Simultaneously improved thermoelectric performance and thermal stability for n‐type Mg 3 Sb 2 ‐based alloys via synergy of elemental Mg and Co doping

Abstract N‐type Mg 3 Sb 2 ‐based alloys have recently attracted considerable attention due to the high thermoelectric performance. However, the performance degradation occurs because of Mg loss at high temperature. Elemental Mg plays a significantly critical role in thermoelectric performance and thermal stability, where most studies on these compounds have thus far concentrated on the nominal Mg content which heavily depends on the fabrication methods, with few attentions devoted to the essential issue of actual Mg content, resulting in the unclear mechanism of improving their stability, severely limiting their practical applications in thermoelectric power generation. Here, we systematically analyzed the thermoelectric performance, thermal stability, and changed microstructures before and after in situ electronic thermoelectric performance measurement at 750 K, for n‐type Mg 3 Sb 2 ‐based alloys with different Mg and Co content. It was found that elemental Mg and Co have a similar effect on adjusting the electron transport characteristic, and the peak values of power factor and ZT are up to 32.4 μW cm −1 K −2 and 1.8, respectively. Thermal stability is more sensitive to the Mg content of material matrix than thermoelectric performance, and the effects of Mg‐poor condition on thermal stability cannot be compensated via cationic Co doping. We also proved the route of Mg loss in experiments. By balancing Mg content and Co doping, the optimized sample showed good stability, in which it reduced only by 10% over 170 h of measurement at 750 K. Density functional theory calculation showed that the bonding strength of Co–Mg is stronger than Mg–Mg, also explaining the enhanced thermal stability.