Post-calcination, a novel method to synthesize cobalt oxide-based thermoelectric materials

Cobalt oxide-based Ca₃Co₄O₉ ceramics are known for their good thermoelectric (TE) performance and chemical stability at high temperatures (<900 °C); however, the crystalline phase of these ceramics is unstable beyond 926 °C due to thermal decomposition. This low limit (926 °C) on the sintering temperature produces a very low-density material with poor TE properties. Here, a novel method (post-calcination) to synthesize high-density and single-phase Ca₃Co₄O ₉ ceramics is investigated by high-temperature X-ray diffraction analysis. The post-calcination method, which includes cooling and reheating of the phase decomposed Ca₃Co₄O₉ ceramics, synthesizes high-density (relative density = 92%) and single-phase Ca ₃Co₄O₉ ceramics via a solid-state reaction at a high sintering temperature of 1200 °C, and improved TE properties (power factor = 0.245 mW K^-2 m^-1 at 800 °C) are observed. The post-calcination method is expected to be applicable to Ca₃Co ₄O₉ and other materials that are difficult to obtain in high-density form due to thermal decomposition. Furthermore, this technique improves the sinterability and production efficiency without using complicated processes.