Thermoelectric Properties of Na_xCo₂O₄ Prepared by the Polymerized Complex Method and Hot-Pressing

Abstract

The polycrystalline Na_xCo₂O₄ thermoelectric materials were synthesized by the polymerized complex (PC) method and subsequently hot-pressed. The microstructure and thermoelectric properties of these samples were evaluated, and their differences between the sample prepared by the conventional solid-state reaction (SSR) method and the PC method were investigated. The shape of the calcined powders prepared by the PC method was flat and plate-like. For the annealed PC sample after hot-pressing at 1073 K for 3.6 ks with the nominal composition of Na_1.6Co₂O₄, no impurity phases were observed based on a X-ray diffraction analysis and scanning electron microscopy (SEM). The crystal grain size of the sample was as small as the calcined powders and was remarkably smaller than that of the sample prepared by the SSR method. The degrees of c-axis orientation of the samples prepared by both methods were equivalent. The electrical resistivity increased and the lattice contribution to the thermal conductivity decreased compared with the sample prepared by the SSR method because of the enhancement of the carrier and phonon scattering caused by its small grain size.