Investigation On The Preparation And Properties Of NaCo₂O₄ Based Thermoelectric Materials And Its Functionally Gradient Material, Electrode

With the decrease of energy sources that can be exploited and the severity of the environment pollutions caused by coal, oil and natural gas, it has become urgent that saves the energy and cleanly produces. In spite of the low transition efficiency, the thermoelectric transition equipments have a lot of characteristics such as no moving parts, pol-lution and noise, long life span and safety, which also becomes the research hot spot. Thermoelectric material is a functional material that realizes the exchange between thermal energy and electrical energy. As the key part of the thermoelectric generation equipment, thermoelectric material becomes the important research object.In this work, the thermoelectric effects, the history, and the recent development progress of thermoelectric materials are reviewed in detailed at first.We have selected sodium cobalt oxide (NaCo₂O₄) as our research object, which is a layered compound consisting of the alternate stack of CoO₂ layer which is responsible for the electric conduction and the insulated layer Na_0.5⁺.In the experiments, analytically pure Co(NO₃)²·6H20, NANO₃ and citric acid were used as raw materials, NaCo₂O₄ samples were fabricated by sol-gel method. Then NaCo₂O₄ block were fabricated by hot-pressed sintering. The microstructures of the samples were characterized by XRD,SEM, et al. The thermoelectric properties were determined by measuring electrical resistivity, Seebeck coefficient and power factor. The effect of processing parameters to thermoelectric propeties were analyzed. Results show that the appropriate processing parameters are: sintering at 800℃, forming pressure is 2.4MPa and holding time within 40 min. Used the same method, NaCo₂O₄ electrode were fabricated. In order to fabricated block materials with better properties, the device of hot-pressed sintering was design and manufactured. This equipment has the structure simply, practical, reliable, might realize the power steplessly continuously controllable, the temperature control was accurate and so on the merit. After a nearly year repeatedly practical application examination, finally indicated is a reliable practical installment.The microstructures and electricity properties of NaCo₂O₄-based oxides doped by two kinds of dopants (Ag, La) have been investigated. Results indicate that the dopants affect the thermoelectric properties of the samples greatly and they can improve the thermoelectric properties in different extent. It also exists an appropriate range of doping values to improve the thermoelectric properties of the samples, It can be concluded that the thermoelectric properties doped Ag is better than that of La and the appropriate range of doping values is x=0.10.In order to increase the thermoelectric conversion efficiency and output power, NaCo₂O₄/Ca₃Co₄O₉ function gradient material were fabricated by hot-pressed sintering, too. Results show that the appropriate processing parameters are: sintering at 800℃, forming pressure is 2.4MPa and holding time within 40 min. Used the same method, NaCo₂O₄ electrode were fabricated.