Temperature-dependent near-surface interstitial segregation in niobium

Abstract

Niobium's superconducting properties are affected by the presence and precipitation of impurities in the near-surface region. A systematic wide-temperature range x-ray diffraction study is presented addressing the effect of low temperatures (108 K-130 K) and annealing treatments (523 K in nitrogen atmosphere, 400 K in UHV) on the near-surface region of a hydrogen-loaded Nb(100) single-crystal. Under these conditions, the response of the natural surface oxides (Nb 2 O 5 , NbO 2 , and NbO) and the changes in the subsurface concentration of interstitial species in Nb are explored, thereby including the cryogenic temperature regime relevant for device operation. The formation and suppression of niobium hydrides in such conditions are also investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species (oxygen and nitrogen) occupying the octahedral sites of the Nb bcc lattice at room temperature, both in the near-surface region and in the bulk. (ii) A decrease in the concentration of interstitials within the first 10 nm from the surface at 130 K. (iii) Hydride formation suppression at temperatures as low as 130 K. These results show that mild annealing in nitrogen atmosphere can suppress the formation of superconducting-detrimental niobium hydrides, while subsurface interstitial atoms tend to segregate towards the surface at 130 K, therefore altering the local concentration of impurities within the RF penetration depth of Nb. These processes are discussed in the context of the improvement of niobium superconducting radio-frequency cavities for next-generation particle accelerators.

An Innovative Approach of Surface Polishing for SRF Cavity Applications

“…Surface treatments such as BCP, EP, and MP cause the pollution of the bulk Nb with hydrogen interstitials [42][43][44]. This scenario leads to poor cavity performance due to the precipitation of niobium hydrides at low temperatures inducing the creation of irreversible surface defects called "H-footprints".…”

Grain boundary segregation and carbide precipitation in heat treated niobium superconducting radio frequency cavities

“…[10][11][12][13] Nitrogen is known to be a trapping center for H-interstitial and, thus, can suppress the precipitation of the hydrides. [14][15][16][17] However, reproduction of the N-infusion of SRF cavities leading to an extra-ordinary high Q remains a big challenge for many laboratories. [18][19][20] Specifically, the role of omnipresent Nb carbide formation even under best high vacuum conditions for the cavity performance is unclear.…”