# Fileset

[Experimental_Study_on_the_Critical_Current_Properties_of_Flexible_Nb3Al_Superconducting_Wires_in_Cryocooler_System (final draft).pdf](https://mdr.nims.go.jp/filesets/2a802878-8289-4eeb-9f3e-ebfd8c4e17e3/download)

## Creator

[Hibiki Fukuda](https://orcid.org/0009-0002-5773-2686), Riku Onoue, Tomoya Sakamoto, [Ryota Inoue](https://orcid.org/0000-0001-5486-2288), [Hiroshi Ueda](https://orcid.org/0000-0003-4976-8638), [SeokBeom Kim](https://orcid.org/0000-0002-2946-5763), [Akihiro Kikuchi](https://orcid.org/0000-0002-5044-7156), Yasuo Iijima

## Rights

© 2023 IEEE.  Personal use of this material is permitted.  Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[Experimental Study on the Critical Current Properties of Flexible Nb3Al Superconducting Wires in Cryocooler System](https://mdr.nims.go.jp/datasets/f39674ba-6897-42f0-a9e5-a917192ea8c7)

## Fulltext

Microsoft Word - ASC2022-Fukuda-Kim21 Experimental Study on the Critical Current Properties of Flexible Nb3Al Superconducting Wires in Cry-ocooler System Hibiki Fukuda, Riku Onoue, Tomoya Sakamoto, Ryota Inoue, Hiroshi Ueda, SeokBeom Kim, Akihiro Kikuchi, and Yasuo Iijima Abstract— In recent years, Jelly-rolled Nb/Al composite monofil-ament Nb3Al superconducting wires with an outer diameter of 30 m, and multi-stranded Nb3Al wires were successfully fabricated by NIMS in Japan. So, it is necessary to measure the temperature dependence of the critical current of Nb3Al wires in cryocooler sys-tem for development of superconducting applications cooled by con-duction cooling method. The single and multi-stranded ultrafine Nb3Al superconducting sample wires with different outer diameter were prepared to evaluate the critical current properties. The criti-cal currents of sample wires were evaluated by the current sweep method and constant current method. In this study, the sample wires were soldered to the copper wire to prevent burnout of the sample wire during critical current measurements. Therefore, the amount of current shunted to the copper wire was estimated to accurately evaluate the critical current of the superconducting wire. Also, the critical current of Nb3Al wires due to bending strain (bending radius of 5 to 15 mm) was experimentally investigated. From the critical current measurement results of Nb3Al wires with different cross-sectional structures, it was confirmed that the superconducting properties of the wires were improved by reducing the wire diame-ter. It was shown that the critical current density of the developed Nb3Al wire is lower than that of REBCO wire, but higher than that of commercial NbTi and Nb3Sn wires.  Index Terms— ultrafine Nb3Al wire, conduction cooling, critical current characteristics, Ic measurement method I.  INTRODUCTION B3AL has better strain and stress tolerance than that ofNb3Sn and exhibits promising high field Jc properties inthe liquid helium temperature [1]–[3]. Therefore, Nb3Al wires have the potential to be promising candidates for use the high magnetic field applications. However, it is difficult to manufacture as a wire due to the diffusion problem between Nb/Al and Cu and the high heat treatment temperature. Jelly-rolled Nb/Al composite monofilament wires with an outer di-ameter of 30 m, and multi-strand wires were successfully fab-ricated [4]. Since the developed Nb3Al wire has flexibility, it is expected that a superconducting magnet can be developed by the React & Wind method. Therefore, we are planning to use this high performance Nb3Al wires for the development of su-perconducting applications operated in cryocooler system. To design the superconducting applications cooled by cryocooler, it is necessary to clarify the critical properties of Nb3Al wires This work was partially supported by the JSPS KAKENHI under Grant No. 21H04477 (Corresponding author:SeokBeom Kim) H. Fukuda, R. Onoue, T. Sakamoto, R. Inoue, H. Ueda, and S.B. Kim are with the Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan. (e-mail: kim@ec.okayama-u.ac.jp).  in conduction cooling conditions. In this study, ultrafine Nb3Al superconducting single wires and stranded wires with different outer diameters and cross-sectional structures were fabricated by the Jelly-rolled method, and their critical properties were measured by conduction cooling system. The bending strain characteristics of the critical current, which is necessary to de-termine the bending radius of the coils, were also studied. II. PREPARED NB3AL WIRESIn the early stages of the development of Nb3Al wire using the Jelly-rolled method, Ta was used as the core material (Type A) because it does not react easily with Nb/Al during the heattreatment process. However, since Ta is mechanically hard, it was difficult to make the long length wire, so Nb was selected as the core material in the next stage (Type B). A structure with-out Nb barrier was proposed to further reduce the wire diameter of Nb3Al superconducting wire (Type C). A sample wire with a copper core (Type D) was developed to realize flexible and A. Kikuchi and Y. Iijima are with National Institute for Materials Science, Tsu-kuba, Ibaraki 305-0047, Japan N Fig. 1.  The cross-sectional view of four types of Nb3Al wires with differ-ent cross-sectional structures and wire diameters fabricated by the Jelly-rolled method. 2 long length Nb3Al superconducting wire. In the case of Type D, Nb barriers are required to suppress the reaction between Cu and Nb/Al. Fig.1 shows the cross-sectional view of Nb3Al sam-ple wires, and two sample wires of Type C were prepared with wire diameters of 33 m and 50 m. The Cu/nonCu ratio of Type C and D are 0.5 and 1, and 7-stranded Nb3Al wires with different twist pitch (5 and 8 mm) using Type C and D were prepared. The critical current characteristics of 7-stranded Nb3Al superconducting wires using Type A and Type B were experimentally investigated in previous study [5].  III. IC MEASUREMENT METHODSFig. 2 shows the 4 K-class GM cryocooler system and sam-ple stage to measure the critical current properties of Nb3Al wires. The sample stage is composed of copper blocks for cur-rent leads and aluminum nitride (AlN) block. AlN plate and block were used for electrical insulation and thermal conduc-tion. The temperature of AlN block, current lead and sample wire was measured using small size Cernox sensor (CX series). Cernox sensors were thermally contacted with APIEZON grease and then mechanically clamped to measure the sample temperature as accurately as possible. The sample wire under conduction cooling system is adia-batic state except for the current leads. In addition, we experi-enced burnout of the sample wire when measuring the critical current due to the small amount of stabilizer in the sample wire. It was difficult to completely prevent burnout even when the current source was shut down using the threshold value of the voltage between taps. Therefore, in this study, the sample wires were soldered to a 1.2 mm diameter copper wire to prevent the sample wire from burning out during critical current measure-ments. Fig. 3 shows the measured Tc of Type D wires by 4-probe method and DC currents of 200 mA, 1 A, 2 A and 3 A. The transition to the superconducting state begins at the same temperature of 15.5 K for each current, but the transition width and residual voltage increase in proportion to the magnitude of the current. The difference in transition width is due to the tem-perature dependence of the critical current and the magnitude of the self-field, and the residual voltage indicates that current is flowing in the soldered copper wire. Figs. 4 and 5 show the typical generated voltage between taps when the measuring the critical current by current sweep and constant current methods. From Figs. 3 to 5, it is found that the current is shunted to the copper wire even when the sample wire is in the superconduct-ing state, and it can be predicted that a resistive component (contact resistance, etc.) is connected in series with the super-conducting wire. Therefore, in order to determine the critical current of the Nb3Al wires, it is necessary to know the amount of current flowing in the copper wire. To estimate the current flowing in the copper wire, we measured the electrical resistiv-ity of the copper wire below 30 K. Fortunately, the electrical resistivity of copper wire was constant at 1.0110-10 m below 15 K. In Figs. 4 and 5, the critical currents obtained by calcu-lating the shunt currents to the copper wires are 6.3A and 3.1A, respectively. When measuring the critical current under the conduction cooling system, the constant current method can be evaluated more correctly than the current sweep method if the Fig. 5. The typical generated voltage between taps when measuring the crit-ical current by constant current method and the transport current of 5 A. 051015202510 12 14 16Voltage (µV)Temperature (K)1.9A5AIc=3.1AFig. 4. The typical generated voltage between taps and temperature of sample wire when the measuring the critical current by current sweep method.  10.610.710.810.911012340 2 4 6 8 10Temperature (K)Voltage (μV)Current (A)1.7A10.82KIc=6.3AFig. 2. Photos of (a) 4 K-class L-shaped GM cryocooler system, (b) Cop-per block for thermal conduction and sample stage, and (c) close-up of sample stage for critical current measurement. Fig. 3. Measured Tc of Type D wire by 4-probe method and DC currents of 200 mA, 1 A, 2 A and 3 A under conduction cooling. 00.511.522.513 14 15 16 17Voltage (μV)Temperature (K)15.5 K2 A1 A3 A200 mA3 critical current is relatively large. In this study, the critical cur-rent of sample wires was evaluated by both the current sweep and constant current methods. In both cases, the critical current is evaluated based on the measured temperature of the sample wire during current transport. IV. MEASURED IC PROPERTIES A. Ic Properties of Single and Stranded Nb3Al Wires Fig. 6 shows the measured critical current (Ic) and calculated critical current density (Jc) of single Nb3Al wires (Type C) with diameter of 33 m and 50 m. Ic of 50 m with a large super-conducting area is higher than that of 33 m diameter, but Jc is lower than that of a diameter of 33 m. It is considered that the reaction distance between the Nb and Al foils was shortened by the wire drawing process, and the superconducting properties of Nb3Al wires were improved. This result demonstrates the im-provement of the critical current density due to the ultrafine Nb3Al wire. Fig. 7 shows the results of comparing the super-conducting properties of two sample wires with the same wire diameter (50 m) but different cross-sectional structures (Type C and Type D). The superconducting area of Type C is larger than that of Type D, and the Ic is also larger. However, the Jc of Type C is lower than that of Type D. This means that in the case of Type C, there is no barrier between the Cu outermost layer and Nb/Al, which may have reacted with Cu and reduced the superconducting properties. Therefore, this result shows the usefulness of the barrier and the copper core. A comparison of the Ic and Jc of the four sample wires (Type A, B, C and D) is shown in Fig. 8. The Ic increases in the order of increasing wire diameter, but the highest Jc was obtained with Type D.   Fig. 9 shows the measured critical current of single and 7-stranded Nb3Al wires (Type C and Type D) with different twist pitch (5 and 8 mm). In these figures, the calculated the Ic of a single wire multiplied by 7 is also shown, and this value is larger than that of the 7-stranded wire. This is because the Ic of 7-stranded wire is larger than that of a single wire, and the mag-nitude of the self-field applied to itself is also larger. The dif-ference between the Ic value of the 7-strand wire and that of the single wire multiplied by 7 is smaller for the Type D wire, and it is easy to predict that the Jc-B characteristic of Type D wire is higher than that of the Type C wire. In Fig. 9, no decrease in the critical current due to the twist pitch (5 and 8 mm) of the 7-strand wire was observed. Fig. 10 shows the magnetic field de-pendence of Jc for Type C and Type D wires measured in liquid helium.  As expected, it was confirmed that the Jc-B character-istics of the Type D wire are better than that of Type C wire.  B. Bending Strain Characteristics The bending radius characteristic of the critical current of a superconducting wire is a very important parameter in its application. Therefore, we measured the bending strain charac-teristics in the bending radius from 5 mm to 15 mm using Type Fig. 6. The measured critical current and calculated critical current density of Type C wires with diameter of 33 m and 50 m.  Fig. 7. The measured critical current and calculated critical current density of Type C and Type D wires with diameter of 50 m. 01234567891010 11 12 13 14 15Critical current (A)Tempreture (K)Type C (50φ)Type C (33φ)01E+092E+093E+094E+095E+096E+097E+098E+099E+091E+1010 11 12 13 14 15Critical current density (A/m2 )Tempreture (K)Type C (50φ)Type C (33φ)01234567891010 11 12 13 14 15Critical current (A)Tempreture (K)Type C (50φ)Type D02E+094E+096E+098E+091E+101.2E+101.4E+101.6E+1010 11 12 13 14 15Critical current density (A/m2 )Tempreture (K)Type C (50φ)Type DFig. 9. The measured critical current of single and 7-stranded Nb3Al wires with different twist pitch (a) Type C (33 m) and (b) Type D. 0510152025303510 11 12 13 14 15Critical current (A)Temperature (K)single wire×77strands,TP=8mm7strands,TP=5mmsingle wire(a)010203040506010 11 12 13 14 15Critical current (A)Temperature (K)single wire×77strands,TP=8mm7strands,TP=5mmsingle wire(b)Fig. 10. The magnetic field dependence of the critical current density of Type C (33 m) and Type D wires measured in liquid helium. 0.0E+001.0E+092.0E+093.0E+094.0E+095.0E+094 8 12 16Critical current density (A/m2 )Magnetic Field (T)Type C (33φ)Type DFig. 8. The measured critical current and calculated critical current density of Type A, B, C (33 m) and D wires to compare the effect of wire diameter. 0510152025309 10 11 12 13 14Critical current (A)Temperature (K)Type AType BType C (33φ)Type D02E+094E+096E+098E+091E+101.2E+101.4E+101.6E+109 10 11 12 13 14Critical current density (A/m2 )Temperature (K)Type AType BType C (33φ)Type DFig. 11. Photographs of the pre-bended copper wire and sample stage to measure the bending strain characteristics of Type D wire. 4 D wire. Fig. 11 shows the photographs of the pre-bended copper wire and sample stage to measure the bending strain character-istics of Type D wire. In the experiment, the Nb3Al wire was soldered along the inside of the pre-bent Cu wire as shown in Fig. 11. Fig. 12 shows the measured bending radius dependence of the Ic in Type D wire annealed at 850°C. When the bending radius is 15 mm (the bending strain ratio is 0.17%), the Ic de-creased by 65% compared to the value of straight line. However, almost no decrease of Ic was observed in the range of bending strain ratio from 0.17% to 0.33%. The superconductivity of the Nb3Al wire was destroyed when the bending strain ratio was less than 0.5% (the bending radius is 5 mm). The magnitude of the critical current of Type D wire annealed at 800°C was lower than that of the wire annealed at 850°C, but it was only 23% lower than that of the straight line when the bending radius was 15 mm. Therefore, we expect that the critical current and bend-ing properties of Nb3Al wires can be improved by optimizing the annealing process. C. Jc Performance of the Developed Nb3Al Wires To demonstrate the performance of the developed Nb3Al wire, a comparison of critical current densities of Nb3Al, REBCO, NbTi and Nb3Sn wires at 4.2 K and 0 T is shown in Fig. 13. The critical current density of the REBCO wire was taken from reference [6]. The thickness of the REBCO wire is 130 µm, the width is 4 mm and the thickness of the superconducting layer is 1.9 µm. The critical current densities of NbTi and Nb3Sn wires were taken from the reference [7] and [8] and fitted to the comparison conditions. The critical current densities of Nb3Al wires at 0 T and 4.2 K are slightly higher than that of commercial NbTi and Nb3Sn wires, but much lower than that of REBCO wires. As shown in Fig. 14, the critical current density in magnetic field of the developed ultrafine Nb3Al wire is more than one order of magnitude lower than that of the REBCO wire. Therefore, further studies are needed to improve the critical current characteristics and flexibility of ul-trafine Nb3Al wires fabricated by the Jelly-rolled method. V. CONCLUSION In this study, the critical current properties of Nb3Al wires with different cross-sectional structures fabricated by the Jelly-rolled method were investigated using a conduction cooling system. The current sweep and constant current methods were used to measure the critical current, and the current flowing in the copper wire soldered to the Nb3Al wire to protect the sample wire was estimated to evaluate the correct critical current value. The Nb3Al wire fabricated by the Jelly-rolled method showed improved critical current by reducing the wire diameter and the reaction distance between Nb and Al. The usefulness of the copper core used for the long length wire and flexibility of the Nb3Al wire was confirmed. The critical current densities of Nb3Al wires are slightly higher than that of commercial NbTi and Nb3Sn wires, but much lower than that of REBCO wires. So, it is necessary to improve the Jc and Jc-B characteristics of Nb3Al wires.  REFERENCES [1] T. Takeuchi et al., “Status and perspective of the Nb3Al development.” Cryogenics, vol. 48, p.371-380, 2008. [2] N. Banno, D. Uglietti, B. Seeber, T. Takeuchi, R. Flükiger, “Strain de-pendence of superconducting characteristics in technical Nb3Al supercon-ductors.” Supercond. Sci. Technol., vol. 18, p. 284-288, 2005. [3] A. Kikuchi et al., “Cu stabilized Nb3Al strands for the high field accelerator magnet.” IEEE Trans. Appl. Supercond., vol. 18, p.1026-1030, 2008. [4] A. Kikuchi et al., “Trial manufacturing of Jelly-Rolled Nb/Al monofila-mentary wire with very small diameter below 50 microns.” IOP Conf. Ser., Mater. Sci. Eng. 756 art no. 012016, 2020. Fig. 12. The measured temperature dependence of the critical current of Type D wire with the straight line and bending radius of 7.5, 10 and 15 mm. 024681011 12 13 14 15Critical current (A)Tempreture (K)Straight line15 mm (0.17%)10 mm (0.25%)7.5 mm (0.33%)Fig. 13. The comparison of critical current density of Nb3Al, REBCO, NbTi and Nb3Sn wires at 4.2 K and 0 T. 05E+101E+111.5E+112E+112.5E+11Critical current density (A/m2 )Type C Type DFig. 14. The critical current densities in magnetic field of developed Nb3Al wires and REBCO wire at 4.2 K. 1.0E+061.0E+071.0E+081.0E+091.0E+101.0E+111.0E+124 6 8 10 12 14 16 18Critical current density (A/m2 )Magnetic Field (T)REBCO (pararell)REBCO (perpendicular)Type DType C (33φ)5 [5] S.B. Kim et al., “Critical characteristics of ultrafine Nb3Al superconducting wires under conduction cooling conditions.” IEEE Trans. Appl. Supercond., vol. 32, 6001005, 2022. [6] https://www.fujikura.co.jp/eng/products/newbusiness/superconductors/01/superconductor.pdf (Products No. “FYSC-SCH04”) [7] T. Boutboul et al., “Critical current density in superconducting Nb-Ti strands in the 100 mT to 11 T applied field range.” Trans. Appl. Supercond., vol.16, no.2, pp.1184-1187, June 2006.  [8] https://www.csj.or.jp/handbook/index.html