1. What are the Limits of Jc in Nb3Sn Strands?
The Ohio State University
M. D. Sumption
X. Xu
E. W. Collings
Low Temperature Superconductor Workshop
Feb 16-18, 2015
Napa Valley, 2015
Hyper Tech Research, Inc.
X. Peng
This work was supported by the U.S. Department of Energy, High Energy Physics university Grant No. DE-FG02-95ER40900 (OSU), DE-SC & DE-SC (OSU) (University Program)

2. The development of Jc of Nb3Sn strands:
Before 2003, the non-Cu Jc of Nb3Sn strands had been improved continuously, mostly driven by optimizing the Nb/Cu/Sn ratio, using the right sort and amount of additions, and proper heat treatments.
The improvement in Jc of Nb3Sn strands has been flat last 10 years. The state-of-the-art Nb3Sn strands achieve a 4.2 K, 12 T non-Cu Jc of ~ A/mm2, and a 15 T value of ~1600 A/mm2.
What’s the ideal grain size for Jc:
So, has the Jc of Nb3Sn reached its limit?
15-30 nm nm
The non-Cu Jc of Nb3Sn strands depends on three parameters:
Nb3Sn fraction in subelements
Birr, T (4.2 K)
Grain size, nm
State-of-the-art RRP
60%
23-26
Limits/Optimal level
62-65%
27-28
15-30
Potentials for improving Jc
Little
Modest
Huge!
(Dietderich’s thin film experiment)

3. Several attempts to improve pinning capacity
Methods
Effect
Problems
Lower reaction temperature (<625°C)
From 650 to 615 °C, d is refined from 110 to 90 nm
reaction time↑, Sn content↓
Introducing Y or Gd particles by using Nb-Y or Nb-Gd alloy a
At 750 C, 0.75 % Y refines grain size from nm to nm
Y or Gd particles harden Nb alloy, and Nb-Y or Nb-Gd alloy is not commercially available
Introducing nanometric Cu-Sn APCs b
It is claimed that Fp,max was shifted to 0.5Birr.
During heat treatment the Cu-Sn APCs agglomerate, reducing effectiveness
12 T layer Jc was around 1000 A/mm2.
A Motowidlo et al., IEEE Trans. Appl. Supercon. 19, 2568 (2009)
b R. Durval

4. The internal oxidation method
First, “Internal oxidation”: oxygen diffuses in an A-B alloy, and selectively oxidizes the solute B.
Requirement: B is less noble than A.
As Nb-Zr is oxidized, fine ZrO2 particles are formed. Used in Nb3Sn films to refine grain size in 1960s.
Key point of this method:
Oxygen must already dissolve in Nb-Zr before Nb-Zr reacts with Sn.
How to realize this in Nb3Sn strands?
B. Zeitlin,
SnO2 powder as oxygen source?
Cu matrix
Barrier
Nb1Zr fila Cu
Sn+SnO2
powders
Pre-dissolve O in Nb-Zr alloy?
How to realize this in Nb3Sn films:
Nb-1at.%Zr foil
Anodization to form Nb2O5
Annealing at 1000 C to decompose Nb2O5
Sn coating
Reaction at 1050 °C
Oxygen hardens metals, making process hard.
L. E. Rumaner et al., Metall. Mater. Trans. A 25, 213 (1994)
B. A. Zeitlin et al., IEEE Trans. Appl. Supercon. 15, 3393 (2005)

5. Why no grain refinement in Zeitlin’s wire?
Zeitlin’s MEIT wire:
1. The “inert” Cu layer blocks the path of oxygen transfer.
Cu matrix
Barrier
Nb-Zr fila Cu
Sn+SnO2
powders
2. The diffusion of oxygen in Cu layer:
Assuming CO(O)=Cs, CO(Nb1Zr)=0:
T, 600 °C
O in Cu
O in Nb
D, μm2/s
100
0.6
Cs, at.%
10-5 3
The O amount supplied is far from sufficient.

6. Can the (external) internal oxidation method work for Nb3Sn wires?
During heat treatment:
Pure Ar atmosphere: no oxygen supply
Ar-O mixture: sufficient O supply
Cu matrix
Nb1 at.%Zr Cu
Sn core
Tc of Nb drops by 0.93 K for 1 at.% O.
Reacted in pure Ar
Reacted in Ar-O
Inter-granular ZrO2 particles
Intra-granular ZrO2 particles
X. Xu et al., Appl. Phys. lett. 104, (2014)

7. An experimental wire for (internal) internal oxidation
A tube type mono-filament:
For control, we also made an analog with NbO2 powder, which cannot supply oxygen.
Both: 650 °C/150 h
SnO2
200 nm
NbO2
SnO2-625 °C/800 h
SnO2-625 °C/800 h
Ave: 36 nm
What if we tweak HT?

8. Layer Jc of Internal oxidation strands
Transport obtained 10.5 kA/mm2 at 10 T for SnO2-650x400h, (quench )
Magnetization vs. field (M-B) loops were measured using a VSM at 4.2 K.
A15 layer outer and inner diameters determination.
1, use the BSE image to calculate do of FG A15 layer.
2, use the fracture SEM to calculate the FG layer thickness, l.
3, Calculate di=do-2l.
images from different places, 6 lines on each, then average: #
l, um 1
10.312
8.68
8.132
6.356
7.72 2
9.453
9.126
8.077
5.44
7.952 3
11.401
9.15
9.38
7.436
7.605 4
9.322
8.479
9.321
8.571
8.233 5
9.077
8.798
8.963
9.501
8.619 6
7.173
9.262
9.315
9.712
9.056

9. What’s the functional Form of Fpmax vs inverse grain size??
80 nm ?
15-30 nm nm

10. Fp-B curves of Internal oxidation strands
For SnO2-625x800h, two causes for increase in 12 T Jc:
Increase in Fp,max.
Some shift of Fp-B peaks.
Birr was obtained by fitting the Fp-B curve using Fp=Kbp(1-b)q, where b=B/Birr.
Birr, T
Grain size, nm
Fp-B peak
NbO °C
20.9
~90
0.22Birr
SnO °C 23 45
0.26Birr
SnO °C
~20 36
0.34Birr
Further improvements:
1. Ti additions => Birr↑
2. Higher Zr contents => smaller grain sizes?

11. What can we do to further improve the Jc:
Increase in Birr:
The Nb-1Zr & SnO2 wire is based on binary Nb3Sn phase: Birr can be improved by Ta or Ti addition.
Birr, T
Grain size, nm
Fp-B peak
SnO °C
~20 36
0.34Birr
Goal 25
0.5Birr
The Nb-1Zr & SnO2 wire has a large diameter (0.22 mm) and is under-reacted after 800 h at 625 °C: if the subelement is processed down and fully reacted, the Birr should be improved by several tesla.
Birr increases with reaction time, and plateaus when fully reacted.
2. Further refinement of grain size:
If we refine the grain size to 25 nm, the Fp,max will be: ~250 GN/m3.
So, how can we refine the grain size down to 25 nm?
Use Nb-Zr alloy with higher Zr content (e.g., 1.5%) to to produce a greater density of ZrO2 particles.
Use a lower reaction temperature (e.g., 605 °C).
Use other solid solution alloys
If we refine the grain size to 25 nm, the Fp-B curve will peak at: 0.5Birr.
15-30 nm nm

12. What if we can further optimize the internal oxidation strand, by both improving its Birr and further reducing its grain size?
Fp-B curves for five different cases:

13. What are the limits of Jc in Nb3Sn? Maybe these ……
Engineering Jc and Ic for the five different cases:
I. Present state-of-the-art RRP strands
II. The wire with SnO C / 800h
III. Only improve Birr to 25 T by Ti doping, etc.
IV. Only refine the grain size to 25 nm
V. Both improve the Birr to 25 T and refine the grain size down to 25 nm
Grain size, nm 36 25
Fp-B peak
0.2Birr
0.34Birr
0.5Birr
Fp,max, GN/m3
~90
180
~250
Birr, T 20
12 T
Layer Jc, A/mm2
5,000
9,600
16,400
20,000
20,800
Non-Cu Jc, A/mm2
3,000
5,760
9,840
12,000
12,480
Engineering Jc, A/mm2
1,600
3,050
5,200
6,360
6,600
Ic, A
800
1,530
2,620
3,200
3,320
15 T
2,700
3,800
7,800
12,500
16,000
2,280
4,680
7,500
850
1,210
2,480
4,000
5,100
430
610
1,250
2,000
2,560
Note: Assuming all the five cases have the same Nb3Sn area fraction with the state-of-the-art RRP strands:
the Nb3Sn area fraction in a subelement is 60%,
the non-Cu area fraction in a strand is 0.53,
the wire diameter is 0.8 mm.

14. Lets put this on Peter Lee’s plot, and compare to YBCO and over-pressure processed Bi:2212 wires

15. Conclusion
Grain size of 35 nm (as compared to nm) has been demonstrated
Layer Jcs of 9600 A/mm2 at 12 T, 4.2 K, and 3800 A/mm2 (extrapolated) at 15 T, 4.2 K have been seen
Fpmax of 180 GN/m3 has been shown (previous max was 90 GN/m3), with peak shift from 0.2Birr -> 0.34 Birr
Fill factors are low– must be demonstrated in conductor with good fill factor – some ideas in development
More important than this specific approach:
Fp,max vs grain sizes does not saturate below 90 nm!
It is possible to get fine grains in wires!

16. Assertions and Questions
Pushing grain sizes down to 25 nm might (should) push the peak of the Fp curve to 0.5 Birr, should give Fp,max = 250 GN/m3
Such grain sizes seem possible, but may require the use of non-commercial alloys
If a Je of 1000 A/mm2 is the criterion for use in a magnet, and conductors with A15 fractions the same as present day RRP could be made …
Could Nb3Sn could be used in magnets of 18 or even 23 T?
Can this be realized in a practical conductor?
If it can, it should be very important for HEP