Synthesis, Microstructure And Thermoelectric Properties Of Binary Compounds In The Ca₃Co₄O₉ System

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 develops new clean energy sources. In spite of the low transition efficiency, the thermoelectric transition equipments have a lot of characteristics such as no moving parts, pollution and noise, long life span and safety, which stirs people to research them with great enthusiasm. As the key part of the thermoelectric generation equipment, thermoelectric material becomes the important research object. In my work, Ca3Co4O9 semiconductor was synthesized and its thermoelectric properties was improved by different dopants.Traditionally, the thermoelectric oxides can be prepared by solid state reaction method, but the whole process usually needs very long time, and it is easy to make the reactants become impure and inhomogenous during preparing the samples. These problems result in sintering slowly, and the material performances to become bad. To be compared, the sol-gel method has no these flaws. In the experiments, nanometer powders were prepared by the sol-gel method. Results show that some primary factors affect sol-gel transition, gelation formation and quality. These factors include the temperature of solation formation and sol-gel transition, drying time. In order to obtain high property gelation, some principles should be followed: the temperature should keep at 70 during solution evaporation, the temperature should be 120 and time should be 12 hours during drying gelation. After dry gelation is calcined at 400 , nanometer powder with low sintering temperature and great performance can be successfully prepared.Based on nanometer powders, CajC CVbased oxides were synthesized by constant sintering, cold isostate pressure and hot pressure methods. It can be found that sintering methods have a great influence on thermoelectric performances: the sample synthesized by hot pressure method is most dense andhas the best thermoelectric performances.The microstructures and thermoelectric properties of CasCog-basedoxides doped by four dopants have been investigated. Results indicate that thesamples are comprised of slate-like grains growing up in different directions, and that grain size and pore play a domain role in electrical conductivity and thermal conductivity besides Seebeck coefficient. The thermoelectric properties of the samples greatly depend on the doping elements and constituent concentration. The sample with La-dopant shows better thermoelectric performances than those with other three dopants. It should be pointed out that there exists an appropriate range of doping values to improve the thermoelectric properties of the samples.