Fabrication and thermoelectric properties of highly textured NaCo2O4 ceramic

Highly textured NaCo2O4 polycrystalline sample was fabricated by means of the cold high-pressure compacting followed by the solid-state reaction. X-ray diffraction and scanning electron microscope were employed to show that the plate-like grains within the sample are aligned along the pressing direction. The resistivity p and thermoelectric power S along the preferred {001} plane were measured in the whole temperature range from 15 to 973 K in air and the correlation between thermoelectric properties and texture was investigated. It was found that both p and S exhibit metallic behavior in the whole temperature range and the above sample exhibits lower p and higher S due to high texture and density. The power factor exhibits a steep rise above 400K and reaches 761 mW m-1 K-2 at 973 K, suggesting a promising candidate for thermoelectric application at higher temperature. The change of slope in both resistivity and thermoelectric power curves at about 450 K might arise from the spin-state transition of Co ions in the CoO7 blocks.