Detective work: Tracking hydrogen absorption in real time

In a joint multi-disciplinary study, MLZ-collaborators have analyzed in real time how hydrogen enters thin layers of metal. Main author, Dr. Laura Guasco, and her colleague and co-author Dr. Sabine Pütter talk about the importance of their findings.

Laura Guasco at the Instrument NREX of the MLZ. © Jan Greune, MCQST

Laura Guasco at the Instrument NREX of the MLZ. © Jan Greune, MCQST

What did you want to find out with your experiments?

Laura: The question we posed was: What does a hydrogen atom do when it enters niobium?

Why is it important to know what happens with hydrogen in niobium?

Laura: We can now tell in a level of detail what happens if hydrogen goes into the layer. This is for example important for sensors, hydrogen storage materials or even fusion reactors, where the material gets infiltrated by hydrogen and alters its properties. Niobium was of course just a model material for other transition metals, but is also used in some alloys.

What is new in your study?

Laura: We already know what the final status is when hydrogen enters another material. It changes the material’s thickness, makes it brittle, degrades the quality, and in the end, there might even be cracks. However, it was difficult to observe the process of entering. And that is what we did.

Sabine Pütter at the Molecular Beam Epitaxy System. © Bernhard Ludewig, FRM II / TUM

Sabine Pütter at the Molecular Beam Epitaxy System. © Bernhard Ludewig, FRM II / TUM

What did you find out?

Laura: We tracked the hydrogen absorption in real time. We found four different stages happening one after the other. First, hydrogen penetrates the material through already existing voids in the atomic lattice. From these voids, hydrogen enters the grains and changes the crystal structure of the material. The material expands because the spacing between the crystal planes increases. This expansion triggers large stress, which is visible through deformations. Finally, because there are new voids, more hydrogen enters the thin layer.

We learned that you have developed extra samples for this study. What is so special about them?

Sabine: We manufactured them in-house with our Molecular Beam Epitaxy (MBE) setup. You need to put brains into preparing the process: How do I design the samples so that they are magnetic and visible to neutrons and X-rays? And of course, they have to be pure and have the necessary crystalline structure. It is like baking a cake: Everybody can do it, but the quality can be different.

Why did you simultaneously use X-rays and neutrons at the instrument NREX?

Laura: This is unique about the instrument NREX: It can measure using neutrons and X-rays at the same time. Neutrons and X-rays are complementary probes: the first are highly sensitive to hydrogen atoms, the second more sensitive to heavier elements such as metals. And this is also a measurement you can only do at a reactor like the FRM II.

The study was an example of the good collaboration between the partners of MLZ. In what respect?

Sabine: The instrument NREX is operated by the Max-Planck Institute for Solid State Research in Stuttgart, where Laura is a Post-Doc. Her colleagues Thomas Keller and Franz Tralmer designed the cell for the measurements. The samples were prepared by me and I am working for Forschungszentrum Jülich at the MLZ. Then, also X-ray diffraction experiments were performed in the Physics Lab run by FRM II at the Technical University of Munich, namely by Markos Skoulatos and Bastian Veltel.