Thermoelectric properties of NaxCo2O4 prepared by the polymerized complex method and spark plasma sintering

NaxCo2O4 powders were synthesized using the polymerized complex (PC) method and then the polycrystalline samples; were prepared by the spark plasma sintering (SPS) method. The microstructure and thermoelectric properties of these samples were evaluated, and their differences due to the preparation process of the polycrystalline Na2Co2O4 samples were investigated. The crystal g-rain sizes of the annealed PC samples after SPS at 1073 K and 1173 K were much smaller than that of the sample prepared by the conventional solid-state reaction (SSR) method and the sintered PC sample without pressure. Their relative densities were greater than 99%. We have obtained the dense sintered NaxCo2O4 samples with a finer microstructure. In the case of the annealed PC sample after SPS at 1073 K, the thermoelectric power, a, wits remarkably higher than that of the SSR sample, and the a value was as high as that of the PC sample without pressure. The electrical resistivity, p, increased because of its finer crystal grains compared to the SSR sample and the sintered PC sample without pressure. In the case of the annealed PC sample after SPS at 1173 K, the p was the lowest of all samples in spite of its filler microstructure. Based on these results, the power factors of the annealed PC samples after SPS at 1073 K and 1173 K were markedly higher than that of the SSR sample over the entire temperature range. For the annealed PC sample after SPS at 1073 K, the maximum power factor, 1.4 x 10(-3) W/mK(2) was obtained at 1073 K.