Solid-state synthesis and thermoelectric properties of Bi-doped Mg2Si compounds

Bi-doped Mg2Si (Mg2Si:Bim, m = 0, 0.005, 0.01, 0.02, 0.03) was prepared by solid-state synthesis. The electronic transport properties (Hall coefficient, mobility and carrier concentration) and thermoelectric properties (Seebeck coefficient, electrical conductivity, thermal conductivity and figure-of-merit) were examined. Mg2Si was synthesized successfully by a solid-state reaction at 673 K for 6 h and Bi-doped Mg2Si powders were obtained by mechanical alloying for 24 h. They were fully consolidated by hot pressing at 1073 K for 1 h. All the Mg2Si:Bim samples exhibited an n-type conduction, indicating that the electrical conduction is mainly due to electrons. The electrical conductivity increased significantly and the absolute value of the Seebeck coefficient decreased with increasing Bi content because the electron concentration was increased considerably from 1016 to 1020 cm−3 by Bi doping. The thermal conductivity was not changed significantly by Bi doping due to the much larger contribution of lattice thermal conductivity over electronic thermal conductivity. The thermal conduction in Bi-doped Mg2Si was attributed dominantly to the lattice vibrations (phonons). A maximum thermoelectric figure-of-merit of 0.7 was obtained for Mg2Si:Bi0.02 at 823 K.