Thermoelectric properties of Ca3Co4O9+δ-based polycrystalline oxides

Oxide ceramics have recently attracted much attention as candidate thermoelectric materials due to their special potential for applications in power generation using waste heat. In this paper, (Na_xCa _1-x)₃Co₄O_9+δ (x=0.05-0.2) polycrystalline samples with a layered structure were prepared by a sol-gel method followed by a low-temperature sintering procedure. The electrical conductivity, the power factor and thermal conductivity basically increase with increasing temperature. The figure of merit is smaller than that of traditional thermoelectric alloys, which is about 1.1×10^-4 K^-1 at 700°C in the sample (Na_xCa_1-x)₃Co₄O _9+δ (x=0.05-0.2)