Defect studies of hydrogen-loaded thin Nb films

Abstract

Hydrogen interaction with defects in thin niobium (Nb) films was investigated using slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Thin Nb films on Si substrates were prepared using cathode beam sputtering at room temperature. Initially, the microstructure of the virgin (hydrogen-free) films was characterized. Subsequently, the films were step-by-step electrochemically charged with hydrogen and the evolution of the microstructure with increasing hydrogen concentration was monitored. Hydrogen loading leads to a significant lattice expansion which was measured by XRD. Contrary to free-standing bulk metals, thin films are highly anisotropic. The in-plane expansion is prevented because the films are clamped on the elastically hard substrate. On the other hand, the out-of-plane expansion is substantially higher than in the bulk samples. Moreover, an enhanced hydrogen solubility in the a-phase was found in nanocrystalline Nb films. It was found that most of positrons in the films are trapped at open-volume defects at grain boundaries (GBs). These defects represent trapping sites also for hydrogen atoms. Hydrogen trapping at vacancy-like defects like GBs leads to a local increase of the electron density and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. In addition, it was found that new defects are introduced at higher concentrations of hydrogen due to the formation of NbH (b-phase) particles. #

Effect of Hydrogen on the Mechanical Properties of Stainless Steels

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Investigation of nanocrystalline Gd films loaded with hydrogen

“…Preferential hydrogenation of GBs was confirmed by positron annihilation spectroscopy [8,9]. Precipitation of the hydride phase in the grain interiors takes place only when GBs are saturated with hydrogen.…”

Positron annihilation study of hydrogen trapping at open-volume defects: Comparison of nanocrystalline and epitaxial Nb thin films

“…The asymmetric shape of the profile indicates that it is a superposition of two contributions from the Nb layer with slightly different inter-planar spacing; see Ref. [4] for details. Taking into account the TEM observations, it indicates that the inter-planar spacing, d 1 1 0 , of the {1 1 0} planes in the "first generation" and in the "second generation" sub-columns differ.…”

“…In the film's cross-section the columns are divided horizontally into two "generations" of sub-columns with a height being approximately the half of the film thickness [4]. The "first generation" sub-columns are situated close to the Si substrate, while the "second generation" sub-columns grow on the top of the "first generation".…”