Study of hydrogen migration mechanisms in materials with complex crystal textures

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The hydrogen permeability measurements in Pd, Fe, Ni membranes have been done by the gas permeation method by an ultrahigh vacuum apparatus. The use of an ultrahigh vacuum apparatus enables us to measure at low pressure and temperature and at thin foil. It is shown that the hydrogen permeation through Pd membrane is exceedingly affected by the surface contamination of the specimen. The contaminants are taken off from the Pd surface by short aging at low temperatures. After this surface cleaning, the amount of hydrogen permeation increases and shows linear-like Arrhenius relation.From above results, our permeation method is suitable for hydrogen permeation experiment of multi-layers specimen. Therefore, the permeation of hydrogen is measured for various number of layers and thickness of multi-layers in Pd-Fe and Pd-Ni systems on Pd base plate.Even after cleaning the Pd surface, the amount of hydrogen permeation through Pd-Ni multi-layers does not show linear-like Arrhenius relation and is larger than that from model calculation in which the bulk property values of Ni are used. From the experimental results and model calculations, it is concluded that the permeation through Pd-Ni multi-layers is not affected by inter Pd layers and boundary layers. Enhanced hydrogen permeation is caused by high grain boundary permeation through small grained Ni layers.After cleaning the Pd surface, the permeation amount through Pd-Fe multi-layers showed linear-like Arrhenius relation, but the dependence for the thickness and number of Fe layers was complex. Further investigation must be needed for this system. The amount through Pd-Fe multi-layers decreased than those obtained from model calculation different from the case of Pd-Ni system.Surface analysis of Pd foil is done by Auger electron spectroscopy and hydrogen gas permeation through Pd is suppressed by C atoms which may come from hydrocarbons.