Stability And Oxygen Transport Property Of Dense Ceramic Membranes

Oxygen-permeable dense ceramic membranes hold promise of finding applications in separation of oxygen from air and upgrading of oxygen-involved processes such as production of syngas through the partial oxidation of methane and combustion of fossil fuels. The oxygen permeability of this type of membrane arises from its mixed oxygen ionic and electronic conductivity at elevated temperatures. For practical applications, the membrane is required to possess high oxygen permeability and retain its integrity under operation conditions. This thesis presents a number of studies on the stability and oxygen transport properties of single-phase perovskite-type (Chapters 2 & 3) and dual-phase ceramic membranes (Chapters 4-6).Chapter 1 introduces the concepts and theories of oxygen permeation for dense ceramic membranes, presents a brief overview of membrane materials, and describes the scope of this thesis.In Chapter 2, the effects of iron doping on the structure, electric and oxygen transport properties of SrCo_1-xFe_xO_3-δ (x=0, 0.05, 0.1, 0.2) are investigated. The perovskite structure of the as-prepared SrCo_1-xFe_xO_3-δ is shown to change in the sequence of hexagonal→Brownmillerite→cubic+Brownrnillerite→cubic as iron content x increases. Along with this structural evolution is the increase in the electric conductivity and oxygen permeability. This is explained as a result of reduced structural distortion, increased Co/Fe-O-Co/Fe bond angle, improved overlapping of the Co/Fe's 3d orbital and oxygen's 2p orbital and gradual disordering of oxygen vacancies with increasing x. It is also found that both the high-temperature electric conductivity and oxygen permeability of the cubic-perovskite SrCo_1-xFe_xO_3-δ decrease as x increases, which is attributed to the difference between the Co-O and Fe-O bond.In Chapter 3, the oxygen transport and stability of perovskite-structured membranes are investigated in atmosphere containing CO₂ and H₂O. It is known that most of perovskite oxides contain basic alkaline earth metals and rare earth elements, and may react with CO₂ and H₂O in the atmosphere, leading to degradation of the...