Molecular Dynamics Simulations on the Chemical Responses of Some Typical Energetic Materials

幾種典型含能材料化學響應的分子動力學模擬研究

Student thesis: Doctoral Thesis

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Award date4 Jun 2020

Abstract

High energy explosives are widely used in defense and other applications (for instance, rocket propellants and airbag inflators). Their energy and safety performances are important concerns, which are influenced by multiple factors, such as crystal structures, defects, and crystal morphology. A fundamental investigation on the initial decomposition and responses to external stimuli using reactive molecular dynamics (MD) simulations is presented in this thesis, which involves the thermal decomposition of perfect 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) and benzotrifuroxan (BTF), as well as shock decomposition of defective β-cyclotetramethylene tetranitramine (β-HMX) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) nanoparticles.

Traditional explosives are a kind of molecular crystal, whose thermal decomposition is a chain process, and their responses under high temperatures are influenced by crystal structure and products. TATB has extraordinary stability under external stimuli, while BTF is very impact sensitive. A comparison of thermal decomposition during heating, expanding, and cooling processes between TATB and BTF crystals was conducted to find the reasons for their different sensitivity. The decomposition of BTF was found to proceed with a higher reaction frequency compared to that of TATB. The reaction frequency of TATB increased rapidly at the initial stage. This phenomenon was not observed in BTF. A hindered carbon ring cleavage along with initial decomposition path and clustering were suggested to explain the sluggish decomposition of TATB. Besides, clusters with a higher number and mass occur in TATB crystal. Graphitic character, as well as stable geometries of carbon ring and carbon chain, are common in stable clusters.

It remains very difficult to procure perfect bulk explosives, because defects are inevitable in experimental explosives formed during crystallization or under external stimulations, which play key roles in sustaining chemical reactions under shock. The defects in explosives are varied usually including voids and impurities, which can greatly affect the shock sensitivity of explosives and the effects need to be studied for better application of explosives. Defective β-HMX crystals with void (VH), entrained oxygen (OH), and entrained amorphous carbon (CH) were investigated, as well as a perfect HMX crystal (PH) for contrast, to study the effects of microscopic defects on shock sensitivity. The results demonstrate that the shock sensitivity of HMX crystals with different defects is enhanced to different degrees. OH has the highest shock sensitivity, which is slightly higher than that in VH; both OH and VH crystals have much higher shock sensitivity than that in CH. Obvious local high temperature areas occur near these defects. Defects facilitate HMX decomposition and the rise of average reaction temperature and pressure.

To improve the performance of explosives and reduce defects in crystals, nano-explosive crystals are widely prepared and investigated. Compared with traditional energetic materials, nano-explosives possess enhanced insensitivity and mechanical strength due to unique surface energy and chemical reaction dynamics. The shock responses of nano-explosives are complex and inconsistent in experiments. Nonequilibrium reactive MD simulations were conducted to examine the effects of particle size on the shock response of close-packed nanogranular RDX. It is found that larger particles and higher impact velocity facilitate fluid jet generation and temperature heterogeneity. Smaller particles lead to lower local temperature due to smaller voids between particles, while once ignited they possess higher reaction activity which is consisted with previous experimental results. Detailed chemical reaction pathways such as N-N fission, final product evolution, NO2 formation, and oxidization were studied to give a throughout understanding of the initiation and reaction growth under shock wave.

This thesis provides useful insights into the relevance between the microstructure and microchemical properties of explosive crystals under heat and shock loading, as well as insights in methods to better utilize or prevent their deflagration and detonation. Besides, the results can guide the formulation of explosives to improve their energy and safety.