(458l) The Microstructural Evolution and Kinetics of Thermal Decomposition of Selected Energetic Cocrystals

normal"> 107%;font-family:" times new roman>The Microstructural Evolution and
Kinetics of Thermal Decomposition of Selected Energetic Cocrystals

" times new roman>J.T. Dean¹,M. Örnek¹,
T.D. Manship², S.F. Son^1,2

¹School of Mechanical Engineering,
Purdue University, West Lafayette, IN.

²Purdue Energetics Research Center,
Purdue University, West Lafayette, IN.

" times new roman>Abstract:

Energetic
materials are a class of materials which possess significant amounts of stored
chemical energy, which can be released upon stimulation from heat, friction, or
impact. These materials are commonly used in explosives, propellants, and
pyrotechnics, with a wide variety of applications spanning from military
ordnance and aerospace to mining and beyond. The improvement of energetic
material performance and reduction of their sensitivity have been the utmost
importance and often are the driving force behind ongoing research into
energetic materials.

Cocrystallization,
in contrast to its longstanding use in the pharmaceutical industry, has
recently been adopted to energetic materials [Landenberger 2010, 2012;
Vuppuluri 2019]. Energetic cocrystals are mixtures of two or more types of
material, known as coformers, with a unique crystal arrangement that are held
in suspension by weak intermolecular forces [Landenberger 2010]. Because of
this unique crystal arrangement, the cocrystal has the propensity to possess
properties or characteristics which differ from its energetic coformers. While
in the past increased energetic safety led to a decrease in performance, the
intent behind energetic cocrystallization is to optimize both safety and
energetic performance beyond what is currently feasible through the coformers
or even physical mixes. Energetic cocrystals are a relatively new topic when compared
to other aspects of energetic material research, therefore the properties and
behavior of these energetic cocrystals are fairly uncharted.

line-height:107%;font-family:" times new roman>The kinetic parameters
for thermal decomposition of energetic materials are critical to understand the
properties of an energetic material and to determine their usage, transportation,
and storage. With accurate kinetic parameters, the risk of violent reaction or
thermal runaway can be greatly reduced. This knowledge is of particular
importance when considering energetics with military applications, as
energetics will be subjected to a wide variety of conditions, circumstances,
and scenarios.

line-height:107%;font-family:" times new roman>In this study, we
investigated the microstructural evolution and the kinetics of the thermal
decomposition of selected cocrystals by following two primary pathways: Simultaneous
differential-scanning calorimetry/thermogravimetry analysis (DSC/TGA) coupled
with mass spectroscopy (MS) using an open alumina pan under argon flow and
hot-stage microscopy with interspersed micro-computed tomography (Micro-CT)
imaging. Thermal decomposition of four cocrystals (2,4,6-trinitrotoluene (TNT)/
2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL20) in 1:1 molar
ratio; 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX)/CL20 in 1:2 molar
ratio; 1-methyl-3,5-dinitro-1,2,4-triazole (MDNT)/CL20 in 1:1 molar ratio and
4-amino-3,5-dinitropyrazole (ADNP)/diaminofurazan (DAF) in 1:1 molar ratio),
their physical mixtures in the same molar ratios, and their individual
coformers were studied by MS/DSC/TGA.

line-height:107%;font-family:" times new roman>The kinetic parameters
including activation energies (E_a)
and pre-exponential factor (A) have been determined using Kissinger and Ozawa
methods across different heating rates from the DSC/TGA analysis. Figure 1
shows the representative DSC traces of TNT/CL20 cocrystals as a function of different
heating rates. For all the heating rates, an endotherm is visible around 140 ^oC;
this correlates to the delayed melting of TNT from its usual melting point of
approximately 80 ^oC. This delay in the melting of TNT demonstrates
the potency of cocrystallization to alter the thermal behavior of energetic
materials. The behavior displayed on this figure agrees with the literature [Zhang
2018; Jia 2019].

Figure

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style='font-size:12.0pt;font-family:"Times New Roman",serif;font-style:normal;
mso-bidi-font-style:italic'>: DSC Data of TNT/CL20 Cocrystals at Selected
Heating Rates

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line-height:107%;font-family:" times new roman>The activation energies determined
through Kissinger’s and Ozawa’s kinetics analysis methods for the TNT/CL20
cocrystal are listed below in Table 1. While these values differ from the
literature [Zhang 2018; Jia 2019], this difference can be attributed to different
testing conditions; specifically, Zhang et al [2018] and Jia et al [2019] used
a different purge gas and sealed aluminum pans.

font-family:" times new roman>Table

style='font-size:12.0pt;font-family:"Times New Roman",serif'> style='mso-element:field-begin'> SEQ
Table \* ARABIC 1

style='font-size:12.0pt;font-family:"Times New Roman",serif'> style='mso-element:field-end'> 12.0pt;font-family:" times new roman>: Activation energies calculated
using Kissinger and Ozawa methods for TNT/CL20 cocrystal

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line-height:107%;font-family:" times new roman>Using the insight gained
from the DSC/TGA regarding the decomposition reaction, we then visually observed
the material at critical points in the decomposition process, such as phase
changes, melting points, and decomposition onset, using hot-stage microscopy and
micro-CT imaging. A series of images taken from the heating of HMX/CL20
cocrystals at a heating rate of 10 ^oC/min to 300 ^oC are
presented below (Figure 3). The cocrystals closely resemble the structure and
size of HMX, which is unique considering the molar ratio is skewed towards CL20.
During the decomposition, cocrystals were observed to change color and begin to
form agglomerates. Towards the end of the decomposition, the camera was
obscured as molten cocrystal surged over and covered the viewing window.

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Figure 2: Microscopy Images of HMX/CL20 Cocrystal;
(Left) Prior to heating, (Center) During heating, (Right) Moments before molten
cocrystal obscured the microscope

" times new roman>The more time spent on the study of energetic
cocrystals only discovers there is much more to learn. There are unique
characteristics of energetic cocrystals which do not align with current mental
models regarding energetic materials, such as the HMX/CL20 cocrystal’s
appearance resembling that of HMX, despite HMX being the minority coformer.
More research and work must be done before the potential held by energetic
cocrystallization is unlocked.

Keywords:
Cocrystal; Thermal decomposition; Energetic materials

" times new roman>

References:

1.     K.B.
Landenberger, A.J. Matzger. “Cocrystal Engineering of a Prototype Energetic
Material: Supramolecular Chemistry of 2,4,6-Trinitrotoluene.” Crystal Growth and Design 10 (2010)
5341-5347.

2.     K.B.
Landenberger, A.J. Matzger. “Cocrystals of 1,3,5,7-Tetranitro-1,3,5,7-Tetrazacyclooctane
(HMX).” Crystal Growth and Design 12
(2012) 3603-3609.

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Vuppuluri, J.C. Bennion, R.A. Wiscons, I.E. Gunduz, A.J. Matzger, S.F. Son. “Detonation
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Zhang, Y.L. Xu, Q. Jia, S.J. Zhang, N. Liu, H.X. Gao, and R.Z. Hu.
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5.     Q.
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44 (2019): 1-10.