Groove-rolling as an alternative process to fabricate Bi-2212 wires Andrea Malagoli CNR-SPIN Genova, Italy WAMHTS-1, DESY, May 2014

Coauthors and acknowledgements V. Braccini, C. Bernini and C. Ferdeghini CNR-SPIN, Genova, Italy M. Putti Physics Department, University of Genova, Italy X. Chaud and F. Debray CNRS LNCMI UJF-UPS-INSA, Grenoble Cedex 9, France High field measurements at LNCMI-Grenoble have been supported by the EU contract number WAMHTS-1, DESY, 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 cm. Issues F. Kametani, … A Malagoli et al. Supercond. Sci. Technol. 2011, 24, WAMHTS-1, DESY, May 2014

Issues Malagoli A. et al. Supercond. Sci. Technol. 2011, 24, Malagoli A. et al. Supercond. Sci. Technol. 2013, 25, WAMHTS-1, DESY, 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, 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, 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 method Atmosphere: pure O 2 Powder: Nexans Granulate Sheaths: pure Silver Architecture: 55 filaments Any optimization yet in terms of tubes thickness WAMHTS-1, DESY, May 2014

250  m 500  m Samples Drawn sample SPIN_D Rolled sample SPIN_G Architecture: 55 x 7 filaments Diameter: 0.8 mm Architecture: 55 x 7 filaments Size: 1.1 x 1.1 mm WAMHTS-1, DESY, May 2014

Heat Treatment The heat treatment schedule was the one optimized by OST WAMHTS-1, DESY, May 2014

Transport Critical Current Density SPIN_D Ha D W et al, 2007 IEEE Trans. Appl. Supercond SPIN_D Rolled Drawn SPIN_G SPIN_D WAMHTS-1, DESY, May 2014

SEM longitudinal cross sections of as-deformed wires WAMHTS-1, DESY, May 2014

SEM longitudinal cross sections of fully-reacted wires WAMHTS-1, DESY, May 2014

Experiment II 37 filaments groove-rolled wire 37 x 18 filaments restacked in a Silver tube SPIN_1 500  m SPIN_2 500  m SPIN_3 500  m SPIN_1 drawn down to  = 1.1 mmSPIN_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, May 2014

Transport Critical Current Density SPIN_3 OST WAMHTS-1, DESY, May 2014

Open & Closed samples 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 mm 2 J C reduction of 28% For as drawn OST the reduction was 70% D. Larbalestier et al., Nat. Mat. 13, 375–381 (2014) WAMHTS-1, DESY, May 2014

Rectangular wire f.f. only ≈ 16 % ! 1.2 x 0.8 mm 2 WAMHTS-1, DESY, May 2014

Bending strain & radius 1.5 x 1 mm 2 Critical bending radius ~ 9 cm WAMHTS-1, DESY, May 2014

Summary We found that it is possible to obtain a three-fold increase in J C 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 J C 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, 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, May 2014

OST-wire

5  m Starting Powder Granulometry By SEM presence of large grains or agglomerates of them which are of 2-5  m in size

CIP and Over Pressure J. Jiang et al. Supercond. Sci. Technol. 2011, As-received CIPped Ic (4.2K, 5T) = 155 AIc (4.2K, 5T) = 385 A D C Larbalestier et al. arXiv: