Concentration dependence of hydrogen diffusion in clamped vanadium (0 0 1) films

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 to 0.5 eV at 0.2 in H V. 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.

Finite size effects: deuterium diffusion in nm thick vanadium layers

“…As seen in figure 2, a clear minimum in the chemical diffusion coefficients is obtained. The observed trend is consistent with that in both bulk and thin films of vanadium [12,25]. At low concentrations, the diffusion coefficients for the n Fe V 2, 3, 4 n n 7 = ( )superlattices approach the same value, which is consistent with the absence of finite size effects of the chemical diffusion in the low concentration limit [23].…”

“…The diffusion constants were extracted from the concentration profiles, as described in [12]. Briefly, Fick's second law…”

“…The analysis of the data is restricted to the times for which the actual boundary and initial conditions hold [12].…”

“…The final diffusion coefficients are obtained as the average value of the diffusion coefficients from the different deuterium concentration profiles in the time window 0-200 s, at the corresponding concentration c. Here the concentration range is divided into intervals of width of c 0.01 D = [D/V]. More details can be found in reference [12].…”

“…However it is highly desirable to have access to the spatial concentration profiles of hydrogen in the host material itself as a function of time as described by Pálsson [11]. This technique has been applied recently to the diffusion of hydrogen in 50 nm vanadium films, where the activation energy was found to change profoundly with concentration, that was linked to a change in site occupancy, even though the measurements were performed above the known phase boundaries [12]. When combining two materials in the form of a superlattice, one can choose the spatial modulation of the absorption potential by the choice of composition, and the spatial distribution of hydrogen concentration is decided by the difference of the absorption potential in the composition.…”