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

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 fabrication 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 annealing. 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 (zT_max) of approximately 1.1, whose performance at the module level exceeded those of commercial single-crystal counterparts. This study provides 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.