Oxygen exchange kinetics of mixed conducting oxide ceramics covered by dendritic surface particles

Abstract

The oxygen exchange process between the gas phase and a mixed conducting solid oxide was simulated by application of finite element modeling (FEM). The solid oxide was covered by inert surface particles of different shape and size. The effect of spherical, ellipsoidal, rectangular, and dendritic surface particles on the oxygen exchange kinetics was elaborated in detail. Relaxation curves for the total amount of exchanged oxygen upon an instantaneous change of the oxygen activity (partial pressure) of the surrounding atmosphere were calculated. One-dimensional analytical solutions of the diffusion equations were fitted to the numerically calculated relaxation curves, yielding apparent chemical diffusivities and surface exchange coefficients. The FEM simulations of the oxygen exchange process were carried out as a function of surface coverage for the various particle types. The apparent surface exchange coefficient decreased with increasing surface coverage. In case of small spherical and ellipsoidal particles with a fairly small extension of the shorter axis (large aspect ratio) the surface exchange coefficient decreased almost linearly with increasing surface coverage caused by the reduction of the active (uncovered) surface area. An additional decrease of the surface exchange coefficient (non-linear variation with surface coverage) was observed with respect to dendritic particles. This peculiar behavior could be interpreted in terms of flux constriction effects, i.e. lateral diffusion processes underneath the inert surface particles. The surface area blocked for the exchange reaction seems to be significantly larger than the cross-section of the dendritic surface particles.