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Stochiometric oxides exhibit low dissociation rate and solubility of hydrogen. Therefore are these suitable as diffusion barriers for minimizing either uptake of hydrogen or outgassing in vacuum environment. On the other hand, stochiometric oxides can be a major obstacle for energy storage based on hydrogen. Most metal hydrides form oxides, preventing the hydrogen uptake of the underlying material. It is therefore of large interest to understand the effect of oxide overlayers to enable effective tailoring of materials properties.
Here we describe experimental exploration of the transport rate through extremely thin Al2O3 layers. The thicknesses of the amorphous oxide layers were 1, 2 and 3 nm. Significant changes in the transport rate were observed when changing the thickness. By coating the layers with Pd, which is catalytically active, we enable fast dissociation of the hydrogen, allowing the separation of the limitation imposed by dissociation and the transport through the oxide.
The main limitation of the transport was determined to be the dissociation of hydrogen at the oxide surface. However, the thickness of the oxide layer influenced the transport rate significantly. By increasing the layer thickness from 1 to 3 nm, the uptake rate is reduced by 3 orders of magnitude at the same temperature. These effects are equally important for hydrogen uptake and release, which is exceedingly important for the outgassing of materials in ultra
high vacuum ambient.
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