Thermoelectric Properties of Highly Textured NaCo_2O_4 Ceramics

NaCo2O4 thermoelectric oxide ceramics with high crystallographic anisotropy were synthesized successfully by combining the traditional solid-state reaction and cold high pressure compacting method. XRD and SEM measurements show that the sintering after cold-high-pressure compacting is helpful to align the c-axis of layered grain within the sample along the pressing direction, i.e. the sample is highly textured. The thermoelectric properties of NaCo2O4, measured below room temperature exhibit a metallic behavior in both the resistivity σ and Seebeck coefficient S. The sign of S is positive, indicating a p-type thermoelectric material. Compared with the samples prepared with other methods, although the thermal conductivity is higher due to the suppression of the phonon scattering by grain boundary or point defect, the above sample exhibits lower σ and higher S which leads to the improvement of the ZT value that reaches 0.013 at 300K.