OXYGEN DIFFUSION IN Bi_2M_4O_9 (M = Al, Ga, Fe) SYSTEMS AND THE EFFECT OF Sr DOPING IN Bi_(2-2x)Sr_(2x)M_4O_(9-x) STUDIED BY ISOTOPE EXCHANGE EXPERIMENTS AND IR ABSORPTION

摘要

Compounds Bi_(2-2x)Sr_(2x)M_4O_(9-x) (M = Al, Ga, Fe) possessing mullite type structure could be interesting candidates for the use as electrolytes in SOFCs at intermediate temperatures (600-800°C). A central role for enhanced oxygen diffusion could be related to the vacancy formation on the site of bridging oxygen of M_2O_7 units related to the effect of Sr doping. Polycrystalline samples of average crystal sizes between 20 and 500 nm were synthesized using precursor gels from corresponding nitrate-glycerin solutions. The systems were characterized using XRD, infrared absorption and SEM/EDX techniques. Oxygen tracer diffusion coefficients could be estimated from isotope exchange experiments at 800°C using well separated infrared absorption peaks for the bridging oxygen. The estimated oxygen diffusion coefficients turns out to be in the range 10~(-16) to 10~(-18) cm~2/s for the basic systems and could not be improved with a nominal increase in Sr content, too. This could be explained by the fact that Sr could not be substituted as to form a thermodynamically stable single phase but shows a clear phase separation in acid leaching experiments.