NMR Studies of Hydrogen Diffusion in Palladium and ß-LaNi5 Hydrides
Nuclear magnetic resonance (NMR) techniques have been employed extensively to study the dynamics of hydrogen diffusion in metal hydrides. The pulsed field gradient (PFG) spin-echo technique has been usedl to deduce the hydrogen diffusion coefficient, D, in transition metals. To overcome difficulties encountered with conventional PFG spin-echo technique in β-LaNi5 hydride, an alternating pulsed field gradient technique2 has also been introduced. Proton spin-lattice relaxation time T1 and the rotating frame, T1ρ measurements2–5 have also been used to obtain the correlation time which corresponds to the mean proton diffusion jump time, τD. However, the T1 and T1ρ data for many hydrides including LaNi5H7, Hf2RhH2.2, Hf2CoH3.8, PdHx, NbHx, Th4H15 etc. are anomalous and difficult to interpret. The slope on the high temperature side of the T1 or Tlρ minimum is always steeper than the slope on the low temperature side. The frequency dependence on the low temperature side is not quadratic and the departure can be large.2–5 The reported attempt frequencies as deduced from T1 and Tlρ data for different hydrides differ from each other by orders of magnitude. Several suggestions have been made to explain these anomalous results. These include: (a) two separate activation energies2 governing two types of hydrogen motion which is discarded subsequently4 because it leads to a ω2 dependence; (b) translational diffusion relaxation4 of nuclear spin model of Torrey6 but unfortunately no comparison of Torrey’s model predictions with data has been made; and (c) the reference made by Jones et al.3 and by Chang et al.4 to a recently proposed unified low frequency response model7 for possible explanation; but again no comparisons have been made.
KeywordsHydrogen Diffusion Metal Hydride Nuclear Magnetic Resonance Study Naval Research Laboratory Pulse Field Gradient
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