| Ceramic membranes made from mixed oxygen ionic and electronic conducting oxides can selectively separate oxygen from air at the temperature higher than 700℃.The membranes have great potential in pure oxygen production,oxygen-fuel combustion and oxygen-related catalytic processes owning to its oxygen transport feature.However,it is still slow to proceed with the practical application of the membranes prepared by the conventional wet ceramic techniques.The reason lies in the low mechanical strength,sealing difficulty and joining difficulty.Asymmetric membranes comprisied a dense thin film on a porous metallic support has been proposed to solve these problems.Successful preparation has been realized by a newly developed plasma spraying-physical vapor deposition（PS-PVD）technique.But since the process has to be proceeded in a vacuum system,the resultant high preparation cost and complex operation would be burdens for membrane preparation.As a result,a low-cost supersonic air-plasma spraying technique（SAPS）is thought to be used in preparing the membrane.In the paper,a typical perovskite La0.6Sr0.4Co0.2Fe0.8O3-δmaterial is chosen to study.The corresponding gas-tight membranes were prepared by use of SAPS and PS-PVD.Afterwards,comparisons on microstructures,compositions,high temperature stabilities,oxygen permeabilities,oxygen ions and electron conducting properties of the two membranes were carried out to judge the feasibility of the SAPS method.The contrast results of the two membranes’microstructures and compositions show that SAPS membrane contains more nanocrystals,while the PS-PVD membrane mostly consists of large-size grains.According to the SAED results,the lattice constant of the SAPS membrane is about 0.265nm,while that of the PS-PVD membrane is about 0.245nm.This means the barrier diameter of the SAPS membrane should be larger,which leads to an easier migration for oxygen ions.The contrast results of high temperature stabilities show that lots of vertical through cracks formed in the PS-PVD membrane when heated.But for the SAPS memrbane,its masses of nano-cracks casued by quenching stress prevent the formation of through cracks.Therefore,the membrane can keep its dense structure after 25 hours’heat treatment at 850℃.Besides,the nanocrystals in the SAPS membrane grow preferentially in heat treatment at air or inert atmosphere.Precisely because of this grain growth process,the decomposition of the perovskite would be slow down,which makes it more stable at the above atmospheres.The performances of the two membranes at the reaction atmospheres（reducing gas,carbon oxide and steam）are not very good.The PS-PVD membrane is found to have a slight advantage on thermal chemical stability at these atmospheres.The reason is thought to be its lesser lattice constant and the stable CoO phase generated in preparing process.In the work,a large-size tubular porous metal supported membrane has been prepared for the first time,and a novel-designed oxygen separation reactor was also set up to characterize the oxygen permeability of the two membranes.The results show that both membranes possess the function of oxygen permeation.Among of which,the SAPS membrane could increases the O2/N2 volume ratio of sweeping air from 1/4.6 to 1/2.3,its highest oxygen permeation flux（JO2）is 0.34×10-4ml/cm2·min.For the PS-PVD membrane,it can increases the O2/N2 ratio to 1.2/1,its JO2 can reach to 6.1×10-4ml/cm2·min.Further studies demonstrate that the oxidation of porous metallic substrates greatly decrease the oxygen permeation flux.The oxygen ions transport capacities and electron conductivities of the two membranes were compared by use of oxygen temperature programmed desorption technology（O2-TPD）and four-probe method.The results show that the electron conductivity of the SAPS membrane at R.T.is 0.21（Ω·cm）-1,while that of the PS-PVD membrane is much lower,which can only reach to 6.0×10-3（Ω·cm）-1.Besides,the desorptions of the physical,chemical adsorbed oxygen and the lattice oxygen of the SAPS membrane were 2,3 and 6 times respectively more than those of the PS-PVD membrane,which means that the SAPS membrane has better adsorption and dissocation abilities on oxygen,and possesses more intrinsic oxygen vacancies and higher capacity of lattice oxygen transmission.All these above prove that the SAPS membrane should have better oxygen permeation ability than the PS-PVD membrane.And further study demonstrates that the banding distributed CoO in the PS-PVD membrane hinder the migration of oxygen ions and electrons,which is the main reason for causing the membrane’s poor performance on oxygen and electron conductivities. |
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