pulse photo-thermal

initiation allows the use of time-resolved spectroscopic techniques to monitor reaction

41



dynamics, elucidate some of the intermediate species produced during the reaction, and to

observe specific structural changes in the reactive components.

The reactive processes that occur during the laser initiation of aluminum/iron (III)

oxide metastable intermolecular composites (MIC) have been studied by laser-induced

desorption ionization time-of-flight mass spectrometry. The ions observed in the plume

from the aluminum show fragments from the ablation of the oxide coating and from the

metal core. Ablation of the iron oxide component consists primarily of pure iron species

such as [Fe]+ and [Fe2]+ and small oxides such as [FeO]+ and [Fe2O]+ ions. In smaller

quantities, metal oxide clusters that are either oxygen deficient, [Fe(FeO)x]+, or oxygen

equivalent [(FeO)x]+, are observed. When the thermite composite is initiated, mixed metal

species are observed in the plume, which correspond to the aluminum substitution

analogues of the iron oxide clusters, specifically, [FeAl2O3]+, [AlFe2O3]+, and [AlFe2O2]+. Notably, the amounts of these mixed metal products that form are inversely proportional

to the size of the aluminum particles. This suggests that the decrease in ignition time

observed in MIC materials is due to the more facile liberation of reactive metallic

aluminum when the particle size is small.

Experimental

Materials

The 50 and 100 nm aluminum samples were obtained from Argonide, while the

micron aluminum was purchased from Alfa Aesar. All aluminum samples were used as

received. The iron (III) oxide powder, <0.25 microns, was purchased from Aldrich and

was used as received.

The size and morphology of the aluminum samples were characterized by field

emission scanning electron microscopy (figure 23). The nano-scale materials are

somewhat polydispersed, with the average particle size being around 50 and 100 nm for

the two sizes, although both showed some particles above 200 and below 50 nm. Both

samples showed some agglomeration of the particles, which was more pronounced in the

50 nm material. The aluminum, which we designate as “micron” scale, was a very

polydispersed material composed largely of particles between 0.5 and 2.5 µm in size with

42



only a few large particles around 5 µm observable. The average size of the micron

sample, averaged from the particles observable in the SEM, was 1.75 µm. The active

aluminum content and the estimated oxide thickness for the three samples were

determined from weight gain measurements using thermogravimetric analysis on a TA

Instruments Q500 TGA. The active aluminum content for the 50 and 100 nm and the 2.5

µm materials was 71.5, 75.5, and 98.0 %, respectively, while the oxide thickness was

determined to be 2.6, 4.5, and 6.0 nm, respectively.

Figure 23. SEM images of (a) 50 nm, (b) 100 nm, and (c) micron-scale aluminum particles

Preparation of samples

The thermite samples were prepared by mixing Fe2O3 with each of the aluminum

particles at a 1:1 molar aluminum to iron ratio. The samples were thoroughly mixed by

grinding the components in a mortar and pestle to ensure a homogenous distribution.

Analysis of the samples after grinding using powder X-ray diffraction indicated that no

new phases were formed as a result of sample preparation. The samples were then packed

into a sample holder for laser desorption ionization time-of-flight mass spectrometry

(LDI/TOF MS) along with the control samples of Fe2O3 and the 50 and 100 nm and the

micron aluminum.

LDI/TOF mass spectrometry

The LDI/TOF mass spectra were collected using a commercial 1999 Bruker Biflex III matrix-assisted laser desorption and ionization time-of-flight mass spectrometer

(MALDI-TOF) (Bruker Daltronics, Inc.) fitted with a solid sample holder for the

introduction of inorganic material. The instrument was equipped with a pulsed nitrogen

43



laser at 337 nm with a peak width of 4 ns. All spectra were collected in linear mode with

positive ion detection averaging 200 shots from the laser.

In order to thermally initiate thermite, temperatures at or above the melting point

of aluminum must be obtained. For laser pulses in the nanosecond regime, there is

enough time for thermal transport into the material. The temperature attained in each

component of the thermite can be estimated from the absorbed energy per unit volume,

Ev, using equation 3, where J is the fluence at the center of the beam, R is the reflection

coefficient, ΛD is the thermal diffusion length, which is the depth of the sample that is

heated with a single pulse from the laser, C(T) is the heat capacity, and ρ is the density.

The thermal diffusion length is calculated from the thermal diffusivity (D) and the pulse

length of the laser (equation 4).8,9,39

Ev = J (1 - R) / ΛD = ρ C (T )dT T i

T f

∫ (3)

ΛD = ½(2πDt)1/2 (4)

It was found that all of the aluminum was melted when the laser reached an

energy density of 0.158 J/cm2. The volume of aluminum that was heated per pulse was

approximately 7.42 x 10-9 cm3. Although the volume of the sample that was hit by the

laser did not change, the active aluminum content would vary based on the size of the

aluminum particle. The Fe2O3 phase composite also absorbed at 337 nm and was found toreach its melting point of 1565 °C at an energy density of 0.474 J/cm2. Temperatures

required to generate a plume from which species could be detected in TOF-MS were

found to be above these threshold values of melting.

UV-Vis diffuse reflectance

The reflectivity of the independent Al and Fe2O3 components and the

compounded thermite were measured as total reflectance against a calibrated Spectralon

diffuse scattering reference in a Perkin-Elmer Lambda 900 spectrophotometer equipped

with a 160 mm integrating sphere.

44




Aluminum samples in the three particle sizes, 3-5 µm and 50 and 100 nm, were

placed as tightly pressed powders in the laser desorption ionization instrument. Time-of-

flight mass spectra observed for the 50 nm Al as a function of incident laser power are

shown in figure 24. No gas-phase ionic products were observed below a threshold energy

density of 0.948 J/cm2. Above this threshold, the major products included [Al]+, [Al2O]+,

and [Al2]+ at m/z of 26.9, 69.9, and 53.9, respectively. At power densities between 1.74

and 1.90 J/cm2, small amounts of more complex AlxOy products were also observed

including [Al2O2]+, [Al3O]+, and [Al3O2]

+ at m/z 85.9, 96.9, and 113.0, respectively

(figure 24b). Since the system was evacuated, the oxide species must have originated

either directly or indirectly from the native oxide layer on the aluminum powder. Laser desorption of Al2O3 produced a large number of neutral species, with the dominant ones

Figure 24. LDI-TOF mass spectra of 50 nm aluminum at a laser energy density of (a)1.74 and (b) 1.90 J/cm2

being AlO, Al2O, and Al2O2.40,41 In fact, AlO has been observed in the emission spectrum

of combusting Al/MoO3 thermite materials.10 Since the observed aluminum oxide ion

clusters are oxygen deficient, they are likely formed from ion-molecule reactions in the

plasma between Al+ and neutrals such as those given in equations 5-7. Clearly, other

pathways may also be operating. The species AlO+ and Al2O2+ are only observed at a

higher laser power and may represent direct ion yields from ablation of the Al2O3 layer.

45



Al+ + AlO Al2O+ (5)

Al+ + Al2O Al3O+ (6)

Al+ + Al2O2 Al3O2+ (7)

Studies by Granier and Pantoya on the fuel particle size dependence of laser ignition of

Al/MoO3 MIC materials showed a general trend of faster ignition times as the Al particle

size dropped to nanometer dimensions.15 This effect was attributed to melting point

depression that accompanies the size reduction in the aluminum. Also important in the

laser ignition process is the presence of a native oxide layer on the aluminum. The oxide

layer is thickest on micron-size aluminum and decreases to approximately 2-4.5 nm as

the aluminum particle size reaches the nanometer dimension. Notwithstanding the thinner

oxide layer, the percent of active aluminum metal available will also decrease (i.e, Al2O3 represents a progressively higher percent of the mass of the particle). A direct effect of

this is a reduction of the burn rate at very small particle sizes. The process of laser

initiation will involve melting of the aluminum, the density change of which is sufficient

to critically stress and break the oxide layer.42 As indicated above, the ions that are

observed in the plasma can be associated with both the oxide and the aluminum phase of

the nanoparticles. Since the oxide has an extremely high melting point and a weak optical

absorption, most of the heating during the pulse will be of the metal itself. Above the

energy density threshold, the three dominant species are [Al]+, [Al2]+, and [Al2O]+, which

reflect contributions from the volatilization and ionization of the aluminum metal and the

breakdown of the aluminum oxide shell. The amounts of these species observed for all

three Al particle sizes at an energy density of 1.90 J/cm2 are shown in figure 25. The 100

nm particles show a large yield of [Al]+ and a small yield of [Al2O]+, while the converse

is true for the 50 nm particles, which show a larger yield of [Al2O]+ than [Al]+. This

difference likely arises from the higher percent of oxide present relative to the active

aluminum content in the 50 nm material, the neutrals of which will deplete more of the

available [Al]+ in the plume, according to equation 5. The 100 nm sample has a higher

active aluminum content relative to the amount of oxide present, which gives a higher

[Al]+ ion yield. Interestingly, the amount of each Al species observed for the micron-size

sample is between that of the 50 and 100 nm samples. It yields more [Al2O]+ than the 100

46



0

5000

10000

15000

20000

25000

30000

Intensity

[Al]+ [Al2]+ [Al2O]+

micron Al

50 nm Al

100 nm Al

Figure 25. Aluminum ions formed at a laser energy of 1.90 J/cm2 for each aluminum particle size

nm Al, consistent with its much thicker oxide layer, but significantly less [Al]+, which

seems inconsistent with its high available Al content. The origin of this may lie in its

higher melting point, which means that less of the heat deposited by the laser pulse is

used to vaporize the sample. The power dependence of the ion yields is consistent with

this suggestion. The total yield of the most prevalent ions ([Al]+, [Al2]+, and [Al2O]+) as a

function of energy density indicates that at the threshold, ion yields are low and relatively

independent of particle size (figure 26). As the power increases, the nanometer-scale

particles produce the highest ion yields, which are consistent with the shorter ignition

times observed in the bulk materials. At the very highest energy density, however, the

micron-size sample begins to exceed that of the nano-scale aluminum. The trends

observed are explainable in the context of the free energy of the active aluminum and the

oxide layer as a function of particle size. The free energy of the active aluminum in thevolume becomes more positive (i.e. less stable) as the size decreases, which gives rise to

the melting point depression. Conversely, the free energy of the surface oxide, which

becomes the dominant contribution to the total free energy as the particles become

smaller, becomes more negative, making the oxide layer more stable and, hence, less

easily ablated into the plume.43 Notably, the results are also extremely consistent with the

47



recent melt dispersion model of Levitas et al., which suggests that the stronger oxide

layer on the nanoparticles results in large pressure changes during rapid melting, causing

pressure-induced spallation of the oxide layer followed by a rapid dispersion of

aluminum clusters.44 It is important to note that the laser-induced ignition of a thermite

reaction will necessarily involve many reactions taking place in different phases of the

irradiated materials. In this study, while we are observing a subset of those reactions, the

species we are observing do afford support at a molecular level of the proposed

advantages of nanoscale fuels on factors such as ignition time and burn rate.15

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

Intensity

1.738 1.896 2.054

Energy Density (J/cm2)

micron

50 nm Al

100 nm Al

Figure 26. Major aluminum ions formed ([Al]+ + [Al2]+ + [Al2O]+) for each particle size

at increasing energy densities

Irradiation of the Fe2O3 sample results in ionic products at a threshold power

density of 1.58 J/cm2. At the threshold, two primary species are observed, [Fe]+ and

[FeO]+, along with K and Na impurities (figure 27a). As the laser power is increased,

more complex species are observed, in particular, [Fe2O]+

and [Fe2O2]+

, with minor products including [Fe2]

+, [Fe3O2]+, and [Fe3O3]

+ (figure 27b). Maunit et al. studied the

laser ablation/ionization of iron oxides in considerable detail, and our spectra parallel the

ones obtained by these authors under nonresonant excitation conditions and below the

threshold of Fe-O bond dissociation.45 As suggested in this previous study, neutral FeO is

an important constituent of the plume and contributes to the cluster formation through

48



ion-molecule reactions. The formation of [Fe2]+ was found to be through an ion-molecule

reaction between Fe+ and neutral FeO that generates oxygen (equation 8), and FeO+ and

Fe2O+ are subsequently formed through an ion-molecule reaction with the generated

oxygen (equations 9 and 10). The larger clusters that are formed in the plume are either

Fe+ + FeO Fe2+ + ½ O2 (8)

Fe+ + ½ O2 FeO+ (9)

Fe2+ + ½ O2 Fe2O

+ (10)

oxygen deficient, Fe(FeO)x, or oxygen equivalent, (FeO)x, and are also formed from ion-

molecule reactions between neutrals such as FeO and precursor ions.

Figure 27. LDI-TOF mass spectra of iron(III) oxide at a laser density of (a) 1.58 and (b)1.74 J/cm2

The TOF mass spectrometry characterization of the species forming in the plumes

of Al/Fe2O3 thermite mixtures was carried out over a range of laser powers. Of interest

are species that form between the two components, which represent gas-phase reactions

that are part of the laser ignition process. No products were observed until an energydensity of 1.11 J/cm2 was reached. At this threshold, however, the only products

observed were those produced from the independent components. It was not until an

energy density of 1.58 J/cm2 was reached that products consisting of Al/Fe mixed

components were observed. At this threshold, small amounts of [AlOFe]+ began to

appear in the spectrum (figure 28a,b). As discussed above, in the pure iron oxide, [Fe 2O]+

49



was formed from the reaction of [Fe2]+ with O2 (equation 10). While this reaction is

possible for the formation of [AlOFe]+, only trace amounts of the precursor ion, [AlFe]+,

were observed in the spectrum. This suggests that the [AlOFe]+ likely results from an

ion-molecule reaction between neutral FeO and aluminum ions (equation 11). Obviously,

FeO + Al+ FeOAl+ (11)

since species such as AlO and Fe+ are also present in the plume, their reaction may also

contribute to the mixed metal product. As photo-thermal heating of the thermite mixture

increases, more complex Al/Fe oxide species appear (figure 28d). All of the observed

clusters are aluminum-substituted analogues of the larger ion clusters; specifically,

[Al2FeO3]+ and [AlFe2O3]+ are related to [Fe3O3]+, while [AlFe2O2]+ is an analogue of [Fe3O2]

+. The relative amounts of the pure iron clusters produced from the Fe2O3 control

and from the thermite are shown in figure 29. It can be seen that [FeO] + and [Fe2O]+ are

actually produced in higher quantities in the thermite, while the larger pure iron clusters

are only produced in very small amounts. This is consistent with the formation of the

mixed oxide clusters at the expense of the pure iron clusters due to competition in the

plume between Al+ and Fe+ for reaction with molecular FexOy species. The fact that

[Fe2O]+ appears to be independent of the formation of [AlOFe]+ is consistent with their

respective formation through different mechanistic pathways.

The production of mixed metal products correlates strongly with the size of the

aluminum. The energy density threshold at which mixed metal products are observed is

lower for the nano-scale particles than that for the micron-scale particles (1.58 vs. 1.73

J/cm2). More significantly, the quantities of all of the mixed metal components increase

with decreasing aluminum particle size (figure 30). This provides direct evidence for the

suggestion that the shorter ignition times that accompany smaller particle sizes are due to

the lower melting point of the nano-scale materials and the active aluminum content

available in the particle that, for a given laser power, produces more reactive species.

50



Figure 28. LDI-TOF mass spectra of thermite mixtures at a laser energy density of (a)1.42, (b) 1.58, (c) 1.74, and (d) 1.90 J/cm2

0

2000

4000

6000

8000

10000

12000

14000

16000

Intensity

[FeO]+ [Fe2O]+ [Fe2O2]+ [Fe3O3]+ [Fe3O2]+ [Fe4O4]+

Fe2O3 standard

Thermite Reaction

Figure 29. Relative amounts of pure iron cluster species formed during laser desorptionof the Fe2O3 control and thermite samples at an energy density of 2.054 J/cm2

51



0

2000

4000

6000

8000

10000

12000

14000

16000

Intensity

[AlOFe]+ [AlO2Fe]+ [Al2O3Fe]+ [AlO2Fe2]+ [AlO3Fe2]+

50 nm Al

100 nm Al

micron Al

Figure 30. Mixed Al/Fe oxide species formed at an energy density of 2.054 J/cm2 for 50

and 100 nm and micron aluminum thermite mixtures

Conclusions

This study has provided insight into some of the reactions that occur between

aluminum and iron oxide species in the plasma phase of the binary energetic system

during laser initiation. In particular, ionic species generated directly from the laser desorption and through various ion molecules are observed for both the aluminum and

iron oxide components. When the binary thermite is laser-initiated, mixed metal ionic

species are produced in the plume. These species are mixed metal analogues of the iron

clusters and are believed to form from the competition between Al+ and Fe+. The amount

of mixed metal clusters that form in the plasma increases as the size of the aluminum

particles decreases. This provides a direct verification that the shorter ignition times

observed with decreasing fuel particle size are due to a lower melting point that liberates

more reactive aluminum during laser incidence.

52



CHAPTER 4

PHOTO-THERMAL INIATION PROCESSES OF ORGANIC-INORGANIC

HYBRID METASTABLE INTERSTITIAL COMPOSITE (MIC) MATERIALS

Introduction

The organic high-energy material pentaerythritol tetranitrate (PETN) was

incorporated at low concentrations into Al (100 nm)/Fe2O3 metastable interstitial

composites (MIC) to form a hybrid organic/inorganic high-energy material. Studies of the

dynamics of energy release were carried out by initiating the reaction photo-thermally

with a single 8 ns pulse of the 1064 nm fundamental of a Nd:YAG laser. The reactiondynamics were measured using time-resolved spectroscopy of the light emitted from the

deflagrating material. Two parameters were measured: the time to initiation and the

duration of the deflagration. The presence of small amounts of PETN (16 mg/g MIC)

results in a dramatic decrease in the initiation time. This is attributed to a contribution to

the temperature of the reacting system from the combustion of the PETN that, at lower

loadings, appears to follow an Arrhenius dependence. The presence of PETN was also

found to reduce the energy density required for single-pulse photo-thermal initiation by

an order of magnitude, suggesting that hybrid materials such as this may be engineered to

optimize their use as an efficient photodetonation medium.

Experimental

Materials

The iron (III) oxide powder, <0.25 µm, was purchased from Aldrich, while the

100 nm aluminum was obtained from Argonide. All materials were used as received.

Pentaerythritol tetranitrate (PETN) was prepared according to the literature and stored in

acetone (Aldrich, HPLC grade) for safety purposes.35

53



Preparation of samples

Pentaerythritol tetranitrate (PETN) was deposited onto the iron (III) oxide by a

wet impregnation technique. A small amount, up to 1.053 mL, of PETN standard solution

(~ 0.5 M) was added to 0.5 grams Fe2O3 in order to obtain varying loading of PETN in

the final composite (Table 3). The acetone of the stock solution of PETN was

subsequently removed under a vacuum (10-3 torr) leaving a dry powder. The thermite

samples were prepared by mixing Fe2O3/PETN samples with 100 nm aluminum particles

in a 1:1 molar aluminum to iron ratio. The samples were thoroughly mixed by grinding

the components together using a mortar and pestle in order to ensure a homogenous

mixture. Approximately 0.25 grams of this mixture was compressed in a die at an applied

pressure of 46,000 psi. The resulting pellets were 6.39 mm in diameter by 3.28 mm

giving an average density of 2.38 g/cm3.

Table 3. Effects of PETN on heat and energy given off by MIC composites

mg PETN/g MIC composite Fraction of heat given off

by PETN

Energy PETN/ g composite

(kJ/g)

16.0 0.0343 0.130

32.0 0.0615 0.260

64.0 0.116 0.52195.9 0.164 0.781

128 0.208 1.04

Nd:YAG laser

A Spectra-Physics DCR-3G Nd:YAG laser using single pulses of both 1064 and

532 nm light was used in order to combust the thermite/PETN samples. At 1064 nm an 8

ns 900 mJ pulse was focused through a fused silica 75 mm focal length lens onto the

pellet, while at 532 nm the laser pulse was 6 ns and 360 mJ. The focused beam diameter

was calculated to be 11 µm giving an energy density of 8.8 x 10 5 J/cm2 at 1064 nm and

5.7 µm with an energy density of 1.4 x 106 J/cm2 at 532 nm. The laser beam was focused

through a hole in a protective steel plate, with the hole measuring 2.54 cm in diameter,

54



which was covered by a fused silica window measuring 24x23x2 mm in order to catch

any debris from the combustion of the pellet.

The kinetics of the combustion flash was recorded using three photodiode

detectors (Thorlabs DET210, 0.8 mm2 Si PIN, 1 ns rise time) and recorded by digital

storage oscilloscopes (LeCroy LC 564A, 1 GHz bandwidth, 10-50 µs sampling rate for

slow signals). The photodiodes were placed 45 degrees from the laser beam axis, looking

down at the sample through a fused silica window and filtered by either a heat absorbing

filter (Hoya HA30) to reduce the 1064 nm scattered light (fast signal) or a 520 nm band-

pass filter (Thorlabs FB520-10, 10 nm FWHM) for both fast and slow signals. Spectra

were collected through a 3 mm diameter liquid light guide (Oriel 77554, NA/0.47, >50%

transmittance from 270 to 720 nm) placed above the sample at 45 degrees from the laser

beam axis coupled into a 300 mm spectrograph (Acton Research Corporation Spectra Pro308i, 150 gr/mm grating blazed at 300 nm) using a biconvex fused silica lens (Spex, 1

inch diameter, f = 28.6 mm) and the light was dispersed onto a back-thinned CCD

(1340x400 pixels, 20 µm square, Princeton Instruments LN/CCD-400EB-G1 operated at

-90 ºC). The timing for single shot operation was coordinated by triggering the CCD

shutter, laser, and oscilloscope using a digital delay generator (EG&G PAR 9650) to

trigger the CCD shutter to open for 30 ms and delaying the laser and oscilloscope triggers

for 10 ms. The CCD exposure time, set in the software to 10 ms, was determined to be 30

ms by adjusting the laser trigger delay while observing the second harmonic 532 nm laser

light.


The effect of PETN incorporation on the dynamics of Al/Fe2O3 MIC initiation

and deflagration was investigated using time resolved spectroscopic techniques. In

general, thermal initiation of the thermite reaction occurs when the reaction temperature

reaches or exceeds the melting temperature of the fuel, in this case aluminum. At this

point there is a breakdown of the native oxide layer on the aluminum and the mass

transport between the fuel and the oxide becomes large enough to propagate the

reaction.42 The combustion of the samples in this instance was achieved photo-thermally

55



using a single 8 ns pulse of the 1064 nm fundamental from a Spectra-Physics DCR-3G

Nd:YAG laser, which allows the use of time-resolved spectroscopic techniques. As such,

the dynamics were monitored over the total time regime of the process by light emitted

from the reacting material, collected through a band-pass filter with a λ max = 520 nm.

The time-resolved data for the Fe2O3/100 nm Al MIC materials containing

various amounts of PETN is shown in figure 31. The emissive plume generated by the

laser incidence event on the surface of the sample can be seen as a sharp spike, indicated

by an arrow in the figure, while the irregular intensity that follows in time is the plasma

emission from the deflagration of the sample. An important quantitative parameter of the

reaction dynamics is the ignition time. For photo-thermally initiated binary fuel-oxidant

materials this is defined as the time at which the energy released by the reaction becomes

greater than or equal to the energy put into the composite by the laser. For data collected photonically, the ignition time is generally taken as the time between the laser excitation

and when emitted light can be detected. In the time-resolved system employed here, the

sensitivity and dynamic ranges of the photodiodes is high, allowing the very early stages

of combustion to be observed. In analyzing the data, the ignition time is defined as the

period between the sharp plume emission from the initial laser incidence and the point at

which the intensity of the light emitted from the combusting sample is twice the intensity

of the noise, as measured in the pre-trigger region of the data. A plot of the initiation time

against the amount of PETN in each sample at 1064 nm is shown in figure 32. The

ignition time at 1064 nm for the pure thermite sample was found to be 6.05 ± 0.28 ms,

however, with the incorporation of PETN (16 mg/g thermite) the ignition time drops

precipitously to 3.35 ± 0.28 ms and continues to decrease until a minimum value of 1.60

± 0.354 ms is observed with 64.0 mg PETN/g thermite. Above this amount of PETN

there is no further decrease and, in fact, it appears to plateau or even increases slightly.

For safety reasons it was not possible to obtain data for higher concentrations of PETN.

56



Figure 31. Photo-thermal initiation of Fe2O3/100 nm Al samples with increasingamounts of PETN at 1064 nm (arrow indicates the position of the laser plume emission)

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120 140

PETN (mg/g thermite)

I n i t i a t i o n T i m e ( m s )

Initiation TimeExponential Decay Fit

Figure 32. Initiation time (ms) as a function of PETN coverage in mg at 1064 nm

57



The intense, irregular peak that follows ignition (figure 31) is the time-evolution

of the plasma emission from the deflagration of the sample. The apparent oscillations in

the emission intensity are attributed to modulation of the light by the deflagration process

with the duration of this emission being directly related to the total time it takes for the

sample to deflagrate. From the data, the deflagration duration is measured from the

previously determined ignition point to the time where the emission intensity has returned

to a value of twice the noise level. A plot of the deflagration duration as a function of

PETN loading is shown in figure 33. As can be seen in figures 31 and 33, the

incorporation of a small amount of PETN into the MIC material has a pronounced effect

on the deflagration duration. The deflagration of a control sample of Al/Fe2O3 MIC

materials with no organic phase present shows a weak broad emission with a long (282

ms) duration. The addition of a small amount of PETN (16.0 mg/g thermite) results in asignificant decrease in the deflagration duration to 88 ms. The deflagration duration

continues to decrease as more PETN is added, albeit more gradually, until it plateaus at

31.5 ms for 64.0 mg of PETN per gram of thermite. Scrutiny of the spectral data in figure

31 shows that, in general, the decrease in deflagration duration is accompanied by an

increase in the emission intensity, which suggests that, as expected, more energy is being

released in a short time period.

0

50

100

150

200

250

300

0 20 40 60 80 100 120 140


D e f l a g r a t i o n D u r a t i o n T i m e ( m s )

1064 nm

532 nm

Figure 33. Deflagration duration time (ms) as a function of the amount of PETN at 1064nm

58



The changes observed in both the ignition time and the duration of deflagration as

a function of PETN addition appear to follow the same basic trend: increasing amounts of

PETN result in a decrease in the time to ignition and the deflagration duration, the effect

of which ultimately plateaus at about 64.0 mg per gram of thermite. In general these

effects can be thought of as originating from the contribution of the heat of combustion of

the organic phase to that of the fuel/oxidant inorganic matrix, which acts to accelerate the

binary reaction. PETN is a high-energy molecule with a heat of combustion of 2,572

kJ/mol.46 The amount of energy provided by the PETN and its contribution to the total

energy output of the composite material at various levels of incorporation is given in

Table 3. At the highest loadings studied the PETN accounts for about 20% of the total

energy liberated by the composite during combustion.

The effect of the PETN on the initiation time is likely due to its combustionduring or immediately after the laser excitation event. Specifically, the laser excitation

rapidly heats a volume of the composite defined by the diameter of the beam and the

thermal diffusion length of the material. During the course of the 8 ns pulse both the

aluminum and the iron oxide are heated with the relative temperature attained by each

phase being dependant primarily on its optical absorbance at 1064 nm and its thermal

diffusivity. The PETN is not absorbing at this wavelength so its combustion occurs from

heat flow from the other components. This will happen relatively efficiently since the

combustion temperature of the PETN is 205 ºC while the temperature reached by the

inorganic components will be at or above their melting point. The PETN will contribute

heat back into the inorganic components when combusted, thereby raising the

temperature further and accelerating the initiation processes. Since even at the lowest

incorporation level the combustion of PETN provides sufficient heat to melt all of the

aluminum in the composite, its contribution to the initiation process will be substantial;

this is directly reflected in the data.

The decrease in the initiation time as a function of PETN concentration (figure

32), particularly in the region before the plateau in the time is reached, is not linear but

instead appears to be approximately exponential. This suggests possible Arrhenius

behavior in the rate (lifetime) of the initiation. Earlier studies in the rates of binary fuel-

oxidant systems have suggested that behavior of the rate is Arrhenius in the early stages

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before auto-acceleration of the reaction occurs due to the extreme exothermicity.11-13 This

suggests that Arrhenius-type behavior may also be valid during the initiation process

prior to deflagration. The rate (lifetime) of the initiation in the composite will depend on

the temperature of the material, which is set to an initial value by the laser pulse and is,

subsequently, augmented by the combustion of the PETN. The contribution of the PETN

to the final temperature will be approximately proportional to the product of the mass of

PETN and the heat of combustion: T ΔH* m. An Arrhenius expression may be written

exploiting this and shown in equation 12, where the initial temperature, To, attained from

the laser pulse and the contribution from the PETN is a function of the mass, m, with the

constant, C, representing the heat of combustion of the PETN and the thermal

conductivity and heat capacity of the inorganic components that are being heated by it.

1 )( 0 CmT R

E

init

a

Aet

+−

= (12)

This form of the Arrhenius equation fits well to the initiation time data,

particularly at low PETN loadings, which is consistent with the basic model of how the

PETN affects the dynamics as shown here. The deviation from Arrhenius behavior as

initiation time plateaus at PETN loadings greater than 64 mg/g thermite mixture is

somewhat unclear but may simply be that the maximum rate has been reached and that

the additional heat placed into the melting and vaporization of the inorganic components

will have only a minimal effect.

As discussed, the deflagration duration (figure 33) of the reaction decreases

rapidly with the addition of PETN and, like the initiation time, it reaches a relatively

constant value at ≥64 mg/g. The lifetime of the deflagration is not described by the

Arrhenius temperature dependence; however, this is expected due to the high

exothermicity and concomitant auto-acceleration of the reaction. The leveling off of the

deflagration time may represent a limiting value determined by the PETN combustion

time, initiated in this fashion, as it begins to dominate the energetics of the composites.

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Studies of the ignition time and deflagration duration were carried out with visible

excitation at 532 nm. Since this wavelength is achieved through frequency doubling of

the 1064 nm fundamental the highest energy density that could be realized was 1.4 x 10 6

J/cm2. At this energy density it was not possible to initiate the combustion of the pure

Al/Fe2O3 MIC materials; however the presence of a small amount of PETN (10.0 mg/g

thermite) resulted in reproducible single-pulse initiation. This effect is explainable in

terms of prior discussion. While the energy provided at 532 nm is insufficient to initiate

the pure thermite it does provide sufficient heat to combust the PETN, which then

provides enough heat to drive the combustion. Notably, PETN does not absorb at 532 nm

so there is not optical contribution to the effect. Consistent with this interpretation, the

initiation time for 532 nm excitation (figure 34) is always longer than what is observed

for 1064 nm excitation which is expected from less total heat going into the system andthe fact that initiation is achieved secondarily through the PETN. The deflagration

duration is essentially the same as that observed with 1064 nm initiation (figure 33)

which is consistent with the idea that the duration of deflagration is dictated by the

energy release of the composite system and is independent of the initiation conditions.

0

1

2

3

4

5

6

0 20 40 60 80 100 120 140


I n i t i t a t i o n T i m e ( m s )

Figure 34. Initiation time of Al/Fe2O3 MIC materials with various concentrations of PETN initiated with a single pulse of 532 nm radiation

61



The results at 532 nm excitation suggest that one of the propitious effects of

PETN incorporation is to lower the threshold energy required for single pulse initiation.

This could have an impact on strategies for developing efficient photodetonation systems.

To quantify this effect, the initation threshold energy was determined for the range of

PETN loadings using 1064 nm initiation. The threshold energy density required to initiate

pure Al/Fe2O3 MIC samples at 1064 nm is quite high at 4.7 x 105 J/cm2. The addition of

PETN results in a relatively linear decrease in threshold energy required for initiation at

1064 nm up to 64.0 mg PETN/g thermite with an energy reaching a minimum value of

5.03 x 104 J/cm2 at 95.9 mg PETN/g thermite (figure 35) which represents almost an

order of magnitude decrease in the necessary energy density.

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

0 20 40 60 80 100 120


E n e r g

y D e n s i t y ( J / c m

2 )

140

Figure 35. Energy density (J/cm2) required in order to initiate the thermite/PETNmixture at 1064 nm

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Conclusions

In conclusion, the addition of high energy materials, in this case pentaerythritol

tetranitrate (PETN), to metastable interstitial composites (MIC) such as thermite-tyoe

Al/Fe2O3 compositions changes the energy release dynamics compared to that of

traditional MIC compositions. This is observed as a decrease in the initiation time and the

duration of deflagration of the sample. This effect is attributed to the contribution to the

overall heating of the sample due to the combustion of the PETN. Since the PETN does

not absorb light, the process occurs through the indirect heating of the PETN from the

inorganic phases to the point where combustion occurs. The initiation time and

deflagration duration decreases with increasing PETN loadings but reaches a minimum at

about 64.0 mg PETN/g thermite. Compositions containing PETN were found to require amuch lower threshold energy for photo-thermal initiation. These results indicate that the

incorporation of an organic phase, even at low concentrations, can have a profound effect

on the reaction dynamics of MIC materials. This affords the possibility of tailoring the

organic phase to better optimize desired properties of the materials.

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SUMMARY

This study has provided some insight into the stability and degradation processes

of traditional energetic materials, such as 2,4,6-trinitrotoluene (TNT) and pentaerythritol

tetranitrate (PETN) when placed in physical contact with metals or metal oxides. It was

found that in the presence of the metal oxides MnO2 and CuO, a clean demethylation of

TNT occurred, leaving the product trinitrobenzene (TNB). A rapid decomposition of

PETN, on the other hand, to gaseous species primarily made up of NO2 along with N2O4,

N2O, and CO2 occurred when placed on the surface of MoO3. These results indicate that

an interfacial surface effect occurs, which was observed as a change in the normal

thermal decomposition pathways of the molecule and may or may not have a net effect

on the stability or energy release in bulk composite materials.Also of interest was the reaction processes occurring during the initiation of

binary inorganic metastable interstitial composite (MIC) materials of aluminum and

iron(III) oxide as well as the energy release dynamics of hybrid organic/inorganic MIC

materials through laser initiation techniques. A laser desorption-ionization (LDI)

technique coupled with a time-of-flight mass spectrometer was used in order to show any

species occurring during the initiation process of MIC materials as ions reacting in the

plasma phase. Mixed metal oxides, such as [AlOFe]+, form from analogues of the iron

clusters the competition between Al+ and Fe+ in the thermite reaction.

The energy release dynamics of the classic thermite reaction in combination with

traditional high-energy materials, such as pentaerythritol tetranitrate (PETN), were

studied by time-resolved spectroscopy in order to monitor the light emitted from the

plasma of the reacting material using a Nd:YAG laser. A study of the initiation time,

measured as the time between the plume of the reacting materials brought on by the laser

pulse and the point where the intensity of the light becomes twice that of the noise, was

performed. This study shows a decrease in the initiation time as a function of the amount

of PETN present. Also, the total deflagration duration, measured as the total light emitted

from the reacting plasma, decreases with increasing amounts of PETN. The threshold

energy required in order to initiate the thermite reaction was also found to decrease

significantly as the amount of PETN present increases. Since PETN does not absorb light

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at the wavelengths used, but enough energy is provided to melt the inorganic material,

heat must be transferred from the Al/Fe2O3 mixture to the PETN, allowing combustion to

occur. The heat of combustion of the PETN then provides enough heat back into the

inorganic mixture to affect the energy release dynamics of the system. This effect

suggests that possible tailoring of the organic phase may be possible in order better

optimize the properties of these materials.

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BIOGRAPHICAL SKETCH

The author was born in Springfield, Illinois on August 2, 1981. She attended

Butler University in Indianapolis, Indiana as an undergraduate where she received her

Bachelor of Science degree in Chemistry in May 2003. She began her graduate studies in

Chemistry at Florida State University in the fall of 2003 in the Department of Chemistry

and Biochemistry under Dr. Albert E. Stiegman.

Melissa Mileham- Stability and Degradation Processes of Energetic Materials

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