First-principle Study Of High-pressure Spectra,Mechanical And Thermal Properties Of Typical Energetic Materials

Energetic materials(EMs),including explosives,propellants,and propellants,is known as high energy density materials.When subjected to a small perturbation,EMs would trigger initial reactions and release huge energy in a short time.EMs have been widely used for military and civil purposes,such as ore mining,civil engineering,rocket propulsion,and so on.Therefore,the basic research of energetic materials is important for national defense industry and economic production.In this thesis,the performance of dispersion correction methods,mechanical properties,vibrational properties under hydrostatic and uniaxial compressions as well as the thermodynamic behavior of several typical EMs at finite temperatures are studied within density functional theory.Most EMs are molecular crystals,dominated by weak interactions such as hydrogen bonds and van der Waals.Therefore,an accurate description of non-bonding interactions is essential for the study of EMs.The performance of dispersion correction methods within density functional theory(DFT),including the semi-empirical dispersion correction DFT-D scheme and the non-local correlation functional vdW-DF scheme,is assessed carefully for six EMs.At ambient condition,calculated volumes by optPBE-vdW,DFT-D3 and vdW-DF2 are in reasonable accordance with experimental data,while DFT-D3 and vdW-DF2 give satisfactory for lattice energies.Under high pressure,the results by PBE-D3 have smaller deviation from experiment than vdW-DF2 in the entire pressure range.Furthermore,the DFT-D3 method is used to calculate the bulk modulus of EMs,which is in good agreement with the experimental value.Therefore,the DFT-D3 method could describe reasonably the non-bonding interaction of EMs under ambient and high-pressure conditions.Mechanical properties of EMs could describe quantitatively the linear and nonlinear mechanical responses,which is helpful to understand the anharmonic behavior,such as thermal expansion,thermal conduction,and phonon-phonon interactions.Homogeneous deformation method combined with first-principles total energy calculations are performed to calculate a complete set of SOECs and TOECs for RDX,PETN and ?-HMX.Bulk modulus,shear modulus,Young's modulus and Poisson's ratio deduced from SOECs reveal quantitatively the anisotropic mechanical properties along different crystal orientations.For example,the x-axis direction of RDX and PETN and the z-axis direction of ?-HMX are not easy to deform when subjected to external tension.Furthermore,the number of complete TOECs of RDX,PETN,and ?-HMX are predicted.Among them,due to the relatively high symmetry,the components of TOECs of ?-HMX satisfies the symmetry relationship,indicating that the current calculation method is reliable.These results provide guidance for experimental measurements and lay the foundation for more accurate calculations.The extreme high-pressure and high-temperature conditions would lead to the changes of crystal and molecular structure,which is the focus of the study of EMs.Based on dispersion corrected DFT,the crystal structure and spectroscopic properties of several typical EMs under different pressure loading conditions are studied.Under hydrostatic compression,most vibrational frequencies show a blue-shift trend with increasing pressure,but the frequency of several modes show red-shift or subtle changes,which could be related to potential structural change.Under uniaxial compression,the Raman spectra of EMs show abnormal changes and anisotropy as the pressure increases,such as the discontinuity and red shift of TKX-50 in the[100]and[001]directions,etc.The above changes are related to the hydrogen bonding network and chemical groups of crystal,which would trigger a possible phase transition under uniaxial compression.The structure and properties at a finite temperature are critical to understand the temperature effects of EMs.Combining the dispersion corrected DFT and the quasi-harmonic approximation,the lattice dynamics of EMs,such as NM,PETN,HMX and TATB,are studied.The introduction of zero-point energy and temperature effect correction could improve the accuracy of calculations.The calculated deviation of the unit cell volume at room temperature from the experimental value is within 0.62%.The calculated lattice parameters and thermal expansion coefficient with increasing temperature show strong anisotropy,which is closely related to the crystal type and molecular configuration.Further,the calculated isobaric heat capacities are associated with the experimental values and provide the evolution of heat capacity in the wide temperature range.The bulk modulus at room temperature is accordance with the experimental values,which gradually decreases with increasing temperature.These results could provide a reference for the experimental measurement of the thermodynamic parameters of EMs.