Abstract
In this work, a technique for hydrogen diffusion measurements through thin films is proposed and demonstrated. A yttrium film, which changes its refractive index upon hydrogen absorption, is used as an optical sensor to detect hydrogen. The yttrium sensor is coated with a thin (up to 12 nm) layer of test material and exposed to atomic hydrogen. To ensure that the calculated diffusion constant is not artificially changed by surfaces processes, the test layer is coated with palladium. Hydrogen diffusion through test layers of Si, Al, Ag, Ru, Mo, Al2O3 and SiO2 were measured and compared with existing data. The hydrogenation time (time to form YH2) was found to exponentially scale with the enthalpy of hydrogen solution in the test material. Comparison between measured diffusion coefficients for different film thicknesses, as well as previously reported results, highlights the strong dependence of the diffusion constant on sample fabrication conditions, and hydrogen exposure conditions. It is concluded that diffusion through thin films can be reliably compared only when specimen form and exposure conditions are the same. The relevance of this study for applications is discussed.
Highlights
Understanding the interactions between hydrogen and solids has been a focus of many studies
Before comparing the diffusion of hydrogen through thin films of different materials, we first demonstrate results showing the relevance of the Pd capping layer and atomic hydrogen source used in this work
The Pd cap ensures that the test layer is subjected to the same atomic hydrogen concentration
Summary
Understanding the interactions between hydrogen and solids has been a focus of many studies. Shielding materials for hydrogen is relevant to applications such as nuclear fusion reactors [5], extreme ultraviolet lithography [6, 7] and space engineering, since uptake of hydrogen can lead to, for example, hydrogen embrittlement [8] or blister formation [9] It is, highly desirable to develop a method that makes a direct comparison between hydrogen permeation through thin films of different materials possible. Together with our previous calibration of ellipsometry signatures of different Y hydride phases [12], this allowed us to study diffusion through a larger set of materials and compare these results to literature data and our previous publication on H diffusion through Ru [13]
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