First-principles Study Of Thermoelectric Properties Of BiCuChO(Ch=Se,S)

Thermoelectric materials can implement the conversion between thermal energy and electrical energy,and have broad application prospects in the fields of waste heat utilization and solid-state refrigeration.Thermoelectric performance can be described by the thermoelectric figure of merit ZT,which depends linearly on the electrical conductivity,proportional to the square of the Seebeck coefficient,and inversely proportional to the thermal conductivity.Therefore,improving the thermoelectric properties of thermoelectric materials usually reduces the thermal conductivity of the materials,and improves the electrical conductivity and Seebeck coefficient of the materials.Due to its unique layered structure,the quaternary chalcogenide compounds BiCuChO(Ch=Se,S)have a very low thermal conductivity,exhibiting great potential in thermoelectric applications.However,the conductivity of BiCuChO and its associated power factor still need to be improved.Therefore,based on the first-principles calculation method and semi-classical Boltzmann transport theory,this thesis has systematically studied the effects of out-of-plane uniaxial strain,in-plane biaxial strain,and doping on the thermoelectric properties of BiCuChO materials.It is found that the application of strains and doping have a significant effect on the thermoelectric properties of BiCuChO materials.The reason is that these factors change the electronic structure of the BiCuChO,which affects the conductivity and power factor of BiCuChO.In addtion,the optimal thermoelectric properties are obtained.The main research contents and conclusions are summarized as follows:(1)A method for enhancing the thermoelectric properties of p-type BiCuChO by out-of-plane uniaxial compressive strain and enhancing the thermoelectric properties of n-type BiCuChO by out-of-plane uniaxial tensile strain are proposed.The effect of out-of-plane uniaxial strain on the thermoelectric properties of BiCuChO materials was studied.For the BiCuSeO,the results show that the valence band near the Fermi level under compressive strain along the Z-(38)direction becomes steeper and the slope of the density of states near the top of the valence band decreases,which is beneficial for enhancing the thermoelectric performance of p-type BiCuSeO.The enhancement of the thermoelectric performance of p-type Bi Cu SO is due to the increase in its Seebeck coefficient under under out-of-plane uniaxial compressive strain.The conduction band near the Fermi level along the(38)-Z direction is flattened under the tensile strain,the slope of the density of states near the top of the conduction band is increased,and the effective mass of carriers is increased,resulting in the enhancement of thermoelectric performance of n-type BiCuChO.In addition,we also calculated the optimal carrier concentration under different out-of-plane uniaxial strains.(2)The effects of in-plane biaxial strain on the thermoelectric properties of BiCuChO materials was studied.It was found that in-plane biaxial compressive strain can improve the thermoelectric properties of n-type materials,while in-plane biaxial tensile strain can enhance the thermoelectric properties of p-type materials.The reason is that the in-plane biaxial strain lead the change of electronic structure of the BiCuChO materials.Under the in-plane biaxial compressive strain,the slope of the density of states near the conduction band bottom increases,which is beneficial for improving the Seebeck coefficient of the n-type material.Under the in-plane biaxial tensile strain,the valence band near the Fermi plane in the M-(38)direction becomes steeper,which is beneficial for improving the conductivity of the p-type BiCuSeO materials,while the increase in the effective mass of carriers can enhance the Seebeck coefficient of p-type BiCuSeO.(3)The effects of Te atom doping on the thermoelectric properties of BiCuSeO was studied.The calculation results of electronic structure show that density of states curve of Te-doped BiCuSeO shifts to a lower energy direction,and the corresponding peak of the density of states of the valence band near the Fermi level decreases.Although the electrical conductivity of BiCuSeO material has been reduced by Te atom doping,the Seebeck coefficient has been improved,especially the n-type BiCuSeO,resulting in Te atom doping improving the power factor of n-type BiCuSeO material.