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Hydrogen diffusion; Palladium–silver alloys; Spin lattice relaxation; Pulsed field gradient nuclear magnetic resonance
Abstract:
The hydrogen dynamics in the α-phase of Pd1−xAgxHy has been studied by proton nuclear magnetic resonance (NMR) measurements. Jump frequencies of the hydrogen atoms in the metal lattice have been deduced from the dipolar spin-lattice relaxation rates Γ1d by applying the BPP model. In PdHy, the temperature dependence of Γ1d is well described by a single jump process for all hydrogen atoms. In contrast, in the palladium–silver alloys Pd1−xAgxHy, the Γ1d data indicate more than one jump process with different activation energies. Two and three processes could be separated for x=0.1 and x=0.3, respectively. In both alloys, the diffusion process with the lowest activation energy (Ea=226 meV) is comparable to that in pure palladium. The jump processes with higher activation energies occur more frequently in Pd0.7Ag0.3 than in Pd0.9Ag0.1. This indicates higher energy-barriers for jumps between O-sites in silver-rich environments. Direct measurements of the long-range diffusion coefficients D were performed by pulsed-field-gradient (PFG)-NMR at temperatures up to 450 K. For both Pd1−xAgxHysamples, D(T) is well represented by a single Arrhenius law with the diffusion parameters D0=3.0×10−7 m2 s−1 and Ea=230 meV for x=0.1 and D0=2.1×10−7 m2 s−1 and Ea=260 meV for x=0.3. The diffusivities calculated from the jump frequencies of the hydrogen atoms are in good agreement with these PFG results and also with measurements using the time-lag technique.
