Hydrogen-vacancy complexes in electron-irradiated niobium

Abstract

The aim of the present work was to investigate the microstructure of bulk niobium irradiated by 10 MeV electrons. Positron-annihilation spectroscopy was employed as a principal technique for the characterization of irradiation-induced defects. Experimental results were compared to first-principles theoretical calculations of positron characteristics. In addition to extended positron-annihilation studies, the specimens were characterized also by x-ray diffraction. It was found that irradiation-induced vacancies are surrounded by hydrogen. Complexes consisting of a Nb vacancy surrounded by one and two H atoms were identified in the irradiated specimens. The concentration of these vacancy-hydrogen complexes was estimated to be ͑18-24͒ ϫ 10 −5 at. %. Vacancy-2H complexes are found to represent the dominating type of defects. Hydrogen atoms surrounding a Nb vacancy cause a shortening of the lifetime of trapped positrons. Moreover, it was demonstrated that hydrogen attached to Nb vacancy can be identified by coincidence Doppler broadening technique. The effect of a thin Pd ͑or Cr͒ overlayer on the irradiation-induced defects was investigated also. It was found that the relative fraction of vacancy-2H complexes is higher in the specimens irradiated with such an overlayer.

Defect studies of H+ implanted niobium

“…Thus the s 1 -component is originated by delocalised positrons at a virtual absence of positron trapping. Such an interpretation is supported by an agreement of the experimental s 1 -values of Table 1 with the ab-initio calculations of positron lifetime in the perfect Nb lattice [2].…”

“…the vacancy not attached with a hydrogen atom. Combining again the available experimental data with the ab-initio calculations, authors [2] came to conclusions that positron lifetimes for the V + 1H and V + 2H defects amount s V+1H % 0.204 ns and s V+2H % 0.182 ns, respectively.…”

“…This situation is qualitatively similar to the irradiation of Nb by energetic (10 MeV) electrons, except for the depth of irradiation damaged region, which is significantly shorter in the case of protons compared to energetic electrons. The irradiation-induced vacancies attached with one or two hydrogen atoms (V + 1H and V + 2H defects, respectively) were identified in the electron irradiated Nb [2] as the only trapping centres for positrons. The presence of hydrogen atoms attached to irradiation induced vacancies diminishes open volume associated with these defects and, correspondingly, shortens the positron lifetimes compared to that of an 'empty' vacancy, i.e.…”

“…Trapping of hydrogen atoms at vacancies in hydrogenloaded and electron-irradiated Nb metal has been investigated in Refs. [1,2], respectively. Hydrogen interaction with irradiation-induced defects is important in the view of Nb applications, e.g.…”

Direct observation of hydrides formation in cavity-grade niobium

“…Strong correlation between the concentrations of oxygen and hydrogen near the surface was reported for several metals [23][24][25][26] and, in particular, for niobium Among point defects, interaction of hydrogen with vacancies in metals is an important issue, which gained significant attention lately due to the discovery of so-called ''superabundant vacancies'' [31]. It was found out that niobium is among the metals where the presence of hydrogen may lead to the enormous enhancement of vacancy concentration [32,33] due to the formation of vacancyhydrogen complexes. There are experimental hints [34,35] that these objects may be involved in a 120 C baking effect on cavities, which remains poorly understood for more than a decade.…”