Hydrogen Diffusion in Quasicrystalline ZrCuNiAl
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Hydrogen Diffusion in Quasicrystalline ZrCuNiAl *
T. Apih, M. Klanjsek, Varsha Khare , P. Jeglic, J. Dolinsek J. Stefan Institute, University of Ljubljana, Jamova 39, SI-1000 Ljubljana, Slovenia ABSTRACT The hydrogen diffusion constant D in the hydrogenated quasicrystalline alloy ZrCuNiAl has been determined using the technique of NMR diffusion in a static fringe field of a superconducting magnet. The diffusion constant of partially quasicrystalline Zr69.5Cu12Ni11Al7.5 exhibits a significant decrease with increasing hydrogen-to-metal ratio H/M, owing to creation of defects in the lattice during hydrogen loading, which dominates over the site-blocking effect. The actual alloy structure—the amorphous, icosahedral or approximant—appears to be less important for the hydrogen diffusivity. INTRODUCTION There exist many methods to determine the hydrogen diffusion constant in metal hydrides [1-3], among which the Nuclear Magnetic Resonance (NMR) has probably been applied to more hydrides than any other technique. Various NMR techniques, including spin-lattice relaxation in the laboratory–, rotating– and dipolar frames, NMR line width and diffusion in a magnetic field –12 –4 gradient are suitable to characterize diffusion coefficients in the range between 10 and 10 2 cm /s. Out of these techniques, only the diffusion in a field gradient gives the direct, modelindependent value of the diffusion constant D, whereas all other techniques yield an indirect information, the correlation time τ c of the fluctuating local magnetic fields. Assuming that τ c fluctuations arise predominantly from the translational diffusion of hydrogen atoms (which is in many cases a crude and not well justified assumption), the diffusion coefficient may be estimated from the relation D = fT a 6τ c . Here a is the mean jump distance and fT is the structure– sensitive "tracer correlation function", ranging usually between 0.5 and 1. Except for some simple structures, a and fT are not easily obtained. Regarding the NMR diffusion technique in a magnetic field gradient, the most commonly employed method is that of Stejskal and Tanner [4], which uses pulsed field gradient (PFG). The technically available PFG values limit the 2
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sensitivity of this technique to the range D ≥ 10 cm / s . The PFG method was successfully applied to simple-structure metal hydrides, like the elemental fcc PdHx and TiHx [3] and the Pd1– xAgxHy alloys [5], where D was determined at temperatures where its value was in the range 10 6 –8 2 – 10 cm /s. The more complicated structure and/or structural and chemical disorders make 2
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the hydrogen diffusion in quasicrystals (QC's) slower with D < 10 cm / s , so that no direct determination of D in QC's by the PFG method was reported—to the best of our knowledge—in the literature so far. In this paper we present a direct determination of the hydrogen self-diffusion coefficient in a hydrogenated ZrCuNiAl partially quasicrystalline alloy by using the method of NMR diffusion in a static fringe field (SFF) of a superconducting
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