Thermoelectric Performance Enhancement for Ca₃Co₄O₉ Ceramics Co-Doped with Ag and Tb

In this study, high-density polycrystalline Ca_3−x−yTb_xAg_yCo₄O₉ (where x = y = 0, 0.01, 0.05, or 0.08) ceramics were successfully prepared through a citrate acid sol–gel method in combination with spark plasma sintering, and the thermoelectric performances of the corresponding materials were investigated. The electrical performance within the tested temperature range of 200–800°C for the x = y = 0.08 sample was found to be optimal, with a power factor of 0.35 mW/mK² at 800°C, owing to the reduction in carrier concentration. However, thermal conductivity analysis showed that the x = y = 0.01 sample has the lowest κ value of 1.65 W/mK at 800°C and relatively low values at other test temperatures. Accordingly, the final dimensionless figure of merit (ZT) values for the x = y = 0.01 sample are optimal among the samples considered, with the highest ZT value being 0.22. Thus, this study demonstrates that appropriate Ag and Tb doping have a positive effect on the high-temperature performance of Ca₃Co₄O₉ ceramics.