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

由于人们对能源和环境问题的兴趣日益浓厚，热电材料有望投入实际应用。另一方面，用软化学方法合成的材料被认为是易于环境的，吸引了许多研究人员。本研究采用柠檬酸配合物与水热压制（HHP）工艺相结合的方法制备了具有良好微观结构的热电材料，并对其热电性能进行了研究。HHP是在饱和蒸汽压下，在较短的时间和较低的温度下生产硬质固体粉末的工艺。对HHP制备的固体进行后续烧结，烧结体致密，电阻率（ρ）低。主要研究了一种氧化物热电材料Na_xCo_2O_4。采用柠檬酸与HHP工艺相结合，可获得相对密度为99.1%、Rotgering因子为0.87的烧结良好、取向良好的烧结体。当高温高压处理的温度和压力较低时，所得样品的塞贝克系数(S)较小，电阻率较大。高温高压处理样品的塞贝克系数较大，电阻率较低，在523K和200MPa条件下的功率因数（S^2/ρ）略大于未处理样品。

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.