Thermoelectric Transport Properties of N-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> Compound Prepared by Oxide-Reduction Process

Abstract

We report the thermoelectric transport properties of n-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> compounds prepared by oxide-reduction process. The oxide-reduction process is a recently developed method that can synthesize Bi2Te3-based compounds using only oxide starting materials, which is advantageous in terms of the diversity of the starting materials and cost efficiency. In this study, starting materials of Bi<sub>2</sub>O<sub>3</sub>,TeO<sub>2</sub> and SeO<sub>2</sub> were ball-milled for homogenous mixing, and then oxidized to prepare mixed oxide powders. The mixed oxide powders were reduced at different reduction temperatures (573, 603, 633 and 663 K) for 7 h under hydrogen atmosphere. A single phase of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> could be achieved when the reduction temperature was higher than 603 K, and the shape of the particles changed from granules to flakes with increasing reduction temperature. Furthermore, the formation of antisite defects of Bi<sub>Se</sub> was promoted by increasing reduction temperature, which strongly affected the electrical and thermal transport properties of the Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> compounds, which were prepared by spark plasma sintering of the reduced powders. The anisotropic microstructures in the sintered bodies were significantly influenced by the shape of the reduced powders, which was also controlled by the reduction temperature. The thermoelectric transport properties were characterized at room temperature, and the detailed effects of the reduction temperature on these properties are discussed in terms of the anisotropic microstructure and antisite defects.