Theoretical Studies On The Structure And Properties Of Cocorystal Explosives

Co-crystallization can cause changes in the structures and properties of energetic materials,which will affect the application of explosives.Hence,for the ongoing development of explosives and the expansion of their applications,a systematic and in-depth research of the changes in the structure and characteristics of co-crystals explosives as well as the underlying reasons of these changes is of utmost importance.In this thesis,density functional theory（DFT）,ab initio molecular dynamics（AIMD）,density functional tight-binding MD（DFTB-MD）and MD methods were used to systematically study the structures,intermolecular interactions,stability,detonation performances,and thermal decomposition mechanisms for different types of cocrystal explosives.The effects of different solvents on the crystal structures,interactions,and impact sensitivity of BTATz（3,6-Bis（1H-1,2,3,4tetrazol-5-ylamino）-s-tetrazine）cocrystals were analyzed.The variations of thermal decomposition mechanisms and the generation of nitrogen gas caused by pyrazine in BTATz-based cocrystal were compared.The crystal structures,interactions,and electronic properties of the cocrystals formed by DNAN（2,4-dinitroanisole）combined with DNB（1,3-dinitrobenzene）and NA（2-nitroaniline）were discussed and the melting-points were predicted.The thermal decomposition mechanisms,decomposition products,and reaction kinetic parameters of pure DNAN crystal and its cocrystals were studied.The non-covalent intermolecular interactions,decomposition mechanisms,gas product evolution,and reaction rate constants of three CL-20（2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaaza-isowurtzitane）host-guest cocrystals under high temperatures were investigated.The main contents of the dissertation are as follows:1.Structures,intermolecular interactions,and detonation properties of BTATz-based solvate cocrystalsThe structure,intermolecular interactions,and detonation properties of BTATz crystal and its solvent cocrystals were studied by DFT and MD.H×××O-C hydrogen bonding is the main force for maintaining the stability of BTATz/2-pyridone and BTATz/DMF cocrystals,while N-H·N hydrogen bonding is the main molecular forces for BTATz crystal and BTATz/pyrazine cocrystal.The former two cocrystals are cross-type crystal packing mode and the hydrogen bond strength is relatively weak,so leading to their poor stability.The BTATz/pyrazine cocrystal has a high degree of electron delocation,which is the intrinsic cause of the widespread presence of largeπbonds between its molecules,van der Waals interactions,and the formation of face-to-face crystal packing patterns.BTATz solvent cocrystals have lower impact sensitivity than pure BTATAz crystal,but retain the relatively high detonation performance of BTATz.2.Decomposition mechanisms and N₂ release mechanisms of BTATz/pyrazine cocrystal at high temperaturesThe decomposition processes of BTATz crystal and BTATz/pyrazine cocrystal were simulated using AIMD.The initial decomposition paths of BTATz in the BTATz crystals include N-H bond breaking on the tetrazolium ring,N-N bond breaking on the tetrazolium ring,and N-H bond formation by the H radical with the tetrazine ring.The three initial decomposition paths of BTATz in the BTATz/pyrazine cocrystal are different from those of the BTATz crystals.N₂ was produced by the reaction of the tetrazine ring or the tetrazolium ring in the two crystals,but their generation speeds of N₂ are different.The formation of the BTATz/pyrazine cocrystal can accelerate the decomposition of BTATz and the energy release,so promoting the release of N₂.3.Structure,molecular interactions,stability,and melting point prediction of DNAN-based cocrystalsDFT and MD were used to explore the crystal structure,molecular interactions,electronic structure,and stability of DNAN/DNB and DNAN/NA melt-cast cocrystals.Face-to-face crystal stacking pattern and relatively strong layer-to-layer interactions make DNAN/NA present low impact sensitivity.The amino group of the NA molecule is a strong electron donor and has small HOMO-LUMO energy gap,so leading to the decrease of the melting point of the DNAN/NA cocrystal.4.High-temperature decomposition mechanisms of DNAN-based melt-cast cocrystalsThe decomposition processes of pure DNAN crystal,DNAN/DNB and DNAN/NA melt-cast cocrystals under high temperatures were simulaed by DFTB-MD.The effects of the cocrystalization on the decomposition mechanisms,products,and rates of DNAN crystals were analyzed.The initial decomposition paths of DNAN in the three crystals are dominated by denitrification and demethylation and the cocrystalization has little effect on the initial decomposition path and the complete decomposition path of DNAN.However,the cocrystalization changed the decomposition rate of DNAN:DNAN/NA>DNAN/DNB>pure DNAN crystal and will increase the amount of gas decomposition products CO and NO.In addition,the release energy during the decomposition process of the cocrystals is higher than that of the pure DNAN crystal.This shows that the performance of the cocrystals is superior to that of the pure DNAN crystal.5.Decomposition mechanisms of CL-20 host-guest cocrystals at high temperaturesUsing DFT,changes in the ESP distribution of CL-20 in cocrystals with three guest small molecules,CO₂,H₂O and N₂O,topological values of the IGM and AIM bond critical points were calculated and weak interactions between the guest molecules and CL-20 molecules were analysed.DFTB-MD was used to simulate the decomposition processes of three CL-20host-guest cocrystals（CL-20/CO₂,CL-20/H₂O,and CL-20/N₂O）at 2000,2500,and 3000 K.Under the decomposition temperature,the weakest intermolecular interactions were identified involved in CL-20/N₂O.The decomposition of the CL-20-based host-guest cocrystals will be accelerated with the increase of the temperature.They release a large amount of energy and six gas small molecules H₂O,NO₂,N₂O,NO,CO,and N₂ were maintained during the decomposition processes.The order of gas production and heat release of the three host-guest cocrystals is CL-20/N₂O>CL-20/H₂O>CL-20/CO₂.The decomposition of the CL-20/N₂O cocrystal has the lowest reaction activation energy.