| Highly asymmetric Lao.6Sro.4Coo.2Feo.803-δ(LSCF) hollow fiber ceramic membrane was prepared by a new phase-inversion and the sintering technique with a mixture of N-methyl-2-pyrrolidone (NMP) and water as internal coagulant, tap water as external coagulant. The gastight LSCF hollow-fiber membranes with a highly asymmetric structure comprising a dense layer of thickness ca.88μm integrated with a porous substrate. Oxygen permeation fluxes through the obtained hollow fiber membranes were measured under air/He gradients at different conditions. The results indicate that the highly asymmetric hollow-fiber membranes possess an oxygen permeation flux of 0.11-2.19 mL·cm-2min-1 in the temperature range of 650-1000℃, which is 2.6-10.5 times higher than that of the sandwich-structured hollow-fiber membranes. Oxygen permeation in the hollow fiber membranes is limited primarily by the surface exchange reactions at lower temperatures, but ionic bulk diffusion will have a rate-limiting effect at temperatures higher than 900℃.LSCF hollow fiber ceramic membrane, which was prepared by original phase-inversion and the sintering technique, exhibited low oxygen fluxes due to the existence of three fully densified layers unfavourable for oxygen permeation. The membrane was further improved by surface modification via an acid-etching technique. The effect of acid concentration and etching time on the microstructure of membrane surface and oxygen permeability was researched. The fibres were characterised by SEM and XRD and tested for air separation. Experimental results revealed that HC1 or H2SO4 etching can create a new membrane structure and improve the oxygen fluxes by a factor of up to 5.9 or 18.6, respectively. Comparatively, H2SO4 was a better acid for controlling the reaction to yield a porous surface with higher surface area. For example, at helium sweep rate of 44.6 mL/min, the oxygen fluxes through the membrane etched by 98wt%sulfuric acid for 40min had the highest oxygen flux of 0.509 mL·cm-2 min-1 as the temperature was 800℃.LSCF hollow fiber membrane precursors were prepared by spinning a starting suspension containing 68.75 wt% LSCF powders,6.25 wt% polyethersulfone (PESf), and 25.0 wt%N-methy1-2-pyrrolidinone (NMP) at room temperature using deionized water and tape water as the internal and external coagulants, respectively. High temperature sintering was carried out in a range of 1200-1500℃and 2-8 h to study the influences of the sintering process on the properties of the LSCF hollow fiber membranes including the microstructure, crystalline phase, mechanical strength, as well as the oxygen permeability. Mechanical strength of the LSCF hollow fibers increased with increasing sintering temperature and reached a maximum of 115 MPa at 1500℃sintering temperature. To obtain gastight LSCF hollow fiber membranes, the sintering temperature must be higher than 1250℃, and the sintering time must be longer than 2 h. However, higher than 1350℃sintering temperature would facilitate the formation of sulfate impurity phases, resulting in noticeable reduction of oxygen permeation flux. The optimum sintering temperature should be around 1300℃, and the sintering time should be within the range of 2-4 h to obtain the gastight and high flux LSCF hollow fiber membranes.
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