Effect of Grain Size on the Mechanical and Thermoelectric Performance of Mg3SbBi Alloys

Mg3Sb2-based compounds have emerged as promising candidates for low-temperature thermoelectric applications due to their excellent n-type thermoelectric properties. In Mg3(Bi,Sb)2, it has been well demonstrated that a large grain size and the resulting suppression of ionized impurity scattering are critical for enhancing carrier mobility and thermoelectric performance. However, practical thermoelectric devices also require robust materials, and the relationship between grain size and mechanical properties in Mg3(Bi,Sb)2 materials remains underexplored. Here, we investigate the influence of grain size, modulated by varying sintering temperatures and methods, on the thermoelectric and mechanical properties of n-type Mg3+σY0.02SbBi samples. As the sintering temperature increases from 773 to 1073 K, the average grain size grows substantially from ∼12.9 to ∼56.6 μm, leading to an improved weighted carrier mobility of up to 142 cm2 V–1 s–1 and power factor of 18.2 μW cm–1 K–2. However, the enlarged grain size simultaneously results in a decrease in Vickers hardness by ∼40% and elastic modulus by ∼24%, consistent with the Hall–Petch relationship. By balancing the mechanical and thermoelectric performance, the sample sintered at 873 K achieves a favorable combination of high hardness and an average zT of 0.92. This work highlights the critical role of microstructural control in advancing Mg3Sb2-based thermoelectric materials and provides a mechanical performance criterion for evaluating their potential in practical device applications.