Structure and property relationships in mixed-conducting Sr-4(Fe1-xCox)(6)O-13 +/-delta materials

Mixed-conducting ceramic oxides have potential uses in high-temperature ele ctrochemical applications such as solid-oxide fuel cells, batteries, sensor s, and oxygen-permeable membranes. The Sr<INF>4</INF>(Fe<INF>1-x</INF>Co<IN F>x</INF>)<INF>6</INF>O<INF>13+/-delta</INF> system combines high electroni c/ionic conductivity with appreciable oxygen permeability at elevated tempe ratures. Dense ceramic membranes made of this material can be used to separ ate high-purity oxygen from air without the need for external electrical ci rcuitry or to partially oxidize methane to produce synthesis gas (syngas, C O + H<INF>2</INF>). Samples of Sr<INF>4</INF>(Fe<INF>1-x</INF>Co<INF>x</INF >)<INF>6</INF>O<INF>13+/-delta</INF> (where x = , 0.1, 0.2, 0.333, and 0.46 7) were prepared by a solid-state reaction method in atmospheres with vario us oxygen partial pressure (p<INF>O2</INF>) and were characterized by powde r X-ray diffraction, scanning electron microscopy, and electrical conductiv ity testing. The proportion of phase components in the samples is dependent on both cobalt content and p<INF>O2</INF>. The total conductivity increase s with both temperature and cobalt substitution in the material. Current-vo ltage characteristics determined in a gas-tight cell indicate that a bulk e ffect, rather than a surface exchange effect, is the main limiting factor f or oxygen permeation through membranes made of Sr<INF>4</INF>Fe<INF>4</INF> Co<INF>2</INF> O<INF>13+/-delta</INF> (x = 0.333 sample). Oxygen permeabili ty measurements at various temperatures showed that, as expected, permeabil ity increases with increasing temperature. At 900 degrees C, an oxygen perm eation flux of 2.5 scc.cm<SUP>-2</SUP>.min<SUP>-1</SUP><INF></INF> was obta ined from an Sr<INF>4</INF>Fe<INF>4</INF>Co<INF>2</INF>O<INF>13+/-delta</IN F> disk membrane that was 2.9 mm thick. (C) 1998 Academic Press.