Detection of surface contamination on niobium materials used in superconducting radio frequency (SRF) applications is difficult due to quantitative sensitivity and near-atomic depth resolution needed. Inspection of samples known to have experienced surface contamination was found to have inconsistent nitride coverage after high-temperature nitrogen gas exposure (“doping”). We compare contaminating species found on samples treated in several different vacuum furnaces, both “exposed” directly in the chamber and “protected” by containment shielding from evaporative sources with “furnace caps.” Typically, furnace caps are used to impede contamination from reaching the interior surface of cavities during the high-temperature vacuum bake that immediately precedes exposure to nitrogen gas. Although, to date, little is known about the effectiveness of these caps, SIMS results showed that they were effective in limiting contamination arising from the furnace environment. Inspection of sample surfaces by SEM showed a lack of nitrides present on contaminated specimens. TEM with energy dispersive spectroscopy performed on these samples revealed that a carbon-rich layer now existed, indicating that a relatively high contaminant load prevents the nucleation and growth of surface nitrides, while thus inhibiting interstitial nitrogen uptake. Except in extreme cases, subsequent removal of the top several micrometers of the surface via electropolishing appears to effectively eliminate any strong influence on the subsequent SRF cavity performance. With the absence of furnace cleaning, carbon contamination was found to be nearly 10× higher for protected nitrogen-doped and electropolished samples, with minimal metallic contamination detected for both processes. SIMS analysis was also performed to compare the cleanliness of samples fully prepared by such nitrogen “doping” with those prepared by a related process, involving the dissolution of niobium surface oxide and diffusion of oxygen into the surface. This oxygen doping or alloying process offers attractive advantages.
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Research Article|
May 12 2023
Analysis of furnace contamination on superconducting radio frequency niobium using secondary-ion mass spectrometry
Jonathan W. Angle
;
Jonathan W. Angle
(Methodology, Writing – original draft)
1
Nanoscale Characterization and Fabrication Laboratory, Virginia Polytechnic Institute and State University
, 1991 Kraft Dr., Blacksburg, Virginia 24061
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Eric M. Lechner
;
Eric M. Lechner
(Methodology, Software, Writing – review & editing)
2
Thomas Jefferson National Accelerator Facility
, Newport News, Virginia 23606
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Charles E. Reece
;
Charles E. Reece
(Formal analysis, Investigation, Resources)
2
Thomas Jefferson National Accelerator Facility
, Newport News, Virginia 23606
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Fred A. Stevie;
Fred A. Stevie
(Conceptualization, Methodology, Resources, Validation)
3
Analytical Instrumentation Facility, North Carolina State University
, Raleigh, North Carolina 27695
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Michael J. Kelley
Michael J. Kelley
a)
(Funding acquisition, Methodology, Resources, Writing – original draft, Writing – review & editing)
1
Nanoscale Characterization and Fabrication Laboratory, Virginia Polytechnic Institute and State University
, 1991 Kraft Dr., Blacksburg, Virginia 240612
Thomas Jefferson National Accelerator Facility
, Newport News, Virginia 23606a)Author to whom correspondence should be addressed: mkelley@jlab.org
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a)Author to whom correspondence should be addressed: mkelley@jlab.org
J. Vac. Sci. Technol. B 41, 034005 (2023)
Article history
Received:
February 28 2023
Accepted:
April 18 2023
Citation
Jonathan W. Angle, Eric M. Lechner, Charles E. Reece, Fred A. Stevie, Michael J. Kelley; Analysis of furnace contamination on superconducting radio frequency niobium using secondary-ion mass spectrometry. J. Vac. Sci. Technol. B 1 May 2023; 41 (3): 034005. https://doi.org/10.1116/6.0002624
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