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Electrical Properties and Defect Structure in The Sr-Fe-Co-O System

Published online by Cambridge University Press:  10 February 2011

B. Ma,
C.-C. Chao,
J.-H. Park,
C. U. Segret and
U. Balachandran
B. Ma
Affiliation:
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439 USADepartment of Physics, Illinois Institute of Technology, Chicago, IL 60616 USA
C.-C. Chao
Affiliation:
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439 USA
J.-H. Park
Affiliation:
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439 USA
C. U. Segret
Affiliation:
Department of Physics, Illinois Institute of Technology, Chicago, IL 60616 USA
U. Balachandran
Affiliation:
Energy Technology Division, Argonne National Laboratory, Argonne, IL 60439 USA
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Abstract

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 (P02) as low as 10−16 atm. Total and ionic conductivities of Sr-Fe-Co-O have been studied as a function of P02 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 ≈0.4. Experimental results show that ionic conductivity decreases with decreasing P02 at high P02 (≥10−6 atm). This suggests that interstitial oxygen ions and electron holes are the dominant charge carriers. At 800°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 P02 is reduced. By using the conductivity results, we estimated oxygen penneation 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.

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