A study of Oxygen transport in mixed conducting oxides using isotopic exchange and conductivity relaxation
Prof. Dr. Ir. H. Verweij
Dr. H.J.M. Bouwmeester
The thesis presents theories and experiments regarding transport of oxygen in oxygen ion conductors. Several chapters are purely theoretical, in which new additions to theories regarding of oxygen isotope exchange and conductivity relaxation are presented. Oxygen isotope exchange experiments were performed on yttria stabilised zirconia doped with terbia and La1‑xSrxCo1‑yFeyO3‑δ(LSCF). Conductivity relaxation was performed on LSCF and LSC (y=0).
Oxygen isotope exchange on solid oxides is applied to obtain information on the reaction mechanism of the incorporation reaction of oxygen molecules at the surface of the ionic conductor. Den Otter derived a theory which involves the amount of oxygen molecules participating in the surface reaction and two independent stochastic parameters representing the chances of exchange of either atom of a molecule with the oxide. The theory was successfully applied to interpret all oxygen isotope exchange experiments.
A detailed description of conductivity relaxation experiments is presented. These and similar experiments are widely applied to obtain surface reaction rates and chemical diffusion coefficients of many compounds by monitoring the recovery of thermodynamical equilibrium after a small perturbation is applied. Usually, this is achieved by changing the oxygen partial pressure of the sample ambient. Although the general mathematical treatment implies that the perturbation is imposed very quickly, this condition may not be true under experimental conditions. Equations are derived which take the effect of reactor flushing into account. General conditions are derived which should be obeyed in relaxation experiments in order to correctly interpret the transients. Further more, it is indicated that the surface and bulk transport parameters may not be evaluated from a single transient; the experiment might be fully determined by only one of these parameters. Strict equations are presented to distinguish determinable from undeterminable transport parameters.
Isotope exchange and conductivity relaxation techniques were applied to measure the oxygen diffusivity and surface reaction rate of La0.6Sr0.4Co1-yFeyO3-δ compounds for y = 0.2, 0.5 and 0.8. The chemical diffusion coefficient decreases with decreasing oxygen partial pressure below 10‑2 bar, which is associated with the hindering of the motion of oxygen ions by ordering of oxygen vacancies.