Tracer diffusivity studies were made of monocrystalline 2/1-mullite. The rare stable isotopes, 18O and 30Si, and the artificial pseudo-stable isotope, 26Al, were used as tracers. Secondary-ion mass spectrometry was used to analyze the depth distribution of the tracer isotopes after diffusion annealing. An essential result of these tracer diffusivity studies was a very low diffusivity of 30Si as compared with the diffusivities of 26Al and 18O; which were almost equal. Based upon this observation, a reaction model was proposed, for the diffusion-controlled formation of mullite in the solid state, which assumed that the growth kinetics of a mullite layer were controlled mainly by the diffusion of Al ions and O ions.

A Diffusion-Controlled Mullite Formation Reaction Model Based on Tracer Diffusivity Data for Aluminium, Silicon and Oxygen. P.Fielitz, G.Borchardt, M.Schmücker, H.Schneider: Philosophical Magazine, 2007, 87[1], 111-27