Mechanically Reinforced Bi-2212 Strand

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 6400404 Mechanically Reinforced Bi-2212 Strand P. Tixador, C. E. Bruzek, B. Vincent, A. Malgoli, and X. Chaud Abstract—Bi-2212 offers a lot of opportunities for very high fields at low temperature. The current density is large under high fields, particularly with recent enhanced results. The Bi-2212 conductor may be a round strand: a very favorable shape to wind and to make high-current Rutherford cables required for protection. There is no satisfying high-current cable with YBCO. One drawback of Bi-2212 is their low mechanical properties. Large fields and current densities indeed induce high mechanical stresses. To improve the mechanical properties of Bi-2212 strands, Nexans has proposed to reinforce it with a metal sheath wrapped around using their process of shaping and welding. The sheath is wrapped around the strand and laser welded, and the whole is drawn to a diameter of 0.9 mm. Several materials for the sheath were studied to determine their resistance to thermal treatment of Bi-2212 and their mechanical properties after treatment. We choose Inconel 601. A method of perforating the sheath has been developed to enable the oxygenation during the heat treatment. A 6 + 1 conductor has also been produced around an Inconel core and inserted in a tube. The 6 + 1 reinforced conductor was then drawn to a diameter of 2.7 mm. The Ic measurements at 4 K show that our mechanical reinforcement does not significantly lower the transport capacities. They therefore validate the method and the heat treatment. Index Terms—Bi-2212 strand, mechanical reinforcement, very high field magnets, 1G wire. I. I NTRODUCTION T HE discovery of the Higgs boson was a worldwide event in 2012. It was possible thanks to the Large Hadron Collider (LHC) and its Low Temperature Superconducting (LTS) NbTi magnets with a rated magnetic flux density of 8.3 T. But the quest for the fundamental law of Universe does not stop. Numerous questions such as Super Symmetry remain not solved. To get a response, colliders with higher energies so higher magnetic flux densities are required [1]. 20 T is a target. The magnetic flux density (B) is limited by the critical characteristic Jc (B) of superconductors. An order of magnitude of the minimum current density required for accelerator magnets is about 400 MA/m2 due to space (tunnel) constraints. Fig. 1 shows that the maximum magnetic flux density is then limited to 15–16 T using Nb3 Sn. Manuscript received August 12, 2014; accepted October 23, 2014. Date of publication December 2, 2014; date of current version February 6, 2015. This work was supported in part by the French ANR Project “SUPER-SMES.” P. Tixador is with the University Grenoble Alpes - CNRS, 38000 F-Grenoble, France (e-mail: [email protected]). C. E. Bruzek is with Nexans, 92587 Clichy, France (e-mail: christian_eric. [email protected]). B. Vincent and X. Chaud are with University Grenoble Alpes-CNRSINSA-UPS-LNCMI, F-38000 Grenoble, France (e-mail: benjamin.vincent@ g2elab.grenoble-inp.fr) A. Malgoli is with the CNR-SPIN, I-16152 Genova, Italy (e-mail: andrea. [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2014.2373642 Fig. 1. Engineering critical current density characteristics (4 K for HTS, 1.8 K for LTS). Data for Y (YBCO) come from CERN [4] and data for Bi come from [3]. Nb3 Sn is the best commercial LTS for high magnetic fields. Fig. 1 also shows that HTS (High Temperature Superconductor) open the way for much higher magnetic flux densities, from theoretical point of view at least. In addition HTS, even at 4 K, show much higher stability against external perturbations. The energies required to quench them are several orders of magnitude higher compared to LTS [2]. Two HTS conductors are available for very high field magnets: BSCCO and YBCO. Among the BSCCO compounds Bi-2212 (Bi2 Sr2 CaCu2 O8+δ ) suits particularly since Bi-2212 wires may be round strands. This is a huge advantage to wind a magnet as we will see. Furthermore at the contrary to YBCO, they especially show isotropic transport properties under magnetic field. The best Bi-2112 round wires reacted under 100 bar pressure [3] show engineering current densities higher than YBCO in transverse fields. The current capacity is a necessary condition but not sufficient for a wire. Since it experiences both high magnetic flux densities and current densities the electromagnetic stresses are very large and the wire should also withstand stresses of several hundred of MPa. For protection reasons in case of a quench, the operating current of magnet should be as high as possible, 10 kA is an objective even if for insert magnets lower currents are certainly possible. Such a current is only possible with a cable assembled from several wires. The Rutherford cable used in accelerator magnets suits well but it is based on round strands so the strong interest of Bi-2212 strands. Bi-2212 Rutherford cables have been successfully tested [5]. Since YBCO wires are thin tapes, they cannot form a Rutherford cable. Other ways are investigated such as Roebel cable or stack of tapes [6] but there is no really satisfying solution so the interest for Bi-2212 round strands. However Bi-2212 strand shows too low mechanical properties: even with 1051-8223 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 6400404 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 2. Sketch of the heat treatment performed at CNR-SPIN. Fig. 3. a MgAg matrix the maximum stress remains below 100 MPa. That is far too low to operate accelerator magnets. We present in this paper a possibility to improve the mechanical properties thanks to an external dedicated jacket. Two reinforcements have been studied: a single strand reinforced with the “shape and weld” process developed by Nexans and 6 + 1 (6 around 1) cable inserted in a drawn tube. Different Inconel foils with Bi-2212 strand before heat treatments. TABLE I C HARACTERISTICS OF THE T WO R EINFORCED Bi-2212 C ONDUCTORS II. Bi-2212 S TRAND AND R EINFORCEMENT M ATERIALS A. Bi-2212 Round Strand Nexans has produced length of round Bi-2212 strand. It comprises 7 sub elements with each 85 filaments. The Bi-2212 content is about 10%. The matrix is pure silver and an alloy (MgAg external sheath) to improve the mechanical properties. The heat treatment (HT) remains fundamental for high critical characteristics. They have been performed in a first time at Grenoble and in a second step at the University of Genova. In Genova straight wires and cables as well as wound cable were heat treated in 0.1 MPa flowing O2 in a tubular furnace with a diameter of 70 mm and a homogeneity zone (±0.5 K) of 30 cm using the heat treatment schedule optimized by Oxford Superconducting Technology (OST) [7] shown in Fig. 2. The critical current of bare wires at 4.2 K under self field conditions (1 µV/cm) varies significantly with values from a minimum of 210 A up to a maximum of 350 A. A change of only 1 or 2 K for the HT peak temperature may affect the Ic value. For these short samples we think that any Jc reduction by end-effect occurred, while regarding the VAMAS sample, which was longer, its Jc reduction could be due also to the endeffect i.e., bubbles agglomerations within the filaments [8]. The lengths were 0.2 to 0.25 m for the straight samples. B. Reinforcement Materials The material used to mechanically reinforce the Bi-2212 wire should meet several requirements: high mechanical properties and no reaction with the Bi-2212 strand during the heat treatment. Inconel alloys family has been chosen (600, 601, and 625). These austenitic nickel-chromium based super alloys form during heat treatment a passivating oxide layer protecting from reaction with Bi. The final selection of the reinforcement composition has been based on five criteria: suitability for shaping tube, weldability, availability of strips with 50–100 µm thickness, their mechanical strength after treatment and possible contamination with Bi-2212. All of these alloys show a good weldabilty and an acceptable ductility at room temperature in accordance with the shaping process. Test heat treatments with the Bi-2212 strand wrapped in different Inconel foils (Fig. 3) were carried out with two bare strands as a reference to study any possible contamination of the Inconel elements during the Bi-2212 HT. Inconel 600 shows the lowest Ic degradations and Inconel 625 the highest. Inconel 601 has been selected for its commercial availability whereas the Ic degradations remain limited. The mechanical performances (stress σ) of the reinforced have been calculated according to the proportion (f) and nature of each element: σcomposite = freinforcement σreinforcement +fstrand σstrand Bi−strand σ0.2% = 80 MPa; Inconel σ0.2% = 500 MPa Of course the reinforcement lowers the engineering current density. Table I shows some characteristics of the two conductors. The Jc reduction assumes only the ratio of the SC strand to the overall area with no degradation of the strand Ic . The 6 + 1 cable shows more Ic degradation but also higher mechanical stress. For the same Ic degradation (53%), it leads to a 150 µm thick jacket for the single strand and to a mechanical stress of 280 MPa. These values remain below the high field insert requirements but the improvement is significant. The thickness of the jackets can be increased: 150 µm leads to 280 MPa for the single strand. We have to look at reinforcement materials with higher mechanical properties. There are better materials than Inconel (σ0.2% = 500 MPa) but they do not necessarily fulfil the other requirements (non contamination, weldability . . .). TIXADOR et al.: MECHANICALLY REINFORCED Bi-2212 STRAND Fig. 4. 6400404 Patterns of the hole locations on the strip. Fig. 6. Bi-2212 strand with the external Inconel jacket. Fig. 5. Holes in the strip and the tube for heat treatments. Barriers can be imagined to reduce the risk of contamination of the Bi-2212 strand. III. M ECHANICAL R EINFORCEMENT BY T UBES Two ways of reinforcement were selected: a single wire and a small cable approach (structure 6 + 1): • The single wire is reinforced by a “shape and weld” external jacket positioned around the wire. • The 6 + 1 cable has an external reinforcement made out with an Inconel tube with an additional central reinforcement core. A. Round Strand: “Shape and Weld” Process In order to enable the indispensable Bi-2212 oxygenation during the heat treatment, reinforcement tubes must allow oxygen exchanges. For this, we carried out perforations of 0.3 mm diameter with a “zig-zag” patern (Fig. 4). This structure limits the mechanical degradation of the tapes while maintaining a sufficient distribution of the oxygen. These perforations are obtained by punching with matrices. It was possible to perforate the 601 Inconel 50 µm thick strip. 200 m of tape have been produced in line for the project demonstrating that the stamping technology is possible for long wires. Fig. 5 shows the perforated strip and the tube. As expected, perforations decrease significantly the ultimate stress and strain but have only a very low impact on σ0.2% . After HT, the ultimate stress and strain at 77 K measured at CEA Saclay remain respectively at 570 MPa (higher than the value considered (500 MPa) for Table I) and 6.1% high enough to reinforce Bi-2212 wires for high field magnets. The mechanical properties should be performed and verified at 4 K but in general there is no big difference in term of ultimate stress at 77 K and 4 K, even often a slight enhancement. The principle of the online manufacturing process developed by Nexans to make small welded tubes is to cut the metallic strip at a very precise width, called edge trimming, to form Fig. 7. 6 + 1 cable with Bi-2212 strands and Inconel core (top right). the perforated strip with trimmed edges to a tube shape around the Bi-2212 wire. Finally laser welding of the edges of the cylindrical formed strip on the top is performed to close the tube. The external diameter of the manufactured tube is 1.8 mm. It is then drawn to 0.9 mm as illustrated by Fig. 6 to put in contact the jacket with the Bi-2212 wire. This drawing step recloses the tube perforations. But they are the seeds of elongated slots visible on the jacket surface. We can assume that they will enable atmosphere exchanges through them during the HT confirmed by Ic measurements. B. 6 + 1 Cable A small and compact 6 + 1 cable has been produced with an industrial planetary cabling machine. It consists in six Bi-2212 wires around one Inconel central core wire. The transposition pitch is 120 mm. This is a compromise between a low strain for the wire but a good mechanical holding of the cable. The cable was then introduced within a Inconel tube with a 150 µm wall thickness. It was drawn down to 2.7 mm to adjust the gap between the cable and the tube (Fig. 7). This tube is not perforated. We have assumed that on short test samples (< 2 m) considered within this project, the atmosphere exchanges can be done through the sample ends and through the gap between the wires. For long piece lengths, shape and weld process can be used to produce perforated tube from punched tapes with a new shaping tooling adapted to the diameter. 6400404 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 25, NO. 3, JUNE 2015 Fig. 8. Electric field current characteristic for the reinforced strand (4 K, 0 T). Fig. 10. Electric field current characteristic for the straight 6 + 1 reinforced cable (4 K, 0 T). V. C ONCLUSION Nexans has successfully proposed a solution to improve the mechanical properties of round Bi-2212 strands, their weak point. Based on Inconel leaky tubes for oxygenation, no real degradation has been observed. The mechanical stress has been enhanced by 212% and 287% with a reduction of 21% and 53% for the engineering critical current density for the single tape and the 6 + 1 cable respectively. Fig. 9. 6 + 1 straight cable and the Vamas sample. IV. P ERFORMANCES OF R EINFORCED C ONDUCTOR A. Single Strand The preparation of the sample for Ic measurement was not simple. The Inconel tube has been removed with great care to inject the current directly in the Bi-2212 strand. The heat treatment with the Inconel forms an isolating layer around the SC strand. This has been removed as well. The Ic (1 µV/cm) at 4.2 K in self field conditions is 236 A (Fig. 8). The reinforcement implementation does not degrade the SC properties: bare strands show similar Ic (240 A and 210 A (4.2 K, 0 T) measured). B. 6 + 1 Cable Two types of samples have been measured: 6 + 1 cable mounted on a Vamas type sample holder and straight sample (Fig. 9). Vamas sample shows a Ic higher than 1 kA (4.2 K, 0 T) (limit of the current supply). Other measurements were not successful. The straight sample has shown a Ic of about 1.1 kA (4.2 K, 0 T (Fig. 10)). We tried to cut five of the six strands to measure a single strand but the implementation has degraded the strand, which shows a resistive behavior. ACKNOWLEDGMENT The authors would like to thank the CEA-Saclay for the mechanical measurements and B. Dardel, S. Morice, M. Vilcot C. F. Theune from Nexans. S. Papu and J. Marpaud are acknowledged for some Ic measurements. R EFERENCES [1] E. Todesco, L. Bottura, G. De Rijk, and L. Rossi, “Dipoles for highenergy LHC,” IEEE Trans. Appl. Supercond., vol. 24, no. 3, Jun. 2014, Art. ID. 4004306. [2] Y. Miyoshi et al., “Performance tests of prototype high field HTS coils in Grenoble,” presented at the Appl. Supercond. Conf., Charlotte, NC, USA, Aug. 10–15, 2014, Paper 2LOr2A-01. [3] D. C. 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