Conductors of Nb3Sn: What are the Key Issues? David Larbalestier Applied Superconductivity Center Department of Materials Science and Engineering Department of Physics University of Wisconsin Recent references: M. Takayasu, R. G. Ballinger, R. B. Goldfarb, A. A. Squitieri, P. J. Lee, and D. C. Larbalestier, “Multifilamentary Nb3Sn wires reacted in hydrogen gas environment”, to appear in Adv. In Cryo. Eng. 2002 L. D. Cooley, P.J. Lee and D.C. Larbalestier “Changes in investigation of flux pinning curve shape for flux-line separations comparable to grain size in binary powder-in-tube Nb3Sn wires” to appear in Adv. In Cryo. Eng. 2002 C. M. Fischer, P. J. Lee and D. C. Larbalestier, “Irreversibility field and magnetization moment as a function of heat treatment time and temperature for a pure niobium powder-in-tube Nb3Sn conductor”, to appear in Adv. In Cryo. M. Naus, P.J. Lee, and D.C. Larbalestier, “Lack of influence of the Cu–Sn mixing heat treatments on the super-conducting properties of two high-Nb, internal-Sn Nb3Sn conductors”, to appear in Adv. In Cryo. Eng. 2002.

Thank you! To my CERN host, Giorgio Cavallari To CERN colleagues – Lucio Rossi, Sergio Calatroni, Luca Bottura, Amalia Ballarino, Philippe Lebrun, Daniel Leroy, Herman Ten Kate, Arnaud Devred –for very interesting discussions To my Madison colleagues, especially Peter Lee, Lance Cooley, Alex Gurevich, Anatoly Polyanskii, Cai Xueyu, Bill Starch, Alex Squitieri, Eric Hellstrom and Chang-Beom Eom To my present students and postdocs – Chad Fischer, Mike Naus, George Daniels, Jermal Chandler, Sandy Liao, Sang Il Kim, Olaf van der Meer (Twente), Matt Feldmann, Jianyi Jiang, Xueyan Song and Satyabrata Patnaik AFOSR, DOE (EE, Fusion and HEP), and NSF for support

Apologies - Topics not covered BSCCO-2212 Round wires possible, cables, R&W coils (Showa/LBNL/BNL collaboration) Nb3Al Complex to make, but some strain advantages over Nb3Sn YBCO-Coated Conductors Very interesting but conductors in prospect 2-3 years from now BSCCO-2223 current leads CERN home to most advanced work

Nb3Sn issues: The first high field Superconductor in 1961 Which way to make it? Bronze Internal Tin Powder-in-tube How high can Jc go? Jc in package or in A15 layer? What about Tc, H* and Hc2? What about flux pinning? What about cost?

Different Routes Sn activity is highest in Powder In Tube, lowest in bronze Reactions are slowest at low Sn A15 phase volume fraction is highest for PIT Bronze and Internal Tin are most produced All made by diffusion that does not equilibrate

Niobium Tin conductor routes “Classical” bronze Sn:Cu ~1:12 Internal tin : Sn:Cu: Nb ratio often not well controlled PIT Sn:Cu - very little Cu All are longitudinally homogeneous but radially inhomogeneous Bronze Route, Vacuumschmelze Internal Tin, Wah Chang/OI-ST NbSn2 Powder in Tube, SMI Holland

US HEP R&D Program Targets Jc (noncopper,12T,4.2 K): 3000 A/mm2 Effective filament size: 40 microns or less Piece length: > 10km at 0.3-1.0 mm dia Heat treatment times: >200 hr; target 50 hr for wind and react Wire cost: Less than $1.50/kA-m (12 T,4.2 K) Cost and Jc targets limit choice to high Sn routes – internal tin and powder-in-tube (PIT)

UW Goals Understand microstructure development Quantify composition gradients and associated Tc gradients – infer Jc and Hc2 gradients Understand flux pinning effects Role of grain boundaries as pinning centers – segregation of Cu Weakening of GBs at higher fields, in favor of Tc, composition and Hc2??

Composition, Tc and Hc2 effects in Nb3Sn Devantay et al. J. Mat. Sci., 16, 2145 (1981) Charlesworth et al. J. Mat. Sci., 5, 580 (1970) Non-equilibrium produces a composition gradient! Data compiled by Devred from original data of Flukiger, Adv. Cryo. Eng., 32, 925 (1985)

High Sn can still be Sn-deficient! I. Low Cu internal Tin Recent strategies: remove Cu from stack, to increase Nb and Sn and A15 fraction High Sn can still be Sn-deficient! Naus et al. Adv. Cryo Eng. 2002

Barrier breakthrough

A15 filaments are Sn-poor Tc is only 16 K in this 2200 A/mm2 at 12 T conductor H*(4.2K) = 24.3T (Nb1wt.%Ti) A15 composition 21-24at.%Sn Naus et al. Adv. Cryo Eng. 2002

H* defined by Kramer extrapolation is not Hc2 H*Kramer CRe1912, 180h/655°C 12 K Hc2 ORe137, 180h/675°C 15 K

Significant distinction between H* and Hc2 dHc2/dT  2.4 H*Kramer H*Kramer, CRe1912, 256h/750°C Hc2, CRe1912, 256h/750°C H*Kramer, ORe137, 180h/675°C Hc2, ORe137, 180h/675°C Naus et al. Adv. Cryo Eng. 2002

II. Reaction is “inside out” in PIT filaments Sn-rich regions are lighter Fischer et al. Adv. Cryo Eng. 2002

Tc Inhomogeneity in PIT A15 Nb3Sn Tc gradient flattens with increasing time, but does not disappear Highest Tc is on inside making filaments magnetically transparent SMI-PIT Nb tube conductor Fischer et al. Adv. Cryo Eng. 2002

H*, Hc2 trends for PIT Nb3Sn at 12K Fischer et al. Adv. Cryo Eng. 2002

H*, Hc2 trends for PIT Nb3Sn at 4.2 K H*(4.2K) achieves 23.5T at 675 C, 25.5T at 750 C in binary PIT and >27T in ternary (Nb,Ta)3Sn PIT

Significant difference between H* and Hc2 in PIT too Even after 50 hours H* ~0.75 Hc2

H* and Tc directly coupled Increasing HT Temp Internal Sn PIT Big benefit for Tc and H* in reacting longer! Naus and Fischer unpublished

III. Flux Pinning Issues Old literature Jc values correlate well with increasing grain boundary area at low fields Limiting grain size of most A15 is 100-150 nm Vortex spacing at 15T is 12 nm Many fewer pins than vortices quite unlike Nb-Ti!

Grain Size influence on Pinning ~1980 Fpmax (4.2 K) vs. inverse grain size for Nb3Sn and SnMo6S8. Values for SnMo6S8 fall on the same curve, albeit in the regime of larger grain size and smaller pinning force, suggesting that smaller SnMo6S8 grain size would result in greater pinning forces. Unfortunately there does not seem to be any feasible way to reduce the grain size of SnMo6S8 by a significant amount. SMS: Dependence of Jc(4.2 K) on grain boundary line length per unit area, Lgb. At low fields, Jc increases monotonically with increasing Lgb but, at high fields, is independent of grain size. Note that the 2 lowest Jc values at 10T result from samples with somewhat depressed Tc values

Limits to Flux Pinning by Grain Boundaries If there is 1 grain boundary for each flux line, does summation change and high-field pinning improve? What is the critical pin separation (relative to the flux-lattice constant a0) for a summation change? B Fp 50 20 15 nm 5 10 15 20 T 10 nm a0 » (f0/B)1/2 Cooley et al. Adv. Cryo Eng. 2002

TEM used to be our standard method for Nb3Sn microstructural images M 88 (170 hrs/700 °C) TWCA MJR Nb3Sn ASC ‘84 500 nm Peter Lee

Intergranular Fracture of Nb3Sn Yields Grain Boundary Contrast Dark Contrast

Microstructural Features of Interest

Large Magnification Range Nb3Sn Layer Wire fractured by bending to reveal grain size. This sample heat treated for very short time (1hr@675°C) Use stereo glasses if possible! PIT-Nb3Sn Superconducting Strand

Shape change of Fp(H) Few (or 1) flux line per g.b. at H* 57 nm grains! Few (or 1) flux line per g.b. at H* Cooley, Lee and DCL ICMC 2001

PIT Nb3Sn strand at early stages of HT a) 57 nm Nb tube ¬ ® Core Nb6Sn5 Nb3Sn Powder Core 1 µm b) 70 nm c) 77 nm 1 µm d) 89 nm e) 113 nm Rapid A15 grain growth

Many flux lines per grain at H* K 113 nm grains

Route(s) to higher Jc? More A15 in the cross-section? Better composition of A15? Better pinning strength of grain boundaries? Finer grains for now not the answer

Jc of A15 layer AND Pinning Force of GBs

Significant Differences in H-T plane PIT composites have highest Tc and H* Internal Tin often compromised by being Sn-deficient (wrt Nb) No uniform optimization yet! Big spread in ternary wires

US Conductor Program 11/01 - Scanlan Goals Jc = 3000 A/mm2 Deff= 40 microns or less Piece length > 10,000 m Heat treatment < 400 hr Cost: < $1.50/kA-m(12 T) Progress Jc = 2400-2900 A/mm2 Deff = 100-120 um (MJR,IT); 50 um (PIT) (SMI ~15 um) 250-1500m (MJR); 250-1000 m (high Jc IT) 150 hr $ 6/kA-m (MJR) Projections from cost study: $1.00--1.50/kA-m

V. Summary and Open Questions - 1 Strong progress in Jc 2900 A/mm2 12T, 4.2K achieved at OI-ST with MJR In spite of lowered Tc, Hc2 and barrier leakages Single barrier internal tin favored for scale up But, at such small Cu fractions inside the barrier, whole filament bundle is coupled Small deffective requires design such as PIT with very uniform filaments Cost and piece lengths have many drivers Regular production is one High Jc – and especially high field performance depend critically on maximizing Tc and Hc2/H* NMR and high field magnets require this so piggy-back strategy has many benefits

V. Summary and Open Questions - 2 Raising Jc is a complex issue Getting more A15 phase out of Cu-Sn-Nb-diffusion barrier package Constraints provided by raw materials producers Nb tube, alloyed Nb etc Reducing composition gradient in A15 layer Without causing too much grain growth and throwing away flux pinning Increasing flux pinning effect of grain boundaries First signs that this is possible

V. Summary and Open Questions - 3 Will the accelerator laboratories throw down the challenge again? I hope so!

Collaborators and Acknowledgements At UW: Peter Lee, Lance Cooley, Alex Squitieri, Bill Starch, Michael Naus, Chad Fischer Wider US Conductor Development Community led by Ron Scanlan, Bruce Strauss, Dave Sutter and Peter Limon Manufacturers, especially Jeff Parrell and Seung Hong (OI-ST), Jan Lindenhovius (SMI) and Andries den Ouden (Twente) and Eric Gregory (formerly IGC).