Optimization of Sintering Temperature for the Synthesis of n-type Mg<sub>3</sub>SbBi<sub>0.99</sub>Te<sub>0.01</sub> Thermoelectric Materials

Abstract

Mg<sub>3</sub>Sb<sub>2</sub>-based materials are promising candidates to replace n-type Bi<sub>2</sub>Te<sub>3</sub> 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 (<i>T<sub>s</sub></i>) is a critical one. In this study, n-type Mg<sub>3</sub>SbBi<sub>0.99</sub>Te<sub>0.01</sub> polycrystalline samples were fabricated by mechanical alloying and spark plasma sintering (SPS), and the effects of varying <i>T<sub>s</sub></i> (923 – 1073 K) on the thermoelectric properties were investigated. Sintering Mg<sub>3</sub>SbBi<sub>0.99</sub>Te<sub>0.01</sub> 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 <i>T<sub>s</sub></i> 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 <i>T<sub>s</sub></i>, resulting in the maximum power factor (<i>PF<sub>max</sub></i>) of 2.2 × 10<sup>-3</sup> W m<sup>-1</sup>K<sup>-2</sup> and the maximum dimensionless figure of merit (<i>zT<sub>max</sub></i>) of 1.20 in the sample sintered at 1073 K. Therefore, when <i>T<sub>s</sub></i> was raised from 923 K to 1073 K, the <i>PF<sub>max</sub></i> and <i>zT<sub>max</sub></i> 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).