Phase-Inversion Tape Casting And Structure-Property Relationship Of Perovskite-type Oxygen Transport Membrane

Perovskite-type metal oxides with the capability to transport oxygen ions and electrons,can be used as oxygen separation membrane,solid oxide fuel cells(SOFCs)and gas sensors.The dense ceramic oxygen-permeable membrane holds promise to reduce the oxygen production cost by-30%compared to the traditional cryogenic distillation process.Meanwhile,perovskite-type oxygen separation membrane can be used to prepare oxygen-enriched air and O2/CO2 mixture by using air and CO2 as sweep gas,respectively.The key point for application of perovskite-type metal oxides is to obtain materials with high oxygen ion conductivity and long-term stability.Whereas,there exists as trade-off between the oxygen permeability and structural stability.People still never find a perovskite-type material which meet these requirements.Metal ion doping is an important approach to regulate the perovskite performance.Structural insight the influence of metal ion doping on the regulatory mechanism of performance is of great significance for developing novel materials.Synchrotron radiation X-ray absorption fine structure(XAFS)is a strong tool for probe the local structure of materials.This thesis is mainly devoted to studying the structure-performance relationship of perovskite materials,using XAFS technique.Also,a novel method to prepare the asymmetric oxygen separation membrane was developed,and the membrane was utilized to prepare oxygen-enriched air.Chapter 1 introduces the perovskite lattice structure,the concepts and theories of oxygen separation membrane,as well as the factors affect the oxygen conductivity of perovskite.The research ideas on this thesis is present as well.In chapter 2,the synchrotron radiation XAFS technique is briefly introduced.This chapter focuses on the'fingerprint,technique in the pre-edge,X-ray near edge structure(XANES)and extended X-ray absorption fine structure(EXAFS),which will be used in our research.In chapter 3,the local structure of Sb doped SrFeO3 is investigated.The oxygen vacancy concentration as well as the basicity of perovskite as reduced,whereas the CO2 tolerance was enhanced.There is no obvious reaction between 20 mol%Sb doped SrFeO3 and CO2.From the aspect of structure-performance relationship,the coordination number around Fe atoms increased with Sb,leading to an improved lattice symmetry and enhanced average metal-oxygen bond energy.DFT calculations indicates the oxygen vacancy formation energy increased with Sb,and the energy barrier in Fe-O-Fe configuration was lower than that in Fe-O-Sb configuration.SrFe0.8Sb0.2O3-? has a preferable oxygen permeability and a superior tolerance against CO2,which can be expected to be used to prepare O2/CO2 mixture.In chapter 4,the influence of A-site and B-site doping on the local structure of SrFeO3 is further investigated.According to the XAFS analyses,the valence state of Fe was correlated with the charge of doping cations and oxygen nonstiochiometry(?).The oxygen vacancy concentration can be reflected form the intensity of pre-edge peak.Sb5+ and Mo6+ doped SrFeO3 hold highest symmetry,followed by Sn4+,Al3+,Nb5+ and Ti4+ doped SrFeO3.Ba2+ and Zn2+ doped SrFeO3 have lowest symmetry.The coordination number is correlated with ionic potential(Z/r),the coordination number increases with ionic potential.This research suggest that the ionic potential is an important criteria to judge the effect of cation doping on the local structure of perovskite materials,and this study is particularly significant as it provide guidance for design of novel materials.In chapter 5,a phase inversion tape casting combined hot pressing technique was applied to prepare asymmetric La0.6Sr0.4Co0.2Fe0.8O3-?(LSCF)oxygen transport membrane.Graphite sacrificial layer was used to eliminate sponge layer.Moderate amounts graphite was added into the slurry of support layer to obtain a support with open finger-like pores.The gas permeability of the support with open finger-like pores was dramatically improved compared with the support prepared by conventional tape casting method.When air was applied at support side and helium was used as sweep gas(air/helium gradient),the oxygen permeation flux was 1.54 ml cm-2 min-1 at 900 ?,remarkably higher by a factor of 2.5 in comparison of the membrane prepared by conventional tape casting method.In a feed gas of pure oxygen,the oxygen fluxes of the two membranes were differ by only 10%.These results clearly demonstrated that concentration polarization was severely present in the membrane made by conventional tape casting in air feed gas,but was notably reduced in the phase inversion derived membrane.Also,this phase inversion tape casting/hot pressing technique can be generalized for the preparation of other ceramic membranes.In chapter 6,an asymmetric Zr0.84Y0.16O1.92-La0.8Sr0.2MnO3-?(YSZ-LSM)oxygen separation membrane was prepared by phase inversion tape casting.The membrane was moditfied by coating a porous YSZ-LSM layer on the surface of the dense layer side ad depositing Sm0.2Ce0.8O2(SDC)nano-particles on the both surface of the membrane.The membrane exhibited desired oxygen permeability after depositing SDC nano-particles.An oxygen permeation flux of 4.6×10-7 mol·cm-2·s-1 was observed under air/helium gradient at 900 ?.Owing to its desired oxygen permeability and good stability,the membrane was applied to prepare oxygen-enriched air.An production rate of 1.8 ml cm-2 min-1 bar-was observed with an O2 concentration of 25 vol%,under pressure difference of 3.0 bar at 900 ?.We consider the O2 concentration can be further increased by increasing membrane area.In chapter 7,the summary of this dissertation is present,and further research is proposed.