Rapid synthesis of Bi substituted Ca₃Co₄O₉ by a polyacrylamide gel method and its high-temperature thermoelectric power factor

A series of polycrystalline (Ca_1-xBi_x) ₃Co₄O₉ ( x = 0.0~ 0.075 ) powders were synthesized rapidly by a polyacrylamide gel method. The dense ceramics were fabricated using the spark plasma sintering ( SPS ) technique. Effects of Bi substitution on high temperature thermoelectric properties of Ca ₃Co₄O₉ were evaluated. Both the electrical conductivity and Seebeck coefficient increased with increasing Bi content up to x = 0.05, thus leading to an enhanced thermoelectric power factor. The Bi substituted sample with x = 0.05 obtained in this study has the highest thermoelectric power factor in the measured temperature range. It reaches 4.8×10_-4 Wm_-1K_-2 at 700 °C, which is 26 % higher than that of Ca₃Co₄O₉ without Bi substitution, and is by up to 15 % larger as compared to the Bi substituted sample synthesized by the solid state reaction method and the SPS technique due to the high chemical homogeneous powder prepared by the polyacrylamide gel method.