Controlling Cu–Sn mixing so as to enable higher critical current densities in RRP^® Nb₃Sn wires

Journal Article · 21 March 2018 · Superconductor Science and Technology

DOI: https://doi.org/10.1088/1361-6668/aab8dd · OSTI ID:1436690

Sanabria, Charlie  ^[1]; Field, Michael ^[2]; Lee, Peter J.  ^[3]; Miao, Hanping ^[2]; Parrell, Jeff ^[2]; Larbalestier, David C. ^[3]

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1. Florida State Univ., Tallahassee, FL (United States). Applied Superconductivity Center. National High Magnetic Field Lab. (MagLab); Lawrence Berkeley National Laboratory
2. Bruker OST, Carteret, NJ (United States)
3. Florida State Univ., Tallahassee, FL (United States). Applied Superconductivity Center. National High Magnetic Field Lab. (MagLab)

Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb₃Sn 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 Nb₃Sn 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 Nausite 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 (Cu₆Sn₅) and the more desirable ε phase (Cu₃Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb₃Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn₂ on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb₃Sn 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.

• Research Organization: Florida State Univ., Tallahassee, FL (United States); Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: National Science Foundation (NSF) (United States); USDOE Office of Science (SC); USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
• Grant/Contract Number: AC02-05CH11231; SC0012083
• OSTI ID: 1436690
• Journal Information: Superconductor Science and Technology, Journal Name: Superconductor Science and Technology Journal Issue: 6 Vol. 31; ISSN 0953-2048
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: United States
• Language: English