THE SCIENCE AND APPLICATION OF MODULATED PERMEATION EXPERIMENTS

Modulated permeation has been proposed as a means to separate surface and bulk processes in the macroscopic determination of diffusion and s olubility coefficients. By modulating the pressure difference across a thin foil at elevated temperature, a modulated flow can be driven dir ect from the gas phase. This method has been used in connection with t he permeation of metals and other materials by hydrogen. The character istic frequency response of the system in terms of pressure modulation amplitudes and phase can be identified with surface and bulk effects through a relatively simple model based on dissociative chemisorption. Further details of the surface processes can be resolved by invoking a model which incorporates the adsorption of molecules, dissociation i n the surface layer and absorption of atoms in the bulk, together with reverse processes. Six rate constants describe the complete surface i nteraction. Bulk trapping is readily included within these models, alt hough its effects are most apparent when surface processes are not the limiting step in the permeation mechanism. The six rate constant mode l with trapping has been applied to the interpretation of modulated pe rmeation experiments on foils of Pd, Ni, Mo alloys, Cu, TiC on Mo allo y, laminated Cu/Mo and infiltrated Cu/Mo.