First-principles Simulation On Molecular Interactions And Physical Properties Of Several New Kind Of Energetic Materials

Energetic materials, including explosives, propellants and pyrotechnics, have been widely used in national economy, defense industry and other fields. Thus, fundamental researches on energetic materials are very important for development of national economy and defense industry. Experimental studies of energetic materials are very dangerous, so computational simulations have gradually become an important research method for energetic materials (EMs). In this thesis, intermolecular interaction of molecular crystals, structural, electronic and elastic properties of metal-organic framework energetic materials (MOF-EMs) and thermal decomposition of crystalline FOX-7 under ambient and high pressure have been studied by combining density functional theory (DFT) and molecular dynamics (MD).Most EMs are molecular crystals. The structural, physical and chemical properties of molecular crystals are determined by their intermolecular interaction. However, DFT based on local density approximation (LDA) or generalized gradient approximation (GGA) can’t describe the long-distance dispersion interaction well since it can’t reproduce the intermolecular interaction of typical molecular crystals. In order to describe the intermolecular interaction well, varieties of schemes have been developed to correct this disadvantage. The performance of DFT and its schemes corrected by dispersion have been systematically assessed for intermolecular interaction of molecular crystals, which provide a reliable guide for the study of physical, chemical properties and dynamics behavior of molecular crystals. Crystal structure of methane has been calculated firstly using different DFT functionals and their dispersion-corrected schemes. The results show PBE-Grimme (PBE-D2) and PBE-D3 schemes can well reproduce the lattice constants and lattice energy of solid methane compared with experimental values, and does not significantly increase the computational cost. Though all vdW-DF schemes can well reproduce the lattice constants and bulk modulus of solid methane, they overestimate their lattice energies and increase the computational cost.Besides, the performance of DFT plus local atomistic potential (DFT+LAP) has been evaluated for the lattice constants and their lattice energies of twelve molecular crystals compared with experimental values and computational results by PBE-Grimme. It shows that PBE-Grimme scheme performs best among all schemes considered here, and the performance of DFT+LAP scheme is very close to that of PBE-Grimme scheme without extra computational cost. This indicates that DFT+LAP scheme is an efficient and accurate method for the study of molecular crystals and non-bond systems. Subsequently, we have made some further improvement and development about DFT+LAP scheme. We found more effective potential formula is the key requirement for accurate description of intermolecular interaction in molecular crystals.Metal-organic frameworks (MOFs) with high nitrogen content have been synthesized and exhibit excellent detonation performance compared with traditional energetic materials (EMs), and it can be used as potential EMs. The structures of MOF-EMs is very complicated, and their electronic and elastic properties play an important role in their applications. The structural, electronic and elastic properties of 1D,2D and 3D MOF-EMs have been firstly and systematically studied using ab initio calculations. The performances of PBE functional and the dispersion-corrected schemes have been assessed for the crystalline structures of MOF-EMs. The results show that PBE functional underestimates the interaction of MOF-EMs and overestimate their lattice constants. However, all dispersion-corrected schemes can make up the underestimation partially, but some of them overcorrect that effect, such as PBE-D and optB88-vdW. Of all schemes here, optPBE-vdW can reproduce the best crystalline structure compared with experimental values. The analysis of electronic structures for MOF-EMs suggests that NHN is metallic, while others are semiconducting or insulating. The relation between electronic structure and the sensitivity of MOF-EMs has also been discussed and it is in agreement with experimental observation. Besides, the bulk moduli of MOF-EMs have been also predicted, which are between 15.1-35.0 GPa and higher than those of traditional energetic materials.In addition to the physical and chemical properties of energetic materials, understanding their detonation mechanism is also one of "hot spots" in this area. FOX-7 is an insensitive explosives. The study on its mechanism of thermal decomposition not only has important meaning for its storage, transport and application, but also is helpful for design of new insensitive explosives. The processes of thermal decomposition of crystalline FOX-7 under ambient and high pressure have been simulated using ab initio molecular dynamics. The initial chemical event of FOX-7 under ambient pressure starts from C-NO2 bond breaking. The products and their evolution of thermal decomposition have been emphasized. Our simulations show that the products contain lots of N2 and H2O, and a little of CO and CO2. Meanwhile, some intermedium species such as NO, NO2 and OH are found to be charged, and these fragments may promote the further reaction of decomposition for FOX-7. Finally, the mechanism of thermal decomposition of FOX-7 is insensitive to hydrostatic pressure, but the high pressure may accelerate the reaction rate of decomposition.