Impurity-free, mechanical doping for the reproducible fabrication of the reliable n-type Bi2Te3-based thermoelectric alloys

Abstract

Precise control of carrier density is essential to synthesize high-performance thermoelectric materials. Doping by impurities is often frustrated in n-type Bi2Te3 alloys by incomplete activation, bipolar doping, the formation of secondary phases, and prevailing intrinsic point defects such as vacancies. This weakens the reproducibility of synthesis processes and reduces the long-term reliability of material's performance, hence aging. Here, we explore an impurity-free doping technique to synthesize n-type bismuth tellurium selenides, combining a cold deformation and a hot extrusion. The cold deformation enables controlling the electron density in the range of similar to 10(19)/cm(3) via the formation of intrinsic point defects, and the hot extrusion allows texturing the microstructure to enhance the electrical conductivity, hence a large power factor of >5 x 10(-3) W-m(-1)-K-2. We confirm that our process is very reproducible, and the properties of the samples are stable without aging even after thermal stresses. Using this method, we can decouple the relationship between bandgap, carrier density, and composition to improve the high temperature thermoelectric property. Moreover, we demonstrate the fabrication of high-performance thermoelectric materials from low-graded, raw materials by modifying the degree of the mechanical deformation to reach an optimum carrier density. Our work provides a promising approach to synthesizing n-type thermoelectric materials in the reproducible and adaptable way. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.