Investigation On Structure And Properties Of Perovskite-type BaFeO_3-?-based Oxygen Permeation Materials

Oxygen production by air separation is of great importance in both environmental and industrial processes as most of large scale clean energy technologies require oxygen as feed gas.Compared with the conventional low efficient and energy consuming cryogenic air separation unit,dense ceramic oxygen permeation membrane is envisaged as promising alternative due to the great advatages of energy saving,high efficiency,simple process and so on.Recent progress in oxygen permeation membranes have led to the membrane material selection or design to a judicious compromise between the oxygen flux and chemical stability of membranes under operating conditions.Thus,it is indeed significance to develop novel membrane materials with both excellent structural stability and high oxygen permeability.According to the theories of defect chemistry and crystallography,doping strategy with In,Gd and Ca cations at the B-site of BaFeO_3-? was applied to tailor the lattice parameters and improve oxygen migration property with the aim of increasing the oxygen permeability.The effects of dopant on lattice parameters?e.g.lattice free volume,critical radius,bond energy?,oxygen vacancy,electrical conductivity,oxygen surface exchange and bulk diffusion,chemical stability,oxygen permeability and long term stability were systematically investigated.With first principles calculation,the influences of dopants on the oxygen vacancy formation and migration mechanisms were investigated to ensure a better understanding of oxygen transport processes.The relationship between crystal structure parameters,oxygen permeability and chemical stability was established.These works would provide theoretical basis and technical support for the development of low cost and high performance BaFeO_3-?-based mixed conducting permeation materials.From both aspects of theoretical computation and experimental work,the structural and property evolution of In,Gd and Ca doped BaFeO_3-?-based oxygen permeable materials were systematically investigated.The detailed results are shown as follows:The cubic perovskite structure of BaFeO_3-? could be stabilized to room temperature as In³⁺ doping content higher than 10 mol%.The 1 mm thick BaFe_0.9In_0.1O_3-? membrane performed a flux of 1.11 ml cm^-2 min^-1 at 950°C.However,with increasing doping content,the oxygen permeability was decreased,due to the decreased oxygen vacancy concentration and reduced lattice free volume,which would result in high oxygen migration energies.With these influences of dopant on lattice structure,a much larger cation of Gd³⁺ was selected to substitute for the B-site atom of BaFeO_3-?to decrease necessary doping content for stabilizing cubic structure.It was found that just 2.5 mol%of Gd³⁺ could fully stabilize the cubic structure of BaFeO_3-?,and at the same time,it expanded the lattice structure and promoted the oxygen migration,leading to some favourable oxygen migration pathes with low activation energies.As a result,a high flux for 1 mm BaFeO.975Gd0.025O_3-?sample of 1.37 mL cm^-2min^-1 was observed at 950°C,which is even higher than that of the most reported Ba0.95La0.05FeO_3-?membrane.This membrane showed a much stable long-term oxygen permeability and structural stability.Although doping rare-earth element can improve the performance of oxygen permeation membrane,it brings much more pressure on membrane cost.To address this problem,the cheap and abundant alkaline earth metal Cawas selected as B-site dopant.The results revealed that Ca doping could not only stabilize the cubic structure of the material,but also expand the lattice structure parameters and thus promote the oxygen ion migration,with comparable oxygen permeability and chemical stability to the Gd doped BaFeO_3-?.Meanwhile,Ca-doping enhanced the corrosion resistance of material against CO₂ and H₂.The Ca-doped BaFeO_3-?oxide is a promising oxygen permeation material.A novel dual-phase oxygen permeation membrane consisting 30 wt%BaFeO.95Sm0.05O_3-?and 70 wt%Ce0.9Sm0.1O_2-?was designed and prepared by a one-pot EDTA/citric acid sol-gel combustion method,which exhibited high chemical stability and superior oxygen permeability.The two phases distributed uniformly and formed electronic and ionic conducting percolated network,respectively.The membrane prepared at optimized condition showed a high oxygen permeability of 0.64 mL cm^-2 min^-1 at 950 ? with stable long-term flux.An asymmetric 30BFS-70SDC membrane,with configuration of a porous layer supported ultrathin dense layer wih surface modification by coating and dipping techniques,was constructed,which delivered a very high permeation flux of 1.88 mL cm^-2 min^-1 at 950 ?.The excellent performance makes the designed asymmetric membrane a promising candidate for oxygen separation application.