Synthesis of Thermoelectric Materials by the Hydrothermal Hot-pressing and its Thermoelectric Properties

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Owing to the rising interest in the energy and environmental problems, it is recently expected to put thermoelectric materials to practical use. On the other hand, the synthesis of the materials by the soft chemical process attracts many researchers because it is considered to be easy to the environment. In this study we have prepared thermoelectric materials with well designed microscopic structure by the combination of the citric acid complex and hydrothermal hot-pressing (HHP) processes, and investigated their thermoelectric properties. The HHP is a process by which hard solid bodies of powders can be produced in a short time and at a relatively low temperature under saturated vapor pressure. The following sintering of the solid body prepared by the HHP may produce the dense sintered body resulting in the low electrical resistivity (ρ). Na_xCo_2O_4, which is one of oxide thermoelectric materials, is mainly examined. By the combination of the citric acid and HHP processes, the well sintered and orientated bodies with the relative density of 99.1% and Rotgering factor of 0.87 were obtained. When the temperature and pressure during the HHP process are relatively low, the obtained samples show smaller Seebeck coefficient (S) and larger electrical resistivity than the samples untreated by the HHP. On the other hand, the samples treated by HHP under high temperature and pressure show larger Seebeck coefficient and lower electrical resistivity, and as a result the power factor (S^2/ρ) of the sample treated by HHP under 523K and 200MPa is slightly larger than that of the sample untreated by HHP.