Effects of Bi on the thermoelectric properties of Mg2Si-Mg2Ge solid solutions

Mg 2 Si 1− x Ge x :Bi m (0.3 ≤ x ≤ 0.7, m = 0 or 0.02) solid solutions were synthesized by using a solid-state reaction (SSR) and were consolidated by hot pressing (HP). In the case of the undoped Mg 2 Si 1− x Ge x specimens, the electrical conduction changed from n-type to p-type at room temperature for x ≥ 0.7 due to the intrinsic properties of Mg 2 Ge. The electrical conductivity rapidly increased with increasing temperature, indicating a non-degenerate semiconducting behavior, and decreased with increasing Ge content. However, all Bi-doped Mg 2 Si 1− x Ge x solutions showed n-type conduction. The carrier concentration was increased from 4.0 × 10 17 to 1.9 × 10 20 cm −3 by Bi doping, and the electrical conductivity was increased from 7.3 × 10 to 4.3 × 10 4 Sm −1 . The absolute value of the Seebeck coefficient increased with increasing temperature, and the Seebeck coefficient ranged from −91 to −224 μ VK −1 for the Bi-doped specimens. Bi doping reduced the thermal conductivities of the Mg 2 Si 1− x Ge x solid solutions at temperatures above 723 K. Mg 2 Si 0.7 Ge 0.3 :Bi 0.02 exhibited a maximum dimensionless figure-of-merit of 0.79 at 823 K.