1. 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

2. 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

3. 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

4. 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?

5. 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

6. 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

7. US HEP R&D Program Targets
Jc (noncopper,12T,4.2 K): A/mm2
Effective filament size: microns or less
Piece length: > 10km at 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)

8. 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??

9. 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)

10. 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

11. Barrier breakthrough

12. 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

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

14. 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

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

16. 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

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

18. 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

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

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Naus and Fischer unpublished

21. 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 nm
Vortex spacing at 15T is 12 nm
Many fewer pins than vortices
quite unlike Nb-Ti!

22. 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

23. 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

24. 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

25. Intergranular Fracture of Nb3Sn Yields Grain Boundary Contrast
Dark Contrast

26. Microstructural Features of Interest

27. Large Magnification Range
Nb3Sn Layer
Wire fractured by bending to reveal grain size.
This sample heat treated for very short time
Use stereo glasses if possible!
PIT-Nb3Sn Superconducting Strand

28. 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

29. 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

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

31. 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

32. Jc of A15 layer AND Pinning Force of GBs

33. 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

34. 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 = A/mm2
Deff = um (MJR,IT); 50 um (PIT) (SMI ~15 um)
m (MJR); m (high Jc IT)
150 hr
$ 6/kA-m (MJR)
Projections from cost study: $ /kA-m

35. 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

36. 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

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

38. 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).