Effects of Microstructure on Oxygen Permeation in Some Perovskite Oxides
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ranging from 930 to 1200 TC for SCFO and 1300 to 1450 TC for LSFCO. The densities of the sintered pellets were determined using Archimedes' method, and were greater than 94% of the theoretical values. To study the sintering characteristics of SCFO, samples were sintered in air at 1100 'C and held from 15 min. to 168 hours. The stability of the microstructure of SCFO was investigated using samples which were sintered at 1100 and 1200 TC for 6 hours, and then exposed at 1050 TC for different times after samples were ground and polished to 1 im. The microstructure of sample used in this investigation is determined using a Nikon AFX-II A optical microscope. The grain size of the samples was determined by measuring the mean linear intercept lengths on micrographs of at least three images. The oxygen permeation fluxes were measured using the apparatus described in Ref. 6. In these experiments, disc samples ground using 600 grit paper were sealed between two quartz tubes with gold rings. Air gas was fed at one end of the sample, while helium gas with a flow rate of about 38 cm 3/min was swept through the other end. The flow rate was controlled by a MKS 247C mass flow controller. A computer controlled Antek 3000 gas chromatograph was connected to the exit on the sweep side, where both oxygen and nitrogen concentrations were measured. The gas chromatograph was calibrated using a standard gas of 02 in helium. The test temperature was measured by a thermocouple inserted in the quartz tube at the air side with the accuracy of ± I 'C. Experiments were conducted in the temperature range of 800 to 900 TC for SCFO (to 930 'C for LSCFO). Samples were first heated up to 900 'C (or 930 0 C) and kept at this temperature until a steady state was reached when the oxygen concentration varied less than 1%. The temperature was then dropped to the next measurement point. The oxygen permeation flux at each temperature was monitored periodically and recorded when the steady-state was reached. The reported oxygen permeation fluxes are the steady-state values. RESULTS AND DISCUSSION Microstructure Characterization While the microstructure of LSFCO samples shows little change in the sintering temperature range with the average grain size about 20 .tm, the microstructure of SCFO is considerably changed with the sintering temperature. Fig. 1 shows the microstructure of SCFO samples sintered at several temperatures, where a bimodal grain size distribution is observed. The grain size as a function of sintering temperature for SCFO is plotted in Fig. 2. It is seen that the grain size increases from 2.0 to 14.8 .tm when the temperature is increased from 930 to 1200 TC. The grain size, however, only changed from 3.4 lim to 12.8 -tm when sintering time is increased from 15 min. to 168 hours at 1100 'C as can be seen in Fig. 3. After the initial sharp increase of grain size, the grain size increases only slightly with further increase of the sintering time, which indicates that the microstructure of this material becomes relatively stable after the densi
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