Sudy of hydrogen-defect interaction in solids using positron annihilation

J. Čížek¹, I. Procházka¹, G. Brauer², W. Anwand², A. Mücklich², R. Kirchheim³, A. Pundt³

, C. Bäthz⁴,
M. Knapp⁴

¹Faculty of Mathematics and Physics, Charles University, CZ-18000 V Holešovičkách 2, Praha 8, Czech Republic

²Institut für Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf, Postfach 510119, D-01314 Dresden, Germany

³Institut fur Materialphysik, Universität Göttingen, D-37073 Tammannstr. 1, Göttingen, Germany

⁴Institute of Materials Science, Darmstadt University of Technology, D-64287 Petersenstr. 23, Darmstadt, Germany

Nanocrystalline thin Nb films loaded with hydrogen were studied in the present work. Thin Nb films were prepared on (100) Si substrate at room temperature by cathode beam sputtering. Microstructure observation by transmission electron microscopy (TEM) revealed that the films exhibit elongated column-like grains. Width of the columns is smaller than 100 nm. Two “generations” of grains can be distinguished in the columns: (i) “first generation” grains attached directly on the Si substrate and (ii) “second generation” grains which grow on the top of the “first generation” grains. X-ray diffraction (XRD) studies revealed that the Nb films are characterized by a strong (110) texture. However lateral orientation of grains (i.e. in the plane of the substrate) is random. Defect studies were performed by slow positron implantation spectroscopy (SPIS) with measurement of Doppler broadening of the annihilation line. Shape of the annihilation line was characterized by the S parameter which represents a fraction of positrons annihilating with the low-momentum delocalized electrons. It was found that the virgin Nb films (i.e. free of hydrogen) contain a high density of defects. Nanocrystalline grain size leads to a significant volume fraction of grain boundaries containing open volume vacancy-like defects. Thus, most of positrons annihilate from trapped state in the open volume defects at grain boundaries.

Subsequently, the films were step-by-step electrochemically charged with hydrogen and evolution of 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, the lattice expansion is highly anisotropic in thin films. The in-plane expansion is prevented because the films are clamped to an elastically hard substrate. On the other hand, the out-of-plane expansion is substantially higher than in the bulk samples. Moreover, we have found an enhanced hydrogen solubility in the a-phase in the nanocrystalline Nb films. Formation of the b-phase (NbH) starts at hydrogen concentration x_H = 0.25 [H/Nb atomic ratio], i.e. it is » 4 times higher than in bulk Nb. Using SPIS it was found that hydrogen is trapped in the vacancy-like defects at grain boundaries. Hydrogen trapping leads to a local increase of electron density in these defects and is reflected by a pronounced decrease of the S parameter in the hydrogen-loaded samples. Subsequently, when hydrogen concentration exceeds x_H = 0.02 [H/Nb], all available traps at grain boundaries are already filled with hydrogen and the S parameter does not change anymore. Formation of the b-phase particles leads to introduction of new defects, which is reflected by an increase of the S parameter at x_H > 0.25 [H/Nb].

This work was financially supported by The Czech Science Foundation (contract 202/05/0074) and The Ministry of Education, Youth and Sports of The Czech Republic (project No. 1K03025).