Characterization of creep behavior of SrCo0.8Fe0.2O3-X

In order to achieve commercial utilization, oxygen separation membranes and solid oxide fuel cells must be able to maintain their mechanical integrity over acceptable life-time periods. This imposes a need for characterization of mechanical behavior of the materials used under operating conditions (high temperature and low stress), in order to identify mechanisms that lead to mechanical degradation during operation. In this study, a series of compressive creep tests on rectangular specimens of SrCo0.8Fe0.2O3-x of different grain sizes (2.4 to 6.8 microns) is performed in air, covering a temperature range of 850 to 975 °C and stress range of 10-20 and 40-50 MPa. In the high stress range the material has been found to render temperature dependent stress exponents (2.2 to 2.9) and high steady-state strain rates. In the low stress range the stress exponent has been found to be close to unity (1.1 to 1.3) indicating diffusion controlled creep. The activation energy appears to assume two different values depending on operating temperature region, 457 kJ/mole below 925 °C and 268 kJ/mole above 925 °C. These values, being much higher than that for oxygen diffusion (<100 kJ/mole) indicate cation diffusion controlled creep. The shift in the value of activation energy indicates a shift in the slowest moving (rate-controlling) species between A and B cation sites. The inverse grain size exponent has been found to be close to unity.