Exceptional thermoelectric performance in Mg3Sb0.6Bi1.4 for low-grade waste heat recovery

Abstract

Bi₂Te₃ alloys have been the most widely used n-type material for low temperature thermoelectric power generation for over 50 years, thanks to the highest efficiency in the 300–500 K temperature range relevant for low-grade waste-heat recovery. Here we show that n-type Mg₃Sb_0.6Bi_1.4, with a thermoelectric figure-of-merit zT of 1.0–1.2 at 400–500 K, finally surpasses n-type Bi₂Te₃. This exceptional performance is achieved by tuning the alloy composition of Mg₃(Sb_1−xBi_x)₂. The two primary mechanisms of the improvement are the band effective-mass reduction and grain size enhancement as the Mg₃Bi₂ content increases. The benefit of the effective-mass reduction is only effective up to the optimum composition Mg₃Sb_0.6Bi_1.4, after which a different band dominates charge transport. The larger grains are important for minimizing grain-boundary electrical resistance. Considering the limited choice for low temperature n-type thermoelectric materials, the development of Mg₃Sb_0.6Bi_1.4 is a significant advancement towards sustainable heat recovery technology.