溶液プロセスによるナノコンポジット熱電材料の作製

抄録

γ-Na_xCo₂O₄ is well known as a candidate of high performance thermoelectric materials. γ-Na_xCo₂O₄-Ag composite was obtained by sintering the powders prepared by the citric acid complex (CAC) method. The observation by the scanning electron microscope (SEM) indicated that in this composite fine Ag particles were dispersed in Na_xCo₂O₄ matrix. The addition of Ag was effective for the reduction of the electrical resistivity (ρ) of γ-Na_xCo₂O₄. The composite with small amounts of Ag shows moderate Seebeck coefficient(S)and thermal conductivity (κ), but with increasing Ag content they became unfavorable values to the thermoelectric performance. The maximum ZT(=S²T/(ρ×κ)) of 1.22 was obtained at 950K in the sample with 10 wt.% Ag. We have also consolidated Na_xCo₂O₄ powders by the hydrothermal hot-pressing (HHP) technique. The measurement by the X-ray diffraction and the observation by SEM indicated that the sintered Na_xCo₂O₄ ceramics with the plate-like grains aligned along the pressed plane can be obtained, 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 and electrical resistivity 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 523K and 200MPa is slightly larger than that of the sample untreated by HHP.