Modeling of oxygen exchange of oxide ceramics: effect of inert particles at the surface

Abstract

The impact of inert particles located at the surface of mixed ionically/electronically conducting oxide ceramics on the oxygen exchange properties has been studied by numerical modeling of the underlying transport processes. In particular, the phenomenological transport equations have been solved by application of the finite element method. Relaxation curves of the total amount of exchanged oxygen upon an instantaneous change of the oxygen activity in the surrounding atmosphere have been simulated as a function of surface coverage as well as particle size. Both thick samples (thickness, 0.05 cm) and thin samples (thickness, 5 μm) have been investigated. Apparent kinetic parameters, viz. chemical diffusion coefficient and chemical surface exchange coefficient, have been extracted from the simulated relaxation curves by means of nonlinear least squares fitting of appropriate analytical solutions. Basically, the surface exchange reaction is blocked by the inert surface particles, leading to pronounced flux constriction effects. Moreover, in the case of thin samples, relaxation curves can be found which consist of two separate relaxation times. The first time constant is related to the fast relaxation process at free (uncovered) surface regions (usually surface reaction controlled kinetics), while the second relaxation time is caused by two-dimensional diffusion in the thin sample underneath the blocking surface particles.