High-Performance Y-Based Superconducting Wire and Their Applications

High-Performance Y-based Superconducting Wire and Their Applications

Yasuhiro Iijima1

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 500 m long wires with uniform Ic distribution over 500 A. In this report, we introduce recent improved performance of Y-based superconducting wires and developments of key technologies for their applications.

1. Introduction Yttrium(Y)-based superconducting wire was called Superconductivity is a phenomenon of exactly zero as the second-generation HTS wire compared to the electrical resistance occurring below certain tempera- first one of Bismuth(Bi)-based superconducting wire. ture (critical temperature: Tc), in certain materials It used a HTS cuprate materials of “RE-Ba-Cu-O” called “superconductors” first discovered by Dutch (RE=Y, Gd, Sm, etc .), that has the most strong intrin- physicist H. K. Onnes in 1911. Conventional supercon- sic superconducting properties among HTS materials. ductors, so-called low temperature superconductors It was expected to show highest current transport per- (LTS), could have shown superconductivities just formances especially in magnetic field, and also high above the boiling point of liquid helium (4 K (-296 °C) mechanical strength suitable for a wide range of ap- ). On the other hand, several kind of cuprate-perovskite plications. But unfortunately there was a severe prob- ceramic materials were discovered in 1987 which lem to make the wire practically applicable. Supercon- showed superconductivities at unusually high critical ducting current was so easily interupted at an interface temperatures far above the boiling point of liquid nitro- between the crystals of superconductors, that single- gen (77 K(-196 °C)), being called high temperature crystal like structures should be obtained in Y-based superconductors(HTS). superconducting wire from end to end. LTS wire has already used in devices such as MRI In 1991, Fujikura succeeded in developing the key (Magnetic Resonance Imaging) scanners for medical original technology to fabricate tape-shaped Y-based application and NMR (Nuclear Magnetic Resonance) superconducting wire, which was called as ion-beam- for analysis of life science and materials research, assisted deposition (IBAD) method 1). By means of which reqires high magnetic field. Only superconduct- IBAD method, a functional thin buffer layer could be ing coils could generate such a field in a large space deposited on the surface of polished metal tape. using quite low electric power, but their operation The crystalline axes of the buffer were biaxially con- temperature was limited only near 4 K (-296 °C) us- trolled by irradiation with an Ar ion beam inclined a ing LTS wires. HTS wire is expected to expand the certain degree from substrate normal. A supercon- application of superconducting coils, including rotat- ducting layer can be deposited epitaxially on the buffer ing machine etc. by the increase of operation tempera- layer, resulting in a single-crystal like structure. Fuji- ture up to 77 K(-196 °C). Forthermore, supercon- kura has consistently concentrated the research activi- ducting power cable operating at 77K can also be ties on development of Y-based superconducting wires designed by using HTS wire which transport quite using IBAD method. As a result, we succeeded in fab- large current with extremely small transmission loss ricating a few hundreds long wire with critical current and reasonable cooling cost. (Ic) over 300 A/cm in hole length 2), and Fujikura start- ed selling Y-based superconducting wires in 2009. In

1 Superconductor Business Development Department this report, we introduce recent improved performance

Fujikura Technical Review, 2013 117 of Y-based superconducting wires and developments It is known, however, the performance of supercon- of key technologies for their applications. ducting layer deposited by PLD method depends largely on the temperature. It is important for making 2. Development of Y-based superconducting it practical homogeneous Ic to the longitudinal direc- wire tion of wire as well as higher Ic performance. There- fore, we have developed the unique large PLD system 2.1 Specifications of Y-based superconducting wire with hot-wall heating, it is called ‘hot-wall PLD’, as A photograph of Y-based superconducting wire at shown in Fig. 2. As a result, we have succeeded in de- Fujikura are shown in Fig. 1. HastelloyTM C276 tapes veloping fabrication of long-length superconducting (75 μm or 100 μm thick) are used for metal substrates. layer with high performance at the high rate of 40 nm Several buffer layers, including biaxially textured lay- / s. 4) ers, are deposited on the substrate by sputtering and Fig. 3 shows abstract of wire development status IBAD method. A superconducting layer is deposited from 2009 to 2012 at Fujikura. We have dramatically on the buffer layer by pulsed-laser-deposition (PLD) progressed the higher Ic of superconducting wires by 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 insulation as a stabilizer and double polyimide tapes are wrapped (polyimide tapes) as a insulation layer. A total thickness of the superconducting wire is approximately 150 - 300 μm. The spec- stabilizer(Cu) ification of critical current(Ic) of the wire is over 400 protection layer(Ag) A/cm width at 77 K, self field (s. f.). superconducting layer buffer layers 2.2 Improved performance of Y-based superconducting wire substrate 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-tex- Fig. 1. Photograph of Y-based superconducting wire. tured metal substrate, and also PLD is the other key technique to fabricate a superconducting layer on the Multi-lanes IBAD buffer layer. We have employed a several nm thick substrate thin textured MgO buffer layer using a large IBAD system with a 110 cm x 15 cm ion source. As a result, we successfully fabricated 1 km-long biaxially textured 3) MgO buffer layer with quite high throughput. This is hot-wall very important in terms of commercial aspects such as Excimer heating production cost and mass for superconducting wires. laser There are several deposition methods for fabricating a HTS cuprate layer of, such as a PLD, a chemical GdBCO target vapor deposition (CVD) and a metal organic deposi- Scanning laser tion (MOD). We have consistently concentrated the Fig. 2. Schematic of PLD system with hot-wall heating. 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, 1.2 Technical Target which is generated from the surface of sintered target ~2009 ~2010 Products Target 1.0 ~2011 2015 irradiated by ultraviolet pulsed laser, such as mainly lc > 70 A/mm 0.8 Piece > 1000m excimer laser. In general, there are many advantages width:2,3,4, of this technique, such as the high rate deposition 0.6 6,12mm

among the gas-phase approaches and easy control of l c (kA/cm) Products 2011 0.4 lc > 300 A/cm Products 2012 the composition for a superconducting layer because Piece:100~300m lc > 500 A/cm width:5,10mm Piece:~500m it is scarcely affected by the vapor pressure variations 0.2 width:5,10mm at 77 K, 0 T of target elements. In addition, the cost of raw material 0 is lower than other deposition processes especially in 10-2 10-1 100 101 102 103 fabrication of Y-based superconductor wires because Length(m) the target is just the sintered bulk of material. Fig. 3. Wire development status from 2009 to 2012 at Fujikura.

118 800

600 500 400 Wire A (77 K) (A) c l Wire B 200 Wire C 0 0 100 200 300 400 500 600 700 Position (m) 100 Fig. 5. Longitudinal Ic distribution of production wires 816.4 m 81.64 mV (example data). 50 572 A

End-to-end current Table 1. Details of the superconducting wires shown in Fig. 5. voltage (mV) 0 0 100 200 300 400 500 600 Wire A Wire B Wire C current (A) (1) Length (m) 621 700 587 Fig. 4. (a)Photograph and (b)I-V characteristic of an 800 m (2) Average Ic (A/cm) 649 575 550 long wire. (3) Standard deviation of Ic (A/cm) 14.7 11.0 6.9 (4) Uniformity: (3) / (2) x 100 (%) 2.2 1.9 1.3

the combination of IBAD and PLD technique. Espe- Outer diam. 133mm cially, our original hot-wall PLD system has enabled Inner diam. the rigorous control of temperature during continuous 60mm depositions at a high rate and has a superiority in bobbin thickening superconducting layer without degrading and obtained a high Ic value over 1000 A/cm in a short coil no.1 Copper plate coil no.2 sample. As a result, we have succeeded in fabricating coil no.3 a 8 m long wire with Ic over 950 A/cm and a 110 m coil no.4 coil no.5 long wire with Ic over 706 A/cm in almost hole coil no.6 length. Measurement wires On the other hand, uniformity of longitudinal Ic dis- Fig. 6. Photograph of six-stacked pancake coils (600 turns in tribution is important for practical long wires with high total). performance. However, we have also succeeded in fabricating a 600 m long wire with Ic over 600 A/cm in was demonstrated in the first in the world that the con- hole length in October 2010. 5) This result set the new duction cooled magnet could realize a stable magnet world record of Ic × L value as 374,535 A m (= 609 A × excitation as well as the liquid nitrogen cooled mag- 615 m) in those days. Furthermore, an 816.4 m long net. 7) wire with end-to-end measured Ic of 572 A/cm was Recently, an epoxy impregnated coil fabricated us- successfully fabricated in Janually 2011. Its Ic x L value ing Y-based superconducting wires may occur a deg- updated the new world record of 466,981 Am/cm 6). As radation of the coil because of its radial thermal stress a result of these developments, quite uniform Ic over during cool down. Therefore, it is necessary to evalu- 500A/cm with length over 500-m are routinely ob- ate the voltage(V)-current(I) characteristics of impreg- tained as shown in Fig. 5 and Table 1. nated coils at low electric fields (below 10-7 V/cm) in order to ensure that no degradation occurs during the 3. Developments of key technologies for applica- fabrication of the coil. 8) tions We have succeeded in developing a conduction- We have also developed coil technologies for mag- cooled impregnated pancake coil magnet in 2011 as net applications. In 2004, we developed a liquid nitro- shown in Fig. 6, which magnet fabricated using ap- gen cooling solenoid magnet fabricated using 70 m- proximately 200 m-long Y-based superconducting long Y-based superconducting wire. In addition, we wires. We confirmed that none of the pancake coils developed and evaluated a world’s first cryocooler- impregnated by epoxy-based resin were damaged dur- cooled solenoid magnet fabricated using 110 m-long ing fabrication by measuring V-I characteristics in liq- Y-based superconducting wire in 2006. We confirmed uid nitrogen. In addition, the central magnetic field of that the magnet could be generated a 1.1 T magnetic the magnet achieved 1.27 T at 50 K under conduction- field by operating current of 400 A for 10 min at 35K cooled conditions, when the transporting current was without voltage generation and temperature rising. It 166.4 A. 9)

Fujikura Technical Review, 2013 119 250 12 Measured 0.05T 200 10 calculated (A) c 150 0.1T 8

0.13T V/cm) 0.15T - 6 100 0.2T 6 0.25T 0.3T 50 4 Critical current I Voltage (10 0 2 −40 −20 0 20 40 60 80 100 120 140 0 Angle of magnetic field θ (° ) 1815 22 29 36 43 50 57 64 71 78 85 92 99 Fig. 7. Angular dependence of Ic-B characteristics of a Y-based Number of turns superconducting wire at 77K. Fig. 8. Calculated and measured voltage distributions in a pancake coil at Ic defined with criterion of 10-6 V/cm at 77 K, s. f.

Incidentally, it is important for safety operation of AC loss was depend on the ratio of operating current the magnet to predict the heat generation of a conduc- to critical current of superconducting wires, hence it tion-cooled coil since the thermal runaway that the was expected to reduce AC loss by decreasing the ra- heat generation is greater than cooling capacity may tio using higher Ic Y-based superconducting wires. In cause significant damage to the coil. Therefore, we addition, the fabrication technology of thin Y-based compared the calculated voltage distribution of a pan- wires of optimized widths was employed in order for cake coil with measured one at 77 K by means of calcu- concentric cross-section of a cable closer to a circular lating with Ic-B characteristics as shown Fig. 7. As a shape to avoid the generation of vertical magnetic result, we obtained that the calculated voltage distribu- fields.11)12) tion of a pancake coil was in good agreement with the measured one as shown Fig. 8. 9) This results indicates 4. Conclusion that the heat generation of a conduction-cooled mag- We introduce recent progress of Y-based supercon- net can be predicted by means of calculating with mea- ducting wires and developments of key technologies sured Ic-B characteristics. for their applications. Higher performance and great One of the superconducting applications in the fu- uniformity of Y-based superconducting wire have been ture is a superconducting motor. A superconducting established by two important key technologies of motors can realize more increased power density and IBAD and PLD method. In 2011, an 816.4 m long wire hence smaller size than conventional motors by in- with end-to-end critical current (Ic) of 572 A/cm, cor- creasing currents and magnetic fields within rotor. responding to the world record Ic x L value of 466,981 These advances in a superconducting motor make it Am/cm, was achieved. We have already established possible to obtain high torque characteristics in low- world’s first skills of routinely fabricating 500 m long speed area. Therefore, it is expected to utilize for use wires with uniform Ic distribution over 500 A. in ship and wind turbine. Fujikura will promote the development of Y-based In 2006, we produced the world’s first Y-based su- superconducting wire that realizes even higher perfor- perconducting motor, intended for use in ships, had a mance and the development of a superconducting rated rpm of 360 and a rated output of 15 kW. It used magnet capable of operation at higher temperatures the Y-based superconducting wires in field coils and featuring greater compactness that will be available the copper wires used in the armature. After they for application in devices such as rotating machines, passed motor rotation tests, we attached a screw to the medical and scientific devices for analysis and evalua- motor to further test the characteristics of the motor tion, etc. Furthermore, Fujikura will proactively ex- for ships and tested it under water. As a result, the de- pand the range of applications to include infrastruc- signed operation of 7.5 kW at 360 rpm was successfully tures that contribute to the low-carbon society verified for the rated field current of 60 A.10) including superconducting cables. Superconducting wire has exactly zero electrical resistance for transporting direct current (DC), howev- Acknowledgements er, in the case of transporting alternating current(AC) This work included results supported by the New extremely small transmission loss occurs. In regard to Energy and Industrial Technology Development Or- AC loss reduction of superconducting cable, higher-Ic ganization (NEDO). and cross-sectional configuration were both essential.

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