Hydrogen in nanostructured vanadium-hydrogen systems - art. no. 094307

Nanostructured vanadium-hydrides, beta (2)-VHx, with typical grain sizes of 80 nm (x=0.82), 30 nm (x =0.73), and 10 nm (x=0.67) were prepared by mecha nical milling under hydrogen atmosphere. The final grain size, about 10 nm, does not change any more with increasing milling time, and a homogeneous a morphous phase is not formed in this system, even after milling for 300 min . The hydrogen concentration in the grains x(G) decreases with decreasing g rain size from x(G)=0.82 in 80-nm grains to 0.72 in 10-nm grains. This indi cates a modification of the beta (2)-gamma phase boundary in the V-H system with nanometer-scale grains. The hydrogen concentration in the intergrains , x(IG) approximate to0.5-0.6, is smaller than in the grains, and was found to be nearly independent of the grain size. The hydrogen diffusivity has b een studied by NMR measurements of the proton spin-lattice relaxation Gamma (1). Generally, the measured Gamma (1) consists of contributions that resu lt from both hydrogen in the grains and in the intergrain regions. Due to t he smaller spin-spin relaxation rate Gamma (2) of the protons in the interg rain regions, their contribution to Gamma (1) could be measured separately by the spin-echo technique. The relaxation data indicate that, at a given t emperature, the hydrogen diffusivity in the intergrain regions is substanti ally higher than inside the grains. The frequency dependence of the dipolar contribution Gamma (1,dip) reveals a distribution in the activation enthal py for hydrogen in the intergrain regions. This distribution was found to b e the broader the smaller the grain size. A change in the diffusion mechani sms, presumably arising from the beta (2)-delta phase transition, takes pla ce at about 200 K. The exchange of hydrogen atoms between the grains and th e intergrain regions occurs very slowly and is negligible on the time scale given by Gamma (1).