Theoretical Calculations Study On Vibrational Spectrum And Regulation Of Electronic Structure For Typical Energetic Materials

Energetic materials are widely used in the field of national defense technology and civil industry.As metastable materials,they are attracted much attention by domestic and foreign researchers.The representative compounds include hexahydro-1,3,5-trinitro-1,3,5-triaz（RDX）,octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine（HMX）,1,3,5-triamino-2,4,6-trinitrobenzene（TATB）,1,3-diamino-2,4,6-trinitrobenzene（DATB）and hexanitrohexaazaisowurtzitane（CL-20）.Although numerous studies have been done in the early stage in order to understand the reaction mechanism and safety performance of energetic materials,it is still a scientific issue of great interest to explore their detonation mechanism and sensitivity from the microscopic perspective.Both the detonation mechanism and sensitivity largely depend on the physical and chemical properties of the energetic materials.Therefore,it is important to investigate the microstructures and physical properties under different conditions.Based on the above considerations,five representative materials are chosen as research object,and the microstructure and related physical properties are studied by using the first-principles method.Firstly,the crystal structures,molecular vibrational modes,thermodynamic properties,mechanical properties and electronic properties ofα/γ-RDX,α/β/δ/γ-HMX,TATB,DATB,andε-CL-20 are studied at zero presusure using the first-principles density-functional theory.Comparing the dispersion correction methods of GGA-PBE+G and GGA-PBE+TS,it is found that the results obtained by using GGA-PBE+G are closer to the experimental values of previous studies.The calculated vibration frequencies are consistent with the experimental and theoretical values.The vibration modes in different frequencies are analyzed.Because the modes in the low frequency region are mainly assigned to-NO₂ groups,it is supported that the C-N or N-N bond fracture first when the matreials decompose.From the elastic constants and 3D diagrams of Young’s modulus,we can see that the stiffness and Young’s modulus are anisotropy.The band gaps of RDX are also different in different crystal directions,it is indicated that the impact sensitivity shows anisotropy in different directions.In addition,the density of states（DOS）near the Fermi level is mainly contributed by the O-p and N-p orbitals,indicating that the electrons around the-NO₂ group of energetic materials are more active.Subsequently,using the GGA-PBE+G correction,the effect of pressure on the structures and related physical properties ofα-RDX,α/β/δ/γ-HMX,TATB andε-CL-20,as well as the response of impact sensitivity to pressure are investigated.The compression of the calculated crystal structures under pressure is consistent with the previous research results.For HMX,there is no sudden change near 27 GPa in the P-V diagram,which shows that HMX has no phase transformation point near 27 GPa.Then,the change of bond length and atomic position are discussed.The largest change in the chemical bond length is the N-N or C-N bond,and the largest change in the atomic position is the N and O atom,indicating that-NO₂ group is the most unstable under pressure.With the increasing pressure,the band gaps decrease,and the widths of their DOS increase.Therefore,the electrons have stronger activity due to the nonlocality increase.Moreover,the density of states near the Fermi level is mainly contributed by O-p and N-p orbits,indicating that the electrons around the-NO₂ group are most active under pressure.Therefore,the N-N or C-N bond of the molecule is the easiest to rupture,verifying that the initial decomposition is breakage of the N-N or C-N bond.The elastic constants and the modulus of elasticity increase with the increasing pressure.From the 3D map of Young’s modulus,the anisotropy of Young’s modulus for TATB decreases with the increasing pressure,while the anisotropy forε-CL-20 increases.In addition,effective mass decreases with the increasing pressure,showing that the electron migration ability is enhanced.It means that the effective mass is related to the impact sensitivity.The smaller the effective mass is,the higher the corresponding sensitivity is.Finally,the（100）,（010）,（001）,（110）,（101）,（011）,and（111）surfaces ofα-RDX,the（001）,（010）,（011）,（100）,（101）,（110）and（111）surfaces ofβ-HMX,and the（001）,（010）,（011）,（012）,（021）,（100）,（101）,（102）,（110）,（111）,（120）,（201）and（210）surfaces ofε-CL-20 are built.The surface structures,electronic structures,excess energies,surface energies and nitro group charges of different surface state structres are investigated using the first principles method.Compared with the bulk structures,the largest change in bond length in the surface states is the N-N bond.All the N-N bond lengths in the surface structures are larger than those in the bulk structures.Meanwhile,the band gaps are defferent in these surface structures,which are smaller than those in bulk structures.It is indicated that the impact sensitivity of surface is larger,showing that the surface states of energetic materials will affect their sensitivity.In addition,the smaller the nitro charge surface is,the larger the surface energy and the excess energy are.Therefore,it is indicated that excess energy can be used as a basis for determining the degree of the impact sensitivity.