Electrical properties and defect structure in the Sr-Fe-Co-O system

The ceramic Sr-Fe-Co-O has potential use as a membrane in gas separation. This material exhibits high conductivity of both electrons and oxygen ions. It allows oxygen to penetrate at high flux rates without other gas components. Electrical properties are essential to understanding the oxygen transport mechanism and defect structure of this material. By using a gas-tight electrochemical cell with flowing air as the reference environment, we were able to achieve an oxygen partial pressure ({sub p}O{sub 2}) as low as 10{sup -16} atm. Total and ionic conductivities of Sr-Fe-Co-O have been studied as a function of {sub p}O{sub 2} at elevated temperature. In air, both total and ionic conductivities increase with temperature, while the ionic transference number is almost independent of temperature, with a value of {approx}0.4. Experimental results show that ionic conductivity decreases with decreasing {sub p}O{sub 2} at high {sub p}O{sub 2} ({ge}10{sup -6} atm). This suggests that interstitial oxygen ions and electron holes are the dominant charge carriers. At 800{degrees}C in air, total conductivity and ionic conductivity are 17 and 7 S/cm, respectively. Defect dynamics in this system can be understood by means of the trivalence-to-divalence transition of Fe ions when {sub p}O{sub 2} is reduced. By using the conductivity results, we estimated oxygen permeation through a ceramic membrane made of this material. The calculated oxygen permeability agrees with the experimental value obtained directly from an operating methane conversion reactor