Comparative H diffusion measurement through metal and non-metal nano-layers using optical sensing

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.