Abstract
The chemical diffusion coefficient of hydrogen in a 50 nm thin film of vanadium (0 0 1) is measured as a function of concentration and temperature, well above the known phase boundaries. Arrhenius analysis of the tracer diffusion constants reveal large changes in the activation energy with concentration: from 0.10 at 0.05 in H V−1 to 0.5 eV at 0.2 in H V−1. The results are consistent with a change from tetrahedral to octahedral site occupancy, in that concentration range. The change in site occupancy is argued to be caused by the uniaxial expansion of the film originating from the combined hydrogen induced expansion and the clamping of the film to the substrate.
Highlights
Hydrogen in vanadium exhibits the fastest diffusion rates of all classical hydrides
The diffusion coefficients were measured in the disordered part of the phase diagram in the concentration range from c = 0 H V−1 to c ≈ 0.45 H V−1 and at temperatures of T = 473 K, 483 K, 493 K, 503 K, 513 K and 523 K
The whole concentration dependence can be determined from a single profile, but to increase the statistical accuracy, the resulting diffusion constants are determined at different times in the interval t = 0–110 s and subsequently averaged
Summary
Hydrogen in vanadium exhibits the fastest diffusion rates of all classical hydrides. This fast diffusion is in part due to the short distances between nearest neighbour interstitial sites where hydrogen resides and its low mass. The short distance between interstitial sites promotes tunnelling, and quantum effects are assumed to be important for the understanding of diffusion at low and intermediate temperatures [1,2,3]. Kleiner et al investigated the concentration dependence of the tracer diffusion (or self diffusion) of hydrogen in vanadium in the α′ phase with NMR. In these experiments they observed an increase in the activation energy with concentration, from
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