groove-rolling as an alternative process to fabricate bi-2212 wires

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

SEM longitudinal cross sectionsof as-deformed wires

WAMHTS-1, DESY, 21-23 May 2014

SEM longitudinal cross sectionsof fully-reacted wires

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

OST-wire

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

CIP and Over Pressure

J. Jiang et al. Supercond. Sci. Technol. 2011, 24 082001

As-received CIPped

Ic (4.2K, 5T) = 155 A Ic (4.2K, 5T) = 385 A

D C Larbalestier et al.arXiv:1305.1269