Investigation of Sublimation and Decomposition by Sanjoy ...

Kinetics of Energetic Materials: Investigation of Sublimation and Decomposition

by

Sanjoy Bhattacharia, M.Sc.

A Dissertation

In

Chemical Engineering

Submitted to the Graduate Faculty

of Texas Tech University in

Partial Fulfillment of the Requirements for the Degree of

DOCTOR OF PHILOSOPHY

Approved

Dr. Brandon Weeks

Chair of Committee

Dr. Rajesh Khare

Dr. Micah Green

Dr. Carol Korzeniewski

Dominick Casadonte

Interim Dean of the Graduate School

August, 2013

Copyright 2013, Sanjoy Bhattacharia

Texas Tech University, Sanjoy Bhattacharia, August, 2013

ii

ACKNOWLEDGMENTS

I’m grateful to a lot people for helping me to finish my PhD degree. It won’t be

possible without the supervision, guidance and financial support of Dr. Brandon

Weeks. I’m grateful to his contribution. I thank all of my group mates for being nice

and friendly with me. I received a lot of helps and instructions from all of my group

mates. My special thanks to Dr. Genxing Zhang who trained me to learn the entire

instruments in our laboratory. I thank Dr. Yen-Chih Liao for helping me to solve AFM

and computer related problems. I thank Xin Zhang for helping me in experiments and

to understand chemistry related problems. I appreciate Dr. Walid Hikal for

collaborating projects and publishing papers. I thank Jay Nunley for helping me in

experiments. I also thank, my group mates in Chemistry departments for helping me

on my experiments on various occasions. Marauo Davis was always cooperative,

helpful and a very good friend.

My special thanks to Dr. Luisa Hope-Weeks for teaching me crystal growth

technique and allowing me to access her laboratory for using TGA and chemicals. I

thank, Dr. Amitesh Maiti and Dr. Richard Gee from Laurence Liversmore National

Labortory for their collaboration with various projects. I thank my committee

members Dr. Rajesh Khare, Dr. Micah Green and Dr. Carol Korzeniewski for their

time.

I thank my wife Swastika for all the supports. My special gratitude goes to my

son, Shwarnim, for being the source of inspiration of my life. I thank my parents and

sister for their part in my life. I thank the students from Bangladesh in TTU for their

help.


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TABLE OF CONTENTS

ACKNOWLEDGEMENT…………………………………………………………….ii

ABSTRACT…………………………………………………………………………..vi

LIST OF FIGURES…………………………………………………………………..vii

LIST OF TABLES…………………………………………………………………….xi

Chapter 1 Introduction…………………………………………………………………1

1.1 Origin of the project ……………………………………………………….1

1.2 PETN, as a standard explosive……………………………………………..3

1.3 Coarsening of PETN ………………………………………………………5

1.4 Decomposition to Detonation: Organic explosive…………………………8

1.5 Motivation of the research…………………………………………………9

References ……………………………………………………………………………14

Chapter 2 Materials and Methodology ……………………………………………….21

2.1 Materials ………………………………………………………………….21

2.2 Crystal growth ……………………………………………………………21

2.3 Thermal Analysis ………………………………………………………...23

2.4 Surface Area Calculation ………………………………………………...24

2.5 Surface Morphology Characterization …………………………………...24

2.6 Gas Phase Characterization ………………………………………………25

References ……………………………………………………………………………26

Chapter 3 Vapor Pressure of Explosive from Single Crystal: PETN ………………..27

3.1 Introduction ………………………………………………………………27

3.2Theory …………………………………………………………………….30


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3.3 Experimental ……………………………………………………………..33

3.4 Discussion………………………………………………………………...34

3.4.1 Sublimation Kinetics of PETN ...………………………………34

3.4.2 Calculation of the Vaporization Coefficient …………………..36

3.4.3 Vapor Pressure PETN single crystals ………………………….38

3.5 Conclusion ……………………………………………………………….42

References ……………………………………………………………………………43

Chapter 4 Sublimation Properties of Pentaerythritol Tetranitrate Single Crystals Doped

with Its Homologs…………………………………………………………………….47

4.1 Introduction……………………………………………………………….47

4.2 Experimental ……………………………………………………………..49

4.3 Result and Discussion…………………………………………………….49

4.3.1 Kinetic Analysis………………………………………………...49

4.3.2 Effect of homolog doping on the vapor pressure of PETN

crystal…………………………………………………………………53

4.4 Conclusion ……………………………………………………………….60

References ……………………………………………………………………………61

Chapter 5 Molecular Impurities in PETN Single Crystal for Controlling Sublimation:

TGA and AFM Study…………………………………………………………………64

5.1 Introduction……………………………………………………………….64

5.2 Experimental……………………………………………………………...66

5.3 Results and discussion……………………………………………………66

5.3.1 Optical microscopy of pure and doped PETN crystal…………..73


v

5.3.2 AFM studies on the surface of pure and doped PETN

crystal surface………………………………………………………...75

5.3.3 Comparing diPEHN and triPEON as dopant for PETN………..76

5.4 Conclusions: ……………………………………………………………...78

References…………………………………………………………………………….79

Chapter 6 Kinetics of the Gas Components from PETN Decomposition...…………..82

6.1 Introduction……………………………………………………………….82

6.2 Theory…………………………………………………………………….85

6.3 Experimental……………………………………………………………...86

6.4 Kinetics of NOX production from PETN decomposition…………………86

6.5 Kinetics of H2O production from PETN decomposition ………………...92

6.6 Conclusion………………………………………………………………..94

References…………………………………………………………………………….95

Chapter 7 Conclusion and Future Works……………………………………………..98

7.1 Overall Conclusion……………………………………………………….98

7.2 Proposed projects for future research……………………………………..98

APPENDIX………………………………………………………………………….101


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ABSTRACT

Explosives are the compounds that store large amount energy because of their

chemical structure. Organic explosives undergo the kinetic processes such as

sublimation, recrystallization and diffusion in the storage, and decomposition in both

the storage and the applications. In the storage, sublimation, recrystallization and

diffusion of molecules combine to undergo a new process called coarsening. The

coarsening of materials is a complex process driven by both the kinetics and the

thermodynamics. These processes cause the reduction of the surface area of the

organic explosives; therefore, influence the physical properties of the stored organic

explosives. The reduction of surface area of organic explosives reduces the initiation

sensitivity. Sublimation properties of molecular crystals of a benchmark organic

explosive were evaluated to understand the coarsening mechanism. This investigation

showed that coarsening of explosive can be controlled by doping explosive with the

homolog compounds of a organic explosive.

The decomposition of explosives is another kinetic process that influences the

intended performances of explosives. The initiation of an explosive leads to the

subsequent processes such as chemical decomposition and detonation. The

decomposition of an explosive to the end products is the first steps to toward the

production of a shock front. A rapid release of the gaseous products along with an

enormous amount of heat produced through the heat of reaction makes the

decomposition of organic explosives difficult to understand. In this part of the

investigation, kinetics of the production of the gas components was evaluated using

mass spectrometry. The activation energy of the investigated gas components were

obtained as a function of extent of reaction.


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LIST OF FIGURES

Figure 1.1 –Molecular structure of PETN showing four nitro groups ………………...4

Figure 1.2: Reaction scheme of PETN synthesis. Pentaerythritol reacts with nitric acid

to form pentaerythritol tetranitrate (PETN)……………………………………………5

Figure 1.3- Change of PETN morphology at 30oC: (image in the left hand side) t= 0

minutes, (image in the right hand side) t=52 minutes [17]…………………………….7

Figure 1.4- Optical images of PETN dendritic growth at 45oC (A) 1min (B) 24

minutes (C) 56 min [18] ……………………………………………………………….7

Figure 2.1: PETN single crystal grown from solvent evaporation techniques. Figure

shows the [110] and [101] surfaces…………………………………………………...22

Figure 2.2: Thermogravimetric analyzer (model i1000) ……………………………..23

Figure 2.3(A): Stereomicroscope (B) Atomic Force Microscope (C) Mass

Spectrometer connected with thermogravimetric analyzer ………............................25

Figure 3.1(A): Determination of kinetic parameter of PETN single crystal and powder

from Arrhenius plot…………………………………………………………………...35

Figure 3.1(B): Face kinetics of PETN crystal at 105 oC. (i) Initial image (ii) after 20

hours (iii) after 30 hours (iv) 40 hours………………………………………………..35

Figure 3.2: Determination of vaporization coefficient using the literature value of

benzoic acid and naphthalene…………………………………………………………37

Figure 3.3: Comparison Vapor Pressure of PETN single crystal with the theoretical

values in the temperaure range of 100oC-135

oC……………………………………...39

Figure 3.4: log P vs 1/T plot for obtaining the coefficients of the modified Antoine

equation log�� (��) = � − �(�) ……………………………………………………40


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Figure 3.5: Clausius-Claperon plot for obtaining the heat of sublimation from the

equation ln � = �� − ∆�� …………………………………………………….41

Figure 4.1: Kinetics of sublimation of pure and diPEHN doped PETN shown in

Arrhenius plot ………………………………………………………………………..51

Figure 4.2: Kinetics of sublimation of pure and diPEHN doped PETN shown in

Arrhenius plot ………………………………………………………………………..52

Figure 4.3: Calibration curve for obtaining the vaporization coefficient using benzoic

acid …………………………………………………………………………………...53


equation log�� (��) = � − �(�) ; (a)1000 ppm diPEHN doped, (b) 5000 ppm

diPEHN and (c) 10000 ppm diPEHN doped PETN crystal ………………………….58


equation log�� (��) = � − �(�) ; (a)1000 ppm triPEON doped, (b) 5000 ppm

triPEON and (c) 10000 ppm triPEON doped PETN crystal …………………………59

Figure 5.1 Mass-loss rates of doped PETN relative to undoped PETN at various

temperatures between 105-125 oC: (A) diPEHN-doped, and (B)

triPEON-doped ………………………………………………………………………68

Figure 5.2: Effects of washing on mass-loss rates: (A) mass-loss rates of washed,

diPEHN-doped crystals relative to unwashed, undoped PETN crystals; (B) mass-loss

rates of washed, diPEHN-doped crystals relative to unwashed crystals of the same

doping ………………………………………………………………………………...71


triPEON-doped crystals relative to unwashed, undoped PETN crystals; (B) mass-loss


ix

rates of washed, triPEON-doped crystals relative to unwashed crystals of the same

doping ………………………………………………………………………………...72

Figure 5.4 Optical images of (a) Pure PETN crystal, (b) 1000 ppm diPEHN doped

PETN crystal, (c) 5000 ppm diPEHN doped PETN crystal, (d) 10000 ppm diPEHN

doped crystal, (e) 1000 ppm triPEON doped crystal,(f) 5000 ppm triPEON doped

crystal, (g) 10000 ppm triPEON doped ……………………………………………...74

Figure 5.5: AFM images of the (110) faces of: (a) pure PETN; (c) diPEHN-doped

PETN ; (e) triPEON-doped PETN crystal. Height profile of the growth layers of (b)

pure PETN; (d) diPEHN-doped PETN; (f) triPEON-doped PETN crystal ………….77

Figure 6.1: (A) TGA-MS plot of NO2 (M/E= 46) production from PETN

decomposition (B) Degree of conversion of NO2 (M/E= 46) with temperature. (C)

Isoconversional curves for NO2 (M/E= 46). (D) Activation energy of the production of

NO2 (M/E= 46) from PETN decomposition as a function of degree of

conversion…………………………………………………………………………….88

Figure 6.2: (A) TGA-MS plot of NO (M/E= 30) production from PETN

decomposition, (B) Degree of conversion of NO (M/E= 30) with temperature (C)

Isoconversional curves for NO (M/E= 30), (D) Activation energy of the production of

NO (M/E= 30) from PETN decomposition as a function of degree of conversion

………………………………………………………………………………………...91

Figure 6.3: (A) TGA-MS plot of H2O (M/E 18) production from PETN

decomposition, (B) Degree of conversion of H2O (M/E 18) with temperature (C)

Isoconversional curves for H2O (M/E 18) (D) Activation energy of the production of

H2O (M/E 18) from PETN decomposition as a function of degree of


x

conversion ……………………………………………………………………………93

Figure A1: TGA curve for standard sample. Calcium oxalate monohydrate was used

as standard sample………………………………………………………………......102

Figure A2: Calibration curve for optical microscope……………………………….103

Figure A3: Images of samples crystal for obtaining surface area; (Top left)(110) face

(Top right) (1�1�0) face (Bottom left) (1�10) face (Bottom right)(11�0)

face…………………………………………………………………………………..104

Figure A4: Molecular structure of diPEHN…………………………………………107

Figure A5: Molecular structure of triPEON………………………………………...107

Figure A6: Calibration curve generated from TGA-MS using calcium

oxalate……………………………………………………………………………….108


xi

LIST OF TABLES

Table 1.1: Properties of some standard explosives [12]……………………………...13

Table 1.2: Properties of PETN [3] …………………………………………………...13

Table 3.1: Parameters of the Equation log�� (��) = � − �(�) for pure and doped

PETN crystal obtained in the temperature of 100oC -135

oC………………………….40

Table 4.1: Vapor pressure of Pure, diPEHN doped and triPEON doped PETN single

crystal…………………………………………………………………………………56

Table 4.2: Parameters of the Equation log �(��) = � − �(�) for pure and doped PETN

crystal obtained in the temperature of 100oC-135

oC …………………………………57

Table 5.1: Activation Energy of the pure and doped PETN single crystal…………...69


1

Chapter 1

Introduction

1.1 Origin of the project

Energetic materials/explosives are the compounds that store large amount

energy because of their chemical structure. These explosives release energy by

exothermic/entropic reaction [1] and contain both fuel and oxidizer in their chemical

structure that enables faster reaction [2]. Energetic materials are classified into two

types (i) primary explosives (ii) secondary explosives [3]. Primary explosives are less

powerful, but very sensitive and can be easily initiated to detonation by applying heat,

flame, and impact. Secondary explosives have a very high energy density. Since

secondary explosives are less sensitive, they are initiated to detonation by applying

strong shock. In a real life application both primary and secondary explosives are used

together. Primary explosives are used as the detonator and secondary explosives as

main the charge. Mining, military and demolition applications consumes 2 million

tons of explosives annually in United States [4].

Secondary explosives are mostly organic compounds made with carbon (C),

hydrogen (H), nitrogen (N) and oxygen (O) [5]. Organic explosives undergo kinetic

processes such as sublimation, recrystallization at storage and decomposition at both

storage and application [6]. At storage, sublimation, recrystallization and diffusion of

molecules combine to undergo a new process called coarsening. Coarsening is a

complex process driven by both kinetics and thermodynamics since the growth of

small nuclei for the formation of small particles is kinetically favored and the

formation of larger crystals from the smaller particles is thermodynamically favored.


2

These small particles have higher surface area to volume which makes them

thermodynamically less favorable compared to larger particles. Small nuclei formed

on the surface by recrystallization coarsen to form thermodynamically stable larger

particles. These processes cause the reduction of the surface area of the organic

explosives; thereby influencing the physical properties of the stored organic

explosives. The reduction of surface area of organic explosives reduces the initiation

sensitivity [6,7]. In this thesis, the sublimation properties of molecular crystal of

organic explosive were evaluated to understand the coarsening mechanism.

The decomposition of organic explosives is another kinetic process which

influences the intended performance of explosives. The initiation of explosives leads

to the subsequent processes such as chemical decomposition and detonation. A certain

degree of decomposition takes place during storage. The decomposition of explosives

influences the shelf life, stability, sensitivity, safety and hazards of handling of

explosives [8]. This decomposition of explosives to the end product is the first step

toward the production of shock front. The heat of detonation is the difference between

the heat of formation of products of decomposition and heat of formation of the

reactants [1]. The decomposition of organic explosives difficult to understand because

of a rapid release of the gaseous products along with enormous amount of heat

produced through the heat of reaction [9]. In this part of the thesis, kinetics of the

production of the fundamental gas component was evaluated to understand the kinetics

of the decomposition of explosives.

Terrorist activities using concealed explosives in the last decades are addressed

for robust detection technology for the security of the airport, air transportations and


3

important public places [10]. Understanding of sublimation properties are also

required to design robust sensors for detecting explosive particles from concealed

devices [11]. Sublimation properties and decomposition products of explosives are

also associated with the environmental aspect. Explosives are exposed to the

environment while applying in the mining and military. Release of explosives in the

environment causes a long term effect on soil, water and air. Since most of the

secondary explosives are organic and sublimate in the room temperature,

understanding the sublimation properties of the pure organic compound are required to

study the partitioning of explosive in the environment. The uses of explosives also

release their decomposition products in the environment. A sudden release of heat and

gaseous products causes short term and long term impact in the areas of application by

intoxicating the ambient air. This is a major concern for the environment in the mining

areas. Understanding the kinetics of the products of the decomposition is required to

evaluate how decomposition products transport in air and water, and deposits in the

soil.

1.2 PETN, as a standard explosive

Pentaerythritol tetranitrate (PETN) is considered as the benchmark secondary

explosive since it has similar physical and chemical properties to other organic

explosive, some are popularly known as TNT (2,4,6 trinitrotoluene), HMX

(Cyclotetramethylene-tetra nitramine) and RDX (Cyclo-1,3,5-trimethylene-2,4,6-

trinitramine). A comparison among the organic explosive is presented in Table 1 [12].

PETN fall into a class of nitrate ester. Other common nitrate ester explosives are

nitrocellulose (NC), nitroglycerine (NG). PETN is more thermally stable compared to


4

other military explosives. Figure 1.1 shows the molecular structure of PETN which

contains four nitro (-NO2) groups in the skeleton. Since nitro groups dissociate easily

from the molecular structure of PETN, oxidizer in the form of nitrogen and oxygen are

readily available for combustion which leads to detonation.

Figure 1.1 –Molecular structure of PETN showing four nitro groups

A very high specific surface area can be obtained for PETN particles. The

specific surface area of PETN particles ranges between 5000-30000 cm2/g which can

be obtained by controlling the kinetics of crystallization [6]. PETN single crystals

have body centered tetragonal structure with the space group symmetry P-421C , cell

angles � = � = = 90", lattice constants � = # ≠ % [13]. The experimental value of

lattice constants are � = # = 0.938 and % = 0.671. [13]. The other crystal habits are

needle like crystals with very high aspect ratio. PETN forms needle like crystal at

higher temperature and deposition rate. PETN undergo structural transition with the

application of very high pressure of 6GPa [14].

Pentaerythritol and concentrated nitric acid are used as the precursor for PETN

synthesis [15]. The reaction mechanism for the synthesis of PETN is shown in Figure

1.2. PETN with impurities precipitated out as a product from the nitration of

pentaerythritol. PETN is dissolved into acetone for recrystallization. PETN has very


5

high solubility in acetone [16]. To purify recrystallized PETN, it is washed with

sodium carbonate solution and cold water. Pure crystalized PETN is white in color.

Impurities in the PETN are mostly its homolog forming dimer and trimer during the

synthesis.

Figure 1.2: Reaction scheme of PETN synthesis. Pentaerythritol reacts with nitric acid

to form pentaerythritol tetranitrate (PETN)

1.3 Coarsening of PETN

A good numbers of literature were published studying various aspect of

coarsening. An investigation on the (110) face of PETN showed that PETN molecules

is highly mobile at room temperature [17]. The effective radius of nanoislands on

PETN surface started decreasing at 30 oC which is shown in Figure 1.3. A kinetic

analysis on the shrinkage of nanoislands showed similar kinetic parameters as found

for the sublimation of the bulk PETN. In a separate study, it was reported that

branches of PETN dendrite crystal grow with a rate of 0.15 µm/s at 30 oC [18]. Figure

1.4 showed dendrite growth of PETN. This study also revealed that only growth rate

increased up to 45oC. PETN branches started to shrink and then disappeared with

further increase of temperature. The proposed mathematical model explained the

contribution of diffusion, sublimation and recrystallization in the branch growth of

PETN [19]. Other theoretical studies on the surface diffusion of PETN molecules

were reported [20]. It was shown that the rate of surface diffusion of PETN molecule


6

on a (110) facet is faster compared to that on a (101) facet. PETN molecules can

diffuse from one facet to another, and diffusion from (110) facet to (101) facet is the

kinetically favorable. Energy barrier for surface diffusion of PETN molecule on (110)

and (101) surface is 17.5 and 4.4 time lower than that of sublimation process. The

diffusion of PETN on (110) surface were also studied using force field based

molecular dynamics [21]. According to this study, PETN molecules diffuse along

<111> directions on the (110) surface. The surface morphology of PETN thin film is

influenced by the rate at which PETN molecule deposits on the surface [22]. An

increase of molecular flux by one order of magnitude modifies the dendritic branch to

grain growth. This study explains the effect of the rate of recrystallization on the

coarsening of PETN in absence of the sublimation process.

In another study, surface morphology evolution of PETN was investigated on

16-mercaptohexadecanoic acid (MHA) self-assembled monolayer (SAM). This study

showed that bulk surface diffusion of PETN molecules cause surface evolution of

PETN on MHA SAM [23]. Surface evolution was also explained with long term

coarsening model using the accelerated aging data of PETN [24]. Coarsening of PETN

particles caused by the formation smother surfaces from the rough particles by the

evaporation and condensation. The coarsening process becomes slower for smother

particles because of its low surface energy.

PETN have four faces ((110) facet) along the long axis and 8 faces ((101)

facet) in the end caps [25]. A theoretical investigation on habit and size evolution of

PETN showed that adatom potential of energy of (110) is higher than that of (101)

faces. In another theoretical investigation on the kinetics of evaporation from the (110)


7

surface, step velocity and activation energy in sublimation were investigated to

understand the coarsening of PETN [26]. This study reported surface properties,

equilibrium growth morphology, and the theoretical values heat of sublimation [27].

All these studies showed similar values of activation energy and heat of sublimation,

and it is ~ 35 kcal/mol.

Figure 1.3: Change of PETN morphology at 30

oC: (image in the left hand side) t= 0

minutes, (image in the right hand side) t=52 minutes [17].

Figure 1.4: Optical images of PETN dendritic growth at 45oC (A) 1min (B) 24

minutes (C) 56 min [18].


8

In a report from Lawrence Livermore National Lab (LLNL), factors that

influence the aging process of PETN was investigated [6]. According to the report, the

driving forces that affect the sublimation/recrystallization and surface diffusion

process are temperature, stress, radiation, residual solvents, and impurities. These

process changes the crystal morphology, particle size distribution, powder surface

area, crystal density, internal defects, powder compact density and density gradients.

In another LLNL report, theoretical frame work for investigating grain coarsening of

PETN was presented [28]. They explained the morphological evolution of PETN with

the aging process. In these reports, it was recommended that addition of impurities in

PETN crystal might control the coarsening process. Experimental investigations are

not available in literature on the effect of controlled addition of impurities in the

coarsening process.

1.4 Decomposition to Detonation: Organic explosive

Secondary explosives are initiated by applying shock, friction, spark, laser,

shear stress, flame, or by initiating the primary explosive [29-38]. The application of a

shock wave compresses the explosives. When pressure reached a critical value (known

as Von-Neuman point), a reaction zone is created which starts the thermal

decomposition and subsequently results a rapid conversion of reactants into products

[12, 39]. Thermal decomposition is an exothermic reaction for most of the explosives.

There are few explosives that show entropic decomposition by releasing a very high

volume of gas [40]. When initiated, these explosives release less amount of heat

compared to the high explosives like PETN.


9

The initiation of an explosive is influenced by the porosity, void and

dislocations of the secondary explosives. Both formation of voids, porosity and

dislocations is related with the coarsening process because sublimation,

recrystallization and diffusion of molecule changes the porosity; and recrystallization

causes to form new surface morphology by forming new dislocations in organic

explosives. The thermal decomposition of secondary explosives is controlled by the

rate at which heat is transferred from the void to bulk explosives. These voids are

responsible for the formation of hot spot in the explosives [41-43]. Heat production in

the void is faster than the transfer of heat to the adjacent bulk explosives from the

collapse of voids. This results the production of hot spots of in bulk explosives. The

critical size and time of the hot spot was reported to be 0.1 to 10 µm and 10-5

to 10-3

seconds respectively [44]. The density of hotspots in the bulk explosive determines

rate of heating for thermal decomposition. Understanding the kinetics of

decomposition of explosives will give insight to the event “hot spot” which is still a

less understood phenomenon in explosives science and engineering. Moreover kinetic

analysis of the explosives is required to predict the performance and possible hazards

associated with the explosives.

1.5 Motivation of the research

Previous investigations on PETN coarsening explained the mechanism and

theoretical framework of coarsening of particles. Very few experimental efforts were

also reported to support the proposed theories and mechanism. Theoretical and

experimental values of kinetic parameters associated PETN surfaces mobility were

reported to explain the coarsening mechanism. No detailed experimental


10

investigations were reported on the sublimation properties of single crystal that could

explain the bulk properties. Since sublimation and recrystallization drive the complex

coarsening process, understanding the kinetics of the sublimation from a single crystal

is necessary to understand the coarsening process at the bulk scale and to develop the

life time prediction model. Anomalies of reported sublimation properties such as

vapor pressure from PETN powder also merited the necessity of the investigation of

the sublimation properties from single crystals. Part of this thesis will focus on the

investigation of the sublimation properties of the pure single crystals. Sublimation

properties such as vapor pressure, activation energy, heat of sublimation of PETN

single crystals were obtained from thermogravimetric analysis. Sublimation properties

of PETN powder were also investigated for comparison with both the theoretical and

the experimental values of PETN single crystal.

According to the discussion presented in the PETN coarsening section,

previous studies recommended a controlled addition of impurities in the crystal matrix

of PETN. Theoretical studies showed that saturation of kink site of PETN with a

heavier molecule such as triPEON inhibits on the shrinkage of kink site. Experimental

data was not reported to support/disprove the inhibition effect of a heavier molecule

on the shrinkage of kink sites of a PETN crystal. Commercially used PETN powder

contains impurities such as diPEHN and triPEON that are produced in the synthesis of

PETN. This motivates to investigate the effect of doping diPEHN and triPEON on the

sublimation properties of PETN single crystal. To this end, a detail investigation on

the sublimation properties of diPEHN and triPEON doped PETN crystals was


11

conducted. Necessary parameters were obtained from this investigation to explain the

effect of doping on coarsening of PETN.

Simulation studies showed that the activation energy of desorption of PETN

molecules is similar to that of the experimental value of PETN. The simulation on

sublimation of PETN also showed that the rate limiting step in the sublimation process

is desorption of PETN molecule from the kink sites of a single crystal. The effect of

impurities on sublimation depends on the saturation of kink sites with the doping

compounds. The understanding the distribution of impurities in crystal is required to

explain the effect of impurities on the sublimation process. In this research, impurity

distribution is qualitatively explained using thermogravimetric data and surface

morphology. The effect of impurity on sublimation is explained with the quantitative

analysis on the rate of sublimation. Then, the mechanism of inhibition of the rate of

sublimation because of the presence of impurities is also explained.

Various kinetic processes are intertwined for obtaining the efficient application

of explosives. Previously this chapter explained how kinetic processes influenced the

physical properties; thereby affecting the safe and efficient performance of explosives.

The decomposition of explosives is the kinetic process which is associated with shelf

life, safety and applications. Numerous studies were reported on PETN decomposition

[45-57]. Most of the research focused on the evolution of decomposition product of

PETN. The kinetic parameters of PETN decompostion were obtained from various

studies. Activation energy of PETN decomposition was reported to be 30-70 kcal/mol

with a first order decomposition kinetics [58] . Kinetics of the gases resulting from

thermal decomposition and its correlation with the initiation and detonation are least


12

understood. Kinetic parameters for each component of the decomposition products can

help to develop mathematical model for better explanation of both decomposition and

detonation process. It can help to design improved technology of explosive initiation.

Previous studies on decomposition did not explain the kinetics of the gas phase

kinetics more vividly. In this part of the research, nonisothermal kinetics of gaseous

products was explained using TGA-MS.


13

Table 1.1: Properties of some standard explosives [12]

Name Chemical

Formula

Molecular

Weight

Density

(g/cm3)

Melting Point

(oC)

Heat of

Detonation

(kcal/g)

PETN C5H8N4O12 316.2 1.78 142 1.51

TNT C7H5N3O6 227.1 1.65 80.9 1.29

HMX C4H8N8O8 296.2 1.90 285 1.48

RDX C3H6N6O6 222.1 1.84 205 1.48

TATB C6H6N6O6 258.2 1.94 Decompose

before melting

1.08

Table 1.2: Properties of PETN [3]

Property Value

Empirical Formula C5H8N4O12

Color Colorless

Energy of Formation -385 kcal/kg

Enthalpy of Formation -407.4 kcal/kg

Specific Energy 1205 kJ/kg

Density

1.76 g/cm3

Heat of Fusion 36.4 kcal/mol

Specific Heat 0.26 kcal/kg

Detonation Velocity 8400 m/s


14

References

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[2] V Boddu, P Redner, Energetic Materials: Thermophysical properties, predictions,

and experimental measurements, CRC Press, 1st ed.,2010.

[3] R Meyer, J Kohler, A Homburg, Explosives, Wiley-VCH, Inc.,6th

ed., 2007.

[4] M Marshall, J Oxley, Aspects of explosive detection, Elsevier, 1st ed., 2008.

[5] AK Sikder, G Maddala, JP Agrawal, H Singh, Important aspects of behavior of

organic energetic compounds: A review, J. Hazard. Mater. 2001, 84, 1.

[6] MF Foltz, Aging of pentaerythritol tetranitrate (PETN), LLNL-TR-415057, 2009.

[7] PM Howe, Effects of microstructure on explosive behavior, Prog. Astronaut.

Aeronaut. 2000, 185, 141.

[8] CM Tarver, SK Chidester, On the violence of high explosive reaction, UCRL-

CONF-202375, July 2004.

[9] G Zhang, BL Weeks, H Sun, JM Abbott, Engineering the microstructure of organic

energetic materials, Appl. Mater. Interfaces, 2009, 1 (5), 1086.

[10] M Marshall, J Oxley, Aspects of explosive detection, Elsevier, 1st ed., 2008.

[11] JI Steinfeld, J Wormhoudt, Explosives detection: A challenge for physical

chemistry, Ann. Rev. Phys. Chem. 1998, 49, 203.

[12] Y Hori, L Davidson, N Thadhani, High-Pressure Shock Compression of Solids

VI, Springer, 1st ed, 2003.


15

[13] J Trotter, Bond lengths and angles in pentaerythritol tetranitrate". Acta

Crystallogr. 1963, 16, 698.

[14] O Tschauner, B Kiefer, Y Lee, M Pravica, M Nicol, E Kim, Structural transition

of PETN-I to ferroelastic orthorhombic phase PETN-III at elevated pressures, J.

Chem. Phys., 2007, 127, 094502.

[15] J Akhavan, The chemistry of Explosives, RSC.,2nd

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[16] RN Robert, RH Dineger , Solubility of Pentaerythritol Tetranitrate, J. Phys.

Chem., 1958, 62 (8), 1009.

[17] AK Burnham, SR Qiu, R Pitchimani, BL Weeks, Comparison of kinetic and

thermodynamic parameters of single crystal pentaerythritol tetranitrate using atomic

force microscopy and thermogravimetric analysis: Implications on coarsening

mechanisms, J. Appl. Phys., 2009, 105, 104312.

[18] G Zhang, BL Weeks, Inducing dendrite formation using an atomic force

microscope tip, Scanning, 2008, 30, 228.

[19] G Zhang, BL Weeks, RH Gee, A Maiti, Fractal growth in organic thin films:

Experiments and modeling, Appl. Phys. Lett., 2009, 95, 204101.

[20] PH Lin, R Khare, BL Weeks, Molecular modeling of diffusion on a crystalline

pentaerythritol tetranitrate surface, Appl. Phys. Lett., 2007, 91, 104107.

[21] J Wang, T Golfinopoulos, RH Gee, H Huang, Diffusion on (110) surface of

molecular crystal pentaerythritol tetranitrate, Appl. Phys. Lett., 2007, 90, 101906.

[22] G Zhang, BL Weeks, Surface morphology of organic thin films at various vapor

flux, Appl. Surf. Sci., 2010, 256, 2363.


16

[23] G Zhang, BL Weeks, Application of dynamic scaling to the surface properties of

organic thin films: Energetic materials, Surf. Sci., 2011, 605, 463.

[24] A Maiti, RH Gee, PETN Coarsening- Predictions from accelerated aging data,

Propellants Explos. Pyrotech., 2011, 36, 125.

[25] LA Zepidda-Ruiz, A Maiti, R Gee, GH Gilmer,BL Weeks, Size and habit

evaluation of PETN crystal-a lattice Monte Carlo study, J.Cryst.Growth,2006,291,461.

[26] L Zepeda-Ruiz, GH Gilmer, A Maiti, R Gee, A Burnham, Evaporation of the

(110) Surface of PETN, J. Cryst. Growth, 2008, 310, 3812.

[27] A Maiti, RH Gee, Modeling growth, surface kinetics, and morphology evolution

in PETN, Propellants Explos. Pyrotech., 2009, 34, 489.

[28] AK Burnham, R Gee, A Maiti, R Qiu, R Pitchimani, B Weeks, L Zepeda-Ruiz,

Experimental and modeling characterization of PETN mobilization mechanisms

during recrystallization at ambient conditions, UCRL-TR-216963, 2005.

[29] RB Frey, The Initiation of explosive charges by rapid shear, DTIC Document,

June 1980.

[30] RM Eason, TD Sewell, Shock-induced inelastic deformation in oriented

crystalline pentaerythritol tetranitrate, J. Phys. Chem. C, 2012, 116 (3), 2226.

[31] SV Zybin, WA Goddard III, P Xu, J Budzien, A Thompson, reactive molecular

dynamics of shock- and shear-induced chemistry in energetic materials for future force

insensitive munitions , Department of Defense Proceedings of the High Performance

Computing Modernization Program - Users Group Conference, HPCMP-UGC 2009,

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[32] JJ Dick, RN Mulford, WJ Spencer, DR Pettit, E Garcia, DC Shaw, Shock

response of pentaerythritol tetranitrate single crystals, J. Appl. Phys., 1991, 707, 3572.

[33] D Sorescu, Theoretical studies of the hydrostatic compression of RDX, HMX,

HNIW, and PETN crystals J. Phys. Chem. B, 1999, 103(32), 6783.

[34] D Sorescu, Theoretical predictions of energetic molecular crystals at ambient and

hydrostatic compression conditions using dispersion corrections to conventional

density functional (DFT-D), J. Phys. Chem. C, 2010, 114(14), 6734.

[35] BP Aduev, GM Belokurov, SS Grechin, AV Puzynim, Detonation of PETN

single crystals initiated by an electron beam, Comb. Explos. Shock Waves, 2010,

46(6), 712.

[36] BP Aduev, GM Belokurov, SS Grechin, VN Shvaiko, Investigation of early

stages of explosive pentaerythritol tetranitrate crystal decomposition initiated by

pulsed electron beams, Russian Phys. J., 2007, 50(2), 99.

[37] ED Aluker, AG Krechetov, AY Mitrofanov, DR Nurmukhametov MM Kuklja,

Laser initiation of energetic materials: Selective photoinitiation regime in

pentaerythritol tetranitrate, J. Phys. Chem. C, 2011, 115(14), 6893.

[38] EM Hunt, High speed study of drop weight impact ignition of PBX 9501 using

infrared thermography, ISRN Mechanical Engineering 2011.

[39] A Tokmakoff, MD Foyer, DD Dlott, Chemical reaction initiation and hot spot

formation in shocked energetic molecular materials, J. Phys. Chem.,1993,97,1901.


18

[40] F Dubikova, R Kosloff, J Almog, Y Zeiri, R Boese, H Itzhaky, A Alt E Keinan,

Decomposition of triacetone triperoxide is an entropic explosion, Am. Chem. Soc.,

2005, 127 (4),1146.

[41] DL Bonnet, PB Butler, Hot spot ignition of condensed phase energetic material,

J. Propulsion. Power., 1996, 12(4), 680.

[42] JE Field, NK Bourne, SJP Palmer, SM Walley, J Sharma, BC Beard , Hot spot

ignition mechanisms for explosives and propellants, Phil Trans. R. Soc. Lond. A,

1992, 339 (1654), 269.

[43] AM Mellor, DA Wiegand, KB Isom, Hot spot histories in energetic materials,

Combust. Flame, 1995,101(1-2),26.

[44] JE Balzer, WG Proud, SM Walley, JE Field, High speed photographic study of

the drop-weight impact response of RDX/DOS mixtures, Combust. Flame, 2003, 135,

547.

[45] P Politzer, JS Murray, Energetic materials part 1. decomposition, crystal and

molecular properties, Elsevier, 1st ed, 2003.

[44] J Kimura, Chemiluminescence study on thermal decomposition of nitrate esters

(PETN and NC), Propellants, Explos. Pyrotech., 1989, 14, 89.

[46] ZA Dreger, YA Gruzdkov, YM Gupta, JJ Dick, Shock wave induced

decomposition chemistry of pentaerythritol tetranitrate single crystals: Time-resolved

emission spectroscopy, J. Phys. Chem. B, 2002, 106 (2), 247.


19


decomposition chemistry of pentaerythritol tetranitrate single crystals: Time-resolved

emission spectroscopy, J. Phys. Chem. B, 2002, 106 (2), 247.

[48] BD Roos, TB Brill, Thermal decomposition of energetic materials 82.

Correlations of gaseous products with the composition of aliphatic nitrate esters,

Combust. Flame, 2002, 128, 181.

[49] CM Tarver, TD Tran, RE Whipple, Thermal decomposition of pentaerythritol

tetranitrate Propellants, Explos. Pyrotech., 2003, 28(4), 189.

[50] X Ruijuan, L Hong, S Luo, J Liu, A study on thermal decomposition of PETN by

PGC/MS, Proc. 3rd Int. Autumn Semin. Propellants, Explos. Pyrotech. Chengdu,

China, 1999,153.

[51] WL Ng, JE Field, HM Hauser, Thermal, fracture, and laser-induced

decomposition of pentaerythritol tetranitrate, J. Phys. Chem.,1986, 59(2),3945.

[52] HN Volltrauer, Real time low temperature decomposition of explosives – PETN,

J. Haz. Mater., 1982, 5, 353.

[53] KK Andreev, BI Kaidymov, Thermal decomposition of nitrate esters. II. thermal

decomposition of pentaerythritol tetranitrate, J. Phys. Chem.,1961,35,1324.

[54] CC Huang, MD Ger, YC Lin, IC Chen, Thermal decomposition of mixtures

containing nitrocellulose and pentaerythritol tetranitrate, Thermochim. Acta., 1992,

208(C), 147.


20

[55] WL Ng, JE Field, HM Hauser, Study of the thermal decomposition of

pentaerythritol tetranitrate, J. Chem. Soc. Perk. Trans., 1976, 6, 637.

[56] FJ Dicarlo, JM Hartigan, GE Phillips, Analysis of pentaerythritol tetranitrate and

its hydrolysis products by thin layer chromatography and radio scanning, Anal. Chem.,

1964, 36, 2301.

[57] AJB Robertson, Thermal Decomposition of Pentaerythritol Tetranitrate,

Nitroglycerin, Ethylenediamine Dinitrate, and Ammonium Nitrate, J. Soc. Chem. Ind.,

1948, 67, 221.

[58] DM Chambers, Perspectives of pentaerythritol tetranitrate (PETN)

decomposition, UCRL-JC-148956, July 2002.


21

Chapter 2

Materials and Methodology

2.1 Materials

PETN powder was used in this research as the standard secondary explosive.

PETN powder was obtained from Lawrence Livermore National Laboratory. Purity

was the collected PETN powder was >99.9%. Acetone was used as the solvent for

dissolving PETN. DiPEHN and triPEON were used as the sublimation inhibiting

doping compounds. Both diPEHN and triPEON with purity >99% were also obtained

from the Lawrence Livermore National Laboratory. Absolute ethanol was used for

washing the surface of the PETN crystal. Both ethanol and acetone were obtained

from the Sigma Aldrich Inc. Benzoic acid was used as a standard material for

obtaining the coefficient of vaporization. Naphthalene was used to test the consistency

of the curve for obtaining coefficient of vaporization. Benzoic acid and naphthalene

were obtained from the Scholar Chemistry Inc..

2.2 Crystal Growth

Two basic process of crystal growths are (i) melt growth and (ii) solution

growth [1]. Since most of the organic materials decompose near melting point and

sublimates before the melting point, “solution growth” process was preferred over

melt growth for the growing crystals. Another advantage with solution growth is that a

controlled amount of doping can be added in the matrix of a crystal by solution

growth. In this research, single crystals were grown following method described by

Pitchimani et al.[2]. PETN powder (wt. 120 mg) was dissolved in 5ml acetone in a

scintillation vial. The solvent from PETN solution was allowed to evaporate for


22

forming a supersaturated solution from which single crystal of PETN grows. Figure

2.1 shows a typical PETN crystal grown for this study using the described method.

Single crystals with size <15mg were used in the TGA for sublimation and

decomposition studies.

Figure 2.1: PETN single crystal grown from solvent evaporation techniques. Image

shows various faces of the crystal

(110)

(1�10)

(11�0)

(101) (101�)

(011) (011�)


23

2.3 Thermal Analysis

Thermogravimetric analysis is conducted by measuring the mass of the sample

as a function of the sample temperature. Thermogravimetric analyzer (TGA) can

record mass, temperature and time of the sample simultaneously. This feature of TGA

enables to study isothermal and nonisothermal kinetics of any process involving phase

changes such as sublimation of volatile compounds, decomposition of material etc.

Other applications of TGA include: (i) heterogeneous chemical reactions with the

desired purge gas (ii) curie transition of ferromagnetic material (iii) oxidation of

material etc [3]. In this research, isothermal sublimation kinetics of PETN single

crystal was investigated using the TGA. PETN was annealed isothermally from 100 oC

to 135 oC in steps in a platinum pan. Nitrogen gas was used as the purge gas with a

flow rate 10cc/min. TGA was calibrated by using calcium oxalate monohydrate.

Standard sample was annealed nonisothermally from room temperature to 1000 oC at

10 oC/min and isotherm is shown in figure A1 (in appendix). TGA (model- i1000) was

supplied by Instrument Specialist Inc. Figure 2.2 showed TGA used in this research.

Figure 2.2: Thermogravimetric analyzer (model i1000).


24

2.4 Surface Area Calculation

Surface area of single crystals was obtained from the optical images which

were collected using a stereomicroscope. Figure 2.3 (A) shows the stereomicroscope

used for taking images. Stereomicroscope was obtained from Olympus Inc. Digital

camera used in the microscope was obtained from AmScope Inc. Optical images were

analyzed for obtaining surface area using “Image J” Software[4]. Optical Microscope

was calibrated using a standard sample. Calibration curve for the optical microscope

was shown in figure A2 (in appendix). To calculate the surface area of a crystal,

images of 4 different (110) faces were collected. Surface area of (110) faces were

obtained directly from the images. Surface area of the 8 faces in the edge cap, defined

as (101) faces, were obtained from the length of the sides of each faces. Since (101)

faces are rectangle like, area of each face can be calculated from the lengths and width

of the rectangle. Length and width of all the (101) faces can be obtained from the

images of (110) faces. In appendix, images of 4 different (110) faces of one sample

crystal were shown in figure A3 (in appendix). Surface area of the sample was

obtained to be 20.82 mm2.

2.5 Surface Morphology Characterization

Surface morphology was characterized by Atomic Force Microscopy (AFM) in

contact mode at a scan rate of 0.5 Hz. AFM was obtained from Veeco Instrument Inc.

(now known as “Bruker Nano Inc.”). Figure 2.3(B) shows the AFM used in this

research. AFM images (256 X256 pixels) were processed by using WSxM software

[5].


25

2.6 Gas Phase Characterization

Gas phase characterization was conducted by a mass spectrometer (MS)

connected with thermogravimetric analyzer (TGA). Figure 2.3(C) showed TGA-MS

used in this research. Mass spectrometry is an important tool of characterization in

engineering and fundamental scientific research. Analysis of gas phase components in

various processes is one of the most important applications [6]. Main functional

components of a mass spectrometer (MS) are (i) analyzer unit with ion source and

detector (ii) high frequency generator (iii) a pulse preamplifier (iv) control unit (v)

control and evaluation software. Samples are annealed in TGA in helium environment.

Gas components produced due to the decomposition of sample were pumped in MS

through a capillary tube. Vacuum was maintained in MS at >1X10-6

mbar. TGA was

calibrated using calcium oxalate monohydrate. Heating rate was controlled in the

temperature program of TGA. In the present set up in the TGA, temperature can be

increased from 0.1 to 100 oC/min. TGA (model: Q20) was supplied by TA instrument.

Mass spectrometer (model: GSD 320) was purchased from Pfeiffer Vacuum.

Figure 2.3: (A) Stereomicroscope (B) Atomic Force Microscope (C) Mass

Spectrometer connected with thermogravimetric analyzer

TGA MS

C

A B C


26

References

[1] KA Jackson, Kinetic process, Willey-VCH, Weinheim, 2004.

[2] R Pitchimani, W Zheng, SL Simon, LJ Hope-Weeks, AK Burnham, BL

Weeks, Thermodynamic Analysis of Pure and Impurity Doped Pentaerythritol

Tetranitrate Crystals Grown at Room Temperature, J. Therm. Anal. Calorim.,

2007, 89, 475.

[3] P Gabbot, Principles and applications of thermal analysis, Blackwell

Publishing 2008.

[4] TJ Collins, ImageJ for microscopy, BioTechniques, 2007, 43(1 Suppl), 25.

[5] I Horcas, R Fernández, JM Gómez-Rodríguez, J Colchero, J Gómez-Herrero,

AM Baro, WSXM: A software for scanning probe microscopy and a tool for

nanotechnology, Rev. Sci. Instrum. 2007, 78, 013705.

[6] Pfeiffer Vaccuum Mass Spectrometer, user manual, 2007.


27

Chapter 3

Vapor Pressure of Explosive from Single Crystal: PETN

3.1 Introduction

Organic energetic materials are the most commonly used explosive for both

military and mining applications. Explosives are exposed to the ambient environment

during storage and application. The fate, transport and distribution of organic

explosive whether it is stored, used or exposed to environment are related with amount

of mass coming out from the bulk of material [1-4]. The emission of explosive from

its sources takes place by three processes, they are (i) diffusion of molecule on the

surface of the material (ii) desorption molecule from the surface of the explosive and

(iii) evaporation into the air. Evaporative mass transfer is explained by the following

equation

,-,. = /0(�1 − �2) (3.1)

here, ,-,. is the rate of evaporation,/0is the emission coefficient, �1 is the vapor

pressure at the surface and �2is the vapor pressure at ambient condition. The equation

(3.1) illustrates that the vapor pressure is a very important parameter of the explosive

for studying the material characteristics as well as the interaction with the

environment. Vapor pressure is the term explaining the process of solid/liquid to gas

phase transition. It indicates the maximum number of molecules (or amount of mass)

available in the gas phase when the material is in equilibrium with its solid/liquid

phase [5]. To study the effects of released material on the environment such as air

water and soil, the thermodynamic equilibrium properties of the explosive such as

vapor pressure, water solubility, must be known [5].


28

In recent years, the uses of improvised explosive devices (IEDs) by the

terrorist are one of the biggest security issues in public places. Trace and bulk

detection of explosives at room temperature and pressure is the biggest challenges for

scientist and engineers [6-8]. Detection of explosives at trace level and vapor phase

depends on the volatility of the compounds. Understanding the vapor pressure of the

compounds is required to develop robust technologies for detection of explosive at any

level [9]. Modeling the plume of the concealed explosives required the information of

heat of sublimation which is a function of vapor pressure [10]. Desorption of material

from the source to detector plays a key role to the ability of a detectors sensing the

suspected material. Thermal desorption is the primary mechanism by which explosives

is transported from a point source of explosives to the detector [10]. A complete

understanding of vapor pressure and heat of sublimation provides the platform to

increase the robustness of the available detection technique and to design new

technology. Sampling strategies also depends on the desorption characteristics of the

explosives.

The vapor pressure of material is also an important parameter to describe the

kinetics of the atom or molecule involving sublimation process, thin film growth,

dendritic crystal formation [11]. Organic explosives sublimate and recrystallize

during storage. At equilibrium, the rate of sublimation of the condensed phase is equal

to the recrystallization of the vapor phase. Simultaneous sublimation and

recrystallization process causes the changes in surface morphology. A complete

understanding of the vapor pressure of the explosives helps to understand the kinetics

which leads to altering the surface morphology in the long run and helps to regulate


29

the storage condition. Military explosives, those are generally stored for long-term,

before being used in the battle field, undergoes changes of surface properties, particles

size distribution [12]. To predict the surface morphology of the material with time,

vapor pressure and heat of sublimation data are required.

A wide ranges of values for vapor pressure and other kinetic and

thermodynamic parameter are available for commonly used explosives like PETN in

literature [13-19]. All those data presented in literature were measured using the

powder sample. It was shown both theoretically and experimentally that the materials

start shrinking from the edge of the growth layer [20-21]. Vapor pressure of the

material depends on the availability of the kink site. Only single crystals truly expose

the actual edges of kink sites. Experimental data of the vapor pressure of explosives

from a single crystal were not reported for any explosives material. It is important to

get the idea of the actual value of vapor pressure from a single crystal since the single

crystals exhibit the ideal material behavior. Although there were theoretical data

reported in literature for the most common organic explosives like PETN, HMX and

RDX. These theoretical data were presented considering those explosives as a single

crystal.

In this work, Vapor pressure data of one of the bench mark explosive,

pentaerythritol tetranitrate (PETN), were presented. Vapor pressure data were obtained

from single crystals. Experimentally obtained vapor pressure data from single crystals

was compared with theoretical values. Coefficients of the modified Antoine equation

for vapor pressure were presented. Kinetics of the sublimation process from the


30

surface of the single crystal was explained. The heat of sublimation was obtained for

PETN single crystal.

3.2Theory

For a solid-vapor equilibrium of a molecular crystal, the desorption of

molecule from the kink sites of the molecular crystal is explained by Arrhenius

equation in following form [21]

/(3) = 4 exp( 89:;� ) (3.2)

where 4 is the vibrational frequency of atom, /is the Boltzmann constant, T is the

temperature and <2 is the energy associated with the desorption process from the kink

site of a molecular crystal. The term exp( 89:;� ) is defined as the probability of the atom

release from the kink site of a molecular crystal. <2 is the activation energy defined as

the following

<2 = => − =; (3.3)

where => is the energy of the molecule at vapor phase and =; is the energy of the

molecule in the kink site.

Thermogravimetric analysis (TGA) is one of the simplest ways to study the

sublimation of a compound. Kinetics of the sublimation process during the isothermal

heating in TGA was analyzed by the Arrhenius equation. To explain the isothermal

kinetics of sublimation in TGA by Arrhenius expression, equation (3.1) is written in

the following form

/(3) = �?@�(89:�� ) (3.4)

where A is the Arrhenius parameter known as the pre exponential factor and R is the

gas constant. For a zero order process, the equation (3.4) can be rearranged as


31

A� ,-,. = A�� − 9:�� (3.5)

where ,-,. is the rate of mass loss per unit surface area. Slope from the plot of A� ,-,. vs

1/3 gives the activation energy for sublimation.

TGA can also be used to measure the vapor pressure of various compounds

[22-27]. Vapor pressure of materials can be calculated from the rate of sublimation

and kinetic theory of gases [28]. Langmuir proposed the model relating vapor pressure

and rate of sublimation in vacuum. According the proposed model, rate of sublimation

from a solid surface (,-,. ), average velocity of the molecule (Ω), and density of the gas

(D) are related like following

,-,. = �EΩD (3.6)

For an ideal gas, D = FG�� where � is the vapor pressure, H is the molecular weight, T

is the absolute temperature and R is the gas constant.

Pressure and average velocity of the molecule is related as

� = IJ DΩK (3.7)

Ω = LJ��IG (3.8)

Comparing equation 1 and 3,

,-,. = �L GKI�� (3.9)

In a gaseous environment, sublimating molecules collide with gas molecules. To add

the effect of the interaction of sublimating molecule and foreign gas molecule, an

additional parameter, , the vaporization coefficient, is added in the equation


32

,-,. = � L GKI�� (3.10)

The vaporization coefficient that is assumed to be 1 in vacuum, M is molecular weight

of the vapor in kg mol-1

. In the presence of a foreign gas, the vaporization constant, ,

cannot be assumed to be 1. To obtain the vaporization constant, a standard material

with known vapor pressure is used. Benzoic acid and naphthalene are used as the

standard material for this study. Since cannot be determined a priori, a modified

form of the Langmuir equation is used

� = MN (3.11)

where M = √KI�P and N = ,-,. L�G. The vaporization constant is calculated in the form

of ‘V’. The value of V does not depend on the material or rate of mass loss, it only

depends on the coefficient of vaporization constant which is dependent on the

experimental set up. So value of V obtained from various standard materials should be

the same in each experimental set up. The vapor molecular weight, M, was assumed to

be the molecular weight of the PETN since the vapor phase is assumed to contain very

little amount of doping compound and will not affect the average molecular weight of

the gas phase.

The relation between vapor pressure and temperature is mathematically

explained by the Antoine equation in the form

log � = � − �QR (3.12)

where A, B and C are constants, T is the absolute temperature and P is the vapor

pressure [29]. However, for low vapor pressure compounds like PETN, a modified

form of Antoine equation is used


33

log � = � − � (3.13)

Coefficient A and B were calculated using equation (3.9) for pure and doped PETN

single crystal for the temperature range of this study.

Enthalpy of sublimation is calculated using the Clausius - Clapeyron equation

which relates vapor pressure and temperature with the enthalpy associated with mass

loss. The Clausius - Clapeyron equation is written as following

ln � = �� − ∆�� (3.14)

where P is the vapor pressure, ∆S is the enthalpy of sublimation and slope from the

plot of A�� vs 1/3 gives enthalpy of sublimation.

3.3 Experimental

PETN powder was collected from Lawrence Livermore National Laboratory.

PETN crystals were grown in acetone solution by solvent evaporation. The solution of

PETN was allowed to evaporate slowly to make the supersaturated solution from

which single crystal were grown. Optical images were collected by Olympus SZH

stereomicroscope and were analyzed to calculate geometric surface area using Image J

software [30]. A thermogravimetric analyzer was supplied by Instrument Specialist

Incorporated (WI, USA). Mass loss of PETN powder and single crystal at various

temperature of the single crystal was collected using a TGA. Mass loss data was

collected in temperature of 100℃ to 135℃ in steps of 5℃ isothermally. Nitrogen gas

was used as the purged gas in the TGA and the flow rate was 15cc/min. Benzoic acid

and naphthalene were used as standard materials for calculating coefficient of

vaporization in TGA and they were obtained from Scholar Chemistry.


34

3.4 Discussion

3.4.1 Sublimation Kinetics of PETN

Kinetic mechanism of the mass loss of PETN single crystal and powder with

isothermal annealing was studied using the Arrhenius equation. Arrhenius plot of

PETN crystals are presented in Figure 3.1 (a). The evaporation rate for PETN crystals

and powder increased consistently with the increase of temperature. Linearity of the

curves explains the rate of mass loss is a zero order process for all samples. Zero

order kinetics also suggests that the only process taking place during the mass loss is

sublimation. Therefore, it can be assumed that no decomposition or recrystallization

taking place in these temperature ranges. The flow of nitrogen in the TGA and zero

order kinetics also ensured that recrystallization process did not take place during the

sublimation process. As a result this kinetic mechanism can be extrapolated to the

room temperature. The experimental technique used in this study cannot measure very

low mass loss of PETN at room temperature regions for. So, zero order kinetics of the

sublimation process can be used to predict the sublimation process at the room

temperature and any storage conditions.

Arrhenius parameter for both PETN single crystal and powder are presented in

figure 3.1(a). The calculated activation energy calculated from the plot is found to be

34.5 and 35.7 kcal/mol for the PETN crystal and powder respectively. The activation

energy for both PETN crystal and powder are consistent with the literature values.

From the work carried out in our group showed that activation energy lies between 32-

37 kcal/mol. The activation energy from the single crystal should be considered as the

actual value of PETN. The frequency factor A for crystal and powder were calculated


35

to be 1.5x10-13

and 4.5X10-14

kg/m2.sec respectively. The determined frequency factor

of PETN powder was one order of magnitude less than that of crystal. This suggested

a lower vapor pressure for PETN powder compared to a PETN crystal. A detailed

discussion about this will be presented in later section of this chapter.

Figure 3.1(b) shows the qualitative transformation of various faces of PETN

crystal with annealing at 105 oC. Figure 3.1 (b) shows that faces on the edge caps,

Figure 3.1(A): Determination of kinetic parameter of PETN single crystal and powder

from Arrhenius plot.

Figure 3.1(B): Face kinetics of PETN crystal at 105 oC. (i) Initial image (ii) after 20

hours (iii) after 30 hours (iv) 40 hours.

-18

-17

-16

-15

-14

-13

-12

0.00244 0.00249 0.00254 0.00259 0.00264

ln K

1/T

PETN crystal

PETN powder

i ii iii iv


36

(101 faces), disappears faster as compared to the flat faces, (110) faces. The PETN

crystal gradually shrinks from the edges. A simulation studies on surfaces energy of

the PETN crystal showed that surface energy of (101) faces (0.27 kcal/mol.A) are

higher than that of (110) surface (0.21 kcal/mol.A). A higher surface energy of the

(101) faces makes them thermodynamically unstable compared to the (110) faces that

influences (101) faces to form a more thermodynamically favorable smoother surface.

Figure 3.1(B) also showed qualitatively that kinetics of sublimation of PETN is

dominated by (101) faces until their disappearance. To understand more about the

kinetics of sublimation of PETN, the effects of annealing on dislocations of (101)

faces have to be investigated.

3.4.2 Calculation of the Vaporization Coefficient

The vaporization coefficient in the form of ‘V’ was calculated using equation

(3.4). The vapor pressure (P) and N (N = ,-,. L�G) for the standard materials (benzoic

acid and naphthalene) were fitted in equation (3.7) to get figure 3.2 and ‘V’ value is

the slope of the straight line of the plot. The vapor pressure of benzoic acid was

calculated using the Antoine equation. The coefficient of Antoine equations for

benzoic and vapor pressure data of naphthalene were obtained from Literature [31,

32]. Vapor pressure for the standard materials presented here was in the sublimation

range of PETN. As figure 3.3 shows, data points of benzoic acid and naphthalene

followed the same trend. ‘V’ value from benzoic acid is was used to calculate the

vapor pressure of the material of interest in this study and an average ‘V’ value was

calculated as 175641 Pa kg0.5

mol0.5

sec-1

m-2

. Naphthalene was used to check the


37

consistency of the ‘V’ values. The results showed that ‘V’ was similar for both

materials.

Figure 3.2: Determination of vaporization coefficient using the literature value of

benzoic acid and naphthalene.

0

100

200

300

400

500

600

700

800

900

0.0E+00 1.0E-03 2.0E-03 3.0E-03 4.0E-03 5.0E-03

P (

Pa)

φ (kg K0.5mol 0.5m-2s-1)

Benzoic Acid

Naphthalene


38

3.4.3 Vapor Pressure PETN single crystals

Figure 3.3 shows a comparison of vapor pressure among PETN crystal and the

theoretical values obtained from this study and theoretical value proposed by the

Burnham et al [33] at the same temperature range of 100 o

C to 135 o

C. The coefficient

of the equation proposed by Burnham et al was obtained in the temperature range of

30oC to 90

oC. The theoretical values presented here were calculated using the

proposed equation. The results showed that the vapor pressure obtained from PETN

crystal is consistent with the predicted value. The vapor pressure of PETN crystal at

the temperature (~135oC) close to melting point (~142

oC) deviated from the predicted

value by almost 100%. However in this study, we did not investigate the surface

evolution kinetics. The likely reason for deviation from the predicted value is the rapid

disappearance of dislocations which smoothen surface faster during annealing. Vapor

pressure <120 oC is consistent in the presented temperature range. These vapor

pressure data can be extrapolated in the lower temperatures.

A single crystal exhibits higher kink sites per unit surface area compared to

powders, although the powder have higher surface area per unit mass. It was shown

both experimentally and theoretically that the desorption process starts from the edges

of the growth layers of a crystal. High kink densities in a single crystal might be the

reason for showing the consistent result. For PETN powder, the vapor pressure is

depends on the density of voids. Voids in PETN powder is not uniform because of the

recrystallization and diffusion of PETN molecules. Higher surface area per unit mass

for the powder does not guarantee the availability of higher area of the kink sites

because powder does not have the well-developed growth layers. The probability of


39

exposing the kink edges in thermal treatment is higher from the surface of a single

crystal without having the effect of the density of voids. The morphology of the

surface of the crystal influences the desorption of molecule since kink density is

influenced by surface morphology.

Figure 3.3: Comparison Vapor Pressure of PETN single crystal with the theoretical

values in the temperaure range of 100oC-135

oC.

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

18.00

20.00

100 105 110 115 120 125 130 135 140

P(P

a)

T (oC)

Crystal

Model


40

The vapor pressure data obtained from this experiment was fitted with the

modified Antoine equation as written in equation (3.13). Figure 3.4 showed the plot

(log�� U� ��)for modified Antoine equation. Parameters (A and B) obtained from

Figure 4 using the equation 3.9 is presented in Table 3.1. These parameters are

obtained only in the temperature range of 100oC to 135

oC. As PETN is a very low

vapor pressure compound, these parameters should reasonably predict correct value in

the temperature below 100oC to the room temperature.

Table 3.1: Parameters of the Equation log�� (��) = � − �(�) for PETN crystal

obtained in the temperature of 100oC -135

oC

Parameters PETN crystal

A 19.1

B 7384


equation log�� (��) = � − �(�) .

-1

-0.5

0

0.5

1

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log

P

1/T (K-1)


41

The Clausius-Clapeyron equation was used to obtain the value of heat of

sublimation. Figure 3.5 showed the Clausius-Clapeyron plot of the PETN crystals. The

vapor pressure of PETN crystal was fitted in equation (3.10) to get the Clausius

Clapeyron plot. The heat of sublimation for PETN crystal was obtained as 33.8

kcal/mol. This obtained heat of sublimation for single crystal is consistent with the

value of PETN powder. Moreover, this is reasonable considering that heat of

sublimation is the amount of energy that has to be transferred through heat or work on

the solid surface to break the bonds for releasing the molecule of interest from the

solid surface. The values of the heat of sublimation for both crystal and powder

explain that it requires similar amount of energy to desorb the molecule from the solid.

However, the vapor pressure of single crystal and powder are different due to the

morphology of the exposed surface.

Figure 3.5: Clausius-Claperon plot for obtaining the heat of sublimation from the

equation ln � = �� − ∆�� .

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

ln P

1/T

PETN crystal


42

3.5 Conclusion

The vapor Pressure of explosives is an important parameter for understanding

the environmental effect and aging process of explosive as well as developing new

device for the detection of explosive for security reasons. In this study, the vapor

pressure of pentaerythritol tetranitrate (PETN), a benchmark secondary explosive, is

presented. Vapor pressure data was obtained from the PETN single crystals. The

vapor pressure data from the single crystals are not available in literature as all the

data there, was obtained from PETN powder. To the best of our best of knowledge,

this is the first instance where vapor pressure data of PETN was obtained from the

single crystals. Experimental values of vapor pressure from single crystal and powder

were compared with the theoretical value of PETN. It showed that vapor pressure

from single crystal is closer to the predicted value. The enthalpy of sublimation and

coefficients of a modified Antoine equation for both PETN single crystals were

presented. These data are required to obtain the sublimation properties at lower

temperatures.


43

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of 2,4,6-trinitrotoluene and its metabolites in natural and model soil systems, Environ.

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S Thiboutot, G Ampleman, C Dubois, J Hawari, Detection of explosives and their

degradation products in soil environments, J. Chromatogr. A, 2002,963,411.

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[4] RR Kunz, KE Gregory, MJ Aernecke, ML Clark, A Ostrinskaya, AW Fountain,

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[5] A Delle Site, The vapor pressure of environmentally significant organic

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[6] DS Moore, Instrumentation for trace detection of high explosives, Rev. Sci.

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[7] DS Moore, Recent advances in trace explosives detection instrumentation, Sens.

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[8] GAS John, JH McReynolds, WG Blucher, AC Scott, M Anbar, Determination of

the concentration of explosives in air by isotope dilution analysis, Forensic Sci., 1975,

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[9] D Menning, H ostmark, Detection of liquid and homemade explosives: What do

we need to know about their Properties?, in: Detection of liquid explosives and

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[10] ME Staymates, WJ Smith, E Windsor, Thermal desorption and vapor transport

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in PETN, Propellants Explos. Pyrotech. 2009, 34, 489.

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Thermodynamic analysis of pure and impurity doped pentaerythritol tetranitrate

crystals grown at room temperature, J. Therm. Anal. Calorim., 2007, 89, 475.

[13] H Stmark, S Wallin, HG Ang, Vapor pressure of explosives: A critical review

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[14] RB Cundall, TF Palmer, CEC Wood, Vapor pressure measurement on some

organic high explosives, J. Chem. Soc., Faraday Trans. I, 1978, 74, 1339.

[15] WL Ng, JE Field, HM Hauser, Study of the thermal decomposition of

pentaerythritol tetranitrate, J. Chem. Soc. Perkin Trans.2 , 1976, 6, 637.

[16] BC Dionne, DP Rounbehler, EK Achter, JR Hobbs, DH Fine, Vapor pressure of

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[17] KH Lau, DL Hildenbrand, S Crouch-Baker, A Sanjurjo, Sublimation pressure and

vapor molecular weight of pentaerythritol tetranitrate, J. Chem. Eng. Data, 2004, 49,

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[18] AP Gershanik, Y Zeiri, Sublimation Rate of Energetic Materials in Air: RDX and

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[19] G Edwards, The vapor pressure of cyclotrimethylene-trinitramine (cyclonite) and

pentaerythritol-tetranitrate, Trans. Faraday Soc. 1953, 49, 152.

[20] A Maiti, LA Zepeda-Ruiz , RH Gee, AK Burnham, Vapor pressure and

sublimation rate of molecular crystals: Role of internal degrees of freedom, J. Phys.

Chem. B, 2007, 111 (51), 14290.

[21] S Glasstone, KJ Laidler, H Eyring, The theory of rate processes, MacGraw-Hill:

New York, 1st ed., 1941.

[22] H Félix-Rivera, ML Ramírez-Cedeňo, RA Sánchez-Cuprill, SP Hernández-

Rivera, Triacetone triperoxide thermogravimetric study of vapor pressure and enthalpy

of sublimation in 303–338K Temperature Range, Thermochim. Acta, 2011, 514,37.

[23] D Menon, D Dollimore, KS Alexander, A TG–DTA study of the sublimation of

nicotinic acid, Thermochim. Acta, 2002, 392–393,237.

[24] K Chatterjee, D Dollimore, K Alexander, A new application for the antoine

equation in formulation development, Int. J. Pharm. 2001, 213(1-2),31.

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compounds used in aromatherapy using thermogravimetry, Thermochimica Acta,

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[26] DM Price, Volatilization, evaporation & vapor pressure studies using a

thermobalance, Proceedings of the Twenty-Eigth Conference of the North American

Thermal Analysis Society, October 4-6, 2000, Orlando, Florida, 2000.


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[27] K Chatterjee, D Dollimore, KS Alexander, Calculation of vapor pressure curves

for hydroxy benzoic acid derivatives using thermogravimetry, Thermochim. Acta,

2002, 392-393,107.

[28] I Langmuir, The vapor pressure of metallic tungsten, Phys. Rev., 1913, 2, 329.

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[32] K. Ruzicka, M. Fulem, V. Ruzicka, Recommended Vapor Pressure of Solid

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47

Chapter 4

Sublimation Properties of Pentaerythritol Tetranitrate Single

Crystals Doped with Its Homologs

4.1 Introduction

Pentaerythritol tetranitrate (PETN) is one of the most powerful explosives used

in military application and mining. PETN is normally used as a powder with a very

high surface area and has a high heat of detonation of 1.5kcal/gm compared to other

common explosive. Detonation of PETN depends on the particle size distribution of

the PETN powder, porosity, surface morphology, surface area etc. Changes of the

morphology, surface area and particle size distribution takes place by coarsening

mechanisms such as sublimation and recrystallization during long-term storage [1-7].

Thus the aging process of PETN is controlled by the coarsening that eventually

increases the particle size, and thereby decrease the surface area. Performance of

PETN is affected negatively with the deterioration of surface area.

The properties of the crystal are affected by the crystal growth kinetics,

dislocation and defects in the crystal, temperature, and presence of foreign particles.

The tailored properties of the crystal depend on how these parameters are controlled

during the growth process of the crystal [8]. Doping with impurities is widely used to

modify the property of crystalline products [8-12]. These reports summarize that the

effect of impurities depend on the chemical nature of both impurity and the crystal

face. Sangwal surveyed the adsorption and effect of the impurities on the growth

process of bulk crystals extensively [13-15]. Mathematical models associated with the


48

effect of impurities were also reported in those surveys. At a lower concentration,

impurities modify growth structures like spiral and hillocks by adsorbing at the kink

site, and at higher concentration, impurities are randomly distributed on the surface

and can be the growth center for initiating the growth layers. Pitchimani et al

investigated morphology changes of PETN crystal doped with ions of zinc, calcium,

iron and sodium [8]. They also showed, qualitatively, that doping PETN with zinc ion

can increase the thermal stability at macro and nano scale. Mridha et al conducted the

study on thermal stability of the nanoislands of PETN doped zinc ion [16]. They

reported that presence of zinc ion changed the thermodynamic and kinetic properties

in PETN nanoislands. Simulation study showed that saturating kink site of PETN

growth steps with bigger molecules like PETN homologs can reduce the sublimation

rate [6]. PETN homologs like dipentaerythritol hexanitrate (diPEHN) and

tripentaerytritol octanitrate (triPEON) are produced during synthesis of PETN [17].

DiPEHN and triPEON are the dimer and trimer of PETN and are thermally more

stable compared to PETN [18]. Controlled addition of diPEHN and triPEON in PETN

single crystal should change sublimation properties.

In this study, sublimation properties of PETN single crystals doped with

diPEHN and triPEON were discussed. Single crystals of PETN and doped crystals

were grown by solvent evaporation. Thermogravimetry is used to evaluate

thermodynamic and kinetic parameters. The effect of doping on the kinetics of the

sublimation process was discussed. The vapor pressure data of the pure and the

homolog doped PETN single crystal was also reported. In this chapter, discussion will

be limited to the effect on the sublimation rate and the vapor pressure of the homolog-


49

doped crystal. The mechanism by which impurities is influencing the sublimation of

PETN will be discussed in the next chapter.

4.2 Experimental

Single crystals of PETN, and doped analogs, were grown in acetone by solvent

evaporation. PETN, diPEHN and triPEON were provided from the Lawrence

Livermore National Laboratory. Single crystals were grown from the supersaturated

solution which formed by the controlled slow evaporation from the scintillation vials

capped with parafilm. The doping concentration of diPEHN and triPEON were 1000

ppm, 5000 ppm and 10000 ppm with respect to the weight of PETN. Surface area of

the single crystals was measured using the optical images which were collected using

an Olympus SZH stereomicroscope. Images of the crystals were analyzed to calculate

geometric surface area using Image J software [19]. Rate of sublimation of the single

crystal was collected by a thermogravimetric analyzer (TGA) supplied by Instrument

Specialist Inc WI, USA. Single crystals were heated from 100℃ to 135℃ in steps of

5℃ isothermally from 60 minutes to 260 minutes. Nitrogen gas was used as the

purged gas in the TGA, and the flow rate was 15cc/min. Benzoic acid (from scholar

chemistry, USA) was used as standard materials for calculating coefficient of

vaporization in TGA.

4.3 Results and Discussion

4.3.1 Kinetic Analysis

Sublimation of diPEHN and triPEON doped PETN was shown as a function of

the reciprocal of the absolute temperature in Figure 4.1 and 4.2 respectively. The

Arrhenius equation for zero order kinetics was used to study the kinetics of desorption


50

of PETN molecules from diPEHN and triPEON doped PETN crystals. Equation 3.5

shows the Arrhenius equation for zero order process. It is assumed that the presence of

diPEHN and triPEON does not initiate decomposition of PETN molecules. Both

Figure 4.1 and 4.2 showed that pure and homolog-doped PETN similar linear pattern

of mass loss. Linear plots in Figure 4.1 and 4.2 explains two important aspects of the

sublimation kinetics; (i) consistent increase of the rates of mass loss of doped PETN

crystal with the increase of temperature, (ii) zero order kinetics of the sublimation

process of doped crystal. The addition of a PETN homolog as an impurity in the

crystal did not change the kinetics of the sublimation process since both pure and

doped PETN crystal followed zero order kinetics of the desorption process. Figure 4.1

and 4.2 also showed that the rate of mass loss per unit surface area for diPEHN and

triPEON doped PETN were decreased compared to pure PETN crystal. For diPEHN

doped crystal, the desorption rate decrease with the increase of doping compound upto

5000 ppm. Further increase of the doping compound (10000 ppm) produces similar

desorption rate as 5000 ppm of doping of diPEHN. TriPEON doped PETN produce

similar effect on the desorption rate for all the doping level. This indicates that

incorporation doping compound is not consistent at different doping level and doping

compound. Detail discussion on the incorporation of impurity will be presented in the

later part of this chapter and in the next chapter. Pitchimani et al. showed that doping

with inorganic salt changed the evaporation rate as well as the Arrhenius parameters

[20]. Rogers et al. studied triPEON doped PETN crystal habits using differential

scanning calorimeter (DSC) and differential thermal analyzer (DTA) [21]. They

reported that the presence of triPEON in PETN crystal habit can decrease the


51

evaporation rate at a lower temperature (<120 oC). They also showed that increasing

the triPEON concentration (>mole fraction 0.02) reduced the heat of fusion

significantly.

Figure 4.1: Kinetics of sublimation of pure and diPEHN doped PETN shown in the

Arrhenius plot.

-18

-17

-16

-15

-14

-13

-12

0.00244 0.00249 0.00254 0.00259 0.00264

ln k

1/T

Pure PETN crystal1000 ppm diPEHN doped PETN crystal5000 ppm diPEHN doped PETN crystal10000 ppm diPEHN doped PETN crystal


52

Figure 4.2: Kinetics of sublimation of pure and diPEHN doped PETN shown in the

Arrhenius plot.

-18

-17

-16

-15

-14

-13

-12

0.00244 0.00249 0.00254 0.00259 0.00264

ln k

1/T

Pure PETN crystal

1000 ppm triPEON doped PETN crystal

5000 ppm triPEON doped PETN crystal

10000 PPM triPEON doped PETN crystal


53

4.3.2 Effect of homolog doping on the vapor pressure of PETN crystal

Vapor pressure of homolog-doped PETN was obtained from the rate of

sublimation. The Langmuir equation (,-,. = � L GKI��) was used to calculate vapor

pressure. A detailed discussion on the Langmuir equation was presented in previous

chapter. It is shown in the previous chapter that the Langmuir equation can be written

as � = MN, where V is the vaporization coefficient. The vaporization coefficient was

calculated using the standard material, and benzoic acid. Vapor pressure of benzoic

acid was collected from literature [22]. Figure 4.3 showed the calibration curve for

benzoic Acid and slope of the curve is vaporization coefficient. Average value of the

vaporization coefficient was obtained as 175641 Pa kg0.5

mol0.5

sec-1

m-2

.

Figure 4.3: Calibration curve for obtaining the vaporization coefficient using benzoic

acid

0

100

200

300

400

500

600

700

0.0E+00 1.0E-03 2.0E-03 3.0E-03 4.0E-03

P (

Pa)

φ (kg K0.5mol 0.5m-2s-1)


54

As explained in the experimental section, the doping level was controlled in

the solution where PETN crystals were grown. It was not claimed that the

concentration of doping compound incorporated with the PETN crystal was the same

as in the solution. It is not within the scope of this particular study to investigate how

much of the doping compound can be incorporated with PETN crystal when it is

grown from solution. Vapor pressure data of pure, diPEHN doped and triPEON doped

PETN was presented in Table 4.1. Vapor pressure of 1000 ppm diPEHN doped PETN

crystal was similar to that of pure PETN crystal at temperatures <130oC. With an

increase in diPEHN concentration to 10000 ppm, vapor pressure of the diPEHN doped

PETN crystals was decreased. With an increase of the temperature to the melting point

(142oC), the difference of vapor pressure of pure and diPEHN doped PETN crystal

also increases.

Vapor pressure of the triPEON doped PETN showed that vapor pressure of

PETN crystal grown in 1000 ppm triPEON doped PETN solution was reduced

compared to the pure PETN. With the increase of triPEON doping vapor pressure of

triPEON doped PETN crystal did not significantly change compared to the 1000 ppm

triPEON doping. Zhang et al [24] studied zinc doped PETN crystals by Atomic Force

Microscopy (AFM). They found from their study that zinc ions were incorporated in

the growth steps of crystal. The investigations from Pitchimani et al. [20] and Zhang

et al. [23] showed that the incorporation of zinc ions in PETN was the reason for the

change of the thermodynamic and kinetic properties of the zinc doped PETN.

DiPEHN and triPEON might be incorporated in PETN in the same way as zinc.

Detailed AFM and X-ray diffraction studies are required to support this claim.


55

Venebles et al. [24] explained the relation between the lattice vibration and

vapor pressure of the solid by proposing a mathematical model which is written as the

following

� = (2WH)XY4Z(/3)[\Y ?8]:^_ (4.1)

where 4 is the lattice vibration of the solid and k is Boltzmann’s constant. The

equation shows that the lattice vibration strongly influences the vapor pressure of a

solid at all temperatures. The possibility exists that impurities decrease the 4 value.

This is consistent with the theory that the sublimation process starts with shrinkage of

the kink site [25]. It is likely that the incorporation of impurities in the kink site

reduced the lattice vibration of PETN during the desorption process and thereby

slowed the shrinkage of the kink site. A detailed discussion on the impurity

incorporation in the PETN crystal will be presented in the next chapter.


56

Table 4.1: Vapor pressure of Pure, diPEHN doped and triPEON doped PETN single crystal.

Tempe

-rature

(oC)

Vapor Pressure

Pure PETN

crystal (Pa)

1000 ppm

diPEHN

doped

PETN

crystal

(Pa)

5000 ppm

diPEHN

doped

PETN

crystal

(Pa)

10000 ppm

diPEHN

doped

PETN

crystal

(Pa)

1000 ppm

triPEON

doped

PETN

crystal

(Pa)

5000 ppm

triPEON

doped

PETN

crystal

(Pa)

10000

ppm

triPEON

doped

PETN

crystal

(Pa)

100 0.25±0.03 0.20±0.06 0.15±0.03 0.18±0.04 0.19±0.09 0.18±0.06 0.20±0.01

105 0.35±0.08 0.31±0.15 0.31±0.07 0.27±0.13 0.29±0.04 0.31±0.08 0.37±0.21

110 0.65±0.10 0.59±0.18 0.62±0.07 0.50±0.22 0.51±0.06 0.53±0.59 0.61±0.08

115 1.27±0.22 1.19±0.28 1.05±0.12 1.04±0.44 0.94±0.14 0.94±0.02 1.02±0.31

120 2.29±0.20 2.26±0.58 1.75±0.36 1.50±0.20 1.69±0.27 1.66±0.35 1.96±0.54

125 3.86±0.40 3.74±0.93 3.17±0.17 2.60±0.44 2.91±0.52 2.79±0.65 2.94±0.57

130 6.53±0.40 6.51±1.49 5.42±0.47 4.42±0.47 4.89±0.67 4.64±0.57 5.37±0.70

135 10.74±0.68 9.68±1.25 8.83±1.17 7.64±0.67 8.37±1.37 8.75±1.38 9.04±1.39


57

Vapor pressure data obtained from this experiment was fitted with the

modified Antoine equation. Following equation is the modified Antoine equation

log � = � − � (4.2)

Figures 4.4 and 4.5 showed the plots for the modified Antoine equation for diPEHN

and triPEON doped PETN respectively. Parameters (A and B) were obtained from

Figure 4.4 and 4.5 were presented in Table 4.2. These parameters were obtained only

in the temperature range of 100oC to 135

oC. However vapor pressure of homolog

doped PETN in the temperature below 100oC to the room temperature can be

calculated using the value presented in Table 4.2 since PETN has a very low vapor

pressure.

Table 4.2: Parameters of the Equation log �(��) = � − �(�) for pure and doped PETN

crystal obtained in the temperature of 100oC-135

oC.

Parameters 1000

ppm

diPEHN

doped

PETN

crystal

5000

ppm

diPEHN

doped

PETN

crystal

10000

ppm

diPEHN

doped

PETN

crystal

1000

ppm

triPEON

doped

PETN

crystal

5000

ppm

triPEON

doped

PETN

crystal

10000

ppm

triPEON

doped

PETN

crystal

A 19.82 19.60 18.45 18.82 18.99 18.40

B 7670 7606 7171 7310.4 7376.5 7128


58


equation log�� (��) = � − �(�) ; (a)1000 ppm diPEHN doped, (b) 5000 ppm

diPEHN and (c) 10000 ppm diPEHN doped PETN crystal.

y = -7670.7x + 19.8

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log P

1/T (K-1)

(a)

y = -7606.2x + 19.6

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log P

1/T (K-1)

(b)

y = -7171.0x + 18.4

-1

-0.5

0

0.5

1

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log P

1/T (K-1)

(c)


59


equation log�� (��) = � − �(�) ; (a)1000 ppm triPEON doped, (b) 5000 ppm

triPEON doped and (c) 10000 ppm triPEON doped PETN crystal.

y = -7310.4x + 18.8

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log P

1/T (K-1)

(a)

y = -7279.5x + 18.7

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log P

1/T (K-1)

(b)

y = -7128x + 18.4

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0.00243 0.00248 0.00253 0.00258 0.00263 0.00268

log P

1/T (K-1)

(c)


60

4.4 Conclusion

Sublimation properties of the pure PETN single crystals and PETN homologs

were investigated using TGA. The results showed that the rate of sublimation was

decreased due to doping. Kinetics of sublimation for homolog-doped PETN crystal

was found to be zero order which was the same as the pure one. The incorporation of

impurities did not affect the kinetics of the doped PETN. The vapor pressure data were

obtained for pure and doped PETN crystals in the temperature of 100oC to 135

oC in

steps of 5 o

C. The vapor pressure of doped PETN single crystals was found to be

reduced in general. Although the actual amount of doping incorporation at a specific

doping level was not measured, change in the vapor pressure and rate of sublimation

indicate the presence of a doping compound in the PETN crystals. Further analyses are

required to quantify how much doping compound is incorporated in the crystal

structure, and to indentify the location of doping compounds in the crystal structure.

However, this investigation revealed that controlled doping with impurities like

diPEHN and triPEON, that have a similar chemical structure with PETN, can reduce

the sublimation rate, thereby slowdown the coarsening of PETN.


61

References

[1] GW Browen, MM Sandstrom, AM Giambra, JG Archuleta, DC Monroe, Thermal

analysis of pentaerythritol tetranitrate and development of a powder aging model, 37th

North American Thermal Society Lubbock, Texas, USA, 20-23 September, 2009.

[2] C Siviour, M Gifford, S Walley, W Proud, J Field, Particle size effects on the

mechanical properties of a polymer bonded explosive, J. Mater. Sci., 2004, 39, 1255.

[3] LA Zepeda-Ruiz, GH Gilmer, A Maiti, RH Gee, AK Burnham, Evaporation from

the (110) surface of PETN J. Cryst. Growth, 2008,310, 3812.

[4] A Maiti, RH Gee, Modeling growth, surface kinetics, and morphology evolution in

PETN, Propellants Explos. Pyrotech., 2009, 34, 489.

[5] A Maiti, RH Gee, PETN coarsening – prediction from accelerated aging data,


[6] A Maiti, LA Zepeda-Ruiz, RH Gee, AK Burnham, Vapor pressure and sublimation

rate of molecular crystals: Role of internal degrees of freedom, J. Phys. Chem. B,

2007, 111(51), 14290.

[7] R Pitchimani, AK Burnham, BL Weeks, Quantitative thermodynamic analysis of

sublimation rates using an atomic force microscope, J. Phys. Chem. B, 2007, 111,

9182.

[8] R Pitchimani, LJ Hope-Weeks, G Zhang, BL Weeks, Effect of impurity doping on

the morphology of pentaerythritol tetranitrate crystals, J. Energ. Mater., 2007, 25,

203.


62

[9] TG Diggies Jr., R Shima, The effect of growth rate diameter and impurity

concentration on structure in czochralski silicon crystal growth, J. Cryst Growth,

1980, 50, 865.

[10] N Kubota, JW Mullins, A kinetic model for crystal growth from aqueous solution

in the presence of impurity, J. Cryst. Growth, 1995, 152, 203.

[11] VA Kuznetsov, TM Okhrimenko, M Rak, Growth promoting effect of organic

impurities on growth kinetics of KAP and KDP crystals, J. Cryst. Growth, 1998, 193,

164.

[12] N Kubota, M Yokota, JW Mullin, The combined influence of supersaturation and

impurity concentration on crystal growth, J. Cryst Growth, 2000, 212, 480.

[13] K Sangwal, Effects of impurities on crystal growth process, Prog. Cryst. Growth

Ch., 1996, 32, 3.

[14] K Sangwal, On the mechanism of crystal growth from solution, J. Cryst. Growth,

1998,192, 200.

[15] K Sangwal, Kinetic effect of impurities on the growth of single crystals from

solutions, J. Cryst. Growth, 1999, 203, 197.

[16] S Mridha, BL Weeks, Effect of Zn doping on the sublimation rate of

pentaerytritol tetranitrate using atomic force microscopy, Scanning , 2009, 31, 181.

[17] J Sandoval, W Quinlin, PETN homologs, Report MHSMP-72-26, 1972, US

Atomic Energy Commission, Albuquerque Operation Office, Albuquerque, NM,USA.

[18] G OM Reddy, A Srinivasa Rao, Stability studies on pentaerytritol tetranitrate,

Propellants Explos. Pyrotech. 1992, 17, 307.


63

[19] TJ Collins, ImageJ for microscopy, Biotechniques, 2007, 43(1 Suppl), 25.




[21] RN Rogers, RH Dinegar, Thermal analysis of some crystal habit of pentaerytritol

tetranitrate, Thermochim. Acta.,1972, 3, 367.

[22] http://webbook.nist.gov/cgi/cbook.cgi?ID=C65850&Mask=4#Thermo-Phase.

[23] G Zhang, SK Bhattacharia, BL Weeks, Effect of zinc doping on pentaerythritol

tetranitrate single crystals, Cryst. Res. Technol., 2010, 45(7), 732.

[24] JA Venables, Atomic processes in crystal growth, Surf. Sci.,1994,798, 299.

[25] AK. Burnham, SR Qiu, R Pitchimani, BL Weeks, Comparison of kinetic and



mechanisms, J. Appl. Physic., 2009, 105,104312.


64

Chapter 5

Molecular Impurities in PETN Single Crystal for Controlling Sublimation: TGA

and AFM Study

5.1 Introduction

Organic molecular crystals are a topic of great interest in academia,

government laboratories, and various industries for a multitude of applications ranging

from pharmaceutical and electrical industries to energetic materials. While the

presence of a small amount of unwanted impurities in the crystal growth process is not

unusual, in many cases impurities are added intentionally with the aim of attaining a

material with tailor-made properties. The presence of a small amount (ppm level) of

impurities in the growth medium can affect crystal growth kinetics due to the

incorporation of impurities on the growth surface. This can significantly impact crystal

properties, as has been investigated in detail for various types of organic and inorganic

crystals [1-7]. Atomic force microscopy (AFM) imaging has revealed morphological

changes in the growth center in the presence of impurities [8-10]. Impurities have also

been shown to alter the thermodynamic properties of singles crystals [11].

Interaction between the crystal matrix and impurities depends on the chemical

nature of the crystal compound as well as the impurity in question. Impurities can be

classified into one of several categories, e.g., anions, metallic cations, polyelectrolytes,

surfactants, tailor-made additives, and so on [12]. In addition, the impurity distribution

map within the doped crystal could depend on the impurity concentration. Thus, at low

concentrations impurities might be adsorbed primarily at the kink sites or at the step


65

edges of crystallite surfaces, while at high concentrations they could be randomly

distributed in the bulk matrix [13-14].

Pentaerythritol tetranitrate (PETN) is a nitrate ester, a very powerful energetic

material, and is used as a secondary explosive in military applications and in mining.

The performance and properties of PETN can change upon long-term aging due to

coarsening that is marked by a gradual decrease in its specific surface area [15, 16].

Previous works have demonstrated that doping with appropriate compounds can

control such aging process of PETN [11, 17-18]. For instance, it was shown that

doping with homologs like dipentaerythritol hexanitrate (diPEHN) and

tripentaerythritol octanitrate (triPEON) can lower the vapor pressure of PETN single

crystals [19]. However, the associated effects on the crystal morphology and mass-loss

properties were not addressed.

In this project, we analyzed the mass-loss (i.e. sublimation) rates from a series

of single crystals of PETN, both pure and the ones grown from solutions in which a

controlled amount of diPEHN and triPEON were added. In order to shed light on the

distribution of impurities on the crystal surface and their effect on the surface

morphology/ roughness we also measured mass-loss rates after washing the crystals

with ethanol, with the idea that a comparison between pre-washed and post-washed

crystal should provide relevant insights. Finally, we imaged several doped and

undoped crystal surfaces with AFM in order to characterize differences in surface

features brought about the homolog additives.


66

5.2 Experimental

Pure and doped PETN crystals were grown by solvent evaporation. To do this,

PETN powder was dissolved in acetone in the scintillating vials. The solvent was

allowed to slowly evaporate away forming a supersaturated solution from which

crystals grew in the bottom of the vial. PETN, diPEHN and triPEON were provided by

Lawrence Livermore National laboratory. Pure and doped crystals were used for

collecting the rate of sublimation data in a thermogravimetric analyzer (TGA) from

105 oC to 125

oC in steps of 5

oC in the presence of nitrogen as the purge gas (flow

rate 15 cc/min). A mass loss at each temperature was measured as a function of time in

the TGA (Model- i1000 supplied by Instrument Specialist Inc). The optical images of

the crystals were collected before running the samples in the TGA in order to measure

the surface area of the crystals. “Image J” software was used to analyze the optical

images. The rate of sublimation was normalized by the surface area of the crystal.

Morphology of the crystal surface was studied using a Nanoscope IIIa multimode

scanning probe microscope (Veeco Instrument Inc., Santa Barbara, Ca) operating in

contact mode. WSxM software was used to process the images from the scanning

probe microscope.

5.3 Results and discussion

Figure 5.1 plots the ratio of the sublimation (i.e., mass-loss) rates of doped and

pure crystal within the temperature range of 105-125 oC. For diPEHN doping (Figure

5.1(A)), it appears that 1000 ppm doping leads to a ~ 20% decrease in the mass-loss

rate as compared to pure PETN, while both 5000 ppm and 10000 ppm lead to a ~ 30-

35% decrease. As for triPEON doping (Figure 5.1(B)) the reduction of mass-loss rate


67

appears to be higher at the 1000 ppm doping level as compared to 5000 ppm or 10000

ppm doping levels. Thus at 1000 ppm triPEON doping there is ~ 30-35% reduction in

mass-loss rate, while at 5000 and 10000 ppm doping levels, the reduction is ~ 30% or

slightly lower. These results suggest that the doped crystal can only accommodate a

maximum amount of homolog impurities, i.e., ~ 5000 ppm of diPEHN and between

1000 ppm and 5000 ppm of triPEON. Such a picture is also consistent with a limited

number of studies in which we added 10000 ppm of diPEHN + 10000 ppm of

triPEON. Crystals grown from such solution exhibited mass-loss rates of only ~ 20-

30% lower as compared to pure PETN.

Figure 5.1 also shows that the reduction of mass-loss rate does not have any

systematic dependence on temperature, irrespective of the homolog or the doping

level. This raises the following possibilities: (1) the mass-loss reduction values at these

elevated temperatures could also be extrapolated to room temperatures; and (2) the

mechanism of mass-loss reduction is likely not through an increase in the effective

heat of molecular desorption (sublimation), because if it were, the level of reduction

would be strongly decreasing with increasing temperature.


68

Figure 5.1 Mass-loss rates of doped PETN relative to undoped PETN at various

temperatures between 105-125 oC: (A) diPEHN-doped, and (B) triPEON-doped.

triPEON-doped

diPEHN-

A

B


69

In order to directly test point (2) above, Figure 4.1 (from previous chapter)

plots the observed mass-loss rates (y-axis log-scale) from doped PETN crystals (as

well as pure PETN) as a function of 1/T. The negative slope of such a plot is equal to

the molecular activation energy of sublimation. From the plots in Figure 4.1 it appears

that the activation energy of sublimation remains essentially unaffected by either the

dopant (i.e., diPEHN or triPEON) or the doping level, i.e., 1000, 5000, or 10000 ppm.

Activation energy of pure and doped crystal is presented in Table 5.1 and the value

remains ~ 35 kcal/mol, a well-accepted value for pure PETN. Thus, it would appear

that the homolog-induced changes in mass-loss rates happen through the impedance to

step motion due to the presence of surface impurities.

Table 5.1: Activation Energy of the pure and doped PETN single crystal

Sample Activation Energy

E (kcal.mole-1

)

Pure PETN crystal 34.6

1000ppm diPEHN doped

PETN 34.0


PETN 34.3


PETN 32.1

1000ppm triPEON doped

PETN 34.5


PETN 33.0


PETN 32.9


70

To gain further insight, we gently washed the doped crystals with ethanol and

compared the mass-loss rates from the washed crystals with those of the unwashed

crystals. Figure 5.2 and 5.3 displays: (a) the ratio of mass-loss rates from the washed

crystals to that of pure PETN (left top and left bottom); and (b) the ratio of mass-loss

rates between unwashed and the washed crystals (right top and right bottom). Some

results of note from Figure 5.2 and 5.3 are:

(1) as compared to pure PETN the washed crystals have distinctly higher mass-loss

rates. For diPEHN-doping levels of 5000 ppm the increase in washed crystals is as

much as ~ 80%, while 10000 ppm doping produces ~ 40-60% increase. Doping with

1000 and 5000 ppm triPEON followed by washing produces 40-60% increase in mass-

loss rate relative to pure PETN, while 10000 ppm produces much less increase;

(2) as compared to washed crystals, the unwashed crystals have much lower mass-loss

rates, as low as 30-40% for 5000 ppm diPEHN and 40-50% for 1000 and 5000 ppm

triPEON;

(3) the mass-loss ratio in unwashed vs. washed crystals is close to ~ 1 for undoped (i.e.

pure) PETN crystals. This supports the notion that gentle washing with ethanol

produces topological surface features in the washed crystals similar to those existing

in the pre-washed crystals.


71


diPEHN-doped crystals relative to unwashed, undoped PETN crystals; (B) mass-loss

rates of washed, diPEHN-doped crystals relative to unwashed crystals of the same

doping.

A

B


72


triPEON-doped crystals relative to unwashed, undoped PETN crystals; (B) mass-loss

rates of washed, triPEON-doped crystals relative to unwashed crystals of the same

doping.

A

B


73

The above results lead to the picture that up to a certain doping level (~ 5000

ppm for diPEHN and even lower for triPEON) a sizable amount of these homolog

impurities get incorporated on the crystal surface. The presence of these relatively

immobile impurities impede the motion of PETN steps, potentially leading to a

roughened surface topology characterized by irregular step geometries, bunched steps,

and so on. Prior to washing, the impurities at the surface lower the mass-loss by

impeding the motion of the steps, while after washing the rougher surface without any

impeding impurities lead to a higher mass-loss rate. At the same time, higher doping

levels (e.g., 10000 ppm diPEHN or triPEON) do not necessarily imply higher surface

concentration of impurities – the excess impurities could either get incorporated within

the PETN bulk or possibly segregate as homolog-rich domains outside the crystal. At

this time we can only speculate on this point, and more experiments need to be done to

be more definitive.

5.3.1 Optical microscopy of pure and doped PETN crystal

The optical images of pure, diPEHN doped and triPEON doped PETN crystals

are presented in Figure 5.4. It shows that both pure and doped crystals have tetragonal

structure. Controlled addition of impurities had no effect on crystal structure. Possible

reason behind unaffected crystal structure might be the similarities of their chemical

structure which may leads them to uniformly incorporate into the crystal matrix in the

crystallization process, although optical images cannot verify how impurities were

incorporated.


74

Figure 5.4 Optical images of (a) Pure PETN crystal, (b) 1000 ppm diPEHN doped

PETN crystal, (c) 5000 ppm diPEHN doped PETN crystal, (d) 10000 ppm diPEHN

doped crystal, (e) 1000 ppm triPEON doped crystal,(f) 5000 ppm triPEON doped

crystal, (g) 10000 ppm triPEON doped.

(a)

(b) (c)

(d) (e)

(f) (g)


75

5.3.2 AFM studies on the surface of pure and doped PETN crystal surface

To look for direct evidence of roughening in doped crystals we took AFM

images of the surfaces of both pure and doped PETN crystals. An ex situ AFM

investigation was conducted on the flat surface of pure and doped PETN crystal.

Figure 5.5(a),5.5(c) and 5.5(e) presented AFM images of the (110) face of the pure

PETN, 1000 ppm diPEHN doped, 5000 ppm triPEON doped crystals respectively.

Figure 5.5(b), 5.5(d) and 5.5(e) showed the height profile of the growth layers of pure,

diPEHN and triPEON doped PETN respectively. Height profile showed that doping of

diPEHN and triPEON increased the step height of growth layer. Average steps height

for PETN was found to be around 3nm, whereas average height for both diPEHN and

triPEON doped PETN is around 4.5nm. This is a significant increase of step height

because of the presence of doping compound. However it is not possible to explain the

mechanism of the increase of step height from this study. Adsorption of doping

compounds which are bigger molecule compared to PETN at the edge of the growth

layers might increase the height of the growth layer.

TriPEON doped PETN crystal exhibited a more random stacking of growth

layers compared to the pure one. The growth layers of triPEON doped crystal overlap

extensively. AFM images also showed that the edges of layers in both diPEHN and

triPEON doped PETN crystals are not as smooth as those in pure crystal and are more

disordered pattern. It is a prominent feature in the diPEHN doped crystal. Changes of

the edge morphology of the doped crystal may be attributed to the incorporation of the

homolog dopant at the edge/kink sites which is consistent with the previous work

presented in literature [20-21]. Sublimation of PETN is associated with shrinking of


76

the edges [22]. The presence of impurities on the edges might be slowing the

shrinkage of the edge, thereby slowing down the overall sublimation rate.

5.3.3 Comparing diPEHN and triPEON as dopant for PETN

As it was explained in the above discussion, both the diPEHN and triPEON

can reduce the evaporation rate of a pure PETN crystal. According to Figure 5.1(a)

and 5.1(b), both the diPEHN and triPEON can produce similar effects on the rate of

evaporation. 5000 ppm diPEHN doping produced the highest reduction of evaporation

rate of PETN crystal, whereas 1000 ppm triPEON doping appears to have the biggest

effect on the rate of evaporation at the highest temperatures. According Figure 4(b),

10000 ppm triPEON doped crystals upon washing appear to lead to crystals that

behave closer to pure PETN crystals in terms of evaporation rates. Perhaps, at 10000

ppm (> 2 wt% triPEON) much of the triPEON segregates as triPEON crystals or

triPEON-rich regions. It explains that a single PETN crystal can incorporate small

amount of triPEON (~5000 ppm triPEON). In order to explore the effect of both the

impurities present together, measurements were carried out on samples where 10000

ppm diPEHN and 10000 ppm of triPEON were added together to the solution. The

degree of evaporation rate lowering (as compared to pure PETN) was found to be ~

20-30%. This value is somewhat smaller than with either 10000 ppm diPEHN or

10000 ppm triPEON, indicating the fact that the total amount of impurities was well

above the maximum amount of impurities that the crystal could accommodate, and

perhaps there is some sort of “cancelling” effect of the two impurities.


77

Figure 5.5: AFM images of the (110) faces of: (a) pure PETN; (c) diPEHN-doped

PETN ; (e) triPEON-doped PETN crystal. Height profile of the growth layers of (b)

pure PETN; (d) diPEHN-doped PETN; (f) triPEON-doped PETN crystal.

(a) 4(b)

(c) 4(d)

(e) 4(f)


78

5.4 Conclusions:

In summary, we carried out a systematic study of the mass-loss rates from

single crystals of pure PETN, and those doped with various amounts of PETN-

homologs diPEHN and triPEON at elevated temperatures of 105-125 oC. Our

investigation shows that diPEHN and triPEON doping in PETN can reduce the mass

loss rate by as much as 35% as compared to pure PETN. Arrhenius plots of the TGA

mass-loss data show that homolog-doping at any concentration does not significantly

change the activation energy barrier of the desorption process, while AFM images

provide evidence to the change of roughness on the edge of the growth steps and the

stacking of growth layers due to the presence of impurity molecules. While this altered

morphology leads to lower mass-loss rates in the presence of the impurity, removal of

the impurity with gentle ethanol washing leads to mass-loss rates as much as 80%

higher as compared to pure PETN. From these results we conclude that homolog

doping of PETN involves a complex process, including the distribution of impurities

both in the bulk material and on the surface, as well as possible segregation of excess

impurities outside the PETN crystals. Since the rates of sublimation do not show any

clear trend of increase or decrease with temperature, one could possibly extrapolate

the above results to room temperature, from which one would conclude that either

diPEHN or triPEON doping should increase the stability of PETN powder against

coarsening under ambient conditions.


79

References

[1] TA Ereminaa, VA Kuznetsova, NN Ereminb, TM Okhrimenko, NG Furmanovaa,

EP Efremovaa, M Rak, On the mechanism of impurity influence on growth kinetics

and surface morphology of KDP crystals—II: experimental study of influence of

bivalent and trivalent impurity ions on growth kinetics and surface morphology of

KDP crystals, J. Cryst. Growth. 2005, 273,586.

[2] VA Kuznetsov, TM Okhrimenko, M Rak, Growth promoting effect of organic

impurities on growth kinetics of KAP and KDP crystals, J. Cryst. Growth. 1998,

193,164.

[3] MHJ Hottenhuis, CB Lucasius, The influence of impurities on crystal-growth in-

situ observation of the (010) face of potassium hydrogen phthalate, J. Cryst. Growth.

1986, 78(2), 379.

[4] RA Kumar, N Sivakumar, RE Vizhi, DR Babu, The effect of Fe(3+) doping in

Potassium Hydrogen Phthalate single crystals on structural and optical properties,

Physica. B. 2011, 406, 985–991.

[5] N Kubota, M Yokota, JW Mullin, The combined influence of supersaturation and

impurity concentration on crystal growth, J. Cryst. Growth., 2000, 212, 480.

[6] LP Dang, HY Wei, Effects of ionic impurities on the crystal morphology of

phosphoric acid hemihydrate, Chem. Eng. Res. Des., 2010, 88 (10A), 1372.

[7] KV Kumar, F Rocha, Kinetics and thermodynamics of sucrose crystal growth in

the presence of a non-ionic surfactant, Surf. Sci., 2010, 604, 981.

[8] HH Teng, PM Dove, CA Orme, JJ De Yoreo, Thermodynamics of calcite growth:

Baseline for understanding biomineal formation, Science, 1998, 282, 724.


80

[9] K.J Davis, P.M. Dove, J.J De Yoreo, The Role of Mg2+

as an impurity in calcite

growth, Science, 2000, 290, 1134.

[10] R Pitchimani, LJ Hope-Weeks, G Zhang, BL Weeks, Effect of impurity doping

on the morphology of pentaerythritol tetranitrate crystals, J. Energ. Mater., 2007, 25,

203.




[12] S. Veintemillas-Verdaguer, Chemical aspects of the effect of impurities in crystal

growth, Progress in Crystal Growth and Characterization, 1996, 32(1-3),75.

[13] K Sangwal, On the mechanism of crystal growth from solution, J. Cryst. Growth,

1998, 192, 200.

[14] K Sangwal, Effects of impurities on crystal growth process, Prog Cryst. Growth

Charact. 1996, 32, 3.


in PETN, Propellants Explos. Pyrotech., 2009, 34, 489.

[16] A Maiti, RH Gee, PETN coarsening – prediction from accelerated aging data,


[17] WM Hikal, S K Bhattacharia, Brandon L. Weeks, Effect of porphyrin doping on

thermodynamic parameters of pentaerythritol tetranitrate (PETN) single crystals,

Propellants Explos. Pyrotech., article first published online: 20 Jul,(2012).


81

[18] WM Hikal, SK Bhattacharia, GR Peterson, BL Weeks, Controlling the coarsening

stability of pentaerythritol tetranitrate (PETN) single crystals by the use of water,

Thermochimica. Acta. 2012, 536, 63.

[19] SK Bhattacharia, A Maiti, RH Gee, BL Weeks, Sublimation properties of

pentaerythritol tetranitrate single crystals doped with its homologs, Propellants

Explos. Pyrotech., article first published online: 20 Jul,(2012).

[20] TA Land, TL Martin, S Potapenko, GT Palmore, JJ De Yoreo, Recovery of

surfaces from impurity poisoning during crystal growth, Nature, 1999, 399, 442.

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mechanisms, J. Appl. Phys., 2009, 105, 104312.


82

Chapter 6

Kinetics of the Gas Components from PETN Decomposition

6.1 Introduction

The performance in applications is determined by the decomposition of

explosives. The resulting heat of detonation is a product of the heat of decomposition

of each individual component [1]. These initial stages of detonation depend explicitly

on activation energy, a kinetic parameter, of decomposition since this energy needs to

be supplied to each decomposition product. Moreover, the kinetic parameter of a

thermal decomposition must be known in order to design the initiation process of the

explosion.

The decomposition kinetics is also related with the quality of an explosive.

Other practical issues like shelf life and thermal hazard of explosive are related with

decomposition kinetics [2]. Hence it is important to understand the true kinetics of

thermal decomposition of explosives. The accuracy of the kinetic parameter for the

decomposition process is required to predicts the hazardous potential, properties of

explosive after long term storage and detonation properties [3-5]. Use of inaccurate

kinetic parameter can cause serious accidents in storage and applications [6-9].

Thermal analysis is the most useful way to obtain the kinetic parameters for the

decomposition of explosive [10-13]. Differential scanning calorimetry (DSC) and

thermogravimetric analysis (TGA) are the most popular thermal analysis technique for

obtaining kinetic parameters. DSC provided information about heat associated with

any processes involving phase change such as melting, decomposition, whereas TGA

gives information about the mass loss with heating. These two techniques are


83

considered as complimentary to each other. The advantages with these methods are

small sample size (0.2 to 10 mg of sample), faster and easy to operate. However, these

two techniques do not provide any information about the kinetics of a specific product

coming out from the thermal decomposition of explosives.

A thermogravimetric analyzer combined with a mass spectrometer (TGA-MS)

is a technique which can analyze the real-time gas phase products that are coming out

from a thermal decomposition [2,14]. This technique also enables to monitor a specific

gas continuously in the entire temperature range of a nonisothermal experiment. TGA-

MS provides a unique platform to study the kinetics of the gas phase components.

However, the downside of this technique is that sometimes two gas components with

similar mass number overlaps with each other which affect the value of the kinetic

parameter. This method combined with TG-FTIR (Fourier Transformation

Spectroscopy combined with Thermogravimetric Analyzer) can solve the problems

associated with the gas components of similar mass number [14].

Accuracy of kinetic parameters depends not only on the experimental

technique, but also on the mathematical model used to determine the parameters.

Normal approach for kinetic analysis is to fit experimental data to a model for

obtaining desired parameters or coefficients. An inaccurate kinetic parameter, obtained

from an improper mathematical model, can cause serious accidents and inefficient

performance in real life application of explosives. Selecting the most appropriate

model for explaining the nonisothermal decomposition reaction is very difficult and

sometimes it is not conclusive. However, theoretical studies of decomposition kinetics


84

can be complementary to determination of experimental kinetics parameter and

thereby, it helps to pick the right model for explaining the kinetics mechanism.

Pentaerythritol tetranitrate (PETN) is a very strong secondary explosive.

PETN along other secondary explosive is used as the main components of propellants

and in the military warhead. Numerous studies on PETN decomposition were

reported in the literature [15-22]. The possible pathways and products of PETN

decomposition at various temperatures were presented in those studies. The most

common products from the decomposition of PETN are nitrogen di oxide (NO2), nitric

oxide (NO), carbon di oxide (CO2), carbon monoxide (CO) and water. Apart from

characterizing the decomposition products, some studies focused on the kinetics of

PETN decomposition [16]. They determined the activation energy for PETN

decomposition using differential scanning calorimetry and thermogravimetric analysis

and the reported value is in the range of 35-70 kcal/mol. However no report was found

on the kinetics of gas phase products of PETN decomposition.

In this chapter, decomposition of PETN was studied with a thermogravimetric

analyzer-mass spectrometer. Then, NO2, NO and H2O from PETN decomposition

waere monitored nonisothermally at various heating rate. An isokinetic analysis was

conducted on the kinetics of decomposition products. Isokinetic analysis was

conducted using Ozawa-Flynn-Wall model. Activation energy of the NO2, NO and

H2O production was presented as a function of the extent of reaction.


85

6.2 Theory

Non isothermal decomposition kinetics is explained by the following equation

,P,. = /(3). `(�) (6.1)

where t is time, T is the absolute temperature, /(3) is the function for Arrhenius

expression and `(�) is the function of extent of reaction (�) which is written as

following

�. = abac (6.2)

where �d is area of the curve at temperature Ti and �. is the total area of the curve.

By replacing /(3)in equation (1) with the Arrhenius expression from equation (3.4)

,P,. = �?89:/e_ . `(�) (6.3)

A linear heating rate � is used for non-isothermal heating, which can be written as

� = ,�,. (6.4)

Combining equation (3) and (4), following expression can be derived

,P,� = af ?89:/e_ . `(�) (6.5)

For nonisothermal decomposition, solution of the equation is derived by Ozawa-

Flynn-Wall (OFW) as the following [2,23,24].

log � = log g a9:�h(P)i − 2.315 − 0.4567 g9:��i (6.6)

Slope of the plot of log β VS 1/T will give 0.4567 g9:� i. Activation energies can be

calculated as a function of extent of reaction.


86

6.3 Experimental

PETN crystal grown from PETN powder was used for decomposition study.

Crystals were isothermally heated in TGA from 25oC to 350

oC. Gas components from

the decomposition product were pumped in mass spectrometer for analyzing the gas

components.

6.4 Kinetics of NOX production from PETN decomposition

Nitrogen-di-oxide (NO2) is the most important decomposition product of

PETN because PETN decomposition starts from the scission of O-NO2 bond by

releasing NO2. The decomposition pathways of PETN to release NO2 from it skeleton

is shown in the following scheme

lm −nmK ∆→ lm ∙ +nmK

The theoretical value of the bond energy RO-NO2 for nitrate ester was reported to 40.7

kcal/mol [16]. In this investigation, energy barrier for NO2 production from PETN

decomposition from PETN skeleton was experimentally obtained using the

thermogravimetric mass spectrometer (TGA-MS). The evolution of NO2 (m/e=46)

from the PETN was monitored in terms of ion current in mass spectrometer. Figure

6.1(A) showed the TGA-MS curve for NO2 (m/e=46). The ion current of NO2 was

plotted as a function of temperature at various heating rate. The NO2 production from

PETN decomposition was monitored in the temperature range of 25oC-350

oC at the

heating rate 5, 10, 15 and 20 oC/min. The NO2 production was very minimal until the

temperature reach at 160 oC. TGA-MS data of NO2 produced a bell shaped curve with

a one peak and the temperature of the peak shifted with the increase of heating rate.


87

The peak temperature was found to be 187, 194, 197 and 201 oC for the heating rate of

5, 10, 15 and 20 oC respectively.

A kinetic analysis was conducted using Ozawa-Flynn-Wall (OFW) model as

presented in the theory section. First step of the calculation procedure of the kinetic

analysis was to obtain the extent of reaction,�, using the data from Mass spectrometer.

Figure 6.1(B) showed the extent of reaction,�, as a function of temperature. The

temperature for a specific � was obtained from the Figure 6.1(B). This value of

temperature was used to plot log β as a function of reciprocal of temperature which is

presented in Figure 6.1(C). A series of curve for the range of extent of reaction 0.1-0.9

is shown in the Figure 6.1(C). The activation energy for each value of � were

calculated from the slope of the line for each level of extent of reaction. Figure 6.1(D)

showed activation energy of NO2 production as a function of extent of conversion.

Apparently, activation energy for the decomposition is similar in the whole range of

extent of reaction. Numerically activation energy ranges from 39.9 to 42.9 kcal/mol.

An average value of activation energy is 41.7 kcal/mol with a standard deviation less

than 3%. As mentioned early in this discussion that theoretical value of lm − nmK

bond energy in nitrate ester is 40.7 kcal/mol [16]. The Experimental value of

decomposition is consistent with theoretical value. To the best of our knowledge this

is the first experimental demonstration of measuring the activation energy of NO2

production from the decomposition of PETN.


88

Figure 6.1: (A) TGA-MS plot of NO2 (M/E= 46) production from PETN decomposition

(B) Degree of conversion of NO2 (M/E= 46) with temperature. (C) Isoconversional curves

for NO2 (M/E= 46). (D) Activation energy of the production of NO2 (M/E= 46) from

PETN decomposition as a function of degree of conversion

5 oC/min 10 oC/min

15 oC/min

20 oC/min

5 oC/min 10 oC/min

15 oC/min

20 oC/min

A B

C D


89

Nitric oxide (NO) is another compound which is produced in abundance during

the decomposition of PETN. Previous studies showed following pathway of the

production of nitric oxide

lm −nmK ∆→ lm ∙ +nmK

lm ∙ +nmK rd0s,1tuuuv lmm ∙ +nm

nmK rd0s,1tuuuv m ∙ +nm

In TG-MS, generation nitric oxide from PETN decomposition was monitored as

m/e=30. Figure 6.2(A) showed the nonisothermal production profile of NO(m/e=30)

from PETN decomposition at 5,10,15 and 20 oC/min. The peak temperature shifted to

right with the increase of heating rate. The peak temperature is 187,198, 200 and 203

oC at 5, 10, 15 and 20

oC/min respectively. However, in TG-MS, there is chance of

overlapping the resolution of NO with CH2O. Continuous production profile of

13CH2O showed that CH2O production is negligible to influence the kinetics of NO

production because there was no continuous production peak was noticed for 13

CH2O.

Another reason might be, the conversion of carbon into carbon-di-oxide in the

continuous production profile.

Figure 6.2(B) showed the plot of extent of reaction as function of temperature.

Extent of reaction progress was calculated using equation (6.2) and area under the

curves of Figure 6.2(B). The Ozawa-Flynn-Wall (OFW) model requires the data for

heating rate (β), extent of reaction (α) and temperature (T) which are obtained from

Figure 6.2(B). Figure 6.2(C) showed OFW plot (log β vs 1/T) for NO production from

PETN decomposition for the extent of reaction (α=0.1 to 0.9). The activation energies

of decomposition for each α was obtained from the slope of the curve and presented in


90

Figure 6.2(D). It showed that activation energy of decomposition is almost similar in

the whole range of α. Average activation energy of decomposition is 40.1 kcal/mol

with standard deviation less than 5%. There was no theoretical or experimental values

available in literature to compare our result. Since our kinetic analysis for NO2 is

consistent with the theoretical, we can accept this analysis is also correct.


91

Figure 6.2: (A) TGA-MS plot of NO (M/E= 30) production from PETN decomposition,

(B) Degree of conversion of NO (M/E= 30) with temperature (C) Isoconversional curves

for NO (M/E= 30), (D) Activation energy of the production of NO (M/E= 30) from

PETN decomposition as a function of degree of conversion.

A B

C D


92

6.5 Kinetics of H2O production from PETN decomposition

A PETN molecule contains 8 hydrogen and 12 oxygen atoms. As a result H2O

is an inevitable product of decomposition and detonation of PETN. The heat of

reaction of PETN decomposition/detonation is significantly influenced by the enthalpy

associated with H2O production. A TG-MS experiment demonstrated continuous H2O

production from nonisothermal PETN decomposition. Figure 6.3(A) showed

nonisothermal H2O production at 5,10,15 and 20 oC/min. H2O production increased

consistently with the increase of temperature as shown in the figure. The Extent of

production (α) of H2O was calculated using area under the curves of Figure 6.3(A) and

presented in Figure 6.3(B) and it showed that extent of reaction shifted to the direction

of increasing temperature with the increase of heating rate. Figure 6.3(C) presented

OFW kinetics of H2O production where logarithm of heating rate is plotted as a

function of the reciprocal of temperature for α=0.1 to 0.9. Activation energies as

function of extent of conversion (α) were obtained from Figure 6.3(C) and presented

in Figure 6.3(D). The activation energy is increased by ~65% and ~50% when

extent of conversion increase from 0.1 to 0.7 and from 0.1 to 0.3 respectively. The

reason for the increase cannot be explained from this study. The numerical value of

activation energies were found to be similar (constant) for the range of α=0.4 to 0.8.

So there is two regime of activation energy. In first regime (α=0.1 to 0.3), activation

energies is increased by~50%, in second regime (α=0.4 to 0.8) activation energies are

almost constant. We are proposing that the activation energy in the first segment is

mainly used to produce S ∙ and O∙, so less H2O is being produced. In the second

segment, H2O forms.


93

Figure 6.3: (A) TGA-MS plot of H2O (M/E 18) production from PETN decomposition,

(B) Degree of conversion of H2O (M/E 18) with temperature (C) Isoconversional curves

for H2O (M/E 18) (D) Activation energy of the production of H2O (M/E 18) from PETN

decomposition as a function of degree of conversion

A B

D C


94

6.6 Conclusion

In summary, PETN decomposition kinetics was studied using TGA-MS. An

isokinetic analysis was conducted on the PETN decomposition product NO2, NO and

H2O by Ozawa-Flynn-Wall model. The activation energy of the investigated gas

components was obtained as a function of extent of reaction. The activation energy for

NO2 remained constant in the whole conversion range. Average activation energy of

NO2 production is 41.7 kcal/mol. This is consistent with the theoretical value RO-NO2

bond energy. Average activation energy required for NO production is 40.1 kcal/mol

and remains almost constant in the whole conversion range. Activation energy for

H2O production was found to have two regimes. Activation energy was increased upto

the extent of conversion 0.4, it becomes constant after that. There was no theoretical

value was found to compared this experimental results.


95

References:

[1] WE Garner, Detonation or explosion arising out of thermal decomposition,

Trans. of the Faraday Soc. 1938, 34, 985.

[2] M Rajic, M Suceska, Study of thermal decomposition kinetics of low-

temperature reaction of ammonium perchlorate by isothermal TG, J. Therm.

Anal.Cal., 2001, 63, 375.

[3] JA Conesa, A Marcilla, JA Caballero, R Font, Comments on the validity and

utility of the different methods for kinetic analysis of thermogravimetric data, J.

Anal. Appl. Pyrol., 2001, 58-59, 617.

[4] M. Suceska, A Computer program based on finite difference method for

studying thermal initiation of explosives, J. Therm. Anal. Cal., 2002, 68, 865.

[5] SV Vyazovkin, AI Lesnikovich, Error in determining activation energy caused

by the wrong choice of process model, Thermochim. Acta, 1990, 165, 11.

[6] N Koga, J Sestak, J Malek, Distortion of the arrhenius parameters by the

inappropriate kinetic model function, Thermochim. Acta, 1991, 188, 333.

[7] JA Caballero, JA Conesa, Mathematical considerations for nonisothermal

kinetics in thermal decomposition, J Anal. Appl. Pyrolysis, 2005, 73, 85.

[8] S Vyazovkin, CA Wight, Model-free and model-fitting approaches to kinetic

analysis of isothermal and nonisothermal data, Thermochim. Acta, 1999, 340-341,

53.

[9] AK Galwey, Perennial problems and promising prospects in the kinetic

analysis of nonisothermal rate data, Thermochim. Acta, 2003, 407, 93.


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[10] MD Ger, WH Hwu, CC Huang, A study on the thermal decomposition of

mixtures containing an energetic binder and nitramine, Thermochim. Acta, 1993,

224, 127.

[11] RN Rogers, ED Morris, On estimating activation energies with a differential

scanning calorimeter, Anal. Chem., 1966, 38, 412.

[12] JS Lee, CK Hsu, CL Chang, A Study on the thermal decomposition behavior

of PETN, RDX, HNS and HMX, Thermochim. Acta, 2002, 392-393,173.

[13] RN Rogers, RH Dinegar, Thermal analysis of some crystal habits of

pentaerythritol tetranitrate, Thermochim. Acta 1972, 3, 367.

[14] L Dong, X Li, R Yang, Thermal decomposition study by synchroton

photoioniation mass spectroscopy, Propellants Explos. Pyrotech. 2011, 36, 493.

[15] FJ Dicarlo, JM Hartigan, GE Phillips, Analysis of pentaerythritol tetranitrate

and its hydrolyis products by thin layer chromatography and radio scanning, Anal.

Chem., 1964, 36, 2301.

[16] D Chamber, Perspectives on pentaerythritol tetranitrate (PETN)

decomposition, Lawrence Livermore National Laboratory, UCRL-JC-148956,July

1, 2002.

[17] GD Miller, LD Haws, RH Dinegar, Kinetics of the thermal decomposition of

PETN”19th

International Symposium on Combustion 1982.

[18] WL Ng, JE Field, HM Hauser, Thermal, fracture, and laser induced

decomposition of pentaerythritol tetranitrate, J. Appl. Phys., 1986, 59, 3945.

[19] HN Volltrauer, Real time low temperature decomposition of explosives –

PETN, J. Haz. Mater. 1982, 5, 353.


97


decomposition chemistry of pentaerythritol tetranitrate single crystals: Time-

resolved emission spectroscopy, J. Phys. Chem. B, 2002, 106, 247.

[21] PS Makashir, EM Kurian, Spectroscopic and thermal studies on

pentaerythritol tetranitrate (PETN), Propellants Explos. Pyrotech., 1999, 24, 260.

[22] DM Roos, TB Brill, Thermal decomposition of energetic materials 82.

Correlations of gaseous products with the composition of aliphatic nitrate esters,

Combust. Flame, 2002, 128, 181.

[23] T Ozawa, A new method of analyzing thermogravimetric data, Bulletin of the

Chemical Society of Japan, 1965, 38(11), 1881.

[24] M Najafi, AK Samangani, Non-isothermal kinetic study of the thermal

decomposition of melamine 3-nitro-1,2,4-triazol-5-one salt, Propellants Explos.

Pyrotech., 2011, 36, 487.

[25] J Kimura, Chemiluminescence study on thermal decomposition of nitrate

esters (PETN and NC), Propellants Explos. Pyrotech. 1989, 14, 89.


98

Chapter 7

Conclusion and Future Works

7.1 Overall Conclusion

In this dissertation, two kinetic processes-(i) sublimation (ii) decomposition,

were investigated for a benchmark organic explosive, pentaerythritol tetranitrate

(PETN). These kinetic processes are very important for the handling, storage and

application of explosives. Since organic explosives are crystalline and theoretical

studies were conducted for a single crystal, the kinetic processes were evaluated using

single crystals that were grown by solvent evaporation. First, a detailed investigation

on sublimation properties was conducted for PETN single crystals. This investigation

showed that (101) faces of PETN crystal disappeared faster than (110) faces in

sublimation process. Sublimation of a PETN crystal follows zero order kinetics with a

activation energy 34.5 kcal/mol. The vapor pressure data of PETN single crystals was

obtained and it is consistent with the theoretically predicted value. The vapor pressure

data was modeled with a simple modified Antoine equation for obtaining vapor

pressure data at a wide range of temperatures. The heat of sublimation was also

obtained for PETN single crystals. Then PETN crystals were doped with diPEHN and

triPEON. The vapor pressure of doped PETN was obtained for the doping level of

1000 ppm, 5000 ppm and 10000 ppm. This investigation showed that vapor pressure

was reduced due to the addition of impurities. This reduction of vapor pressure

indicates that controlled addition of dimer and trimer of PETN can be used to control

the coarsening of PETN powder because it can slow the sublimation process. For

predicting vapor pressure of doped PETN crystals at a wide range of temperatures,


99

coefficients of modified Antoine equation were also obtained from this investigation.

Then distribution of doping compound in PETN single crystal was investigated using

the rate of sublimation data and atomic force microscopy. This investigation showed

that impurities were distributed both on surface and in the crystal matrix. Addition of

impurities also changed the height of the growth steps. This investigation also showed

that the reason of the reduction of sublimation is not due the change in the energy

barrier, it is due to the changes in the surface morphology of the single crystals. In the

last segment of this investigation, kinetics of the gas components from PETN

decomposition was investigated by TGA-MS. The activation energy for the generation

of NO2, NO and H2O were obtained from this investigation. The activation energy of

NO2 is consistent with the theoretically predicted value.

7.2 Proposed projects for future research

This dissertation discussed sublimation and decomposition of PETN single crystal.

To understand overall kinetic process of energetic material, following investigations

are recommended. These ideas are not addressed in literature.

(1) Growth kinetics single crystal: Growth kinetics of pure PETN will explain how

different facet of PETN crystal grows. Thermodynamics and kinetics of PETN

crystal growth will be obtained from this study. This study will explain effect

of recrystallization process on the coarsening mechanism. Crystal will be

grown from a seed crystal by flowing the solution of PETN. Flow of the

solution will be laminar for maintaining the control over the growth rate of

crystal.


100

(2) Growth kinetics of dislocations in PETN crystal: Single crystal grows from

growth of the dislocation in crystal. Growth of dislocation will explain the

kinetics at molecular level. This study will justify the theoretical studies on

coarsening of PETN powder. This study will explain surface evolution during

the growth. This study will also explain how the surface morphology changes

at various growth rates. Atomic Force Microscopy (AFM) will be principle

tool for monitoring the growth of the dislocation. Crystal will be grown

following method described in proposal 1. In-situ AFM scanning will be

conducted on the surface of the crystal.

(3) Growth kinetics single crystal in presence of impurities: Growth kinetics will

be investigated in presence of impurities. Various kind of impurities will be

used to get ideas of how different molecule changes growth kinetics.

(4) Growth kinetics of dislocations in presence of impurities: Since presence of

impurities change the morphology, this study will show how surface

morphology is evolved because of the addition of impurities. It will also

explain the effect of impurities on the growth kinetics of dislocations.

(5) Nanolithography to add impurity in kink site: Impurities can be added on the

kink by using nanolithography. This method will to add impurities on specific

site on the surface of the crystal. Effect of impurity added on the kink site of

crystal on sublimation kinetics will be investigated.

(6) Correlation between growth and sublimation kinetics: This study will explain

how kinetics of crystallization influence the sublimation kinetics. Crystal


101

grown at various flow rate of PETN solution will be used in the TGA to

measure the rate of sublimation.

(7) Sublimation from surface of various surface energy: First part of this study will

focus on the effect of impurity on surface energy of various faces of PETN

crystal. In second part, effect of surface energy on the sublimation of PETN

will be studied. This study will help to understand the correlation between

surface energy and doping concentration for controlling coarsening

mechanism.

(8) Effect of surface morphology on initiation and decomposition: PETN crystal

with different surface morphologies will be used to initiate by laser irradiation.

A high speed camera will be used to collect the images of the initiation

process. Decomposition studies will be conducted by differential scanning

calorimetry and TGA-MS.

(9) Direction measurement of vapor pressure from single crystal: In this study

vapor pressure was calculated from the mass loss of PETN. Direct

measurement of vapor will ensure the consistency of the vapor pressure data.

Knudson effusion technique can be used to measure vapor pressure directly.

(10) Studying kinetics of all the decomposition products using TGA-MS and

TGA-FTIR: In this research, kinetics of three gas components was

investigated. To investigate the kinetics of others gas component, similar

studies have to be conducted using both TGA-MS and TGA-FTIR


102

APPENDIX

Figure A1: TGA curve for standard sample. Calcium oxalate monohydrate was used as

standard sample.


103

Figure A2: Calibration curve for optical microscope.

y = 422.38x + 18.265

0

200

400

600

800

1000

1200

1400

1600

1800

0.00 1.00 2.00 3.00 4.00 5.00

Pix

els

Magnification


104

Figure A3: Images of samples crystal for obtaining surface area; (Top left)(110) face

(Top right) (1�1�0) face (Bottom left) (1�10) face (Bottom right)(11�0) face


105

Satistical Analysis

� = |(@� − @K)|y��K�� + �KK�K

Where “x” is the sample mean and “s” is standard deviation. 1 and 2 refer the

sample1 and sample 2 respectively.

And “df” in the table is degrees of freedom where df= n1+n2-2

Null hypothesis- “Two sample mean has no difference”

Critical values are taken at 95% confidence interval.

Sample 1-Pure PETN crystal

Sample 2-1000 ppm triPEON doped PETN

T(oC) |x1-x2| L=s1

2/n1 M=s

2/n2 (L+M)

0.5 t-(data) t (crit)

100 0.063567 0.0001736 0.001513258 0.041071 1.547729 2.447

105 0.062161 0.0002411 0.000334659 0.023994 2.590695 2.447

110 0.139045 1.261E-05 0.000766538 0.027913 4.981337 2.447

115 0.325434 0.0003033 0.003914983 0.064948 5.010667 2.447

120 0.602944 0.0033814 0.013912111 0.131505 4.584958 2.447

125 0.949371 0.0017292 0.049525373 0.226395 4.193431 2.447

130 1.643251 0.0005284 0.083788878 0.290374 5.659076 2.447

135 2.367708 0.003914 0.346416252 0.591887 4.00027 2.447

t-(data)> t-(critical), so data “rejects” the null hypothesis. So results from Sample 2 are

significantly different from sample 1 at 95% confidence interval.


106

Sample 1-Pure PETN crystal

Sample 2-5000 ppm DiPEHN doped PETNt-(data)> t-(critical)

so data “rejects” the null hypothesis. So results from Sample 2 are significantly

different from sample 1 at 95% confidence interval.

T(oC) |x1-x2| L=s1

2/n1 M=s

2/n2 (L+M)

0.5 t-data t(crit)

100 0.10709 0.0001736 0.00018549 0.01895 5.651247 2.447

105 0.045645 0.0002411 0.000925234 0.034151 1.336573 2.447

110 0.031227 1.261E-05 0.000872909 0.029758 1.04937 2.447

115 0.213951 0.0003033 0.002933019 0.056888 3.760887 2.447

120 0.545405 0.0033814 0.025219993 0.169119 3.224972 2.447

125 0.693829 0.0017292 0.005510171 0.085084 8.154597 2.447

130 1.114198 0.0005284 0.043342288 0.209453 5.319556 2.447

135 1.910766 0.003914 0.274543333 0.527691 3.620998 2.447

t-(data)> t-(critical) so data “rejects” the null hypothesis. So results from Sample 2 are

significantly different from sample 1 at 95% confidence interval.


107

O2NOH2C C

CH2ONO2

CH2ONO2

CH2 O CH2 CH2ONO2

CH2ONO2

CH2ONO2

C

Figure A4: Molecular structure of diPEHN.

Figure A5: Molecular structure of triPEON.


108

Figure A6: Calibration curve generated from TGA-MS using calcium oxalate

monohydrate.

TGA plot

m/e 18

m/e 44

0

50

100

150

200

250

300

Ion Current (nA)

20

40

60

80

100

120Weight (%)

0 200 400 600 800

Temperature (°C) Universal V4.7A TA Instruments

Investigation of Sublimation and Decomposition by Sanjoy ...

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