Theoretical Studies Of The Thermal Decomposition Mechanisms Of Cocrystal,Defective,and Aluminized Explosives

The detonation process of explosives is a complex reaction process that occurs under external stimuli with characteristics such as extremely short reaction time and complex reaction process.The aggregation state and microenvironment of molecules in condensed explosives have a significant impact on their structure and properties.The structure of explosives determines their properties,which determine their use.Therefore,studying the reaction process of different types of explosives under external stimuli is of great significance for revealing their detonation mechanism and synthesizing new explosives.In this thesis,density functional theory（DFT）,density functional tight binding（DFTB）method,ab initio molecular dynamics（AIMD）,DFTB-MD,and reactive MD（RMD）were adopted to study the decomposition mechanisms of cocrystal,defective,and aluminized explosives at high temperatures.And the relationship between the explosive structure and performance was revealed.The contents of the dissertation are as follows:1.Decomposition mechanism of cocrystal explosives at high temperaturesAIMD was used to simulate the thermal decomposition process of a series of energetic cocrystals containing 2,4,6-triamino-1,3,5-triazine-1,3-dioxide（TTDO）and oxidizing molecules（TTDO:H₂O₂,TTDO:HNO₃,and TTDO:HCl O₄）at 3000 K.The initial decomposition path of the TTDO molecule includes N-H bond breaking,intramolecular H transfer,intermolecular H transfer,and C-N bond breaking,while the existence of the HNO₃and HCl O₄ molecules changes its initial decomposition mechanism.The key to the formation of H₂O is the combination of H and OH,that of N₂ is the formation of the-N-N-structure,and that of CO₂ is to form the intermediate CO-R with carbonyl structure and then produce the fragment with the-O-C-O-structure.Moreover,H₂O₂,HNO₃,and HCl O₄ were all involved in the formation of H₂O,N₂,and CO₂.These oxidizing molecules affect not only the types of N_x,but also its formation mechanisms.DFTB-MD simulations were performed to study the thermal decomposition mechanisms of benzotrifuroxan:2,4,6-trinitrotoluene（BTF:TNT）cocrystal at 2400 and 2700 K.The results show that the TNT molecules are easier to decompose at the beginning of the reaction,which promotes the decomposition of the whole cocrystal system.The N-O bond breaking of furoxan ring is the main decomposition path of BTF.The C-NO₂ homogeneous decomposition and the rearrangement of-NO₂ are the main decomposition paths of TNT.The cocrystal produces a series of small molecules and intermediates during thermal decomposition,in which-CNO is an important intermediate product.As the temperature increases,the thermal decomposition rate constants of BTF and TNT continuously increase,indicating that the decomposition of the system accelerated.2.Structure and decomposition mechanisms of explosives containing defectsThe structure and decomposition mechanisms of BTF crystal with vacancy defect were studied by DFTB and DFTB-MD methods.The vacancies tend to be distributed in the BTF crystal in the form of aggregation in thermodynamics.The opening of the furoxan ring by the N-O bond fracture is the most important decomposition path of the BTF molecule.The introduction of the vacancies into the BTF crystal does not change its decomposition path,but its decomposition rate and initial decomposition molecule’location change;and moreover,the BTF molecules around the vacancy defects are easier to decompose.The diffusion coefficients of the C,N,and O atoms in the vacancy-containing systems are greater than those in the ideal system.Among these atoms,the vacancies have the greatest influence on the diffusion of the C atoms.The same methods were employed to study the crystal structure,electronic properties,intermolecular interactions,and decomposition mechanisms of impurity-containing3-nitro-1,2,4-triazole-5-one（NTO）crystals.The impurity molecules tend to be distributed in the form of aggregates.The frontier orbitals of the NTO crystal are mainly distributed in the local area near impurity molecules.The initial decomposition path of the NTO molecule is the same in the impurity-containing and ideal crystals,and its thermal decomposition mechanism is dominated by the single molecule decomposition mechanism.The NTO molecules around the impurity molecules begin to decompose first.In the initial reaction stage,the decomposition rate of the NTO crystal containing impurities is faster than that of the ideal crystal,but the inhomogeneous effect caused by the impurities disappears with the extension of time.3.Decomposition mechanisms of aluminized explosive nanoparticlesThe thermal decomposition of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane（HMX）based aluminized explosive nanoparticle（NP）with core-shell structure at high temperature was simulated by RMD.The decomposition of the HMX@Al NP is earlier than that of the HMX NP.The amount of NO,NO₂,N₂,H₂O and CO₂ produced by the HMX@Al NP is lower than that by the HMX NP.These products are easy to react with Al to form a series of aluminized clusters such as Al_mN_n,Al_mC_n,and Al_mO_n since the Al atoms have high activity.The Al_mO_n clusters have zeolite-like structure with dozens of atoms,the Al_mC_nclusters possess two-dimensional network structure,and the Al_mN_n clusters have shortchain structure.Besides,the C clusters in the HMX@Al system are larger,indicating that the existence of Al is helpful for the growth of the C clusters.The thermal decomposition of core-shell structured aluminized explosive NPs under oxygen atmosphere were simulated by RMD.The decomposition of the aluminized explosive NPs is accompanied by volume expansion and the Al shell undegoes a process of melting-diffusion-reaction-aggregation from the initial surface shell to the block.O₂ mainly reactes with the intermediates and free radicals and rarely directly participates in the initial decomposition of the explosive molecules.A large number of the solid aluminized substances form,and the Al_mO_n clusters are the main components of solid aluminized substances since there is sufficient oxygen in the system,while the Al_mC_n and Al_mN_n clusters distribute sporadically.The Al_mO_n clusters in the TATB@Al O NP have a block structure,while the others have sphere-shaped structures.The block and spherical Al_mO_n clusters have similar unit to the zeolite with three-dimensional structure.