Donor-like effect and thermoelectric properties in n-type Bi<sub>2</sub>Te<sub>3</sub>-based compounds

Abstract

Grain size refinement is the vital stratagem for improving mechanical properties of Bi<sub>2</sub>Te<sub>3</sub>-based thermoelectric material. However, the donor-like effect induced by grain size refinement seriously deteriorates the thermoelectric properties especially near room temperature. Once the donor-like effect is generated, it is very difficult to eliminate the donor-like effect by the simple heat treatment process and other processes. In this study, the influences of particle size on the donor-like effect and thermoelectric properties are systematically studied for Bi<sub>2</sub>Te<sub>3</sub>-based compounds. As the particle size decreases, the donor-like effect is enhanced significantly. The oxygen-induced donor-like effect dramatically increases the carrier concentration from 3.36× 10<sup>19</sup> cm<sup>–3</sup> for 10 M sintered sample to 7.33×10<sup>19</sup> cm<sup>–3</sup> for 120 M sintered sample, which is largely beyond the optimal carrier concentration of 2.51×10<sup>19</sup> cm<sup>–3</sup> and seriously deteriorates the thermoelectric properties. However, when the particle size of the powder is 1–2 mm, the Seebeck coefficient of –195 μV/K and the carrier concentration of 3.36×10<sup>19</sup> cm<sup>–3</sup> near room temperature are achieved, which are similar to those of the ZM sample with the Seebeck coefficient of –203 μV/K and the carrier concentration of 2.51×10<sup>19</sup> cm<sup>–3</sup>. The powders without the obvious donor-like effect can be used as the excellent raw material for powder metallurgy process. A maximum <i>ZT</i> value of 0.75 is achieved for the 18 M sintered sample. The excellent thermoelectric properties are expected to be obtained by enhancing the texture further. This study provides a new way to regulate and effectively suppress the generation of the donor-like effect, and provides an important guidance for the preparation of materials with excellent thermoelectric and mechanical properties by powder metallurgy process.