HYDROGEN AND DEUTERIUM DIFFUSION IN TITANIUM DIHYDRIDES DIDEUTERIDES/

The macroscopic diffusivities Cl of hydrogen in titanium dihydrides, ( TiHx)-H-1 (1.65 less than or equal to x less than or equal to 2.02), a nd of deuterium in titanium dideuterides. (TiHx)-H-2 (x=1.74, 1.98), h ave been measured by means of pulsed-field-gradient (PFG) nuclear magn etic resonance over wide temperature ranges. The effective activation enthalpy for hydrogen diffusion, H-a(1H), obtained bf fitting an Arrhe nius expression to the diffusivities, increases with increasing hydrog en concentration from H-a(1H)=0.55 eV (x=1.65) to H-a(1H)=0.92 eV (x=2 .02). The proton spin-lattice relaxation rate Gamma(1), measured on th e same samples, reveals also a rapid increase in H-a(1H) for x-->2. Th e effective activation enthalpy in the deuterides, H-a(2H)=0.60 eV (x= 1.74) and H-a(2H)=0.67 eV (x=1.98), depends only weakly on x. A compar ison of PFG and Gamma(1) data indicates that hydrogen atoms jump predo minantly between adjacent tetrahedral (T) sites. The observed increase in the effective activation enthalpy with increasing x stands in cont radiction to published H-a(1H) values based on Gamma(1) or NMR line wi dth data. The literature values H(a)(1H)approximate to 0.50 eV. which reflect primarily measurements at lower temperature and lower x-values , are nearly independent of the hydrogen concentration x. The present studies show, however, that the concentration dependence of the diffus ion parameters of hydrogen in TiHx is similar to that in ZrHx, where H -a increases sharply as x approaches the stoichiometric limit. Tf the sublattice formed by the T-sites is almost filled, additional diffusio n processes besides the jumps from occupied to empty T-sites must cont ribute significantly to the hydrogen diffusivity. Several models for s uch an additional diffusion mechanism are discussed. (C) 1997 Elsevier Science S.A.