To explore oxygen permeable materials, oxygen permeation properties of partially A-site substituted BaFenormalO3−δ perovskites were investigated. Ba sites in BaFenormalO3−δ were substituted with cations such as Na, Rb, Ca, Y, and La by 5%. The partial substitution with Ca, Y, and La, whose ionic radii are smaller than that of Ba, succeeded in stabilizing a cubic perovskite structure that is a highly oxygen permeable phase, as revealed by X-ray diffraction analysis. This can be explained in terms of a decrease in the tolerance factor (t) . Among the normalBa0.95normalM0.05FenormalO3−δ (M = Na, Rb, Ca, Y, and La) membranes tested, normalBa0.95normalLa0.05FenormalO3−δ showed the highest oxygen permeability at 600–930°C, owing to the stabilization of the cubic phase without the formation of impurity phases. From chemical analysis, the oxygen permeability of normalBa1−xnormalLaxFenormalO3−δ membranes was correlated with the amount of oxygen defects (δ) in the lattice. The oxygen permeation flux of normalBa0.95normalLa0.05FenormalO3−δ membrane was significantly increased by reducing its thickness. Furthermore, a normalBa0.975normalLa0.025FenormalO3−δ membrane exhibited good phase stability under He flow at elevated temperatures. The obtained results indicate the promising properties of normalBa1−xnormalLaxFenormalO3−δ membranes as a cobalt-free material that has a high oxygen permeability, good phase stability, and low cost.

Partially Zr-substituted BaFe 1−y Zr y O 3−␦ membranes were developed as a Co-free oxygen permeable membrane. In order to stabilize the cubic perovskite structure, Fe sites in BaFeO 3−␦ were partially substituted with Zr 4+ . In the substitution range of y = 0.01-0.1, the cubic perovskite structure was stabilized even at room temperature. Among the membranes prepared, a BaFe 0.975 Zr 0.025 O 3−␦ material ͑y = 0.025͒ showed the highest oxygen permeation flux of 1.30 cm 3 ͑standard temperature pressure͒ min −1 cm −2 at 930°C under an air/He gradient. The oxygen permeation flux was higher than that of partially Ce-substituted BaFe 1−y Ce y O 3−␦ membranes reported previously. From the results obtained by chemical and scanning electron microscope analyses, it appears that the oxygen permeability for BaFe 1−y Zr y O 3−␦ membranes was well correlated with the amount of oxygen defects in the lattice as well as the grain size. In addition, the oxygen permeation flux of the BaFe 0.975 Zr 0.025 O 3−␦ membrane was significantly increased after decreasing the thickness of the membrane from 2.0 to 0.4 mm. For thin membranes ͑0.4-1.0 mm͒, the thickness dependence of the oxygen permeability deviated from the Wagner equation, suggesting that the oxygen permeation of BaFe 0.975 Zr 0.025 O 3−␦ is controlled by not only bulk diffusion of oxide ions but also their surface reactions.Membranes based on mixed conductors can selectively separate oxygen from air at high temperature. 1 The driving force of the separation is only a difference of oxygen partial pressure on both sides of membranes. Hence, the oxygen separation using mixed-conductive oxides attracts much attention as a new energy-saving oxygen separation technology. In addition, mixed-conductive membranes have been applied to another important application, called a membrane reactor that partially oxidizes methane to form hydrogen and carbon monoxide. 2 It was first reported by Teraoka et al. 3 that membranes based on La 1−x Sr x Co 1−y Fe y O 3−␦ with a cubic perovskite structure show oxygen permeability at elevated temperatures. Since then, Co-based perovskite oxides have attracted considerable interest as efficient oxygen permeation membranes. 4-8 Among them, SrCo 0.8 Fe 0.2 O 3−␦ showed high oxygen permeability, although its phase stability is low in a reducing atmosphere. 9,10 Recently, Shao et al. have reported that cubic Ba 1−x Sr x Co 1−y Fe y O 3−␦ has high oxygen permeability and good phase stability, even in a reducing atmosphere. 11,12 Among Co-based membranes, cubic SrCo 0.9 Nb 0.1 O 3−␦ 13 and SrCo 0.95 Sc 0.05 O 3−␦ 14 also show high oxygen permeability. It is known that partial substitution for A or B sites in Co-based perovskite oxide stabilizes the cubic phase at lower temperature, improving the oxygen permeability even at low temperature. 7 This makes it possible to improve the oxygen permeability through the composition control. However, membranes of this kind are not favorable for practical use because of the high cost of Co. Moreover, Co-based membranes u...