Hydrogen uptake kinetics on niobium surfaces

Abstract

On the basis of the two-state model, the kinetics of hydrogen adsorption on a surface was studied as a function of temperature and time. Following this model, we formulated the hybridizing kinetics and obtained the sticking coefficient of the two-surface state in terms of the bulk fraction x by use of the method of Pick and Greene. We calculated the chemisorption energy and the activation energy for chemisorption by use of the theoretical calculation condition of Davenport, Pick and Greene, and Pick. The estimated chemisorption energy is 13.5-14.3 kcal/mol H and the activation energy for chemisorption is 0.8 -1.6 kcal/mol H. The relation between bulk fraction and heat rate was also obtained by application of the flash-desorption theory to this model.

Temperature-dependent multilayer relaxation of clean and hydrogen-dosed Nb(100)

“…The hydrogen contamination of the technically clean Nb͑100͒ was judged low ͑Ͻ1%͒ based on the base pressure ͑ϳ5 ϫ 10 −11 torr͒ and previous experience with evaluating the effects of hydrogen contamination at similar base pressure. 28,29 The hydrogen-dosed surface structural data exhibit temperature-dependent trends in multilayer relaxation that are consistent with prior photoemission, [10][11][12][13][14][15][16] inelastic electron scattering, 17 and adsorption and/or desorption kinetics [5][6][7][8] experiments that probe the hydrogen-Nb͑100͒ surface system. These experiments probed temperature-dependent electronic ͑photoemission͒ and vibrational ͑inelastic electron scattering͒ properties associated with hydrogen-dosed Nb͑100͒ that were interpreted to result from the effects of hydrogen in tetrahedral subsurface sites.…”

“…Both experiments exhibited reversible temperature-dependent effects produced by hydrogen dosing that provided indirect experimental evidence of the self-trapped subsurface state model which has been proposed to account for the kinetics of hydrogen uptake by Nb. [5][6][7][8][9][10] Our LEED results for multilayer surface relaxation of hydrogen-dosed Nb͑100͒ provide more direct evidence for the subsurface hydrogen sites and additional support for the model of uptake kinetics based on these sites. Figure 4 displays a side-view model of the ͑bcc͒ Nb͑100͒ surface with symbols that describe various lattice plane displacements and separations.…”

“…2 The surface properties of Nb͑100͒ offer special interest: unlike ͑bcc͒ W͑100͒ and Mo͑100͒, which exhibit reversible temperaturedependent reconstruction, 4 Nb͑100͒ maintains a p͑1 ϫ 1͒ structure. Hydrogen uptake by Nb͑100͒ has been extensively studied [5][6][7][8][9] because of the hydrogen adsorption and/or desorption behavior manifested by this surface. The chemical effects have been attributed to subsurface hydrogen sites that affect the admission of hydrogen into the bulk and govern surface hydride formation.…”

First Direct Observation of Nanometer size Hydride Precipitations in Superconducting Niobium

“…The finding that nanoscale Nb hydrides completely disappear when heated to 300 K suggests that the segregated Nb-H phases dissolve thoroughly during this process, and H atoms diffuse back to neighboring tetragonal sites. The two-state model 26,27 describes H concentration as being highest near the surface (< 100 nm) rather than the bulk, and hydrogen atoms cannot diffuse through the dense Nb2O5 layer. Thus, if Nb-H precipitates form at the interface between the Nb2O5 layer and Nb matrix and those hydrides protrude through the oxide layer, we can observe the nanoscale features with AFM scan.…”

“…In SRF grade Nb, hydrogen concentrations are known to be highest in ~ 100 nm region closest to the surface 26,27 , underneath the topmost dense Nb2O5 layer 28 . Understanding of the size, morphology, and distribution of the nanometer scale Nb hydride-like features requires the use of additional techniques at cryogenic temperature, which will be helpful for evaluating the extent of localized degradation in the cavity performance.…”