Effects of Bi on the thermoelectric properties of Mg 2 Si-Mg 2 Ge solid solutions

Mg2Si1− 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 Mg2Si1− 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 Mg2Ge. The electrical conductivity rapidly increased with increasing temperature, indicating a non-degenerate semiconducting behavior, and decreased with increasing Ge content. However, all Bi-doped Mg2Si1− x Ge x solutions showed n-type conduction. The carrier concentration was increased from 4.0 × 1017 to 1.9 × 1020 cm−3 by Bi doping, and the electrical conductivity was increased from 7.3 × 10 to 4.3 × 104 Sm−1. The absolute value of the Seebeck coefficient increased with increasing temperature, and the Seebeck coefficient ranged from −91 to −224 μVK−1 for the Bi-doped specimens. Bi doping reduced the thermal conductivities of the Mg2Si1− x Ge x solid solutions at temperatures above 723 K. Mg2Si0.7Ge0.3:Bi0.02 exhibited a maximum dimensionless figure-of-merit of 0.79 at 823 K.