Research For Fabrication And Thermoelectric Properties Of Na_xCo₂O₄

The thermoelectric material has become more and more attractive, which is a kind of the functional material for thermal energy and electric energy conversion. The thermoelectric materials and devices mainly used for thermoelectric generation and refrigeration because it do not pollute environment and can utilize the regeneration able energy. Recently layered cobalt oxides have been extensively investigated as a promising candidate for a thermoelectric material because of their excellent thermoelectric properties. We have selected sodium cobalt oxide (Na_xCo₂O₄) as a research object;designs a apparatus for measuring thermoelectric performance of materials;discuss the effect of synthesis and different dopants on thermoelectric properties of Na_xCo₂O₄.Na_xCo₂O₄ is a layered compound, consisting of the alternate stack of CoO₂ layer, which is responsible for the electric conduction, and the insulated layer. According to the G.A. Slack's theory, Na_xCo₂O₄ is a pattern of "phonon glass electron crystal". The conducting carriers affected thermoelectric properties of Na_xCo₂O₄, such as electrical resistivity, Seebeck coefficient and thermal conductivity.The synthesis of the Na_xCo₂O₄ thermoelectric oxide powders were attempted by using the polymerized complex method and citric acid complex method, both of that are a kind of sol-gel method. The effects of these sol-gel method on the crystallographic anisotropy, composition and thermoelectric properties of the sintered Na_xCo₂O₄ were investigated and compared to each other. The citric acid complex method was quite effective for synthesizing the crystallographically oriented sodium cobalt oxide with compositional homogeneity and a fine microstructure and for enhancing the thermoelectric performance Na_xCo₂O₄ cylindrical specimens were prepared by constant sintering. It can be found the sintering method has a great influence on thermoelectric properties.The crystal structure and thermoelectric properties of Na_xCo₂O₄-based samples with doped by three elements such as Ag, Ca and Ni have been investigated. TheXRD analysis showed that the doped samples were composed of the y -N phase and few impurity phase. The sign of the Seebeck coefficient is positive over the whole temperature range for all samples, indicating P-type thermoelectric materials. The general trend is for electrical resistivity of all samples to decrease with increasing temperature, the Seebeck coefficient of all samples increased with increasing temperature. The Ag-doped sample shows better thermoelectric properties than that of the other sample, the Ag substitution led to a significant increase in the power factor.The result in this thesis shows Na^C^C^ is a promising thermoelectric material. Once it has appropriate synthesis method and suitable dopants, the probability of thermoelectric properties improvements will exist.