Owing to their potential and wide applications in the separation of oxygen from air, [1±4] conversion of natural gas to syngas, [5±9] and highly selective oxidation of light hydrocarbons, [10,11] dense oxygen-permeable membranes with mixed oxygen-ion and electron conductivity have attracted increasing attention. Among the mixed conducting ceramic membranes, perovskite-type (ABO 3 ) oxide membranes exhibit the highest oxygen permeability because of both their high ionic and electronic conductivities. During the past few decades, many oxygen-permeable membrane materials with high oxygen-permeation fluxes have been synthesized based on cobalt doping of the B-sites of a perovskite structure, since cobalt is beneficial to the activation of oxygen molecules. Even though a high oxygen-permeation flux was achieved, only a few membrane materials could be operated steadily because of the easy evaporation and reduction of cobalt. [12] Therefore, the development of cobalt-free perovskite oxygen-permeable membranes with a considerable oxygen-permeation flux is highly desired. Large efforts have been made to develop new cobalt-free materials; for example, a research team at Eltron developed a new cobalt-free material, i.e., La 1±x Sr x Ga yFe 1±y O 3-d . [13] However, the high costs resulting from the use of expensive gallium and the low oxygen-permeation flux would prevent large-scale application of this material. Zhu et al. [14] designed another cobalt-free oxygen-permeable membrane based on BaCe x Fe 1±x O 3±d . This membrane consists of two phases, a Ce-rich phase and an Fe-rich phase, but only the Fe-rich phase contributes to the oxygen permeation, and this membrane is difficult to prepare in the pure perovskite structure. Figueiredo et al. [15] developed the cobalt-free material CaTi 1±x Fe x O 3±d , but the oxygen-permeation flux was also low (0.02 mL min ±1 cm ±2 at 900 C for a membrane thickness of 1.0 mm). In this paper, we present the development of the novel cobalt-free oxygen-permeable membrane material Ba 0.5 Sr 0.5 -Zn 0.2 Fe 0.8 O 3±d (BSZF) with a perovskite structure, based on the fact that doped SrFeO 3±d is a mixed conductor.[16] Upon investigating previously published work, it was found that the partial substitution of Sr by Ba can decrease the permeation activation energy and increase the oxygen-permeation flux because Ba not only enlarges the lattice free volume, but also reduces the average metal±oxygen bond energy in the lattice. [14] It was also found that doping the B-site of (La,Sr)- The X-ray diffraction (XRD) patterns of the BSZF powder and of the membrane sintered at 1175 C for 10 h showed that both the powder and the membrane exhibit a perovskite structure. This has also been proven using high-resolution transmission electron microscopy (HRTEM). The TEM results of a typical powder particle are given in Figures 1a,b. The high-resolution images show the periodic arrangement in a projection close to the [021] zone axis at two different magnifications. From the corresponding selected-area electron...
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