Investigation Of Dense Ceramic Oxygen Separation Membrane And SOFC Electrode Materials

Oxygen permeable membranes have attracted much attention for its potential applications in oxygen production,partial oxidation of methane,and combustion of fossil fuels.Despite years' intensive research and development,no single material has been found that can fulfill all the requirements of oxygen permeability,mechanical strength and chemical stability for the proposed applications.The composite materials show promise to meet these requirements.This dissertation is to investigate the structure and properties of a SrCo_0.8Fe_0.2O_3-δSrZrO₃ composite and to develop the membrane-based processes for production of oxygen.In addition,the novel experimental methods have been developed to determine the oxygen ionic conductivity of zirconia-nickel composite and the maximum La deficiency in La_1-xMnO_3±δ.Chapter 1 describes the potential applications of mixed conducting materials,the concepts and theories of oxygen permeation and the progresses of research on the oxygen permeable materials especially the perovskite-typed materials.In Chapter 2,the oxygen permeability and thermo-mechanical properties of SrCo_0.08Fe_0.2O_3-δ-SrZrO₃ composite are investigated.It is shown that the grain growth of SrCo_0.8Fe_0.2O_3-δ(SCF) is inhibited by the introduction of the SrZrO₃ phase. Compared with the single-phase SCF membrane,the composite exhibits a increase of 74%in bending strength and a decrease of 15%in thermal expansion coefficients, while the oxygen permeability is decreased slightly and the activation energy for oxygen permeation is increased.A prototype oxygen separator was built with a one-end closed membrane tube.By connecting the open-end of the membrane tube to a vacuum pump,oxygen can be separated from air at 900℃.This confirms the feasibility of producing pure oxygen with oxygen permeable membranes.In Chapter 3,the SCF doped with zirconium is studied in more detail.It is revealed that the solubility limit of Zr in SCF lies at 4-5 mol%at 1250℃,and about 3 mol%at 1150℃.The dissolution of zirconium leads to an increase in the temperature at which oxygen losses from the lattice and a decrease in the electric and ionic conductivity.These effects are attributed to the stronger interaction between the cations and anions in the lattice.When the Zr content is greater than the solubility limit,SrZrO₃ phase is formed in the SCF matrix.The presence of the SrZrO₃ phase strongly inhibits the growth of the SCF grains,accounting for the improved thermo-mechanical properties of the membrane.Oxygen(diluted with CO₂) has been proposed for use as the oxidant for combustion of fossil fuel,because this oxyfuel combustion produces a concentrated and cleaner stream of CO₂,enabling efficient separation and capture of CO₂.In Chapter 4,it is proposed to produce the as-required O₂/CO₂ gas stream by using CO₂ as the sweeping gas to remove the oxygen from the permeate side of the membrane. Although the Zr-doped SCF membrane shows a smaller oxygen permeation flux under the air/CO₂ gradient compared with the case of air/He gradient,the membrane remains well-permeable to oxygen.It is found that the Zr-doping significantly improves the performance of the SCF membrane in the presence of high concentration CO₂,which is attributed to the reduction of the basicity of the SCF membrane due to the incorporation of high valence zirconium ions into the lattice.It is expected that membranes with high oxygen permeability and sufficiently large CO₂ resistance can be obtained by proper doping in the perovskite-structured oxides with metal ions of high valence and small radius such as Ti and Zr ions.In Chapter 5,we propose to use the oxygen permeation method to determine the ionic conductivity of the nickel-zirconia composite which is the standard anode material for solid oxide fuel cell(SOFC).An oxygen permeation cell was constructed by exposing one side of the disk-shaped composite to CO₂ and the other side to CO. Under the given CO₂/CO gradient and at elevated temperatures,oxide ions are extracted from CO₂ at one side of the permeation cell and transported through the membrane to the other side to oxidize CO to CO₂.From the oxygen permeation rate through the membrane and the oxygen partial pressures,the oxygen ionic conductivity is derived of 0.059 S/cm at 1000℃.This method can be applied to determine the oxygen ionic conductivity of other potential anode materials.In Chapter 6,we propose and verify an X-ray diffraction-based method to determine the maximum La deficiency in perovskite-structured La_1-xMnO_3±δ. Computer simulation predicts that the intensity ratio of(024) and(012) reflections for La_1-xMnO_3±8 in hexagonal setting increases with increasing the La deficiency x.XRD analysis shows that with increasing x until 0.09,the ratio increases as predicted,then levels off with further increase in x.An abrupt change in electrical conductivity is also observed at x of～0.10.It is concluded that the maximum deficiency lies in between 0.09-0.10 for La_1-xMnO_3±δ.The methodology presented in this paper in principle can be applied to other perovskite-structured materials.In Chapter 7,the research presented in this dissertation is evaluated and future research needs are identified.