| The ceramic membrane holds promise to reduce the oxygen production cost by 30%over the present cryogenic distillation process.The main barrier hindering the development of this novel technology is the lack of membrane materials exhibiting both high oxygen permeability and stability.The composite involving an oxygen ionic conducting phase and an electronic conducting phase exhibits improved stability over the single-phase mixed conducting material,but the oxygen permeability of the former is lower that of the latter.The disadvantage of the dual-phase composite can be largely compensated by fabrication the membrane into the hollow fiber geometry.This is because that the hollow fiber has a small outer diameter,thus a large quantity of membranes can be packed in a module and that it usually has a small wall thickness and thus imposes less resistance to the permeation of oxygen.Based on these considerations,this dissertation is focused on the preparation and characterization of dual-phase composite hollow membranes.Chapter 1 presents an overview of the principle of oxygen permeation through the membrane and state-of-the-art membrane materials as well as the preparation of the membrane especially in hollow fiber geometry.The research needs in the oxygen separation membrane are identified,and the scope of the dissertation is described.In Chapter 2,dual phase composite of Zr0.84Y0.16O1.92(YSZ) and La0.8Sr0.2MnO3-δ(LSM) are explored for oxygen separation application,in which oxygen ions and electrons transport through YSZ and LSM phase respectively.The hollow fiber precursor was prepared by the phase-inversion process,and transformed to a gas-tight ceramic by sintering at 1350℃.The as-prepared fiber exhibited a thermal expansion coefficient of 11.1×10-6 K-1 and a three-point bending strength of 152±12 MPa.The oxygen permeability of the hollow fiber was measured by exposing its shell side to the ambient air and sweeping the tube side with high purity helium or CO2 to carry away the permeated oxygen.An oxygen permeation flux of 2.1×10-7 mol·cm-2·s-1 was obtained under air/He gradient at 950℃for a hollow fiber of length 57.00 mm and wall thickness 0.16 mm.The oxygen permeation flux remained almost unchanged when CO2 was used as the sweep gas.The as-produced O2/CO2 mixture can be used as oxidant for combustion of fossil fuel;this oxyfuel combustion process produces a concentrated CO2 stream and thus enables efficient CO2 capture.The other important feature of the YSZ-LSM membrane is that unlike the single-phase perovskite-structured oxide membrane,the composite membrane does contain any toxic and expensive elements,which is also vital for practical application.Considering the satisfactory trade-off between the permeability and stability and the packing density of the hollow fiber,the YSZ-LSM hollow fiber is promising for oxygen production applications.In Chapter 3,dual-phase composite of Ce0.8Sm0.2O2-δ(SDC) and LSM are investigated.In this composite,oxygen ions are transported through the SDC phase.The reason for choosing SDC is due to its higher oxygen ionic conductivity than that of YSZ.The SDC-LSM hollow fiber was prepared using the phase-inversion/sintering technique.A stable oxygen permeation rate of 3.2×10-7 mol·cm-2·s-1 was measured under air/He gradient at 950℃,and 3.0×10-7 mol·cm-2·s-1 under air/CO2 gradient.The oxygen permeation rate was slightly lower than the value measured at the early stage of the measurement after 700 h.The oxygen permeability of the fiber did not degrade significantly under the given operation condition,thus the membrane is promising for production of O2/CO2 required for combustion of fossil fuels with integrated CO2 capture.It was also found that oxygen permeation through the hollow fiber can be well described by the Wagner equation and assuming that the gas flow in the core of the fiber conforms to the plug flow model. The oxygen production capacity for a membrane unit can be accessed through modeling.Chapter 4 presents a study on dual-phase composite of YSZ and La0.8Sr0.2Cr0.5Mn0.5O3-δ (LSCM).LSCM,as a potential anodic material for solid oxide fuel cells,has been reported to be stable under reducing conditions,thus the composite membrane of YSZ-LSCM is expected to be stable under a large oxygen gradient,i.e.,with one side of the membrane exposed to air and the other side to reducing atmosphere.The composite was fabricated into hollow fibers using phase-inversion/sintering process.The as-prepared fiber shows a three-point bending strength of 279±5 MPa.A stable oxygen permeation rate of 3.3×10-8 mol·cm-2·s-1 was observed under air/He gradient at 950℃,and 3.9×10-7 mol·cm-2·s-1 under air/CO gradient.The membrane was found to remain stable under stringent condition for over 600 h,showing that it is promising for chemical reactor application.Chapter 5 describes a study on SDC-LSCM composite membrane.The composite was fabricated into hollow fibers by an improved phase inversion/sintering process.Instead of using SDC and LSCM as starting materials,individual metal oxides and carbonates were used,thus reducing the number of preparation steps and costs.The thermal decomposition behaviour of the hollow fiber precursor was analyzed using TGA/DTA,and its densification process was investigated using dilatometer.The hollow fiber precursor was converted to a gas-tight ceramic by sintering at 1350℃in the atmosphere of N2 containing 4%H2.An appreciable oxygen permeation flux of 1.4×10-7 mol·cm-2·s-1 was observed for the fiber under air/He gradient at 950℃,and a much larger flux(3.3×10-6 mol·cm-2·s-1) under a large oxygen gradient(air/CO). Examination on the membrane after oxygen permeation measurement shows that the LSCM phase of the composite has been degraded by CO,indicating that it may not possess sufficient stability under highly reducing atmosphere(CO).In Chapter 6,a hollow fiber membrane of SrCo0.8Fe0.2O3-δ in composite with SrZrO3(10 mol%) is investigated.The hollow fiber was prepared using the phase-inversion/sintering method. The as-prepared hollow fiber had a dimension of 0.25 mm in thickness,1.70 mm in outer diameter. An oxygen flux as large as 1.0×10-6 mol·cm-2·s-1 was obtained under the air/helium gradient at 950℃.The permeation flux increased with temperature as expected,and the apparent activation energy was calculated to be 35.3±1.6 kJ/mol in the temperature range of 850-950℃.A plug-flow model in combination with the Wagner theory was used to simulate the oxygen permeation process.The simulation result is in a fair agreement with the measured permeation data.In the last chapter,the summary of this dissertation is presented,and future research need indentified.
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