Influence of the Phase Transformation in Na_xCoO₂ Ceramics on Thermoelectric Properties

Sodium cobalt oxides (Na_xCoO₂, denoted as NCO) have attracted considerable attentions as the promising candidate of high-temperature thermoelectric materials because of their high thermoelectric power, high thermal stability and non-toxicity. The performances of NCO materials are dependent on their crystal structure. In this paper, the crystal structure and thermoelectric properties for NCO samples prepared by the solid state method under different sintering temperatures are discussed. Importantly, a phase transformation is discovered in NCO ceramics by TG-DTA analysis. The structural transformation is probably caused by Na volatilization at high temperature. With increasing sintering temperature, the Na ion concentration decreases, and the valence state of the Co ions increases, which can influence the carriers transport properties. In addition, a large number of grain boundaries generated by the fine grain prepared under low sintering temperature can effectively decrease the thermal conductivity. Thus, the figure-of-merit ZT value can be enhanced by the large Seebeck coefficient and low thermal conductivity in the low-temperature phase.