Effect of shape and particle size distribution of inert surface particles on the oxygen exchange kinetics of oxide ceramics

Abstract The effect of size distribution and shape of inert surface particles on the oxygen exchange kinetics of solid oxides is investigated. In particular, relaxation curves are calculated numerically for both thick (bulk) samples (thickness: 0.05 cm) and thin samples (thickness: 0.5–5 μm) by means of the finite element method. A “bimodal” size distribution, comprising coarse particles and many tiny particles, as well as a distribution of coarse and smaller particles of comparable size (“monomodal” distribution) are taken into account. Apparent kinetic parameters (chemical surface exchange coefficient and chemical diffusion coefficient) are extracted from the calculated relaxation curves. A significant deviation from the linear relationship between the surface exchange coefficient and the surface coverage is found, if the surface is primarily covered by coarse particles owing to flux constriction effects caused by lateral diffusion underneath the surface particles. In the case of thin samples the relaxation process can exhibit two or more time constants. The effect of the particle shape is investigated by a systematic variation of the aspect ratio of particles with a rectangular cross section. The shorter side length is decisive for the occurrence of flux constriction effects which are negligible for large aspect ratios.