Research Of The Mechanism Of Energy Transport In Solid Materials Under Stress

Stress change the atom separation and the overlap of electronic orbital.Hence,for the materials,the electronic structure,the interaction between atoms and the crystal structure are changed by stress.Eventually,there will be a new phase,after all the changes achieve balance.The substances usually have different structures and special physicochemical properties under stress.The numerical simulation of thermal conductivity and the detailed analysis of heat transport mechanisms under stress,are significant in geophysics,high-temperature superconductivity and stress processing of materials.Though,the related researches are rare,in terms of theoretical analysis,numerical simulation and experimental measurement.More studies are needed.The numerical simulation is a cheaper and more feasible method to systematically study the thermal conductivity and the heat transport mechanisms under stress,compared with the experiments.So,it is used in this paper to analyze the effect of pressure on thermal conductivity.Three most popular methods for calculating the thermal conductivity and stydying the mechanisms are used to study cross-linked polyethylene(PEX),solid state argon,silicon and polymorphs of silica under ultra-high pressure,including Molecular Dynamics,First Principle calculation and solving the phonon Boltzmann transport equation.Solving the phonon Boltzmann transport equation combines with Density Functional Theory(DFT)or classical potentials is chosen to study the thermal conductivity of materials under strain.Predicting the thermal conductivity of solid argon with respect to temperature under 50 GPa and the thermal conductivity of silicon under high pressure,is used to test the validation and accuracy of this method.The phonon dispersion curve,phonon spectral thermal conductivity,phonon density of states and phonon lifetime are presented.Molecular Dynamics(MD)simulation with PCFF potential is used to study the effect of strain on thermal conductivity of space network PEX.It is shown that with longer PE chains and higher DC,the thermal conductivity of PEX is higher.The influence of strain on thermal conductivity is discussed.The thermal conductivity varies slightly under different uniform strain.When PEX is only stretched in X direction,the thermal conductivity increases with increasing strain,while generally decreasing in Y and Z direction.Thermal conductivities of polymorphs of silica,including low-phase a-quartz,high-phase stishovite and coesite,are studied by solving Boltzmann transport equation combined with BKS potential.And the result at atmospheric pressure is in reasonable consistent with the experiments.The conclusions are as below.(1)With increasing temperature,the thermal conductivity of low-phase a-quartz decreases,and the anisotropy is weakened.However,with increasing pressure,the thermal conductivity increases,and the anisotropy is enhanced.By analyzing the properties of phonons,it is found that the harmonic interaction of phonons has modest contribution to the change of thermal conductivity caused by temperature and pressure.The inharmonic interactions of phonons are main reason of the variation of thermal conductivity.The contribution of acoustic phonons to the thermal conductivity increases with lower temperature or higher pressure.The contribution of optical phonons to the hexagonal thermal conductivity is not affected.The contribution of optical phonons to the basal thermal conductivity decreases with higher temperature or lower pressure.(2)The effect of temperature and pressure on thermal conductivity of stishovite and coesite is studied.Generally,the conclusion of the effect of temperature is similar to a-quartz.The effect of pressure is related to the stiffness of the material.Pressure has smaller impact on the materials with higher stiffness.Thermal conductivity is anisotropy due to the anisotropy of phonon group velocity and the deviation of phonon distribution function.Besides the change of deviation of phonon distribution function is the main reason of the variation of thermal conductivity influenced by pressure and temperature.With increasing pressure,low-phase a-quartz transfers to high-phase coesite under same temperature.They have same electronic structure and different crystal symmetry.The thermal conductivities of a-quartz and coesite have similar values,but change from uniaxial to biaxial.Under much higher pressure,the phase of silica transfers to stishovite with different electronic structure.The thermal conductivity of stishovite is an order of magnitude higher than the other two.