Study of the relationship between process parameters, volatility of Te, and physical properties in n-type Bi2Te3-based alloys for the reproducible fabrication of high-performance thermoelectric materials

Abstract

Solid compounds containing volatile elements often deviate from the stoichiometric composition, owing to the loss of the volatile species during thermal processes. This leads to a wide range of properties reported for the nominally same materials. It is important to understand how the process parameters of each fab-rication step affect the volatility of the element, and how the physical properties change with the altered stoichiometry. Herein, we address this issue in n-type thermoelectric Bi2Te3-based alloys containing volatile Te. Thermal analysis indicated that mechanical deformation enhanced the volatility of Te by inducing a eutectic reaction through the formation of Te-rich phases. The effective evaporation of Te from the eutectic liquid phase created point defects and micropores, which significantly affected the thermoelectric and physical properties of the specimens. Moreover, Te atoms were transferred from heavily deformed samples to lightly deformed samples along the chemical potential gradients of Te via the vapor phase during an-nealing. The post-annealing temperature was increased above the eutectic temperature by introducing a pre-annealing step, which suppressed the formation of micropores. Therefore, we fabricated high-density n-type Bi2Te3-based alloys with a thermoelectric figure-of-merit (zTmax) of approximately 1.1, whose per-formance at the module level exceeded those of commercial single-crystal counterparts. This study pro-vides a comprehensive relationship between the process parameters, Te volatility, and physical properties of Bi2Te3-based alloys. Our results offer an insight for the reproducibly and tuning of physical properties of other compounds containing volatile elements other than Bi2Te3.(c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).