Modulated permeation was proposed here as a method for separating the surface and bulk contributions to the macroscopic determination of diffusion and solubility coefficients. By modulating the pressure difference across a thin foil at high temperatures, a modulated flow could be driven by the gas phase. The characteristic frequency response of the system, in terms of pressure modulation amplitudes and phase, could be identified with surface and bulk effects via a relatively simple model that was based upon dissociative chemisorption. Further details of the surface processes could be resolved by using a model which incorporated the adsorption of molecules, dissociation in the surface layer and absorption of atoms in the bulk, together with the reverse processes. Six rate constants described the complete surface interaction. Bulk trapping could be easily included in the models, although its effects were most apparent when surface processes were not the limiting step in the permeation mechanism. The 6 rate constant model, with trapping, was used to interpret modulated permeation experiments on foils of Pd, Ni, Mo alloy, Cu, TiC on Mo alloy, laminated Cu/Mo and infiltrated Cu/Mo.

A.K.Altunoglu, N.S.J.Braithwaite: Journal of Alloys and Compounds, 1995, 231, 302-6