Atom-probe tomography analyses of niobium superconducting RF cavity materials

Sebastian, Jason; Seidman, David N.; Yoon, Kevin E.; Bauer, P.; Reid, Thomas; Boffo, C.; Norem, J.

doi:10.1016/j.physc.2006.03.118

Cited by 18 publications

(13 citation statements)

References 9 publications

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“…More recently, two studies performed on niobium tips by means of three-dimensional local-electrode atom-probe tomography came to contradictory results (maybe due to slightly different preparation procedures of the Nb tips). Sebastian et al (2006) observed a smooth transition from near-stoichiometric Nb 2 O to near-stoichiometric Nb 2 O 5 on the surface of the Nb tip, whereas Yoon et al (2007) observed no continuous transition but a surface layer with an almost constant composition that can be interpreted by a mixture of NbO and NbO 2 .…”

Section: Discussionmentioning

confidence: 99%

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

et al. 2009

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A comprehensive electron energy-loss spectroscopy study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO2, Nb2O5) was carried out. In this work (Part I), the plasmons and energy-loss near-edge structures (ELNES) of all relevant Nb edges (Nb-N2,3, Nb-M4,5, Nb-M2,3, Nb-M1, and Nb-L2,3) up to energy losses of about 2600 eV and the O-K edge are analyzed with respect to achieving characteristic fingerprints of Nb in different formal oxidation states (0 for metallic Nb, +2 for NbO, +4 for NbO2, and +5 for Nb2O5). Chemical shifts of the Nb-N2,3, Nb-M4,5, Nb-M2,3, and Nb-L2,3 edges are extracted from the spectra that amount to about 4 eV as the oxidation state increases from 0 for Nb to +5 for Nb2O5. Four different microscopes, including a 200 keV ZEISS Libra with monochromator, were used. The corresponding wide range of experimental parameters with respect to the primary electron energy, convergence, and collection semi-angles as well as energy resolution allows an assessment of the influence of the experimental setup on the ELNES of the different edges. Finally, the intensity of the Nb-L2,3 white-line edges is correlated with niobium 4d-state occupancy in the different reference materials.

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Section: Discussionmentioning

confidence: 99%

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

et al. 2009

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“…Oxygen and oxide formation on pure Nb surfaces were studied utilizing X-ray diffraction techniques (Ma & Rosenberg, 2003; Delheusy et al, 2008). Earlier, LEAP tomographic studies were successfully employed for studying the formation of niobium oxides on Nb surfaces (Sebastian et al, 2006; Yoon et al, 2007, 2008)…”

Section: Introductionmentioning

confidence: 99%

Atom-Probe Tomographic Analyses of Hydrogen Interstitial Atoms in Ultrahigh Purity Niobium

Kim¹,

Seidman

2015

Microsc Microanal

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Atomic-scale characterization of hydrogen and formation of niobium hydrides, using ultraviolet (wavelength=355 nm) picosecond laser-assisted local-electrode atom-probe tomography, was performed for ultrahigh purity niobium utilizing different laser pulse energies, 10 or 50 pJ/pulse or voltage pulsing. At 50 pJ/pulse, hydrogen atoms migrate onto the 110 and 111 poles as a result of stimulated surface diffusion, whereas they are immobile for <10 pJ/pulse or for voltage pulsing. Accordingly, the highest concentrations of H and NbH were obtained at 50 pJ/pulse. This is attributed to the thermal energy of the laser pulses being transferred to pure niobium specimens. Therefore, we examined the effects of the laser pulse energy being increased systematically from 1 to 20 pJ/pulse and then decreasing it from 20 to 1 pJ/pulse. The concentrations of H, H2, and NbH and the atomic concentration ratios H2/H, NbH/Nb, and Nb3+/Nb2+ were calculated with respect to the systematically changing laser pulse energies. The atomic concentration ratios H2/H and NbH/Nb are greater when decreasing the laser pulse energy than when increasing it, because the higher residual thermal energy after decreasing the laser pulse energy increases the mobility of H atoms by supplying sufficient thermal energy to form H2 or NbH.

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“…Indeed, the fact that thermodynamics permits interstitial atom concentrations a few percent suggests that exposure of a fresh Nb surface to air or water should result in substantial contamination. Surface studies [1,13,27,[30][31][32][33] support aspects of this "pollution" model. As discussed further below, a , a criterion where SRF cavity tests are often terminated.…”

Section: Which Is More Important Roughness or Contamination?mentioning

confidence: 97%

Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities

Dzyuba¹,

Romanenko²,

Cooley³

2010

Supercond. Sci. Technol.

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A model for the onset of the reduction in SRF cavity quality factor, the so-called Q-drop, at high accelerating electric fields is presented. Since magnetic fields at the cavity equator are tied to accelerating electric fields by a simple geometric factor, the onset of magnetic flux penetration determines the onset of Q-drop. We consider breakdown of the surface barrier at triangular grooves to predict the magnetic field of first flux penetration H pen . Such defects were argued to be the worst case by Buzdin and Daumens, [1998 Physica C 294 257], whose approach, moreover, incorporates both the geometry of the groove and local contamination via the Ginzburg-Landau parameter . Since previous Q-drop models focused on either topography or contamination alone, the proposed model allows new comparisons of one effect in relation to the other. The model predicts equivalent reduction of H pen when either roughness or contamination were varied alone, so smooth but dirty surfaces limit cavity performance about as much as rough but clean surfaces do. Still lower H pen was predicted when both effects were combined, i.e. contamination should exacerbate the negative effects of roughness and vice-versa. To test the model with actual data, coupons were prepared by buffered chemical polishing and electropolishing, and stylus profilometry was used to obtain distributions of angles. From these data, curves for surface resistance generated by simple flux flow as a function of magnetic field were generated by integrating over the distribution of angles for reasonable values of . This showed that combined effects of roughness and contamination indeed reduce the Q-drop onset field by ~20%, and that that contamination contributes to Q-drop as much as roughness. The latter point may be overlooked by SRF cavity research, since access to the cavity interior by spectroscopy tools is very difficult, whereas optical images have become commonplace. The model was extended to fit cavity test data, which indicated that reduction of the superconducting gap by contaminants may also play a role in Q-drop.

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Atom-probe tomography analyses of niobium superconducting RF cavity materials

Cited by 18 publications

References 9 publications

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

Atom-Probe Tomographic Analyses of Hydrogen Interstitial Atoms in Ultrahigh Purity Niobium

Model for initiation of quality factor degradation at high accelerating fields in superconducting radio-frequency cavities

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