Synthesis of NaxCo2O4 by the hydrothermal hot-pressing and its thermoelectric properties

NaxCo2O4 powders were synthesized by the citric acid complex (CAC) method and consolidated by the hydrothermal hot-pressing (HHP) technique. The measurement by the X-ray diffraction and the observation by the scanning electron microscope indicated that the sintered NaxCo2O4 ceramics with the plate-like grains aligned along the pressed plane can be obtained by the CAC and HHP methods, and the larger pressing pressure and higher treating temperature in the HHP process are favorable to obtaining the samples with high orientation. The relative density of the samples also increases with increasing pressure and temperature in the HHP process. The Seebeck coefficient (S) and electrical resistivity (ρ) of the samples treated by HHP are generally smaller than those of the sample untreated by HHP. As a result, the power factor (S2/ρ) of the sample treated by HHP under 523 K and 200 MPa is slightly larger than that of the sample untreated by HHP.