Hydrogen in metals
Hydrogen in metals
This chapter focuses on special classes of materials. For QENS studies metal/hydrogen systems are particularly suited due to the huge incoherent scattering cross section of H, low concentrated systems and thus single particle behaviour can be studied, the mobility is large and thus well-pronounced line broadening are obtained, and if H is replaced by D, additionally coherent QENS can be performed. Long range diffusion studies are presented for intermetallic hydrides which allow statements about the rate limiting step in the hydrogen absorption kinetics. For NbDx, an extensive study of the collective diffusion is presented; the separation of coherent and incoherent scattering is done both by software (skillful data evaluation) and by hardware (polarization analysis). Then the diffusion mechanisms in metal/hydrogen systems of increasing complexity are discussed, where QENS exhibits its full power because of single-crystalline samples: H in fcc metals, H in bcc metals, hexagonal closed packed α-phase hydrides, β-V2H as an example of an ordered metal hydride, and LaNi5Hα as an example of an intermetallic hydride. Another scientific problem is the nature of the single diffusion step: hydrogen tunnelling in Nb doped with O, N, C, and in α-ScHx. Tunnelling is even more pronounced for the positive muon, an exotic isotope of the proton; muon diffusion studies in Al and Cu are presented for comparison. Finally, examples of hydrogen diffusion in disordered systems (crystalline metals with trapping impurities or even amorphous metals) are discussed.
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