Theoretical Studies On Pressure,Electric Field,Nano,and Temperature Effects On Energetic Materials

External stimuli can cause accidental explosions of energetic materials.Understanding the response mechanism of energetic materials to external stimuli is essential for their safety in the preparation,processing,transportation,storage and application.In this thesis,density functional theory(DFT),density functional tight-binding method(DFTB),DFTB molecular dynamics(DFTB-MD),and MD were used to systematically study the crystal structure,electronic structure,hydrogen bond,population,vibrational properties,and initial decomposition mechanisms of HMX(1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane)crystal at high pressures.We investigated the molecular conformation,structure,energetic properties,andstabilityofcocrystalexplosiveCL-20(2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane)/HMX and its pure components under external electric field.We investigated the energetic properties,electronic structure,and vibrational properties of CL-20,NTO(5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one),and a series of chain-catenated energetic crystal nano particles.The thermal decomposition mechanisms and reaction kinetics of HMX-based cocrystal explosives and a series of chain-catenated energetic crystals at high temperatures were also studied.Main contents of the dissertation are as follows:1.Pressure effects on?-HMX crystal.Periodic DFT was used to investigate the structure,electronic and vibrational properties,and initial decomposition mechanisms of?-HMX crystal in the pressure range of 0-30 GPa.The results show that HMX presents a structural transition at 8 GPa.As the pressure increases,the intra-molecular hydrogen bonding interactions enhance gradually,but they have a sudden decrease at 8 GPa.The pressure-dependent frequency shifts indicate that all the modes move towards higher vibrational frequencies.A comprehensive analysis of density of states and population indicates that the N-NO₂group may act as an active site for the initial decomposition of?-HMX under the compression.As the pressure increases,N-NO₂homolysis becomes more preferential than HONO elimination.2.Electric field effects on cocrystal explosive CL-20/HMX.The effects of external electric field on CL-20/HMX cocrystal and its pure components were investigated by MD simulations.The results show that the conformational change of CL-20/HMX is a gradual process,while there exists sudden conformational changes at 0.3and 0.9 V·?^-1for?-HMX and?-CL-20,respectively.An analysis of N-NO₂bond length distributions,radial distribution functions between inter-molecular O and H atoms,and cohesive energy density reveals that the external electric field can remarkably decrease the N-NO₂bond length and enhance hydrogen bonding interactions.A comprehensive analysis of the sensitivity based on the maximum bond length of the trigger bond,the interaction energy between two N atoms of the trigger bond,and mechanical properties indicates that the external electric field can effectively improve the thermal stability of the systems.3.Nano effects of CL-20,NTO,and a series of chain-catenated energetic compounds.The energetic properties,electronic structure,and vibrational characteristics of CL-20and NTO nanoparticles were studied through a combined strategy based on DFTB and DFT methods.Their nanoparticles possess different characteristics from gaseous molecules and bulk crystals.The excess energies of the CL-20 and NTO nanoparticles decrease with the increasing particle size,and their enthalpies of sublimation increase with the increasing particle size.The energy gap of the CL-20 nanoparticle is higher than that of NTO nanoparticle.The surface-induced surface states of the CL-20 and NTO nanoparticles result in the significant decrease on the energy gaps and the formation of active sites on the surfaces.The vibrational frequencies of the nanoparticles are between those of gaseous molecules and bulk crystals and have wide distributions,which makes the energy easy to transfer to the molecules,thereby facilitating their decomposition.The energetic properties and electronic features of the nanoparticles for a series of chain-catenated high-nitrogen energetic crystals were studied by DFTB and DFT.It is found that the excess energies gradually decrease with the increasing particle size and nitrogen chain length.The surface energies increase with the increasing nitrogen chain length,and significantly vary with the particle size.Their enthalpies of sublimation increase with the increasing particle size,and decrease with the increasing nitrogen chain length.Their melting points increase as the particle size increases,and decreases as the nitrogen chain length increases.Their highest occupied molecular orbital(HOMO)energy levels increase with the increasing particle size,while lowest unoccupied molecular orbital(LUMO)energy levels show a gradual decrease trend.Small-sized nanoparticles are all more reactive than large-sized nanoparticles.4.Thermal decomposition mechanisms and reaction kinetics of HMX-based cocrystal explosives and a series of chain-catenated high-nitrogen energetic crystals.We studied the decomposition processes and and reaction kinetics of a series of N_x(x=4,8,10,11)chain-catenated energetic crystals at various temperatures by DFTB-MD simulations and DFT.The thermal decomposition and reaction kinetics are dependent on both the temperature and nitrogen chain's length.There are two sequential stages in the initial decomposition process for the crystals N₈and N₁₀:(i)competition between cis-trans isomerization and initial unimolecular decomposition;(ii)subsequent complicated global decomposition reactions.Increasing either the temperature or nitrogen chain's length will accelerate the competition and make initial decomposition dominated.Cis-trans isomerization does not occur in the crystals N₄and N₁₁.The dominant initiation paths for N₄,N₈,and N₁₀occur in the heterocycle and in the bond between heterocycle and azo group,while that for N₁₁is ring elimination.The decomposition reactions exhibit a clear first order kinetics character.DFTB-MD and DFT were used to elucidate the decomposition mechanisms and reaction kinetics of cocrystal explosives HMX/DMF(N,N-dimethylformamide)and HMX/DNDAP(2,4-dinitro-2,4-diazapentane)at high temperatures.The decomposition and reaction mechanisms of the two cocrystals show a great dependence on the temperature.In the HMX/DMF cocrystal,HMX involves the conformational change at 2000 K and begins to decompose at 2500 K.At 3000 K,the global decomposition and interactions of HMX with cocrystal molecules occur.The comparative results reveal that the HMX molecules have larger reactivity with DNDAP at low temperatures but with DMF at high temperatures.