Thermoelectric Characteristics of Co3(BO3)2 and NaF-substituted Bi2Sr2Co2Oy Prepared via a Sol-gel route

Abstract The discovery of thermoelectric cobaltites initiated a systematic exploration of these materials for potential applications in thermoelectric generators, which convert waste heat into electricity. This technology may contribute to addressing the current energy crisis. Among the cobaltites studied, p-type Bi2Sr2Co2Oy is a promising thermoelectric material. However, a significant drawback of cobaltites is their low thermoelectric conversion efficiency. To improve thermoelectric performance, it is essential to optimize synthesis techniques, enhance microstructure, and incorporate various dopants and additives, etc. In this work, reference, Co3(BO3)2 and NaF-substituted, as well as Co3(BO3)2/NaF co-substituted Bi2Sr2Co2Oy ceramics were prepared using the sol-gel method. We examined the microstructure and evaluated the power factor (PF) and figure of merit (ZT) through measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (k). The introduction of substituents reduced the resistivity of Bi2Sr2Co2Oy, leading to improvements in both PF and ZT values. The highest PF and ZT values were achieved for Co3(BO3)2–substituted Bi2Sr2Co2Oy. The incorporation of Co3(BO3)2 significantly increased the material's grain size and density. The PF enhanced from 0.124 mW/(m⋅K2) at 973 K for the reference sample to 0.228 mW/(m⋅K2) for the Co3(BO3)2 –substituted sample, while ZT raised from 0.054 to 0.072 at 573 K.