Tellurium-evaporation-annealing for p-type bismuth-antimony-telluride thermoelectric materials

A tellurium evaporation annealing method has been investigated to control the carrier concentration of sintered (Bi,Sb)₂Te₃ compounds. Hot-pressed (Bi,Sb)₂Te₃ bulk alloys and tellurium powders located in an evacuated ampoule, were heated to 673 K and held for 3, 12 and 48 h. The crystal structure and chemical composition in the annealed specimens were preserved, while the carrier concentrations were varied between 1.53 × 10¹⁹ and 2.57 × 10¹⁹ cm ^-3, and the thermal conductivity at 300 K ranged between 1.20 and 1.25 W m^-1 K^-1. The figure of merit at 300 K was enhanced from 0.86 to 1.06 when the specimens were annealed for 3 h. To identify the underlying mechanism, we utilized ab initio density functional theory calculations. These computations indicated that a Te ad-layer on top of the Bi₂Te₃ energetically favors bulk Bi atoms to migrate to the surface. Our experimental measurements and the first-principles validations consistently indicate that the tellurium evaporation annealing method is a novel process for enhancing the thermoelectric performance of Bi-Te compounds by controlling their carrier concentrations, which is particularly useful in dealing with nano-scale composites.