Molecular Dynamics Simulation Of Thermal Conductivity Regulation Based On Defect Structure And Inharmonic Properties

Graphene has a very high thermal conductivity,and the defective structure will destroy the thermal conductivity of graphene.Defects including stacking,doping,and structural shape can have unpredictable effects on the thermal conductivity of graphene.The impact of defects such as Single vacancy,double vacancy and Stone Wales on the reduction of graphene thermal conductivity has been very clear.In this article,we constructed three different defect structures(regular defect,random defect,fractal defect)of graphene to explore the influence of structural defects on the thermal conductivity of graphene by molecular dynamics simulation.The equilibrium molecular dynamics(EMD)simulation showed that the thermal conductivity of graphene gradually decreased with the increase of porosity.Among them,the fractal defect structure has the most significant effect on the thermal conductivity of graphene.The phonon transport function shows that the defects lead to the suppression of phonon transport,the enhancement of phonon-defect scattering,the increase of heat transfer resistance,and the decrease of thermal conductivity.Different defect types and the number of defects are closely related to the thermal conductivity of materials.Through structural regulation,we not only have a more comprehensive understanding of the thermal conductivity of graphene,but also have a clearer understanding of the phonon heat transport mechanism.It enriches the study of the heat transport performance of graphene of various defect types,and provides a huge dataset and reliable feature values for machine learning to predict thermal conductivity.Metal halide perovskites(MHPs)have soft lattices with strong anharmonicity and will undergo entropy-driven solid-solid phase transitions upon heating.Here,we investigate the polymorph stabilities and phase transitions in one of the lead-free MHPs,CsSnI3,by several molecular simulation techniques.Three different phase transitions(γ?β,β?α,and yellow→black)in CsSnI3 have been successfully reproduced by Molecular Dynamics(MD)simulations with a newly developed empirical force field.The heating and annealing MD simulations and free energy calculations with the non-equilibrium thermodynamic integration(NETI)method predict the transition temperatures of 275,385,and 280 K for the γ?β,β?α,and yellow→black transitions,respectively.Lattice dynamics(LD)simulations within the harmonic approximation fail to predict the correct phase stability in CsSnI3 at high temperatures.The quasi-harmonic approximation(QHA)calculations that include the volume dependence of the phonon frequencies and lattice energies correctly predict all phase transitions in CsSnI3.However,the transition temperatures of the γ?β and β?α transitions predicted by the QHA calculations significantly deviate from those by MD simulations.By comparing the Gibbs free energies calculated by the LD simulations within the QHA and MDbased NETI method,we find the differences of 3-30 meV for different polymorphs.Although calculations based on the harmonic model can provide valuable information,the anharmonic terms need to be included for accurate predictions of transition temperatures of phase transitions in CsSnI3 and other MHPs.During the temperature rise or annealing,the tilt of the octahedron can lead to structural phase transition in the perovskite structures.We studied the thermal conductivity of perovskite with structural phase transition by equilibrium molecular dynamics simulation.During the study of each phase,the thermal conductivity of perovskite decreases with the increase of temperature.Near the phase transition point,no different phenomenon was observed,and the thermal conductivity decreased steadily.In addition,non-equilibrium molecular dynamics(NEMD)simulations and first-principles calculations were used for auxiliary verification,and the results were basically consistent.