Experimental Results on Diffusion Coefficients

Extensive compilations of experimental data on hydrogen diffusion coefficients in binary metal-hydrogen systems have been published by Volkl and Alefeld [9] and Wipf [42]. These reviews can be referred to as sources of information on H diffusivities in different M-H systems. In this section we shall discuss some general features of H diffusivity in metals resulting from numerous experimental studies. 

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

We now turn to the discussion of isotope effects in hydrogen diffusivity. The classical diffusion theory predicts that the pre-exponential factor Dq in Eq. (26.17) is inversely proportional to the square root of the mass of a diffusing particle, while the activation energy does not depend on this mass. According to these predictions, the diffusion coefficient of D atoms should be s/2 times lower than that of H atoms over the entire temperature range. However, the isotope dependence of hydrogen diffusivity in all metal-hydrogen systems studied so far shows deviations from the predictions of the classical theory. In particular, the measured effec- 

Note that the vertical scales for the three metals are shifted with respect to each other. 

The isotope dependence of the hydrogen diffusivity in f c.c. metals differs from that in the case of b.c.c. metals. While the ratio of the pre-exponential factors 

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