Molecular Dynamics Study On The Thermal Conductivity Of "Phononic Liquid" Thermoelectric Material ?-Cu_2-xSe

Thermoelectric materials have a great promising prospect in the field of the energy material for its advances on transforming heat to electricity and vice versa based on Seebeck and Peltier effects.The crucial factor in the thermoelectric materials designing is optimizing the transport prosperities of electric and thermal carriers,enhancing the power factor and suppressing the thermal conductivity.The strategy of inducing disordered impurities and"liquid-like"ions makes a great success in the thermoelectric designing.This concept boosts lots of thermoelectric compounds with ultra low thermal conductivities known for“phonon-liquid”materials including"liquid-like"Cu ions.However,the role of"liquid-like"atoms is still in debate.The argument issue is whether the diffusion of Cu ions cause phonon scattering,which leads to a decrease in thermal conductivity.Molecular dynamics simulation is observable for the microscopic structure and characteristics,which is helpful to understand the role of“liquid-like”ions in the?-Cu_2-xSe thermal transport process and phonon transport mechanisms.In the thesis,we first considered the thermal conductivity and structural characteristics of?-Cu₂Se.Thermal conductivities at 500-1000 K were analysed,and the diffusion of"liquid-like"Cu ions at different temperatures was also studied.The results show that?-Cu₂Se undergoes a glass transition at about 800 K,and its mechanical and thermodynamic quantities-related properties will be affected according to this glass transistion.As the temperature increases,the thermal conductivity of?-Cu₂Se gradually decreases.Energy between particles is mainly transported through the non-bonding interaction.Non-bonding force of Cu ions(Van der Waals'force and electrostatic force)are obviously affected by temperature changes,while the Se ions aren't.Diffusivity of Cu ions can be ignored under 700 K,while the diffusivity of Cu ions gradually increases above 700 K,reflecting the"liquid-like"characteristics.However,Se ions always maintain stable.Cu ions tends to move in sites between sublattice,which weakens Cu-Se collision.The"liquid-like"characteristics of Cu ions is responsible for the low thermal conductivity.Secondly,we conducted research on vacancy and doping effect.Diffusivitis of Cu ions are greatly increased from 500 K to 1000 K when there are vacancies inside;Se ions maintain a stable sublattice structure,but MSD curve of Se ions shows that the vibration of Se ions is greatly enhanced at their equilibrium positions,and Se ions may deviate from the equilibrium position and move for a long distance according to the presence of Cu sites in Cu_2-xSe(x>0).The enhancement of Cu ions'diffusivities and Se ions n0vibration are both responsible for the decrease of thermal conductivity.Diffusivity and thermal conductivity of?-Cu_2-xSe are affected by the presence of vacancies,not influenced by quantities of vacancies.On the other hand,Ions'diffusivities and thermal conductivity in the system show no obvious changes when Cu ions are doped into the lattice.Finally,we analysed the thermal transport process inside?-Cu_2-xSe from phonon mechanism.As the temperature increases,the low-frequency peak of the phonon vibration dynamic density VDOS has a slight displacement toward the low-frequency direction,and the peak value decreased,while high-frequency peak has hardly moved,and the peak value has hardly changed,leading to a reduction of the phonon energy(frequencies)during the heat transport process,which decreases the thermal conductivity.The peak of VDOS of Se ions'vibration decreases at higher temperature when there are vacancies inside the lattice.Separation of the acoustic branch and the coupling of the optical branch and the acoustic branch cause the phonon scattering,which lead to a decrease in thermal conductivity.