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 (Nb2O5, NbO2, 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.
Highlights
Niobium is a type II superconductor with the highest critical transition temperature (Tc) of all pure metals
The formation and suppression of niobium hydrides in such conditions are investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species 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, altering the local concentration of impurities within the RF penetration depth of Nb
Based on the temperature and pressure of the hydrogen treatment, the hydrogen concentration was estimated to be 1 at.% at the end of the thermal treatment [21]. In agreement with such estimation, figure 2(c) presents selected grazing-incidence x-ray diffraction (GIXRD) scans through the (211) Nb reflection at a fixed incident angle αi = 0.5◦ corresponding to a scattering depth Λe of ≈85 nm before and after the complete loading experiment, with both measurements obtained at room temperature
Summary
University of Bremen, MAPEX Core Facility for Materials Analytics, MAPEX Center for Materials and Processes, 28334 Bremen, Germany. Interstitial oxygen and nitrogen were discussed to result in active trapping sites for hydrogen atoms, thereby suppressing the formation of niobium hydrides [15, 19, 23, 24] This may explain why annealing under controlled nitrogen atmospheres, such as the so-called nitrogen doping (T = 1073–1273 K) and nitrogen infusion (T = 393–433 K) treatments, aimed to increase the presence of interstitial nitrogen in the nearsurface region of Nb, can drastically improve Q0 [12, 13]. The presence of interstitial nitrogen in the nearsurface region of Nb SRF cavities, allied with the shrinkage of the natural oxides during high-temperature annealing cycles, was reported to increase its superconducting energy gap while significantly reducing its RF surface resistance (Rs) [25, 26]. Complementary atomic force microscopy (AFM) measurements supporting the GIXRD observations are discussed
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