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

随着人们对能源和环境问题的日益关注，热电材料的实用化也越来越受到人们的期待。另一方面，软化学方法由于其对环境友好的特点，吸引了众多的研究者。本研究采用柠檬酸配合物与水热热压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是一种可以在短时间内和在饱和蒸汽压下的相对较低温度下生产硬质固体粉末的方法。通过HHP制备的固体的后续烧结可以产生致密的烧结体，导致低电阻率（ρ）。重点研究了氧化物热电材料Na_xCo_2O_4。采用柠檬酸法和HHP法相结合的工艺，获得了相对密度为99.1%、Rotgering因子为0.87的烧结好、取向好的坯体。当HHP过程中的温度和压力相对较低时，所获得的样品比未经HHP处理的样品具有较小的塞贝克系数（S）和较大的电阻率。高温高压处理后的样品具有较大的塞贝克系数和较低的电阻率，其功率因数（S^2/ρ）在523 K和200 MPa下略高于未经高温高压处理的样品。

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.