First-principles Study Of The Thermoelectric Properties Of Mg₃Bi₂

Thermoeletric materials have excellent application prospects as one of new energy materials,which is attributed to their superb characteristics of being environmentally friendly,renewable and reusable.The only factor limiting the large-scale implementation of thermoelectric materials in practice is that their conversion power is much lower than traditional energy sources,usually not exceeding 15%.Therefore,the improvement of the conversion efficiency of thermoelectric materials is an essential goal in the field of thermoelectric materials research.It is worth nothing that Mg₃Bi₂ has a multi-valley feature in the conduction band near the Fermi surface,demonstrating its potential as an excellent thermoelectric material.The thermoelectric performance of a substance is described by a dimensionless parameter thermoelectric figure of merit ZT.The transport parameters constituting ZT are Seebeck coefficient S,electrical conductivity ? and thermal conductivity ?.Thermal conductivity is divided into electronic thermal conductivity and lattice thermal conductivity.Usually,the ratio of electronic conductivity to lattice thermal conductivity is minimal compared,and it can be ignored.At this point,the electron-related part is represented by the power factor.Besides,the transport parameters interact with each other and cannot be adjusted independently for a particular parameter.Therefore,the adjustment of ZT is a complicated process that requires repeated trial and error.In this thesis,the ground state properties of Mg₃Bi₂ and its thermoelectric properties under low-pressure conditions are studied through first-principles calculations.The ground-state electronic structure information,calculated by the Hybrid functionals method shows that it is a narrow-band semiconductor with an energy gap of0.15 e V.The bottom of the conduction band is located outside the high symmetrical linein the reciprocal space.The hydrostatic pressure that applied to system ranges from 1GPa to 4 GPa,and there is no virtual frequency in the phonon spectrum,which indicates that the structure remains stable during the pressurization process.As the pressure increases,the energy gap gradually widens,and band convergence occurs in the conduction band.At a temperature of 300 K,when the carrier concentration of N-type doping is between 10¹⁹-10²⁰cm^-3,the power factor of the system gradually increases with increasing pressure and reaches a maximum.It can be seen that the thermoelectric performance of the N-doping of Mg₃Bi₂ with low-pressure modulation leads to progressively better thermoelectric performance.