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

Abstract

A detailed study of the near-surface structure and composition of Nb, the material of choice for superconducting radio-frequency accelerator (SRF) cavities, is of great importance in order to understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse x-ray scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex situ during annealing in an ultrahigh vacuum as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to 800°C in a vacuum promotes a partial reduction of the natural surface oxides (Nb 2 O 5 , NbO 2 , and NbO) into NbO. Upon cooling to 120°C, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 h. An oxygen enrichment below the Nb-oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides, was observed upon a stepwise annealing up to 250°C. Nitrogen introduction to the system at 250°C promotes neither N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to 500°C in a nitrogen atmosphere, the growth of a new subsurface Nb x N y layer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.

Anaerobe Syntrophic Co-culture-Mediated Green Synthesis of Ultrathin Niobium Carbide (NbC) Sheets for Flexoelectricity Generation

“…The XPS spectra of C 1s and Nb 3d (Figure a,b) confirm the formation of Nb–C. The deconvoluted peak at ∼283 eV confirms the Nb–C bond on the sample surface . Additionally, the deconvoluted XPS spectrum of Nb3d contains a strong peak centered around 206 eV (for 3d 3/2 ), suggesting the presence of NbC in the sample .…”

Temperature-dependent near-surface interstitial segregation in niobium

“…The oxygen concentration can be estimated from the natural oxide consumption retrieved by XRR and amounts for ≈14 at.% within the first 10 nm of the Nb specimen. Based on previous works, the presence of interstitial nitrogen atoms in Nb with a concentration above ≈1 at.% can be disregarded under the conditions explored here [40,55]. However, it is still possible that nitrogen has diffused into the near-surface region of Nb below this limit, contributing to the detected diffuse intensity.…”

Nitrogen doping and infusion in SRF cavities: A review

“…Even though diffusion of the nitrogen into the bulk of the SRF cavity is limited in depth at these low temperatures (120-200 °C), the introduction of nitrogen is sufficient to modify the cavity surface within the rf penetration depth as seen from cavities' test results, which are similar to those previously reported for high-temperature nitrogen doped cavities. The nitridation and diffusion of nitrogen in to the bulk is expected when the Nb surface is free of Nb2O5 which occurs >300 C [84,85], but the infusion was done when the cavity was fully annealed at 800 C/3hrs and cooldown to desired temperature under UHV conditions before injecting nitrogen in to the furnace. While post-doping electropolishing is required to remove coarse nitrides from the surfaces of high-temperature nitrogen doped cavities, no further processing is required for the low-temperature "infusion" recipe showing a clear benefit in reducing processing steps as well as keeping higher gradient with high Q0 values.…”