Visualizing spatially inhomogeneous hydrogen isotope diffusion by hydrogenography

We developed a novel, recently patented application of hydrogenography, i.e., the change of the optical properties of certain indicator materials (e.g., yttrium) when these are charged with hydrogen isotopes. By applying patterned, discontinuous yttrium films to the back side of permeation samples, we circumvented the issue that the hydrogen isotopes could also diffuse laterally within the yttrium film, which might blur or even prevent detection of laterally inhomogeneous diffusion effects through the actual sample. Thus, we were able to directly visualize the preferential permeation of deuterium along grain boundaries in recrystallized tungsten foils exposed to a low-temperature deuterium plasma. Similarly, we were able to show that permeation of deuterium focuses on the percolating Fe-Ni matrix phase of the tungsten heavy alloy HPM 1850. These findings give a strong incentive to assess whether 1-D diffusion simulations, which implicitly assume lateral homogeneity, are generally sufficient to accurately describe H isotope diffusion in fusion reactor materials. At least under certain conditions, an approach considering the effects of internal 3-D structures of the materials - like those described here - may be necessary.