Optimization of Thermoelectric Properties of Mg3Sb2-xBix Phases

Abstract

Thermoelectric (TE) materials can directly interconvert heat and electricity. Given escalating energy needs and greenhouse gas emissions, TE technology emerges as a promising green solution for recovering waste heat. Material performance is quantified by the dimensionless figure of merit (ZT = S²σT/κ), demanding a delicate balance of low thermal conductivity, high electrical conductivity, and high Seebeck coefficient. Unfortunately, optimizing these properties simultaneously proves challenging. Moreover, the widespread use of TE generators is hindered by their low efficiency, high cost, and environmental impact. This research seeks to develop cost-effective, abundant, non-toxic, and sustainable TE materials. For an n-type material, this study explores the effects of Mg and Bi amounts, Mn doping, ball milling duration, and sintering conditions on the purity and thermoelectric properties of Mg3BixSb1.99-xTe0.01. We found that excess Mg is necessary to achieve an n-type conduction even in the presence of Te, but too much Mg forms impurities that decrease thermoelectric efficiency. Additionally, Mg-rich annealing improves the material's grain size while preserving its n-type properties. Increasing Bi content leads to decreased phase stability and decomposition. Mn doping increases the carrier concentration and electrical conductivity but reduces the Seebeck coefficient. There is also an optimal ball milling time, beyond which decomposition of the material occurs. The highest figure of merit, zT of 1.55 was achieved for the Mg3.27Mn0.03Bi1.3Sb0.69Te0.01 sample at 623K, which is comparable to the performance of Bi2Te3. In the second part, Ni was used to optimize carrier concentration by first replacing Mn in the Mg site, as in Mg3.29Ni0.01Bi1.5Sb0.49Te0.01, and secondly, by acting as an independent dopant, as in Mg3.27Mn0.03Ni0.01Bi1.5Sb0.49Te0.01.