Optimization of Sintering Temperature for the Synthesis of n-type Mg₃SbBi_0.99Te_0.01 Thermoelectric Materials

Mg3Sb2-based materials are promising candidates to replace n-type Bi2Te3 for cooling and power generation at low temperatures. Generally, the thermoelectric performance of a material is sensitively affected by synthesis process parameters, and among them, sintering temperature (Ts) is a critical one. In this study, n-type Mg3SbBi0.99Te0.01 polycrystalline samples were fabricated by mechanical alloying and spark plasma sintering (SPS), and the effects of varying Ts (923 – 1073 K) on the thermoelectric properties were investigated. Sintering Mg3SbBi0.99Te0.01 at an elevated temperature of 1073 K resulted in a notable increase in electrical conductivity at low temperatures below about 423 K. This is ascribed to a sharp reduction in carrier scattering by ionized impurities. For the same reason, the carrier mobility increased sharply at a Ts of 1073 K, which is a critical temperature for sintering in this study. Moreover, the Seebeck coefficient increased and thermal conductivity decreased simultaneously by raising Ts, resulting in the maximum power factor (PFmax) of 2.2 × 10-3 W m-1K-2 and the maximum dimensionless figure of merit (zTmax) of 1.20 in the sample sintered at 1073 K. Therefore, when Ts was raised from 923 K to 1073 K, the PFmax and zTmax increased by 29 % and 64 %, respectively. This improvement in performance is attributed to the annihilation of defects generated during the mechanical alloying process, which was confirmed by microstructure analysis by transmission electron microscopy (TEM).