Point defect engineering and machinability in n-type Mg3Sb2-based materials

Abstract

Approaching practical thermoelectric devices require high-performance and machinable thermoelectric materials. However, the currently available materials are usually brittle. In this work, Nd-doped Mg3Sb2-based compounds exhibit not only excellent thermoelectric performance but also superior machinability. Mg3.2Nd0.03Sb1.5Bi0.5 exhibits a high power factor of 20.6 μW cm−1 K−2 at 725 K and a peak zT of 1.8, which mainly originates from the increased n of ~8 × 1019 cm−3 by Nd/Mg substitutional defects. Defect calculations predict that other rare earth elements (Sm, Gd, Tb, Dy and Ho) have the same effect as Nd on Mg3Sb2 and the predicted highest achievable electron concentrations at 700 K are ~1020 cm−3. The measured hardness, Young's modulus and fracture toughness of Mg3.2Nd0.03Sb1.5Bi0.5 are 1.1 GPa, 49.8 GPa and 1.4 MPa m1/2, respectively. In addition, the sample can be easily machined into the dog-bone shape with external thread at both ends, indicating the excellent machinability of Mg3Sb2-based materials. This work suggests a bright future of Mg3Sb2-based thermoelectric materials for practical applications and device fabrication.