Physical Properties of Oxide Thermoelectric Materials Prepared by the Polymerized Complex Method

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In this Study we have prepared the oxide thermoelectric materials with controlled microscopic structure by the polymerized complex method, and measured their physical properties. It is generally considered that the polymerized complex method is a convenient method for preparing various oxide materials. We have investigated the Zn_<1-x>Al_xO and Layered-Na_xCo_2O_4 systems. Stoichiometric amounts of metal nitrate hydrate and citric acid were dissolved in ethylene glycol. The solution was concentrated and the viscous polymerized complexes were formed. These polymeric products were decomposed to a fine and uniform powder in air. In the case of Zn_<1-x>Al_xO system the obtained powder was sintered by SPS (Spark Plasma Sintering) apparatus at 1100℃ for 10min at a pressure of 10〜30MPa, while in the case of Na_xCo_2O_4 system the obtained powder was sintered by Hot-Pressing at 900℃ for 1〜2h at a pressure of 25MPa. The phase identification of the samples was made by X-ray diffraction and the m … More icroscopic structure of the samples was examined using SEM. The physical properties such as Seebeck coefficient, the electrical resistivity and thermal conductivity of the samples were measured. The crystal grain size of Zn_<1-x>Al_xO prepared by the polymerized complex method is about 1〜2μm, which is much smaller than that by the conventional solid state reaction method, 10μm. The absolute value of the Seebeck coefficient and the electrical resistivity for Zn1-xAlxO generally decreases with increasing Al content of x. Comparing theses values at fixed Al content between the samples prepared by the polymerized complex and solid state reaction methods, the samples by the polymerized complex method have smaller values. There is almost no difference in the thermal conductivity between these samples, although the crystal grain size of the former is much smaller. These results indicate that the more homogeneous samples were obtained by the polymerized complex method and the doping of the carrier into the material can be more easily realized in the samples prepared by the polymerized complex method. On the other hand, NaxCo2O4 ceramics prepared by the polymerized complex method show higher crystallographic anisotropy than that by the solid state reaction method, and have lower electrical resistivity maintaining moderate Seebeck coefficient. As a result, lager figure of merit was obtained on the samples prepared by the polymerized complex method. Less