Thermoelectric characteristics of Co₃(BO₃)₂ and NaF-substituted Bi₂Sr₂Co₂O_y 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 Bi 2 Sr 2 Co 2 O y 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, Co 3 (BO 3 ) 2 and NaF-substituted, as well as Co 3 (BO 3 ) 2 /NaF co-substituted Bi 2 Sr 2 Co 2 O y 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 Bi 2 Sr 2 Co 2 O y , leading to improvements in both PF and ZT values. The highest PF and ZT values were achieved for Co 3 (BO 3 ) 2 –substituted Bi 2 Sr 2 Co 2 O y . The incorporation of Co 3 (BO 3 ) 2 significantly increased the material’s grain size and density. The PF enhanced from 0.124 mW/(m⋅K 2 ) at 973 K for the reference sample to 0.228 mW/(m⋅K 2 ) for the Co 3 (BO 3 ) 2 –substituted sample, while ZT raised from 0.054 to 0.072 at 573 K.