High-Performance Y-based Superconducting Wire and Their Applications

High-Performance Y-based Superconducting Wire and Their Applications Yasuhiro Iijima 1 Yttrium(Y)-based superconducting wires are expected to be applied to various superconducting apparatus. They have high current density and show the high performance in liquid nitrogen, which is much cheaper than liquid helium. In 1991, Fujikura succeeded in developing the key original technology to fabricate Y-based superconducting wire, which was called as ion-beam-assisted deposition (IBAD) method. An 816.4 m long wire with end-to-end critical current (Ic) of 572 A/cm, corresponding to the world record Ic x L value of 466,981 Am/cm, was achieved at Fujikura. Today, we have established skills of routinely fabricating 5 m long wires with uniform Ic distribution over 5 A. In this report, we introduce recent improved performance of Y-based superconducting wires and developments of key technologies for their applications. 1. Introduction Superconductivity is a phenomenon of exactly zero electrical resistance occurring below certain temperature (critical temperature: Tc), in certain materials called superconductors first discovered by Dutch physicist H. K. Onnes in 1911. Conventional superconductors, so-called low temperature superconductors (LTS), could have shown superconductivities just above the boiling point of liquid helium (4 K (-296 C) ). On the other hand, several kind of cuprate-perovskite ceramic materials were discovered in 1987 which showed superconductivities at unusually high critical temperatures far above the boiling point of liquid nitrogen (77 K(-196 C)), being called high temperature superconductors(hts). LTS wire has already used in devices such as MRI (Magnetic Resonance Imaging) scanners for medical application and NMR (Nuclear Magnetic Resonance) for analysis of life science and materials research, which reqires high magnetic field. Only superconducting coils could generate such a field in a large space using quite low electric power, but their operation temperature was limited only near 4 K (-296 C) using LTS wires. HTS wire is expected to expand the application of superconducting coils, including rotating machine etc. by the increase of operation temperature up to 77 K(-196 C). Forthermore, superconducting power cable operating at 77K can also be designed by using HTS wire which transport quite large current with extremely small transmission loss and reasonable cooling cost. 1 Superconductor Business Development Department Yttrium(Y)-based superconducting wire was called as the second-generation HTS wire compared to the first one of Bismuth(Bi)-based superconducting wire. It used a HTS cuprate materials of RE-Ba-Cu-O (RE=Y, Gd, Sm, etc.), that has the most strong intrinsic superconducting properties among HTS materials. It was expected to show highest current transport performances especially in magnetic field, and also high mechanical strength suitable for a wide range of applications. But unfortunately there was a severe problem to make the wire practically applicable. Superconducting current was so easily interupted at an interface between the crystals of superconductors, that singlecrystal like structures should be obtained in Y-based superconducting wire from end to end. In 1991, Fujikura succeeded in developing the key original technology to fabricate tape-shaped Y-based superconducting wire, which was called as ion-beamassisted deposition (IBAD) method 1). By means of IBAD method, a functional thin buffer layer could be deposited on the surface of polished metal tape. The crystalline axes of the buffer were biaxially controlled by irradiation with an Ar ion beam inclined a certain degree from substrate normal. A superconducting layer can be deposited epitaxially on the buffer layer, resulting in a single-crystal like structure. Fujikura has consistently concentrated the research activities on development of Y-based superconducting wires using IBAD method. As a result, we succeeded in fabricating a few hundreds long wire with critical current (Ic) over 3 A/cm in hole length 2), and Fujikura started selling Y-based superconducting wires in 29. In this report, we introduce recent improved performance Fujikura Technical Review, 213 117

of Y-based superconducting wires and developments of key technologies for their applications. 2. Development of Y-based superconducting wire 2.1 Specifications of Y-based superconducting wire A photograph of Y-based superconducting wire at Fujikura are shown in Fig. 1. Hastelloy TM C276 tapes (75 μm or 1 μm thick) are used for metal substrates. Several buffer layers, including biaxially textured layers, are deposited on the substrate by sputtering and IBAD method. A superconducting layer is deposited on the buffer layer by pulsed-laser-deposition (PLD) technique. Ag layers deposited on the superconducting layer by sputtering as a protection layer. In addition, a copper tape is laminated with solder on Ag layer as a stabilizer and double polyimide tapes are wrapped as a insulation layer. A total thickness of the superconducting wire is approximately 15-3 μm. The specification of critical current(ic) of the wire is over 4 A/cm width at 77 K, self field (s. f.). 2.2 Improved performance of Y-based superconducting wire IBAD and PLD method are two important technologies in order to fabricate tape-shaped Y-based superconducting wires. IBAD is the key original technology to obtain a biaxially textured buffer layer on non-textured metal substrate, and also PLD is the other key technique to fabricate a superconducting layer on the buffer layer. We have employed a several nm thick thin textured MgO buffer layer using a large IBAD system with a 11 cm x 15 cm ion source. As a result, we successfully fabricated 1 km-long biaxially textured MgO buffer layer with quite high throughput. 3) This is very important in terms of commercial aspects such as production cost and mass for superconducting wires. There are several deposition methods for fabricating a HTS cuprate layer of, such as a PLD, a chemical vapor deposition (CVD) and a metal organic deposition (MOD). We have consistently concentrated the research activities on development of PLD method. PLD method is the technique that a thin film is grown on substrate by deposition of the particle assemblage, which is generated from the surface of sintered target irradiated by ultraviolet pulsed laser, such as mainly excimer laser. In general, there are many advantages of this technique, such as the high rate deposition among the gas-phase approaches and easy control of the composition for a superconducting layer because it is scarcely affected by the vapor pressure variations of target elements. In addition, the cost of raw material is lower than other deposition processes especially in fabrication of Y-based superconductor wires because the target is just the sintered bulk of material. It is known, however, the performance of superconducting layer deposited by PLD method depends largely on the temperature. It is important for making it practical homogeneous Ic to the longitudinal direction of wire as well as higher Ic performance. Therefore, we have developed the unique large PLD system with hot-wall heating, it is called hot-wall PLD, as shown in Fig. 2. As a result, we have succeeded in developing fabrication of long-length superconducting layer with high performance at the high rate of 4 nm / s. 4) Fig. 3 shows abstract of wire development status from 29 to 212 at Fujikura. We have dramatically progressed the higher Ic of superconducting wires by lc (ka/cm) insulation (polyimide tapes) stabilizer(cu) protection layer(ag) superconducting layer buffer layers substrate 1.2 1..8.6.4.2 Fig. 1. Photograph of Y-based superconducting wire. 1-2 Multi-lanes Excimer laser Scanning laser Technical Target ~29 ~21 ~211 Products 211 lc > 3 A/cm Piece:1~3m width:5,1mm at 77 K, T 1-1 1 1 1 1 2 1 3 Length(m) IBAD substrate hot-wall heating GdBCO target Fig. 2. Schematic of PLD system with hot-wall heating. Products Target 215 lc > 7 A/mm Piece > 1m width:2,3,4, 6,12mm Products 212 lc > 5 A/cm Piece:~5m width:5,1mm Fig. 3. Wire development status from 29 to 212 at Fujikura. 118

8 voltage (mv) 1 5 816.4 m 81.64 mv End-to-end current 1 2 3 current (A) 572 A 4 5 6 Fig. 4. (a)photograph and (b)i-v characteristic of an 8 m long wire. lc(77 K) (A) 6 5 4 2 Wire A Wire B Wire C 1 2 3 4 5 6 Position (m) Fig. 5. Longitudinal Ic distribution of production wires (example data). 7 Table 1. Details of the superconducting wires shown in Fig. 5. Wire A Wire B Wire C (1) Length (m) 621 7 587 (2) Average Ic (A/cm) 649 575 55 (3) Standard deviation of Ic (A/cm) 14.7 11. 6.9 (4) Uniformity: (3) / (2) x 1 (%) 2.2 1.9 1.3 the combination of IBAD and PLD technique. Especially, our original hot-wall PLD system has enabled the rigorous control of temperature during continuous depositions at a high rate and has a superiority in thickening superconducting layer without degrading and obtained a high Ic value over 1 A/cm in a short sample. As a result, we have succeeded in fabricating a 8 m long wire with Ic over 95 A/cm and a 11 m long wire with Ic over 76 A/cm in almost hole length. On the other hand, uniformity of longitudinal Ic distribution is important for practical long wires with high performance. However, we have also succeeded in fabricating a 6 m long wire with Ic over 6 A/cm in hole length in October 21. 5) This result set the new world record of Ic L value as 374,535 A m (= 69 A 615 m) in those days. Furthermore, an 816.4 m long wire with end-to-end measured Ic of 572 A/cm was successfully fabricated in Janually 211. Its Ic x L value updated the new world record of 466,981 Am/cm 6). As a result of these developments, quite uniform Ic over 5A/cm with length over 5-m are routinely obtained as shown in Fig. 5 and Table 1. 3. Developments of key technologies for applications We have also developed coil technologies for magnet applications. In 24, we developed a liquid nitrogen cooling solenoid magnet fabricated using 7 m- long Y-based superconducting wire. In addition, we developed and evaluated a world s first cryocoolercooled solenoid magnet fabricated using 11 m-long Y-based superconducting wire in 26. We confirmed that the magnet could be generated a 1.1 T magnetic field by operating current of 4 A for 1 min at 35K without voltage generation and temperature rising. It Outer diam. 133mm Inner diam. 6mm bobbin Copper plate Measurement wires coil no.1 coil no.2 coil no.3 coil no.4 coil no.5 coil no.6 Fig. 6. Photograph of six-stacked pancake coils (6 turns in total). was demonstrated in the first in the world that the conduction cooled magnet could realize a stable magnet excitation as well as the liquid nitrogen cooled magnet. 7) Recently, an epoxy impregnated coil fabricated using Y-based superconducting wires may occur a degradation of the coil because of its radial thermal stress during cool down. Therefore, it is necessary to evaluate the voltage(v)-current(i) characteristics of impregnated coils at low electric fields (below 1-7 V/cm) in order to ensure that no degradation occurs during the fabrication of the coil. 8) We have succeeded in developing a conductioncooled impregnated pancake coil magnet in 211 as shown in Fig. 6, which magnet fabricated using approximately 2 m-long Y-based superconducting wires. We confirmed that none of the pancake coils impregnated by epoxy-based resin were damaged during fabrication by measuring V-I characteristics in liquid nitrogen. In addition, the central magnetic field of the magnet achieved 1.27 T at 5 K under conductioncooled conditions, when the transporting current was 166.4 A. 9) Fujikura Technical Review, 213 119

Critical current Ic (A) 25 2 15 1 5 4 2 2 4 6 Angle of magnetic field θ ( ).5T.1T.13T.15T.2T.25T.3T 8 1 12 14 Fig. 7. Angular dependence of Ic-B characteristics of a Y-based superconducting wire at 77K. Voltage (1-6 V/cm) 12 1 8 6 4 2 Measured calculated 1 8 15 22 29 36 43 5 57 64 71 78 85 92 99 Number of turns Fig. 8. Calculated and measured voltage distributions in a pancake coil at Ic defined with criterion of 1-6 V/cm at 77 K, s. f. Incidentally, it is important for safety operation of the magnet to predict the heat generation of a conduction-cooled coil since the thermal runaway that the heat generation is greater than cooling capacity may cause significant damage to the coil. Therefore, we compared the calculated voltage distribution of a pancake coil with measured one at 77 K by means of calculating with Ic-B characteristics as shown Fig. 7. As a result, we obtained that the calculated voltage distribution of a pancake coil was in good agreement with the measured one as shown Fig. 8. 9) This results indicates that the heat generation of a conduction-cooled magnet can be predicted by means of calculating with measured Ic-B characteristics. One of the superconducting applications in the future is a superconducting motor. A superconducting motors can realize more increased power density and hence smaller size than conventional motors by increasing currents and magnetic fields within rotor. These advances in a superconducting motor make it possible to obtain high torque characteristics in lowspeed area. Therefore, it is expected to utilize for use in ship and wind turbine. In 26, we produced the world s first Y-based superconducting motor, intended for use in ships, had a rated rpm of 36 and a rated output of 15 kw. It used the Y-based superconducting wires in field coils and the copper wires used in the armature. After they passed motor rotation tests, we attached a screw to the motor to further test the characteristics of the motor for ships and tested it under water. As a result, the designed operation of 7.5 kw at 36 rpm was successfully verified for the rated field current of 6 A. 1) Superconducting wire has exactly zero electrical resistance for transporting direct current (DC), however, in the case of transporting alternating current(ac) extremely small transmission loss occurs. In regard to AC loss reduction of superconducting cable, higher-ic and cross-sectional configuration were both essential. AC loss was depend on the ratio of operating current to critical current of superconducting wires, hence it was expected to reduce AC loss by decreasing the ratio using higher Ic Y-based superconducting wires. In addition, the fabrication technology of thin Y-based wires of optimized widths was employed in order for concentric cross-section of a cable closer to a circular shape to avoid the generation of vertical magnetic fields. 11)12) 4. Conclusion We introduce recent progress of Y-based superconducting wires and developments of key technologies for their applications. Higher performance and great uniformity of Y-based superconducting wire have been established by two important key technologies of IBAD and PLD method. In 211, an 816.4 m long wire with end-to-end critical current (Ic) of 572 A/cm, corresponding to the world record Ic x L value of 466,981 Am/cm, was achieved. We have already established world s first skills of routinely fabricating 5 m long wires with uniform Ic distribution over 5 A. Fujikura will promote the development of Y-based superconducting wire that realizes even higher performance and the development of a superconducting magnet capable of operation at higher temperatures featuring greater compactness that will be available for application in devices such as rotating machines, medical and scientific devices for analysis and evaluation, etc. Furthermore, Fujikura will proactively expand the range of applications to include infrastructures that contribute to the low-carbon society including superconducting cables. Acknowledgements This work included results supported by the New Energy and Industrial Technology Development Organization (NEDO). 12

References 1) Y. Iijima et al., : In-plane aligned YBa2Cu3O7-x thin films deposited on polycrystalline metallic substrates, Applied Physics Letters, Vol.6, No.6, pp.769-771, 1992 2) S. Hanyu, et al., : Progress in Scale-Up of RE-123 Coated Conductors With of 3 A/cm by IBAD/PLD Process, IEEE Transactions on Applied Superconductivity, vol. 19, no. 3, pp.324-3243, 29 3) S. Hanyu et al., : Fabrication of km-length IBAD-MgO substrates at a production rate of km h-1, Superconductor Science and Technology, vol. 23, p.1417, 21 4) K. Kakimoto et al., : High-speed deposition of high-quality RE123 films by a PLD system with hot-wall heating, Superconductor Science and Technology, vol. 23, p.1416, 21 5) K. Kakimoto, et al.: Long RE123 coated conductors with high critical current over 5 A/cm by IBAD/PLD technique, Physica C 471, pp. 929-931, 211 6) M. Igarashi, et al.: Advanced development of IBAD/PLD coated conductors at FUJIKURA, Physics Procedia 36, pp. 1412-1416, 212 7) H. Fuji et al., : Development of Long Y-123 Conductor and Solenoid Magnet by IBAD/PLD Process, IEEE Transactions on Applied Superconductivity, vol. 17, no. 2, pp.3383-3385, 27 8) H. Miyazaki, et al., : Thermal Stability of Conduction-Cooled YBCO Pancake Coil, IEEE Transactions on Applied Superconductivity, vol. 21, no. 3, pp.2453-2457, 211 9) M. Daibo, et al. : Characteristics of Impregnated Pancake Coils Fabricated using REBCO Coated Conductors, IEEE Trans. Appl. Supercond. Vol.22, No.3, 3924, 212 1) M. Iwakuma, et al., : Development of a 15 kw Motor With a Fixed YBCO Superconducting Field Winding, IEEE Transactions on Applied Superconductivity, vol. 17, no. 2, pp.167-161, 27 11) T. Ohkuma, et al., : Recent Progress in the Development of YBCO HTS Power Cables, Cryogenic engineering, vol. 46, no. 6, pp.335-341, 211 (in Japanese) 12) N. Amemiya, et al., : AC Losses in High Tc Superconductors : AC Loss Characteristics of Tapes and Power Transmission Cables, Cryogenic engineering, vol. 45, no. 8, pp.376-386, 21 (in Japanese) Fujikura Technical Review, 213 121