| Mixed ionic-electronic conductors that exhibit high oxygen permeability and good structural stability at elevated temperatures and in reducing atmospheres are of interest for many applications. For example, they can be used as electrodes for solid oxide fuel cells, as oxygen gas separation membranes, and in methane conversion reactors. The ABO3−δ perovskites are the most extensively investigated oxides for use as oxygen permeation membranes due to their high oxygen permeability. Unfortunately, many of the ABO 3−δ perovskites experience oxygen loss at high temperatures and in reducing atmospheres, which can lead to phase transformations and structural instability. A possible solution and alternative to ABO3−δ perovskites are the development of mixed conductors having a perovskite-related intergrowth structure. This dissertation explores the oxygen permeation properties of various intergrowth oxides.; The composition SrFeCo0.5Oy has been reported in the literature to exhibit high electronic and oxide-ion conductivities, high oxygen permeability, and good stability in reducing atmospheres at high temperatures. However, the SrFeCo0.5Oy composition is a three phase mixture consisting of an intergrowth phase Sr4Fe6−x CoxO13+δ (x ≤ 1.5), the perovskite phase SrFe0.75Co0.25O3-δ, and a cubic spinel phase Fe3−xCoxO4. From a systematic study, it is shown that the oxygen permeation properties of SrFeCo0.5 Oy (x = 2 composition in Sr4Fe6−xCO xO13+δ) are dominated by the perovskite phase SrFe 1−xCoxO3−δ, and not the intergrowth phase Sr4Fe6−xCoxO13+δ. The oxygen permeation flux values for the composition Sr4Fe 4Co2O3−δ are two orders of magnitude lower than that for the perovskite phase SrFe0.2Co0.8O 3−δ.; The layered compounds La1−xSr1−xFeO 4, La1−xSr1−xCoO4, Sr 3Fe2−xCoO7−δ, and LaSr 3Fe3−xCoxO10, belonging to the Ruddlesden-Popper series, were synthesized and investigated for use as oxygen permeation membranes. The oxygen permeation properties of the La1−x Sr1−xFeO4 and La1−xSr 1−xCoO4 phases were poor, with flux values on the order of 10−9 to 10−8 mol cm−2 s−1 at 900°C. On the other hand, the oxygen flux values for Sr3Fe2−xCoxO7−δ and LaSr3Fe3−xCoxO10, ranged between 10−8 and 10−7 mol cm−2 s−1 at 900°C, which is about one order of magnitude lower than that exhibited by the perovskite phase SrFe0.2Co0.8O3−δ. Despite the lower oxygen permeation flux values, the layered compounds experienced no phase transitions aid showed no signs of structural instability under permeating conditions, which may make them attractive for use in high temperature electrochemical applications. |
