Hydrogen in nanostructured vanadium-hydrogen systems

Nanostructured vanadium-hydrides, β₂ - VH_x, with typical grain sizes of 80 nm (x=0.82), 30 nm (x = 0.73), and 10 nm (x = 0.67) were prepared by mechanical milling under hydrogen atmosphere. The final grain size, about 10 nm, does not change any more with increasing milling time, and a homogeneous amorphous phase is not formed in this system, even after milling for 300 min. The hydrogen concentration in the grains x_G decreases with decreasing grain size from x_G = 0.82 in 80-nm grains to 0.72 in 10-nm grains. This indicates a modification of the β₂ - γ phase boundary in the V-H system with nanometer-scale grains. The hydrogen concentration in the intergrains, x_IG ≈ 0.5-0.6, is smaller than in the grains, and was found to be nearly independent of the grain size. The hydrogen diffusivity has been studied by NMR measurements of the proton spin-lattice relaxation Γ₁. Generally, the measured Γ₁ consists of contributions that result from both hydrogen in the grains and in the intergrain regions. Due to the smaller spin-spin relaxation rate Γ₂ of the protons in the intergrain regions, their contribution to Γ₁ could be measured separately by the spin-echo technique. The relaxation data indicate that, at a given temperature, the hydrogen diffusivity in the intergrain regions is substantially higher than inside the grains. The frequency dependence of the dipolar contribution Γ_1,dip reveals a distribution in the activation enthalpy for hydrogen in the intergrain regions. This distribution was found to be the broader the smaller the grain size. A change in the diffusion mechanisms, presumably arising from the β₂ - δ phase transition, takes place at about 200 K. The exchange of hydrogen atoms between the grains and the intergrain regions occurs very slowly and is negligible on the time scale given by Γ₁.