Oxygen Vacancy Exchange Diffusion Behavior In Highly Epitaxial Complex Oxide Thin Films

A-site ordered double perovskite cobaltate oxides LnBaCo2O5.5+δ (LnBCO)(Ln lanthanide,-0.5<δ<0.5) are important cathode materials of SOFCs because of their lower working temperature, which can reduce the operating temperature of SOFCs from high temperature from1000℃to the intermediate temperature (IT) range of600-800℃. Oxygen catalysis in SOFC cathode is composed by a number of complex processes involving in oxygen adsorption and dissociation at the surface of cathode materials, oxygen diffusion in films. Therefore, it is important to study the oxygen diffusion in SOFC cathode materials for improving the performance of SOFC, and accelerating the process of IT SOFC practical application.In these thesis, we focus on the the high temperature H/O diffusion behaviors in LnBCO epitaxial thin films. A series of LnBCO single crystal fims are deposited with pulsed laser deposition method. By measuring the resistance of the films at different temperatures during the change of H/O environment, we study the influence of oxidation and reduction on H/O diffusion in LnBCO films. The main results are summarized as below,1. Surface exchange and oxygen vacancy diffusion dynamics were studied in double perovskites LnBCO (Ln=La,Er) single-crystalline thin films by carefully monitoring the resistance changes under a switching flow of oxidizing gas (O2) and reducing gas (H2) in the temperature range of240～800℃. The results have demonstrated that:a giant resistance change△R by three to four orders of magnitude in less than0.1s was found. At temperatures ranging from230to400℃, the resistance change of the LnBCO films as a function of time in both the Co25+→Co3+and the Co3+→Co35+oxidation steps become oscillatory. Because oxygen vacancies of LnBCO are present mainly in the LnO layers rather than in the BaO layers, the change in the average Co oxidation state of a CoO2layer would be greater when its vacancy-exchange involves the LnO layer than the BaO layer hence making the diffusion associated with the LnO layer more readily detectable than that with the BaO layer by resistance measurements. The average Co oxidation state on the film surface would oscillate hence leading to the oscillations of the dR/dt plot. This unprecedented oscillation phenomenon provides the first direct experimental evidence of the layer by layer oxygen vacancy exchange diffusion mechanism.2. A comparative study has been made on surface exchange and oxygen vacancy diffusion dynamics between ErBCO and PrBCO thin films, since Pr3+is larger than Er3+in ionic radius. The results have indicated that, at temperatures between240-400℃, the oscillations in the dR(t)/dt vs. t curve of PrBCO occur in both the Co25+(Co2+/Co3+)→Co3+and the Co3+→Co3.5+(Co3+/Co4+) oxidation steps,this phenomenon is similar to ErBCO films. In both of the oxidation steps, the dR(t)/dt vs. t curves exhibit two oscillations of different magnitudes, the larger one is related to the diffusion through the PrO layer, and the smaller one to that through the BaO layer. Since Pr3+ion is larger than Er3+ion, the vacancies in BaO layers are much more in PrBCO than in ErBCO, leading to the smaller difference of diffusion rates for oxygen passing through the PrO and BaO layers in PrBCO than through the ErO and BaO layers in ErBCO, hence making both of exchange diffusions through the LnO and BaO layers observable for PrBCO.3. The effects of strain on the oxygen vacancy/hydrogen diffusion in the A-site ordered double-perovskites EuBaCo2O5+δ (-0.5<δ<0.5)(EuBCO) thin films were systematically studied. Different strains were induced in EuBCO films on the selected single crystalline substrates of (001) LaAlO3(LAO),(001) MgO(MGO) and (110) NdGaO3(NGO), respectively. The results manifest that the transition temperatures for Co ion reductions from Co3.5+to Co3+and then from Co3+to Co25+under reducing gas (H2) in the temperature range of200～800℃, are extremely sensitive to the strain values of EuBCO films, indicating the interface strain changing the Gibbs energy of chemical reaction (AG).In addition, we study the electrical and optical properties of ZnO thin films deposited on sapphire substrates with PLD method. It is found that annealing in nitrogen atmosphere is an effective method to improve the crystalline quality and the ultraviolet photoluminescence of ZnO films.The conductive atomic force microscopy (C-AFM) reveal that the grains have the electrical conductivity much higher than the grain boundaries in ZnO films. The influence of oxygen pressure on the crystal quality and optical properties of the Zn1-xMgxO films is studied. It is found that the crystal quality of the films decreases with increasing the oxygen pressure from10-4Pa up to10Pa.. Comparing with pure ZnO films, the UV peak of Zn1-xMgxO alloys shows the blue-shifts in the range of3.374to3.332eV, depending on the working pressure. The different blue-shifts are suggested to be due to variation of Mg content in the films deposited at different oxygen pressures.