Kinetic regimes of hydrogen absorption in thin films
As visions of a hydrogen economy are gaining prominence, it becomes increasingly important to understand how penetration of hydrogen into materials can disrupt their structural integrity, and to develop methods to mitigate this degradation. Various mechanisms have been observed in coatings and thin films such as absorption through defects, interstitial incorporation, hydride phase formation, and structural failure and/or peeling-off.
In-situ measurements of the hydrogen concentration during hydrogen absorption are essential to understand which mechanisms are involved, how they are connected to each other, and how they develop in time. Resonant neutron reflectometry (RNR) is a powerful tool to directly quantify the hydrogen concentration with high sensitivity and time resolution, while allowing experiments on any thin layer in a hydrogen atmosphere.
Researchers at MPI-FKF combined in situ RNR with x-ray reflectivity and electrical resistance to understand the relation between hydrogen content and structural and electrical changes in the material in a time-dependent manner. This allowed them to distinguish four different regimes of hydrogen absorption, each a consequence of the preceding one. First, hydrogen penetrates the material through grain boundaries, voids and pre-existing defects. From these defects, hydrogen enters the grains and changes the crystal structure of the material, inducing a large expansion of the spacing between crystalline planes. This expansion induces large stresses in the film, which are released through further plastic deformations and dislocation formation. Finally, due to the presence of new defects, more hydrogen penetrates the thin layer up to concentrations exceeding the amounts that can be attained in the corresponding hydride material.
The combination of in situ RNR with x-ray reflectivity and electrical resistance measurements proves to be a powerful approach for directly tracking the temporal sequence of absorption mechanisms – an access that was not previously possible. The insights gained provide a solid foundation for defining safe operating conditions in hydrogen-related applications such as pipeline coatings, storage vessels, membranes, and proton-based devices and sensors.
