skip to main content


This content will become publicly available on March 22, 2019

Title: Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP ® Nb 3Sn wires

Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb 3Sn wires, in particular a critical current density (J c) of more than 1500 A mm -2 at 16 T and 4.2 K with an effective filament diameter (D eff) of less than 20 μm. The restacked-rod-process (RRP ®) is the technology closest to meeting these demands, with a J c (16 T) of up to 1400 A mm -2, residual resistivity ratio > 100, for a sub-element size D s of 58 μm (which in RRP ® wires is essentially the same as D eff). An important present limitation of RRP ® is that reducing the sub-element size degrades J c to as low as 900 A mm -2 at 16 T for D s = 35 μm. To gain an understanding of the sources of this J c degradation, we have made a detailed study of the phase evolution during the Cu–Sn 'mixing' stages of the wire heat treatment that occur prior to Nb 3Sn formation. Using extensive microstructural quantification, we have identified in this paper the critical role that the Sn–Nb–Cu ternary phase (Nausite) can play. The Nausitemore » forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300 °C–400 °C range—greatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408 °C) η phase (Cu 6Sn 5) and the more desirable ε phase (Cu 3Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb 3Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn 2 on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb 3Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350 °C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ε phase through better Cu–Sn mixing. Finally, at a D s of 41 μm, the Nausite control heat treatment increases the J c at 16 T by 36%, reaching 1300 A mm -2 (i.e. 2980 A mm -2 at 12 T), and moving RRP ® closer to the FCC targets.« less
ORCiD logo [1] ;  [2] ; ORCiD logo [1] ;  [2] ;  [2] ;  [1]
  1. Florida State Univ., Tallahassee, FL (United States). Applied Superconductivity Center. National High Magnetic Field Lab. (MagLab)
  2. Bruker OST, Carteret, NJ (United States)
Publication Date:
Grant/Contract Number:
SC0012083; AC02-05CH11231; DMR-1157490
Accepted Manuscript
Journal Name:
Superconductor Science and Technology
Additional Journal Information:
Journal Volume: 31; Journal Issue: 6; 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); National Science Foundation (NSF)
Country of Publication:
United States
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; RRP wire heat treatment; Nb3Sn wire heat treatment; critical current improvement; Nausite; accelerator magnets; Future Circular Collider; Hi-lumi upgrade
OSTI Identifier: