Optical detection of hydrogen diffusion through thin film barrier materials
Olena Soroka was a PhD student in the research group XUV Optics (XUV), she defended her thesis online on Thursday November 26th 2020. Her supervisor was prof.dr. F. Bijkerk from the Faculty of Science and Technology.
The ability of hydrogen to dissolve and diffuse in solid materials causes accelerated material wear and decreased robustness. Hydrogen permeation into such components can be mitigated in several ways. For instance, metal alloying can be applied in order to reduce the hydrogen solubility. Alternatively, a protective film can be added that acts as a diffusion barrier between the component and the hydrogen environment. Such a barrier is beneficial when a component has a complex structure that is crucial for its functionality and, therefore, cannot be altered.
Most techniques for quantitative detection of hydrogen in materials are not easily accessible. Moreover, the barrier properties of a material depend often on the way of sample preparation. For thin films, an in-situ method of probing is therefore preferred. In this work, an optical sensor for hydrogen diffusion in thin metal and non-metal films was proposed and the relevant physical processes regarding the fabrication and use of such a sensor were investigated. This knowledge was applied for measuring and comparing hydrogen diffusion through a range of potential barrier materials and reference materials. A Y thin film was used as the sensor layer, onto which a test layer, of which the diffusion properties are to be measured, was deposited. The dielectric function of the sensor film changes upon hydrogen absorption, which was monitored with spectroscopic ellipsometry or optical transmission. The design was optimized to enable comparison of hydrogen permeability in different materials.
The work in this thesis resulted in, to our knowledge, the first quantitative measurements of the hydrogen diffusion constant in Ru. Based on a detailed comparison of ellipsometry measurements and X-ray diffraction measurements with synchrotron light as well as lab equipment, an ellipsometry model was developed to quantify the ratio of the hydride phases YH2 and YH3 in a Y film, from which the absorbed amount of hydrogen can be determined. Investigations with different protective capping layers on top of Y showed that the hydrogen adsorption and desorption properties of the cap layer play a decisive role for the stability of the different hydride phases of the Y film, as well as the hydrogen absorption and desorption kinetics. With this knowledge a sensing stack of Pd (cap layer)/test layer/Y/Si substrate was developed, where the efficient hydrogen absorption and diffusion in the Pd guaranteed that the H diffusion through the test film was the limiting factor for Y hydrogenation, at least for films with slow hydrogen diffusion kinetics, which were of interest for this work.
This Y-based sensor was tested with a range of test materials, including metals (Ru, Al, Ag, Mo, Cu), Si and oxides (SiO2 and Al2O3). Hydrogen diffusion kinetics were derived by exposing samples with different test layer thicknesses to atomic hydrogen, monitoring the time required for the Y to YH2 transition. The hydrogenation time (which is inversely proportional to the diffusion constant) roughly scaled with the heat of solution of H in the test material, indicating that the heat of solution can serve as a first indicator for the speed of hydrogen transport through a layer of a certain material. When comparing the hydrogen diffusion constants from this study to values reported in literature, it was found that the reported literature values scatter over orders of magnitudes for a single material, depending on the specimen form and the type of diffusion measurement. This stresses the importance of the sensing approach developed for this thesis, where different materials in the form of thin films can be investigated under identical conditions.