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Groove-rolling as an alternative process to fabricate Bi-2212 wires
Andrea MalagoliCNR-SPIN
Genova, Italy
WAMHTS-1, DESY, 21-23 May 2014
Coauthors and acknowledgements
V. Braccini, C. Bernini and C. Ferdeghini CNR-SPIN, Genova, Italy
M. PuttiPhysics Department, University of Genova, Italy
X. Chaud and F. DebrayCNRS LNCMI UJF-UPS-INSA, Grenoble Cedex 9, France
High field measurements at LNCMI-Grenoble have been supported by the EU contract number 228043
WAMHTS-1, DESY, 21-23 May 2014
• it has been shown that the main obstacle to current flow through the 2212 filament is the presence of bubbles
• the pressure caused by them provokes a Bi-2212 movement and an expansion of the wire during the melt stage that makes it very inhomogeneous over length longer than 10-15 cm.
Issues
F. Kametani, … A Malagoli et al. Supercond. Sci. Technol. 2011, 24, 075009
WAMHTS-1, DESY, 21-23 May 2014
0 10 20 30 40 50 60 70 80 90 100140
150
160
170
180
190
200
210
220
230
Ic [A
]
sample position
Ic
4,42
4,44
4,46
4,48
4,50
4,52
4,54
4,56
tot
al m
ass/
unit
leng
th [m
g/m
m] m/L
Issues
Malagoli A. et al. Supercond. Sci. Technol. 2011, 24, 075016
Malagoli A. et al. Supercond. Sci. Technol. 2013, 25, 055018
WAMHTS-1, DESY, 21-23 May 2014
Motivations
• Recently the density of the filaments of a commercial OST wire after drawing was increased using an “over pressure” process at ASC-NHMFL (D. Larbalestier et al., 2014 Nat. Mat. 13, 375–381) leading to a strong reduction of the bubble density and size and to a strong enhancement of the critical current
• Our different approach lies in enhancing the 2212 density of the conductor and getting improved performances acting on the wire fabrication process – keeping in mind as a key point its straightforward scalability over industrial lengths.
WAMHTS-1, DESY, 21-23 May 2014
Drawing:The strain is shared in a compressive transversal component and in a remarkable stretching longitudinal component
The Idea
Rolling:The strain is almost completely in the compressive transversal direction
WAMHTS-1, DESY, 21-23 May 2014
Columbus Superconductors has developped groove rolling mills for continuous MgB2 wire production from 35 mm to 3 mm in only three steps. A very nice filament homogeneity can be achieved.
Experiment I
Powder In Tube methodAtmosphere: pure O2
Powder: Nexans GranulateSheaths: pure Silver
Architecture: 55 filamentsAny optimization yet in terms of tubes
thicknessWAMHTS-1, DESY, 21-23 May 2014
250 mm 500 mm
Samples
Drawn sampleSPIN_D
Rolled sampleSPIN_G
Architecture: 55 x 7 filamentsDiameter: 0.8 mm
Architecture: 55 x 7 filamentsSize: 1.1 x 1.1 mm
WAMHTS-1, DESY, 21-23 May 2014
Heat Treatment
The heat treatment schedule was the one optimized by OST
WAMHTS-1, DESY, 21-23 May 2014
Transport Critical Current Density
0 1 2 3 4 5 6 7500
1000
1500
2000
J C [A
/mm
2]
Applied Field [T]
Drawn
SPIN_D
Ha D W et al, 2007 IEEE Trans. Appl. Supercond. 17 3099
0 1 2 3 4 5 6 7500
1000
1500
2000
J C [A
/mm
2]
Applied Field [T]
Drawn
Nexans Korea
SPIN_D
0 50 100 150 200 250 300 350
0
1
2
3
AMBI005
E [V
/cm
]
I [A]
AMBI004
7T, 4.2K
Rolled
Drawn
0 1 2 3 4 5 6 7500
1000
1500
2000
Drawn
Rolled
Nexans Korea
J C [A
/mm
2]
Applied Field [T]
SPIN_GSPIN_D
WAMHTS-1, DESY, 21-23 May 2014
Experiment II37 filaments groove-rolled wire
37 x 18 filaments restacked in a Silver tube
SPIN_1
500 mm
SPIN_2
500 mm
SPIN_3
500 mm
SPIN_1 drawn down to = 1.1 mm
SPIN_2 drawn down to = 2.2 mm and then groove rolled to 1.1 x 1.1 mm
SPIN_3 drawn down to = 4.2 mm and then groove rolled to 1.1 x 1.1 mm
WAMHTS-1, DESY, 21-23 May 2014
Transport Critical Current Density
0 1 2 3 4 5 6 7 8 9 10 11 12 130
500
1000
1500
2000
2500
3000
3500
J c
[A
/mm
2 ]
B [T]
SPIN_1
0 1 2 3 4 5 6 7 8 9 10 11 12 130
500
1000
1500
2000
2500
3000
3500
J c
[A
/mm
2 ]
B [T]
SPIN_1 SPIN_D
0 1 2 3 4 5 6 7 8 9 10 11 12 130
500
1000
1500
2000
2500
3000
3500
J c
[A
/mm
2 ]
B [T]
SPIN_1 SPIN_D SPIN_2
0 1 2 3 4 5 6 7 8 9 10 11 12 130
500
1000
1500
2000
2500
3000
3500
J c
[A
/mm
2 ]
B [T]
SPIN_1 SPIN_D SPIN_2 SPIN_3
0 1 2 3 4 5 6 7 8 9 10 11 12 130
500
1000
1500
2000
2500
3000
3500
J c
[A
/mm
2 ]
B [T]
SPIN_1 SPIN_D SPIN_2 SPIN_3 SPIN_G
0 1 2 3 4 5 6 7 8 9 10 11 12 130
500
1000
1500
2000
2500
3000
3500
J c
[A
/mm
2 ]
B [T]
SPIN_3 OST
SPIN_3
OST
WAMHTS-1, DESY, 21-23 May 2014
Open & Closed samples
0 1 2 3 4 5 6 7 80
500
1000
1500
2000
2500
3000
J c [A
/mm
2 ]
B [T]
SPIN_R-Closed SPIN_R-Open• for solenoid type winding, as NMR
magnets, having a rectangular shaped conductor allows a better compaction of the turns drastically reducing the voids space between them and avoids unwanted conductor twisting during the winding process.
• groove-rolling makes the corners smooth and this is a very desirable condition to avoid damages in the insulation.
• the geometric ratio 1.5/1 makes this wire very far from being a tape, maintaining the peculiar Bi-2212 isotropy
Rectangular wire
1.5 x 1 mm2
JC reduction of 28%
For as drawn OST the reduction was 70%D. Larbalestier et al., Nat. Mat. 13, 375–381 (2014)
WAMHTS-1, DESY, 21-23 May 2014
Rectangular wire
f.f. only ≈ 16 % !
1.2 x 0.8 mm2
0 1 2 3 4 5 6 7 8
1400
1600
1800
2000
2200
2400
2600
2800
3000
3200
B [T]
J C [A
/mm
2 ]
250
300
350
400
450
500
Je [A/m
m2]
WAMHTS-1, DESY, 21-23 May 2014
Bending strain & radius
0.0 0.1 0.2 0.3 0.4 0.5 0.60.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
I C/IC
0
bending strain %
1.5 x 1 mm2
Critical bending radius~ 9 cm
WAMHTS-1, DESY, 21-23 May 2014
Summary
• We found that it is possible to obtain a three-fold increase in JC with respect to the drawing through this alternative deformation process.
the increase in the critical current is entirely due to the deformation process which is able to densify the superconducting powders and therefore diminish the bubble density
- the clear effect in terms of powder densification has been shown through the comparison between the critical current densities measured in the open- and closed-ends rectangular wire: the fact that the reduction we register is remarkably less than what observed in drawn commercial wires is a strong indication of a higher density.
• This result is remarkable because we still have a strong margin for improvement, given that the process is not optimized yet in terms of geometry of the restacks, thickness of the external and internal Ag tubes, filament average dimensions, and in general filling factor.
• We believe that the use of groove-rolling instead of drawing to fabricate Bi-2212 wires is an appealing way to obtain denser and long-length wires with JC values suitable for high field applications.
- the application to long length wires and the eventual industrialization of the process is quite immediate and particularly appealing being even faster and cheaper.
WAMHTS-1, DESY, 21-23 May 2014
Perspectives
• Optimization of- density of the starting monofilament wire (use of rolling)- geometry of the restacks, taking into account the “square” shape of the
grooves- thickness of the external and internal tubes- filament average size
• Use of harder Ag-alloy as external sheath to enhance the densification power• Round wire….
• To get funding
WAMHTS-1, DESY, 21-23 May 2014
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00
50
100
150
200
250
pae
rtic
les
dis
trib
ution [a.
u.]
grain size [m]
Nexans Granulate
5 mm
Starting Powder Granulometry
By SEM
presence of large grains or agglomerates of them which are of
2-5 mm in size