Effect of nano-sized carbon sphere on microstructure and thermoelectric properties of Ca₃Co₄O₉ ceramics with binary-pores structure

Oxide thermoelectric ceramics can directly convert heat energy into electric energy, and vice versa. Ca₃Co₄O₉ ceramics is potential high-temperature thermoelectric materials due to the advantages of high-temperature oxidation resistance and environmental protection. Freeze casting method introduced the nano-sized carbon spheres was adopted to prepare porous Ca₃Co₄O₉ ceramics with binary-pores structure. The pore distribution for the sample with 2wt% carbon spheres addition was relatively uniform and the porosity of 69.7% was obtained. The Seebeck coefficient varied from 101.1 μV/K to 211.4 μV/K, and the electrical conductivity ranged from 1.9 S/cm to 57.6 S/cm over all measured temperatures. In addition, the minimum total thermal conductivity of 0.59 W/(m·K) approached. Therefore, the binary-pores structure effectively reduced thermal conductivity. This study focused on the carbon spheres addition of porous Ca₃Co₄O₉ ceramics has outstanding performance in thermoelectric properties, which was considered as the excellent process method for preparing thermoelectric ceramics.