Oxygen transport in mixed conducting perovskite oxides

The electrical conductivity relaxation (ECR) technique was used to investigate the oxygen transport in mixed ionic electronic conducting (MIEC) perovskite oxides. The application of ECR technique on perovskite materials was investigated from the aspects of both theory and experimental method.;The effects of the sample geometry on the sensitivity of ECR technique and on the values of oxygen transport coefficients obtained were investigated as a prerequisite for designing reliable ECR experiments. The experimental system, including the apparatus and the conductivity relaxation measurement, were improved in order that ECR experiments can be carried on MIEC oxides with high oxygen ion conductivity.;The kinetics of oxygen transport in La0.5Sr0.5CoO 3--delta were investigated using the ECR technique in the temperature range from 600 to 700°C and in the oxygen partial pressure range from 0.01 to 1.0 atm. The chemical diffusivity of oxygen, D˜0, and surface exchange coefficient, kchem, were investigated by systematically changing the ratio of the initial oxygen partial pressure, pO2(1), to the final oxygen partial pressure, pO2(2). Results showed that for pO2 (1)/pO2(2) > 20, a linear model for the exchange kinetics is no longer valid. The ECR process was found to have a strong dependence on the magnitude of pO2(2) and it was attributed to the pressure dependence of the surface exchange reaction. The pO2(2) dependence can be explained using a surface exchange reaction model developed in this group.;ECR experiments were carried out on La0.6Sr0.4Fe 0.8Co0.2O3--delta which has application potentials in oxygen-delivery related device, in the temperature range of 840--910°C in 0.01--0.05 atm O2. Long term stability and the effects of CO2 on the oxygen transport, which are concerns in industrial applications, were investigated.;It was found that the measured diffusion coefficient of La0.6Sr 0.4Fe0.8Co0.2O3--delta does not change with annealing time at 900°C. The surface exchange reaction was found to be sensitive to the sample history. The stabilized surface after annealing showed a constant surface exchange coefficient.;Introduction of CO2 did not affect the bulk diffusion coefficient. For the surface exchange reaction, CO2 did not affect the surface exchange rate at 906°C. However, it decreased the surface exchange rate at 750°C. The flow rate experiments confirmed that reliable transport parameters were obtained for La0.6Sr0.4Fe0.8Cr 0.2O3--delta in this work.;ECR experiments were carried out on La0.2Sr0.8Fe 0.8Cr0.2O3--delta. It was found that the ECR process of La0.2Sr0.8Fe0.8Cr0.2 O3--delta became slower when annealed at 821°C. A time dependent slow transformation, which appears to affect the surface exchange kinetics adversely, while not apparently affecting the bulk transport in the sample, was observed. The surfaces of samples were investigated by using angle resolved X-ray photoelectron spectroscopy (XPS) to determine the associated changes in the surface composition. Chromium and strontium surface enrichments were observed. Using a simple model for the surface segregation, the effective diffusivity of the diffusing cations was estimated.