Thermoelectric properties of Mg₂Si_0.7Ge_0.3Bi _m prepared using a solid-state reaction

Abstract

Mg₂Si_0.7Ge_0.3Bi _m (m = 0 − 0.03) solid solutions were synthesized using a solid-state reaction and consolidated by hot pressing. All specimens showed n-type conduction, and the carrier concentration increased from 4.0 × 10¹⁷ to 2.1 × 10²⁰ cm⁻³ with increasing Bi-doping content; the electrical conductivity thereby increased from 7.3 × 10² to 7.0 × 10⁴ Sm⁻¹ at room temperature. The electrical conductivity of the undoped specimen increased with increasing temperature, and the specimen behaved similar to a non-degenerate semiconductor. The absolute value of the Seebeck coefficient of the undoped specimen was very high at low temperature, but it decreased with increasing temperature. Bi-doped Mg₂Si_0.7Ge_0.3 showed a Seebeck coefficient ranging from −235 to −197 μVK⁻¹ at 823 K. The power factor significantly increased with Bi doping and increased with increasing temperature; it were also improved by around 10 times at 823 K. The lowest thermal conductivity was 2.2 Wm⁻¹K⁻¹ for Mg₂Si_0.7Ge_0.3Bi_0.02 at 823 K, and the maximum ZT value of 0.79 was obtained for Mg₂Si_0.7Ge_0.3Bi_0.02 at 823 K.