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Title: Evidence of incomplete annealing at 800 °C and the effects of 120 °C baking on the crystal orientation and the surface superconducting properties of cold-worked and chemically polished Nb

High-purity niobium rods were cold-worked by wire-drawing, followed by various combinations of chemical polishing and high-vacuum baking at 120 °C or annealing at 800 °C in order to better understand changes to the surface superconducting properties resulting from typical superconducting radio-frequency cavity processing. AC susceptibility measurements revealed an enhanced upper transition Tc at ~ 9.3–9.4 K in all samples that was stable through all annealing steps, a value significantly above the accepted Tc of 9.23 K for pure annealed niobium. Corresponding elevations were seen in the critical fields, the ratio of the surface critical field Hc3 to the bulk upper critical field Hc2 rising to 2.3, well above the Ginzburg–Landau value of 1.695. Orientation imaging revealed an extensive dislocation rich sub-grain structure in the as-drawn rods, a small reduction of the surface strain after baking at 120 °C, and a substantial but incomplete recrystallization near the surface after annealing at 800 °C. We interpret these changes in surface superconducting and structural properties to extensive changes in the near-surface interstitial contamination produced by baking and annealing and to synergistic interactions between H and surface O introduced during electropolishing and buffered chemical polishing.
 [1] ;  [2] ; ORCiD logo [1] ; ORCiD logo [1] ;  [3]
  1. Florida State Univ., Tallahassee, FL (United States). National High Magnetic Field Lab. (MagLab)
  2. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Novosibirsk State Univ., Novosibirsk (Russian Federation)
  3. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
Superconductor Science and Technology
Additional Journal Information:
Journal Volume: 28; Journal Issue: 7; Journal ID: ISSN 0953-2048
IOP Publishing
Research Org:
Florida State Univ., Tallahassee, FL (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Country of Publication:
United States
36 MATERIALS SCIENCE; 43 PARTICLE ACCELERATORS; Superconductivity; SRF; Heat Treatment; Niobium; Surfaces; interfaces and thin films; Condensed matter: structural, mechanical & thermal
OSTI Identifier: