Boundary-layer diffusion in a cylindrical sandwich composite was investigated. The system consisted of a thin layer, A (in the nm range), sandwiched between two semi-infinite outer layers, B, with the diffusivity of the diffusant in A (DA) being much greater than that in B (DB). The interdiffusion between layer A and layer B was weak. Two diffusion processes: constant surface concentration source and instantaneous surface source, were treated and their concentration distributions were solved analytically. Numerical results were presented which illustrated the effects of the diffusivity ratio, DA/DB, and of the specimen size upon the concentration distributions. The theory was applied to the study of wet oxidation in cylindrical composites of GaAs/AlAs/GaAs. Theory and experiment were in good agreement.

Radial Boundary Layer Diffusion in a Cylindrical Sandwich Composite with Application to Oxidation of GaAs/AlAs/GaAs. S.C.Ko, S.Lee, Y.T.Chou: Materials Chemistry and Physics, 2009, 115[1], 488-92