Understanding secondary phase inclusion and composition variations in the microstructure design of n-type Bi₂Te₃ alloys via selective dissolution of KCl

This study investigated the effects of KCl treatment on microstructure and thermoelectric properties of n-type Bi₂Te_2.7Se_0.3 (BTS) thermoelectric materials. The innovative KCl treatment was originally intended to introduce nanopores through selective dissolution of KCl from a mixture of thermoelectric materials and KCl. However, it unexpectedly induced substantial variations in material composition and microstructure during the subsequent spark plasma sintering (SPS) process. Hydroxyl groups adsorbed on the powder surface during the dissolution resulted in the emergence of a Bi₂TeO₅ secondary phase within the BTS matrix after the SPS process at 450 ℃. The concentration of Bi₂TeO₅ increased with an increase in the KCl content. Furthermore, a remarkable grain growth occurred at low KCl concentrations, likely due to the liquid-phase formation in a Te-rich composition during SPS. However, excessive Bi₂TeO₅ at higher KCl concentrations hindered grain growth. These variations in the microstructure had complex effects on electrical properties: The Te_Bi antisite defects increased the electron concentration, and Bi₂TeO₅ reduced electron mobility. Additionally, the lattice thermal conductivity decreased due to the presence of Bi₂TeO₅, from 0.8 W∙m⁻¹∙K⁻¹ at 298 K for the pristine BTS to 0.6 W∙m⁻¹∙K⁻¹ at 298 K for BTS treated with 1 wt% KCl. These insights allowed precise adjustments of the electrical and thermal conductivities, leading to an enhancement in the maximum value of figure-of-merit (ZT) from 0.76 to 0.96 through the selective dissolution of KCl approach. We believe that our observations potentially enable advances in thermoelectric materials by engineering microstructures.