Study On The Preparation And Properties Of Transition Metal Oxide Thermoelectric Materials

The transition metal oxide Ca3Co4O9, NaCo2O4 and CaMnO3 based thermoelectric materials were prepared by the sol-gel method followed by pressing and sintering, and doped with metal or rare-earth ions to improve the thermoelectric properties. The structures were characterized by XRD, IR, SEM, TG-DTA, and XPS techniques, and the thermoelectric performance parameters (electrical resistivity, Seebeck coefficient and thermal conductivity) were tested. The influencing factors on the structure and properties were investigated. An apparatus of testing thermoelectric parameters was manufactured for measuring the Seebeck coefficient and electrical conductivity of samples. Based on the research of thermoelectric materials, the oxide thermoelectric devices were fabricated preliminarily.The layered cobaltite oxide NaCo2O4 and doped samples Na(1-X) MxCo2O4 (M= Ca, Mn, Zn, Mg; x= 0.05) were prepared and characterized. The process of phase formation and the electric conduction mechanism were analyzed. The ion-doping of NaCo2O4 increased the power factor by enhancing the electrical conductivity.The layered cobaltite oxide Ca3Co4O9 and doped samples Ca3-xYbxCo4O9 (x= 0.1 0.6), Ca3-xSrxCo4O9 (x= 0.1～0.4), Ca3-xAgxCo4O9 (x= 0.1～0.4) were prepared and characterized. The process of formation of precursor powder and ceramic were investigated. The observation of microstructure and electronic structure and calculation of unit cell parameters were studied to optimize the experimental conditions. The thermoelectric parameters were tested to explore the electrical conduction mechanism and the generating process of thermoelectric force. Comparison of the thermoelectric properties of doped samples, the influence of the valence of doped ions (Yb3+, Sr2+, Ag+)on the electrical resistivity and Seebeck coefficient were investigated. Ag+ and Sr2+ doping reduced the resistivity, and Yb3+ doping improved the Seebeck coefficient significantly. The thermal conductivity and thermoelectric figure of merit of Ca2.8Yb0.2Co4O9 were tested and calculated.The perovskite oxide CaMnO3 and doped samples Ca1-xSrxMnO3 (x= 0.05～0.2) and Ca1-xSmxMn03 (x= 0.01～0.06) were prepared and characterized. The process of formation of precursor powder and crystal phase was analyzed. The impact of experimental conditions on the molding of ceramic was studied. The microstructure and electronic structure were observed and the unit cell parameters were calculated. The thermoelectric properties analysis indicated that Sm3+ doping decreased the resistivity about one order of magnitude, and optimized the Seebeck coefficient effectively. The thermal conductivity and thermoelectric figure of merit were tested and calculated.According to the principle of measuring the performance of thermoelectric materials, an apparatus of testing thermoelectric parameters was manufactured. The apparatus contains two systems of heating-control and data-collection, which can measure the Seebeck coefficient and electrical conductivity simultaneously.We have tried to fabricate the oxide thermoelectric devices based on the preliminary research into p-type Ca2.8Yb0.2Co4O9 and n-type Cao.95Sm0.05MnO3 ceramics. Testing the performance indicated that the output voltage increased with the temperature difference linearly.