Diffusion layer

Abstract: Diffusion in solids is known to occur along grain boundaries and over free surfaces more rapidly than through the interiors of crystals. In order to facilitate quantitative investigation of grain boundary and surface diffusion, a mathematical analysis of the problem has been completed, assuming that grain boundary diffusion is analogous to the diffusion of heat along a thin copper foil imbedded in cork. The calculated diffusion‐penetration relationship for grain boundary diffusion is shown to agree with the experimentally determined grain boundary self‐diffusion of silver.

Abstract: The technique of diffusive gradients in thin films (DGT) provides an in situ means of quantitatively measuring labile species in aqueous systems. By ensuring that transport of metal ions to an exchange resin is solely by free diffusion through a membrane, of known thickness, Δg, the concentration in the bulk solution, C b , can be calculated from the measured mass in the resin, M, after time, t, by C b = MΔg/DAt, where D is the molecular diffusion coefficient and A is the exposure surface area of the membrane. If sufficiently thick (∼1 mm) diffusion layer is selected, the flux of metal to the resin is independent of the hydrodynamics in solution above a threshold level of convection. Deployment for 1 day results in a concentration factor of ∼300, allowing metals to be measured at extremely low levels (4 pmol L -1 ). Only labile metal species are measured, the effective time window of typically 2 min being determined by the thickness of the diffusion layer. Because metals are quantified by their kinetics of uptake rather than the attainment of equilibrium, any deployment time can be selected from 1 h to typically 3 months when the resin becomes saturated. The measurement is independent of ionic strength (10 nM-1 M). For Chelex-100 as the resin, the measurement is independent of pH in the range of 5-8.3, but a subtheoretical response is obtained at pH <5 where binding to Chelex is diminished. The effect of temperature can be predicted from the known temperature dependence of the diffusion coefficient and viscosity. The application of DGT to the in situ measurement of Cd, Fe, Mn, and Cu in coastal and open seawater is demonstrated, and its more general applicability as a pollution monitoring tool and for measuring an in situ flux, as a surrogate for bioavailability, is discussed.

Abstract: A sensitive electrochemical technique, which permits the recording of the instantaneous rate of permeation of electrolytic hydrogen through palladium, is described. Results were obtained under conditions required by theory for the diffusion of hydrogen with the use of electronic potentiostats. Analysis of the results shows the validity of the equations previously deduced for the diffusion of hydrogen. No anomalies in the diffusion have been found under these conditions. Thus the diffusion constant is found to be independent of thickness in the range 0.0035 to 0.054 cm. The permeation has been found to be inversely proportional to thickness as required by theory. The diffusion constant for a hydrogen poor $\alpha$-palladium has been found to be 1.30 $\pm$ 0.20 x 10$^{-7}$ cm$^2$ s$^{-1}$ at room temperature. Reasons for permeation anomalies reported in the literature for diffusion of hydrogen from the gas phase are discussed. Attention is drawn to errors in the classical time lag determination which unless corrected, can give rise to spurious thickness and temperature dependence of the diffusion constant.