Thermoelectric properties of NaxCo2O4 prepared by the polymerized complex method

Na_xCo₂O₄ powders were synthesized by using a polymerized complex (PC) method and then the polycrystalline samples were prepared by a pressureless sintering and hot-pressing. The microstructure and thermoelectric properties were investigated, and the differences between the conventional solid-state reaction (SSR) method and PC method were evaluated. The crystal grain size of the sample prepared by the PC method was remarkably smaller than that of the sample prepared by the SSR method. The degree of c-axis orientation of the samples prepared by both methods was nearly equal from the F-values calculated by the Lotgering´s method. The electrical resistivity increased and the thermal conductivity decreased compared with the sample prepared by the SSR method because of the enhancement of carrier and phonon scattering caused by the reduction in the crystal grain size. Especially, K markedly decreased in the hot-pressed sample because of its small grain size. As a result, the figure of merit was improved over the whole temperature range and showed the maximum value of 6.5×10^-4 (K^-1) at 773 K.