Morphology and thermoelectric performance of bulk sodium tungsten bronze prepared via low energy-consuming route

Metal oxides are one of the promising thermoelectric (TE) materials due to their high-temperature stability, low cost, and non-toxicity. Sodium tungsten bronze (STB), Na_xWO₃, is known to possess high electrical conductivity but the TE performance has not been thoroughly examined. In this study, STB was synthesized via a reduction reaction (RR) and a solid-state reaction (SSR) followed by hot pressing (HP) and spark plasma sintering (SPS). The obtained STB bulk samples have a cubic structure and the composition x of 0.41–0.47. A nanostructure was observed in the sintered RR samples. The RR-SPS sample was composed of several hundred nanometer-sized cubic particles with high porosity. Such morphology resulted in the low thermal conductivity of 6.6 W m^–1 K^–1 at 800 °C. As a result, the dimensionless figure of merit, ZT, of the RR-SPS sample was higher than those of the dense samples sintered by HP with the maximum ZT of 0.019 at 800 °C.