Surface characterization of nitrogen-doped Nb (100) large-grain superconducting RF cavity material

Abstract

(100) oriented niobium (Nb) crystals annealed in the vacuum conditions close to that used in mass production of 1.3 GHz superconducting radio-frequency cavities for linear accelerators, and treated in nitrogen at a partial pressure of 0.04 mbar at temperatures of 800 °C and 900 °C have been studied. The surfaces of the nitrogen-treated samples were investigated by means of various surface-sensitive techniques, including grazing-incidence X-ray diffraction, X-ray photoemission spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy in planar view and on cross-sections prepared by a focused ion beam. The appearance of a dense layer of epitaxial rectangular precipitates has been observed for the Niobium nitrided at 900°C. Increased nitrogen concentration in the near surface region was detected by glow-discharge optical-emission spectroscopy, focused ion-beam cross-sectional images and X-ray photoelectron spectroscopy. Crystalline phases of NbO and β-Nb 2 N were identified by X-ray diffraction. This information was confirmed by X-ray photoelectron measurements, which in addition revealed the presence of Nb 2 O 5 , NbON, NbN and NbN x O y components on the surface. These results establish the near-surface Nb phase composition after high-temperature nitrogen treatment, which is important for obtaining a better understanding of the improved RF cavity performance.

Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

“…However, at 500°C a new layer with a thickness of 1.41 nm was formed between NbO and Nb. The electron density of this layer is similar to the theoretically calculated value for the β-Nb 2 N phase, which is known to precipitate upon high-temperature treatments in a N 2 atmosphere [10,14]. Therefore, it can be concluded that the layer relates to the presence of a possible Nb x N y compound underneath the oxide layer, as the presence of typical β-Nb 2 N precipitates at the surface was not observed by SEM [ Fig.…”

“…Furthermore, a contribution from two peaks at binding energies of 204.3 and 207.0 eV is detected, which can be attributed to a NbN x O y phase, supported by the presence of a peak in N 1s core level [Figs. 8(c) and 8(d)] at the binding energy of ∼397.5 eV, which corresponds to Nb bound to N in niobium nitrides and oxynitrides as reported for the nitrogen-doped niobium samples [10]. The peak at a higher binding energy in the N 1s region at ∼400 eV is assigned to the adsorbed N on the surface.…”

“…For the (100) surface, XPS measurements on singlecrystalline Nb etched by BCP and annealed at around 155°C and 265°C in UHV showed predominantly a , together with an increase in surface roughness [31]. Furthermore, the presence of epitaxial NbO was confirmed by means of GIXRD for cavity-grade Nb (100) specimens annealed at 900°C in a high vacuum and treated with nitrogen at the same temperature [10].…”

“…Such a passivating layer was not present in the sample under test, as confirmed by the regrowth of the oxides NbO 2 and Nb 2 O 5 after it was reexposed to air. Moreover, in contrast to what was observed for cavitygrade Nb upon high-temperature annealing, where rectangular-and triangular-shaped precipitates were observed at the surface [10,14], no precipitates were detected by SEM after infusion [see Fig. S1(a) in Supplemental Material [34]].…”

“…The former treatment, known as nitrogen doping, includes annealing polycrystalline Nb cavities for a few minutes at 800°C in 3 × 10 −2 -8 × 10 −2 mbar of N 2 . Such treatment conditions lead to the formation of β-Nb 2 N inclusions at the surface as well as promoting the growth of NbO [6,10]. By etching approximately 5 μm by EP, therefore removing the aforementioned inclusions, cavities displayed record-breaking quality factors at intermediate accelerating field gradients up to 20 MV=m.…”