Theoretical Studies Of The Structure And Properties Of Cocrystal,Defect,and Nano Explosives

The microscopic environments of the molecules in explosives produce very important effects on its structure,properties,and responses towards external stimuli.In the present thesis,density functional theory(DFT),density functional tight-binding method(DFTB)DFTB molecular dynamics(DFTB-MD)were used to systematically study the structure,energetic properties,electronic structure,optical properties,and mechanical performance of perfect crystal TEX(4.10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo-[5.5.0.05,903,11]-dodecane),cocrystal explosive CL-20(2.4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane):HMX(1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane),and vacancy-containing NTO(5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one)at high pressures.We investigated the structure,energetic properties,electronic structure,and vibrational properties of HMX crystal with twin boundaries,HMX nanoparticles,and TATB(triamino-trinitrobenzene)nanoparticles(NPs).The initial decomposition mechanisms and macroscopical reaction kinetics of HMX nanoparticles at high tempeartures were also studied.The contents of the dissertation are as follows:1.High-pressure behaviors of perfect crystal TEX,cocrystal explosive CL-20:HMX,and vacancy-containing NTO.Periodic DFT was used to investigate the geometries,electronic structure,and optical properties of ideal TEX crystal under hydrostatic pressure of 0-100 GPa.It is found that there is a structural transition in the crystal at 61 GPa.The hydrogen bonds in the system were re-distributed at the structural transition point.The band gap obviously decreases as the pressure increases.This indicates that its sensitivity increases under high pressures.Its frontier orbital energy levels were located at N-NO2.This suggests that the N-NO2 group acts as an active center for triggering the decomposition of TEX.As the pressure increases,the optical activity of TEX strengthens.We performed periodic dispersion-corrected DFT(DFT-D)calculations to compare the structural,electronic,optical,and mechanical properties of the CL-20:HMX cocrystal and its components(β-HMX and ε-CL-20)under hydrostatic pressure of 0-100 GPa.The two constituents produce different effects on the crystal structure of the cocrystal in different directions.The valence band maxima(VBM)and conduction band minima(CBM)of the cocrystal are localized on the N-NO2 groups of the HMX and CL-20 molecules,respectively.The HMX molecules contribute little to the CBM.The absorption pattern of the cocrystal is similar with that of ε-CL-20,but is completely different from that of HMX.The mechanical properties of the cocrystal present great anisotropy.The cocrystal is more capable to resist against the shape deformation or volume change than its constituents at high pressures.The effects of vacancy and pressure on the structure and initial decomposition of crystalline P-NTO were systematically investigated by using DFT-D.It is found that there is a repulsive trend between two adjacent vacancies or vacancy complexes in the solid NTO especially when they are located at different molecular layers.The electronic structure can be strongly manipulated by the vacancy defects,which can lead to the reduction of the band gap.An applied hydrostatic compression will result in the advancing close of the band gap,which further decreases the stability of the solid NTO.When the vacancy defects were introduced into the ideal bulk crystal,its VBM and CBM were located in a localized region surrounding the vacancy complexes,where initiation reactions are energetically favored than in other regions.When the pressure of 50 GPa was applied to the vacancy-containing NTO,only divacancy and trivacancy systems happened to decompose.The initial decomposition of vacancy-containing NTO took place near the vacancies and its initiation mechanisms are the hydrogen migration between two molecules.2.Formation and growth mechanisms of twin boundary in crystalline β-HMX.The dispersion-corrected DFTB(DFTB-D)and DFT-D methods were used to study the formation and growth mechanisms of twin boundary(TB)in crystalline β-HMX.Three kinds of TBs along the[010]axis were considered.The(001)surface was predicted to have the lowest surface energy andhas the largest tolerance to lattice disorder resulted from the twinning among all the surfaces.The TB along(001)surface is energetically more favorable than the(101)surface in the HMX crystal.The TB-induced VBM is localized on the TB core.The HMX molecules on both the inner and outer surfaces can be more active than in the bulk.This indicates that the surface molecules near the defects can trigger the chemical decomposition of the condensed phase P-HMX and play vital roles in increasing its sensitivity.The HMX molecule is most likely to be adsorbed in the concave site on(001)plane through either normal or twinning pathway in a competitive manner.After the grooves on the surface being filled,new grooves merge naturally for further adsorption and so promote the growth of the crystal.3.Structure and properties of HMX and TATB NPs.We studied the energetics,electronic structure,and vibrational properties of a series of spherical(3-HMX NPs using a combination of dispersion corrected DFT-D and DFTB-D methods.It is found that the properties of the surface moleculse in the HMX NPs are different from those of the molecules in the gaseous and solid states due to size effects.The induced surface states remarkably reduce the band gap of the HMX NPs and so lower the material stability.The surface states on VBM are located at the O atoms of NO2 and originate dominantly from the(100)facets.This indicates that the NO2 groups on the(100)facets have very high activity and can trigger the chemical decomposition of the NPs.In general,the frequencies of the internal modes for the HMX NPs present the wide distribution between those for gas phase molecule and bulk crystal,thus leading to the broadening of the IR and Raman bands.This increases the absorption channels for the energy transferring into the molecule from its surroundings which are responsible for triggering the initial chemical decomposition of HMX.The energetics,electronic features,and vibrational properties of a series of TATB NPs were investigated by using DFT-D and DFTB-D.The surface molecules in TATB NPs exhibit higher chemical activities than those in the gas state and in bulk crystal.There is a great anisotropy found between different surfaces exposed on the TATB NPs.The induced-surface states reduce the band gaps of the TATB NPs and so lower the material stability.These surface states on valence band side are found to be localized on the surface TATB molecules with the highest surface energies.This indicates that the surfaces with high tension play a very important role in the chemical decomposition of the systelms.The frequencies of the internal modes for the surface molecules in the TATB NPs present the wide distribution,so leading to the broadening of the IR and Raman bands.As the size of the particle increases,the frequencies of main vibrational modes in the region of 1200-1600 cm-1 increase remarkably.4.Decomposition mechanisms and reaction kinetics of a-HMX NPs at high temperatures.DFTB-MD simulations have been performed to study the thermal decompositions of different a-HMX NPs at high temperatures.The global thermal decompositions of the HMX NPs present great dependence of the temperature and particle size.In the early decomposition stage,the expansion of the HMX NPs is remarkable in competition with the initial chemical decomposition.The smaller NPs have the higher expansion ability.The following thermal decompositions are greatly dependent on the system temperatures.At low temperatures,the predominating decomposition is the isomerization of the HMX molecule,while at high temperatures,the N-NO2 homolysis accompanied with ring breaking dominates.The HONO elimination occurs to a much less extent in the DFTB-MD simulation temperatures.The global reaction rates for different NPs are both size-and temperature-dependent.Increasing either the temperature or particle size accelerates the reaction rate.The thermal decompositions and kinetics of the HMX NPs are quite different from thegaseous or solid phase decompositions.