Hydrogen-induced defects in niobium studied by positron annihilation spectroscopy

Abstract

Changes of the defect structure of Nb induced by hydrogen loading were studied by positron annihilation spectroscopy (PAS). Two sets of samples with different initial microstructure were studied: (i) well-annealed bulk samples, and (ii) thin nanocrystalline films. First, the microstructure of the virgin samples was characterized. Subsequently, the samples were step-by-step electrochemically loaded with hydrogen in the α -phase region, where the Nb–H system represents a single-phase solid solution. Two complementary PAS techniques, namely positron lifetime (PL) spectroscopy and slow positron implantation spectroscopy (SPIS), and in addition X-ray diffraction were applied to investigate the evolution of the microstructure with increasing hydrogen concentration. It was found that new defects were created in the bulk Nb sample due to hydrogen loading. Their concentration increases with increasing hydrogen concentration. A comparison of PAS results with theoretical calculations revealed that complexes consisting of a vacancy surrounded likely by four hydrogen atoms are formed. Hydrogen trapping in open-volume defects at grain boundaries was observed in the thin Nb films.