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Predicting Physical and Chemical Behaviors of Energetic Materials
Materials Science and Engineering
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Energetic materials (EMs) can release a large amount of energy by fast decomposing chemical reactions. The development of new generation EMs with decreased sensitivity, improved detonation performance and environmentally acceptance is of considerable importance for applications in civilian and military fields. Plenty of EMs are synthesized in labs through various ways, but few can satisfy the strict requirements of extended engineering applications. Particularly, the atomistic-level understanding of many fundamental issues related to thermal decomposition mechanisms and detonation properties of newly synthesized or long existing representative EMs remain unclear. Solving these fundamental problems is of great significance for proposing new generation of EMs prior to the difficult task of synthesis and for discovering new EMs that might optimize energy release, sensitivity, and environmental impact before expensive experimental tests. Here we focus on several typical EMs including silicon contained Si-PETN, three dimensional metal-organic frameworks of potassium 4,4’-bis(dinitromethyl)-3,3’- azofurazanate, cyclo-N5-contained EMs of [M(N5)2(H2O)4]·4H2O (M = Mn, Fe, Co, and Zn), two nitro compound of nitromethane and nitryl cyanide, and methane hydrate. The summary of our studies on these materials are listed below. To understand the influence of elemental Si on the detonation properties of EMs, we carried out the combined reactive molecular dynamics (RMD) with quantum mechanics molecular dynamics (QMMD) simulations to predict the thermodynamics parameters of the Chapman-Jouguet (CJ) state as measurements of detonation performance. We found that the detonation temperature for tetrakis(nitratomethyl)silane (Si-PETN) is higher than that for pentaerythritol tetranitrate (PETN) because of high energy release while forming Si-based products. However, lower detonation pressure and detonation velocity for the Si-PETN system than those for PETN system were found because Si atoms attract nearby oxygen atoms from other molecules or fragments resulting in cluster products and leading to less gas product formation. This study uncovers how the specific atoms influence the detonation properties of EMs from the atomic perspective, providing useful information for designing EMs with improved properties. To understand the thermal stability and detonation properties of the new energetic three-dimensional (3D) metal-organic frameworks (MOFs), we carried out quantum mechanics (QM) simulations to examine its initial decomposition mechanism and the Chapman Jouguet state for sustainable detonation. We found that the initial decomposition reaction is to break the C2N2O five-member ring in which K+ ions play a significant role in stabilizing the molecule structure, leading to an excellent thermal stability. Furthermore, this MOF system has a higher detonation velocity than lead azide, a comparable detonation pressure and temperature, and no toxic gases are produced at detonation. The combination of these detonation properties makes it a promising candidate for green EMs. Our results suggest that synthesizing 3D MOFs is an effective approach to develop environmentally acceptable alternatives to toxic EMs with enhanced thermal stability. To elucidate the high stability of the recently synthesized cyclo-N5-contained EMs of [M(N5)2(H2O)4]·4H2O (M = Mn, Fe, Co, and Zn) under ambient conditions, QM calculations were employed. The results from our study indicate that the stability is due to the presence of two types of water (coordinated H2O (c-H2O) and hydrogen bonded H2O (h-H2O)). On the basis of the lower energy cost to remove h-H2O from the materials and the subsequent large decrease in the energy barrier, we propose that h-H2O acts as a “safety device” that prevents the materials from becoming kinetically unstable. To study the influence of additives to the detonation properties of nitromethane (NM), the simplest EM, we employed RMD with QMMD procedure to compare the detonation properties of liquid NM with the 1:1 molar ratio mixture of NM: NCNO2 (recently synthesized). We found that the mixture significantly improved detonation properties including Chapman-Jouguet (CJ) temperature, CJ pressure, and detonation velocity. This is because the number of nitrogen atoms increases the gaseous final products while producing fewer carbon clusters. The detonation properties are also enhanced by the increased initial density leading to higher gas expansion capability. Thus, our results indicate that NCNO2 is a good additive to NM or a suitable replacement for liquid NM due to its simple molecular structure, huge energy release and excellent detonation performance under low temperature conditions. To understand the diffusion and decomposition mechanism of trapped molecules in methane hydrates (MHs), we employed the QM simulations. We found that the initial decomposition reaction in MHs initiates from hydrogen transfer among water molecules. Then the attacks from fragments of O and OH to CH4 molecules are responsible for the destructions of the methane molecules. Next, our simulation results revealed that the methane molecule prefer to escape from the ice cage through the hexagonal face. To suppress the methane diffusion, we demonstrated that the diffusion barrier is significantly enhanced by adding electron or hole carriers. This is because that the extra electrons and holes enhance the electrostatic interaction between methane and water molecules, leading to the increased barriers. Thus, the clathrate hydrates could be stabilized by adding extra free electron or hole carriers