Diffusion coefficient, of hydrogen

The above data relate to very pure iron samples with low dislocation densities. In real steels the trapping effects result in much lower apparent diffusivities, which are dependent on the metallurgical state of the steel, as well as its chemical composition. Typical values for the apparent diffusion coefficient of hydrogen in high-strength alloy steel at room temperature are in the region of 10" mVs. 

Fig. 16. Panorama of values in the literature for diffusion coefficients of hydrogen in silicon and for other diffusion-related descriptors. Black symbols represent what can plausibly be argued to be diffusion coefficients of a single species or of a mixture of species appropriate to intrinsic conditions. Other points are effective diffusion coefficients dependent on doping and hydrogenation conditions polygons represent values inferred from passivation profiles [i.e., similar to the Dapp = L2/t of Eq. (95) and the ensuing discussion] pluses and crosses represent other quantities that have been called diffusion coefficients. The full line is a rough estimation for H+, drawn assuming the top points to refer mainly to this species otherwise the line should be higher at this end. The dashed line is drawn parallel a factor 2 lower to illustrate a plausible order of magnitude of the difference between 2H and H.

Therefore, only now, when we know the meaning of the law of constant total energy, do we know the conditions under which it is valid, as well as its exceptions. Thus, in a lean mixture of hydrogen with air, the diffusion coefficient of hydrogen (which may naturally be considered the carrier of the chemical energy) exceeds the thermal diffusivity of the mixture diffusion carries away ... 

Diffusion of atoms from the point at which they dissociate on a metal surface to the edge of the metal crystallite is one of the component steps of hydrogen spillover. Quasielastic neutron scattering experiments have produced direct evidence for the diffusion coefficients of hydrogen on the surface of catalysts. The mean time between diffusional jumps for hydrogen on a Raney Ni surface has been found to be 2.7 0.5 x 10 9s at 150°C.72 For H on the surface of Pt crystals dispersed within a Y type zeolite the mean time between surface jumps was found73 to lie between 3.0 and 8 x 10-9s at 100 °C. 

The diffusion coefficient of hydrogen in steel is given as a function of temperature as... 

A thin plastic membrane separates hydrogen from air. The molar concentrations of hydrogen in Ihe membrane at the inner and outer surfaces are determined to be 0.045 and 0.002 kmol/m respectively. The binary diffusion coefficient of hydrogen in plastic at the operation temperature is 5.3 X lO mVs. Determine the mass flow rate of hydrogen by diffusion through the membrane under steady conditions if the thickness of the membrane is (a) 2 ram and (b) 0.5 mni,... 

The metal hydride material in these experiments is a 25 micron thick palladium foil ( 99.999 purity, Alfa). The diffusion coefficient of hydrogen through palladium has been measured by electrochemical and g s phase techniques and is approximately 1.6e-7 cm /sec at 20 C (J ). T e exposed palladium membrane surface area is... 

Yeo and McBreen " measured the diffusion coefficients of hydrogen and chlorine in Nafion immersed in HCl solutions, and that of bromine in HCl and HBr solutions as a function of electrolyte concentration and temperature. In concentrated HCl solutions, the order of diffusion coefficients is hydrogen > chlorine > bromine, as expected from the molecular sizes. Activation energies for... 

For most of the studied metal-hydrogen systems, the temperature dependence of the measured tracer diffusion coefficient of hydrogen follows the Arrhenius law,... 

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