Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy

Abstract

Raman microscopy/spectroscopy measurements are presented on high purity niobium (Nb) samples, including pieces from hot spot regions of a tested superconducting rf cavity that exhibit a high density of etch pits. Measured spectra are compared with density functional theory calculations of Raman-active, vibrational modes of possible surface Nb-O and Nb-H complexes. The Raman spectra inside particularly rough pits in all Nb samples show clear differences from surrounding areas, exhibiting enhanced intensity and sharp peaks. While some of the sharp peaks are consistent with calculated NbH and NbH 2 modes, there is better overall agreement with C-H modes in chain-type hydrocarbons. Other spectra reveal two broader peaks attributed to amorphous carbon. Niobium foils annealed to >2000 C in high vacuum develop identical Raman peaks when subjected to cold working. Regions with enhanced C and O have also been found by SEM/EDX spectroscopy in the hot spot samples and cold-worked foils, corroborating the Raman results. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and this may explain the correlation between hot spots in superconducting rf (SRF) cavities and the observation of a high density of surface pits. The origin of localized high carbon and hydrocarbon regions is unclear at present but it is suggested that particular processing steps in SRF cavity fabrication may be responsible.

First-Principles Study of Carbon and Vacancy Structures in Niobium

“…Recent secondary ion mass spectroscopy measurements of SRF niobium samples indicate local carbon concentrations of 10 −1 to 10 −2 at % depending on the specimen history and heat treatment, 17 and recent Raman spectroscopy studies indicate that large amounts of carbon are present in various forms as local clusters on niobium cavity and sample surfaces. 20 How the carbon collects is not known; the previously described reports emphasize clean handling and rule out casual environmental contributions. Dislocations seem to play a role because material working coupled to chemical polishing and annealing is a reproducible pathway toward the previously described results.…”

In-situ plasma processing to increase the accelerating gradients of superconducting radio-frequency cavities

“…Measurements on mechanically polished and buffered chemical polished niobium samples using a 2 mm diameter probe revealed that the work function is typically 4.5-4.8 eV, lower than the expected work function of the niobium pentoxide. The presence of carbon and hydrocarbon contaminants on top of niobium samples and their possible adverse effect on the work function and cavity performance have also been reported in other studies [11,20]. For example, the work function of a top carbonaceous layer estimated through XPS studies was reported at 3.6 eV [11].…”

“…All RGA signals measured during thermal cycling of cavities must come either from released gases that were condensed on the surface at cryogenic temperature or from volatile species produced on the surface during cavity operation for example by the interaction of electrons with surface contaminants [19]. The exact initial chemical composition and bonding strength of hydrocarbons on the SNS cavity surfaces are not known so far, but evidence of hydrocarbons on other cavities have been reported in the literature [11,19,20]. SIMS (secondary ion mass spectrometry) measurements of niobium samples fabricated for plasmasurface studies at the SNS also showed evidence of hydrocarbons at and near the metal surface.…”

Niobium-based semiconductor electrodes for hydrogen evolution reaction

“…Briefly, Tauc plots are generated from Figure 2 and the OBG is obtained by linear extrapolations as shown in Figure S1 [46]. Raman spectra (Figure S2) show the characteristic peaks for Nb2O5 [45] and TiO2 anatase form [46], whilst no signals are detected for Nb3(PO4)5. Specifically, for Nb2O5 the Raman spectrum (Figure S2A) has peaks between 80 -300 cm -1 attributed to bending modes of Nb-O-Nb linkages as well as to the vibrations of the NbO6 octahedra in Nb2O5.…”

“…Specifically, for Nb2O5 the Raman spectrum (Figure S2A) has peaks between 80 -300 cm -1 attributed to bending modes of Nb-O-Nb linkages as well as to the vibrations of the NbO6 octahedra in Nb2O5. [45]. Additionally, Raman signals located in the region between 400 -800 cm -1 are usually assigned to the symmetric and antisymmetric stretching mode of the Nb-O-Nb group.…”