Interface exchange limited transport of O through a polycrystalline film into a substrate with a higher O diffusivity was characteristic for mixed ionic and electronic conductor electrode films on ionic conductors in solid oxide fuel cells. Such films could generally be approximately represented as a homogeneous medium with some effective diffusivity (type A regime), so that the mathematical description of the O diffusion in such films was significantly simplified. The diffusion equations in a film/substrate system were solved for the practically important boundary conditions under which O exchange was limited at the gas/film and film/substrate interfaces. An approximate solution was derived that simplified the deduction of interface exchange coefficients from O isotope exchange depth-profile experiments.

Oxygen Self-Diffusion through Fine-Grained Polycrystalline Films into Single Crystalline Substrates - a Mathematical Description. P.Fielitz, C.A.G.Borchardt: Solid State Ionics, 2005, 176[1-2], 61-72