The Research Of Formation And Application Of Oxygen Vacancy On The Rare Earth Element (Ce、Sc) And Zirconia Material

The main goal of this thesis is to study the type, mechanism and application of theformation of oxygen vacancy of rare earth element （Ce, Sc） and zirconia. Two kindsof material were studied in this thesis: zirconia doped ceria （ZDC） and Scandiastabilized zirconia （ScSZ）. Recently researchers focus on the capacity of releasingoxygen of ZDC on the condition of different temperature and different oxygen partialpressure and the application on the catalysis of automotive exhaust. But few studiesare involved in the field on the quantitative analysis of the thermodynamic parameterof the oxygen vacancy in the process of releasing oxygen. This thesis studied thenonstoichiometric of oxygen vacancy at different temperature and different oxygenpartial pressure and obtained the thermodynamic parameter of ZDC. Simulation ofsolar driving dissociation of water was also studied in this thesis. By far, theinvestigation of ScSZ mainly includes the ionic conductivity at high temperature（800-1000°C） of the electrolyte of solid oxide fuel cell （SOFC）. But few researchfocus on1) the relationship between the oxygen vacancy on the grain boundary andthe specific grain boundary conductivity and the space charge potential2) thelimitation of the application of ScSZ to be the electrolyte of SOFC at so hightemperature. The relationship between the oxygen vacancy on the grain boundary andthe specific grain boundary conductivity and the space charge potential was studied inthis thesis. Ho₂O₃was doped in ScSZ to enhance the ionic conductivity below800°C. Two different Zirconia doping ratio ZDC samples were prepared by sol-gel. Thefirst is ZDC03（Ce0.97Zr0.03O₂），97mol%doped with3mol%Zirconia；The second isZDC20（Ce0.8Zr0.2O₂），80mol%doped with20mol%Zirconia. The samples’ crystalstructures were examined by XRD. The results show that both samples are cubicstructure. The thermodynamic properties of ZDC were studied by thermal gravityanalysis. The TG results show that ZDC becomes more easily to be reduced andpossesses more oxygen vacancies with the increase of Zirconia in the same oxygenpartial pressure and same temperature. It also shows that the enthalpy decides theminimum temperature to split water and entropy decides the temperature scope tosplit water by comparing the thermodynamic properties of both ZDC and water.Solar simulation device for splitting water by ZDC was designed on the basis of theanalyzing of the thermodynamic properties of ZDC. The experiment results show that1500°C is the best temperature for reducing ZDC to release oxygen and split water.At the temperature of1500°C, the releasing amount of oxygen of ZDC is much higerthan that of pure ceria in the same period. But in the case of ZDC, the increase ofcontent of Zirconia has no influence on the increase of the amount of releasingoxygen. During the same time the amount of oxygen released by ZDC03and ZDC20are almost same at the same reducing temperature. While in the process of oxidizing,the increase of doping content of Zirconia decreases the kinetic of releasing hydrogen.At the same oxidizing temperature of800°C, the amount of hydrogen of ZDC03andpure Ceria is higher than ZDC20by61%and58%, respectively, during the first2minutes. Meanwhile, the solar to fuel efficiency of ZDC was calculated in which theefficiency of pure ceria and ZDC03are13.5%and14.1%, respectively.Nano-crystalline ScSZ powders were prepared by the hydrothermal ureahomogeneous precipitation method. The samples are in the molar ratio of Zr4+/Sc3+=95/5（5mol%Sc₂O₃doped ZrO₂,5ScSZ）,92/8（8mol%Sc₂O₃doped ZrO₂,8ScSZ）and90/10（10mol%Sc₂O₃doped ZrO₂,10ScSZ）,88/12（12mol%Sc₂O₃doped ZrO₂，12ScSZ），85/15（15mol%Sc₂O₃doped ZrO₂,15ScSZ）. The samples’ crystalstructures were examined by XRD. The results show that5mol%，8mol%and10mol%Sc₂O₃doped Zirconia could stabilize the high temperature phase cubic to room temperature. Other samples are monoclinic or rhombohedral structure. The threecubic structure powder samples were pressed and sintered to prepare the ScSZ bulkmaterials. XRD results show that5ScSZ and8ScSZ are still cubic crystal structureafter sintering. But the crystal structure of10ScSZ partially transfers to rombohedralstructure after sintering. Impedance spectra analysis was used to study the electricalproperty of ScSZ. The result of impedance fitting shows that the ionic conductivity of8ScSZ is higher than that of other ScSZ. Rare earth element Ho₂O₃is doped intoScSZ to improve the conductivity. The impedance fitting result shows that theconductivity of1mol%Ho₂O₃and7mol%Sc₂O₃doped Zirconia forming7Sc1HoSZis higher than that of8ScSZ. In addition, to investigate the mechanism of theelectrical property of ScSZ TEM experiment, brick layer model and Mott-Schottkymodel were introduced to examine and analyze the grain boundary resistance, specificgrain boundary resistance, space charge potential on grain boundary and grain size.The result shows that the grain boundaries of high-purity8ScSZ are free of anyamorphous phase and without any segregation; the total grain boundary conductivitydecreases but the specific grain boundary conductivity increases with the decrease ingrain size of8ScSZ. As a result, the grain boundary space charge potential decreasesand the concentration of oxygen vacancies in the space charge regions increase withdecreasing grain size.8ScSZ possesses the lowest specific grain boundaryconductivity, highest space charge potential and lowest oxygen vacancy concentrationon the grain boundary compared to the samples of7Sc1HoSZ,7Sc3HoSZ and7Sc5HoSZ. With the increase of the doping content of Ho₂O₃the specific grainboundary conductivity goes down, space charge potential goes down and oxygenvacancy concentration goes up.