Effect of Bi Substitution on Microstructure and Thermoelectric Properties of Polycrystalline [Ca₂CoO₃]_pCoO₂

Partially Bi-substituted [Ca₂CoO₃]_pCoO₂ ceramics were synthesized using a hot-forging technique. Then, the effects of Bi-substitution on microstructure and thermoelectric properties were evaluated. The average grain size of the precursor powder prepared by a solid-state reaction increased concomitant with the increase in Bi content. Furthermore, the electrical resistivity (ρ) of the hot-forged sample was decreased accordingly. Seebeck coefficient (S) was increased slightly by Bi-substitution and was less affected by grain size. Therefore, power factor (PF=S²/ρ) was improved. On the other hand, the ρ of the hot-forged sample was reduced using a large-grained precursor powder prepared by a solution growth method. Different from the case of the former samples made from as-sintered powder, the ρ of the sample made from large-grained powder was increased slightly with the increase in Bi content. These results indicate that the main advantage of Bi-substitution for the decrease in the ρ of the hot-forged sample is the enhanced grain growth during the heat treatment processes.