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

Abstract

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 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 inmore » 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. 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

Authors:

Sanabria, Charlie  ^[1];

• Search DOE PAGES for author "Sanabria, Charlie"
• Search DOE PAGES for ORCID "0000-0001-5017-5309"

Field, Michael ^[2]; Lee, Peter J.  ^[1];

• Search DOE PAGES for author "Lee, Peter J."
• Search DOE PAGES for ORCID "0000-0002-8849-8995"

Miao, Hanping ^[2]; Parrell, Jeff ^[2]; Larbalestier, David C. ^[1]

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+ Show Author Affiliations

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:

Thu Mar 22 00:00:00 EDT 2018

Research Org.:

Florida State Univ., Tallahassee, FL (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), High Energy Physics (HEP); National Science Foundation (NSF)

OSTI Identifier:

1436690

Alternate Identifier(s):

OSTI ID: 1485092

Grant/Contract Number:  

SC0012083; AC02-05CH11231; DMR-1157490

Resource Type:

Accepted Manuscript

Journal Name:

Superconductor Science and Technology

Additional Journal Information:

Journal Volume: 31; Journal Issue: 6; Journal ID: ISSN 0953-2048

Publisher:

IOP Publishing

Country of Publication:

United States

Language:

English

Subject:

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