Effect of cooling rate on the microstructure and thermal conductivity of 5 at% Cu–Bi₂Te₃

The quest to enhance the performance of Bi₂Te₃ material has led to the adoption of various strategies, including (a) the utilization of doping processes and (b) modifying the cooling rate during melt solidification. Therefore, in this work, we selected a Bi-Cu-Te (5 at% Cu- Bi₂Te₃) alloy system and brine-ice ( \(BTC- {B}_{q}\) $BTC-B_q$ ) and food-grade oil ( \(BTC- {O}_{q}\) $BTC-O_q$ ) as the quenching mediums to achieve different cooling rates. 5 at% Cu-Bi₂Te₃ was prepared using flame-furnace-melting followed by quenching in these two mediums. This strategy aids in reducing thermal conductivity and improving mechanical hardness. SEM micrographs of both the \(BTC- {B}_{q}\) $BTC-B_q$ and \(BTC- {O}_{q}\) $BTC-O_q$ alloys show a microstructure containing a Bi₂Te₃ matrix and secondary phase of Cu_2−xTe (Cu_1.25Te for \(\text{BTC}-{B}_{q}\) $\text{BTC}-B_q$ , Cu_1.5Te for \(\text{BTC}-{O}_{q}\) $\text{BTC}-O_q$ ). Further on studying the thermoelectric properties of these quenched materials, cooling rates introduce defects in the microstructures that act as scatterers, resulting in the reduction of thermal conductivity. Compared to the air-cooled alloy ( \(\text{BTC}-AC\) $\text{BTC}-AC$ ), \(\text{BTC}-{B}_{q}\) $\text{BTC}-B_q$ and \(\text{BTC}-{O}_{q}\) $\text{BTC}-O_q$ alloys exhibit a thermal conductivity reduction of – 64% (– 1.396 W/mK) and – 62% (1.48 W/mK) at 300 K, respectively. Additionally, owing to the Cu segregation at the interphases and the formation of secondary phases in these alloys, the highest Vickers hardness value of –1.28 GPa was achieved for brine-quenched compared to the oil, air-cooled, and other Bi₂Te₃-based alloys. Overall, the strategy of utilizing different cooling rates (quenching) ensured in achieving lower thermal conductivity and improving the mechanical hardness in 5 at% Cu-Bi₂Te₃ alloy systems.