Hydrogen diffusion studies using nuclear magnetic resonance

Abstract

Nuclear magnetic resonance (NMR) techniques have been applied in a considerable number of experimental studies of molecular and atomic motion in solids. In most experiments designed to measure diffusion parameters, the power spectrum of the random variation of local magnetic dipolar fields is determined directly from the data. Data usually consists of measurements of the nuclear spin system longitudinal spin‐lattice relaxation time T1 or the transverse spin‐spin relaxation time T2, or both. From these measurements the frequency of atomic jumps can be calculated. The analysis also yields the value of the activation energy of self‐diffusion for the diffusing atom.Two relatively new NMR techniques, the measurement of the spin lattice relaxation time in the rotating frame, T1ρ, and the direct measurement of the diffusion coefficient by observation of the spin echo decay in the presence of an applied magnetic field gradient, are beginning to be utilized in solid state diffusion experiments, but have not yet been applied to metal hydrides.The theory of the effect of atomic motion on nuclear magnetic dipolar interactions is reviewed as the mechanism responsible for spin relaxation.The relationships between T1, T2, and T1ρ and the time between atomic jumps, τc is discussed. The pulsed‐gradient spin echo technique is outlined for direct measurement of the translational diffusion coefficient.NMR experiments utilizing steady‐state and pulsed techniques for studying diffusion in metal hydrides are described for several transition metals including Sc, Ti, V, Y, Zr, Nb, and Ta. Experiments in La, Th, and U‐hydrogen systems are also reported. Diffusion parameters. including Ea, and A in the Arrhenius relation, τ1 = (A−1) exp (Ea/RT), are reported.