High-Temperature Thermoelectric Properties of Sol–Gel Processed Ca_2.5Ag_0.3RE_0.2Co₄O₉ (RE: Y and Rare-Earths) Materials

Herein, dually doped Ca_2.5Ag_0.3RE_0.2Co₄O₉ (RE: La, Pr, Nd, Sm, Gd, Dy, Er, Yb, Eu, Tb, Ho, Lu, Ce, and Y) samples are synthesized by sol–gel technique and consolidated by cold pressing under high pressure to systematically scrutinize the influences of Y and rare-earth dually doping with Ag on transport properties of Ca₃Co₄O₉ for high-temperature thermoelectric (TE) applications. Characterization results reveal that targeted phase is successfully produced, and doping of the compositions is provided. Doping of Y and rare-earth elements together with Ag into the Ca²⁺ site is effective in increasing the Seebeck coefficient and decreasing the electrical resistivity of the samples, thanks to the reduction in carrier concentration. Thermal conductivity of the samples is reduced related to the lower relative densities and alloy scattering originated from dually doping. Among the samples, Ca_2.5Ag_0.3Ho_0.2Co₄O₉ and Ca_2.5Ag_0.3Eu_0.2Co₄O₉ exhibit the highest power factor (PF) values of 0.65 and 0.62 mW m⁻¹ K⁻² at 800 °C, respectively. These results are quite high for bulk oxide TE materials which can be assessed as potential oxide TE materials for high-temperature TE power generation.