Presentation Information
[WBP1-06]Evaluation of Superconducting Transport Properties in (Ba,Na)Fe2As2 Round Wires with Harder Sheath Materials
*Ryosuke Sakagami1,2, Tsuyoshi Tamegai2,3, Junyi Luo1, Akihiro Kikuchi3, Masaru Yamamoto4, Satoshi Awaji1 (1. IMR, Tohoku University (Japan), 2. The University of Tokyo (Japan), 3. National Institute for Materials Science (NIMS) (Japan), 4. Japan Superconductivity Application Development Inc. (JSA) (Japan))
Keywords:
Iron-based superconductor,Superconducting round wire,Critical current,Fusion
[Purpose]
The 122-type iron-based superconductors (IBSs) exhibit a moderate superconducting transition temperature (Tc) of 38 K and a high upper critical field (µ0Hc2) of 70 T. The 122-type IBSs can be operated not only at 4.2 K but also at ~20 K and are suitable for use in higher magnetic fields. IBS wires can be fabricated via the ex situ powder-in-tube (PIT) method, which is relatively simple, low-cost, and scalable. This method allows the fabrication of not only superconducting tapes but also superconducting round wires. The highest reported transport critical current density (Jc) at 4.2 K under 10 T for IBS tapes and round wires are 260 kA cm−2 in a stainless steel (SS)/Ag/(Ba,K)Fe2As2 tape and 71 kA cm−2 in a Cu/Ag/(Ba,Na)Fe2As2 hot isostatic pressing (HIP) round wire, respectively. 122-type IBS round wires are considered promising due to their potential for low-cost fabrication and strong suitability for various kinds of high-field applications. For example, Bykovskiy et al. proposed the use of 122-type IBS wires for quench detection in fusion magnets, noting that a critical current (Ic) of 10 A under operating conditions would be sufficient. For such applications, it is essential to fabricate IBS round wires as thin and long as possible. Indeed, Hasegawa et al. reported quench-detection tests using Nb–Ti/Cu–Ni round wires with outer diameter (OD) of ~0.1 mm. To this end, we focused on Monel, a Ni–Cu-based alloy that is mechanically harder than Cu. Monel has previously been noted for its effectiveness in fabricating ultrafine superconducting wires. It enabled the fabrication of a Monel/Nb/MgB2 round wire with OD of only 0.015 mm.
In this study, 122-type IBS (Ba,Na)Fe2As2 round wires were fabricated by the ex situ PIT method with Monel/Ag sheaths and drawn to ODs of 0.30, 0.46, 0.68, and 1.0 mm.
[Method]
The 1.0-mm wire was cut into ~30 mm segments in an Ar-filled glove box and heat-treated in vacuum at 700°C for 2 h. After removing the Monel sheath with an etchant (Sunhayato H-200A), a magnetization measurement on a ~4 mm segment was conducted. X-ray computed tomography (CT) measurements were also performed on the specimen to investigate the wire core structure. The magnetic Jc was then calculated using the extended Bean model with the core cross-sectional area (S) obtained by X-ray CT measurements. The transport Ic was measured for another 1.0-mm wire specimen heat-treated at 700°C for 2 h. The estimated transport Jc was calculated as transport Ic / Smin, where Smin is the minimum core cross-sectional area obtained by X-ray CT of the magnetization specimen.
[Results]
Fig. 1(a) shows a certain degree of sausaging (longitudinal nonuniformity in the cross-sectional area of the wire core), but the core remained continuous in the examined region of the 1.0-mm wire specimen. As shown in Fig. 1(b), the magnitudes of the magnetic Jc and estimated transport Jc differ by ~1–2 orders.
[Consideration]
The origin of this difference will be addressed with additional experiments in the presentation.
[Conclusion]
The 122-type iron-based superconductors (IBSs) exhibit a moderate superconducting transition temperature (Tc) of 38 K and a high upper critical field (µ0Hc2) of 70 T. The 122-type IBSs can be operated not only at 4.2 K but also at ~20 K and are suitable for use in higher magnetic fields. IBS wires can be fabricated via the ex situ powder-in-tube (PIT) method, which is relatively simple, low-cost, and scalable. This method allows the fabrication of not only superconducting tapes but also superconducting round wires. The highest reported transport critical current density (Jc) at 4.2 K under 10 T for IBS tapes and round wires are 260 kA cm−2 in a stainless steel (SS)/Ag/(Ba,K)Fe2As2 tape and 71 kA cm−2 in a Cu/Ag/(Ba,Na)Fe2As2 hot isostatic pressing (HIP) round wire, respectively. 122-type IBS round wires are considered promising due to their potential for low-cost fabrication and strong suitability for various kinds of high-field applications. For example, Bykovskiy et al. proposed the use of 122-type IBS wires for quench detection in fusion magnets, noting that a critical current (Ic) of 10 A under operating conditions would be sufficient. For such applications, it is essential to fabricate IBS round wires as thin and long as possible. Indeed, Hasegawa et al. reported quench-detection tests using Nb–Ti/Cu–Ni round wires with outer diameter (OD) of ~0.1 mm. To this end, we focused on Monel, a Ni–Cu-based alloy that is mechanically harder than Cu. Monel has previously been noted for its effectiveness in fabricating ultrafine superconducting wires. It enabled the fabrication of a Monel/Nb/MgB2 round wire with OD of only 0.015 mm.
In this study, 122-type IBS (Ba,Na)Fe2As2 round wires were fabricated by the ex situ PIT method with Monel/Ag sheaths and drawn to ODs of 0.30, 0.46, 0.68, and 1.0 mm.
[Method]
The 1.0-mm wire was cut into ~30 mm segments in an Ar-filled glove box and heat-treated in vacuum at 700°C for 2 h. After removing the Monel sheath with an etchant (Sunhayato H-200A), a magnetization measurement on a ~4 mm segment was conducted. X-ray computed tomography (CT) measurements were also performed on the specimen to investigate the wire core structure. The magnetic Jc was then calculated using the extended Bean model with the core cross-sectional area (S) obtained by X-ray CT measurements. The transport Ic was measured for another 1.0-mm wire specimen heat-treated at 700°C for 2 h. The estimated transport Jc was calculated as transport Ic / Smin, where Smin is the minimum core cross-sectional area obtained by X-ray CT of the magnetization specimen.
[Results]
Fig. 1(a) shows a certain degree of sausaging (longitudinal nonuniformity in the cross-sectional area of the wire core), but the core remained continuous in the examined region of the 1.0-mm wire specimen. As shown in Fig. 1(b), the magnitudes of the magnetic Jc and estimated transport Jc differ by ~1–2 orders.
[Consideration]
The origin of this difference will be addressed with additional experiments in the presentation.
[Conclusion]