Study On Lead Chalcogenides And Na_xCo₂O₄ Thermoelectric Oxide Material

Thermoelectric material is o kind of functional material which could convert heat energy to electric energy directly. The current development of thermoelectric material and the fundamental theory about thermoelectric material were reviewed and discussed in this dissertation. It can be found that the alloy compound thermoelectric material has been used in many fields; however the thermoelectric oxides have been recognized as possessing more potential in the research and more value in the application. In the present work, the synthesis and characterization of lead chalcogenides and Na_xCo₂O₄ thermoelectric material were studied. The reaction process and possible mechanism were discussed in this study.PbE (E=Te, Se) microcrystals with different morphology were synthesized by an elemental solvothermal route. Na_xCo₂O₄ thermoelectric oxide ceramics with high crystallographic anisotropy was prepared by auto-ignition combining with the conventional powder metallurgy techniques. X-ray diffraction, TEM and SEM studies were used to analysize the crystal structure and morphology of products. The increase of Na content has some effect on power factor, conductivity and seebeck coefficient of Na_xCo₂O₄ thermoelectric materials.The main achievements of the work are listed as followings:1. The dendritic PbTe and cubic PbSe high crystalline microcrystals have been successfully obtained at 180℃for 12 h via an elemental solvothermal route. The effect of reaction time and temperature on the morphology was discussed, in which it can be found that the morphology can be controlled in an elemental solvothermal route by adjusting the reaction time and temperature.2. PbE (E=Te, Se) microcrystals have the same photoluminescence excitation wavelength, but the obvious different in the PL intensity in the photoluminescence spectrum. These characteristics are mainly corresponding to the diversities in their sharps and defects in the interfacial region, which could indicate that the special structures might have some significant influence on the optical properties of PbE (E=Te, Se) microcrystals.3. The flaky Na_xCo₂O₄ ultra fine powder with mean particle size of about 2μm and narrow size distribution in different nominal x composition has been successfully synthesized by auto-ignition combining with the airflow shatter process. The smaller compaction module M of 4.0369 obtained from Huang Peiyun's equation and Lotgering factor 0.975 indicate the as-synthesized thermoelectric oxide Na_xCo₂O₄ powder possesses higher compactibility, which is beneficial for fabricating the high performance thermoelectric ceramic.4. The increase of Na content improves the power factor and seebeck coefficient, but decrease the conductivity of Na_xCo₂O₄ thermoelectric materials. The porosity of Na_xCo₂O₄ thermoelectric ceramics could sharply decrease the electrical properties. And thus, controlling the Na content and enhancing the density will become the next key research.The present work has focused on the Co-based thermoelectric oxides and metallic thermoelectric compound, and the results are useful for the theoretical instruction and utility value. In the future research, the auto-ignition combined powder metallurgy cold high pressure compacting method could be used to enhance the density of material, which can improve the conductivity, and also is benefit for the obtaining of the high power factor thermoelectric material.