Fabrication of superconducting coils and (Ba,A)Fe₂As₂ (A:Na, K) long round wires with large superconducting cores

Abstract

We fabricated superconducting coils using 10-20 m-class round wires of 122-type iron-based superconductors (IBSs). Round wires and coils are fabricated by powder-in-tube method and hot-isostatic-press technique. Transport critical current (I c) of the whole (Ba,K)Fe2As2 coil is 46 A under the self-field at 4.2 K, and magnetic field at the center of the coil reaches 0.3 T. Although the edge of the long wire in the coil is damaged, the rest of the part is relatively homogeneous. Furthermore, the largest transport critical current density (J c) and I c in (Ba,K)Fe2As2 wires picked up from the coil reach 49 kAcm−2 and 44 A at 4.2 K under a magnetic field of 10 T, respectively. This value exceeds the previous highest transport J c of (Ba,K)Fe2As2 round wires. We also fabricated a (Ba,Na)Fe2As2 coil using long round wire with large superconducting core by react and wind method. Transport I c of the coil is significantly low due to cracks perpendicular to electric current flow direction, although magnetic J c in the round wire picked up from the coil reaches 40 kAcm−2 at 4.2 K under 4 T.

X-ray Characterizations of Iron-based Superconductor Round Wires with Large Critical Current Density

“…In addition, a good correlation between the degree of texturing and Jc was also discovered [8]. Since 2020, we started the preparation of long (Ba,A)Fe2As2 wires over 10 m, and constructed some small coils to generate magnetic fields [9,5,10]. Critical currents (Ic) of these long wires was smaller than that in short segments picked up from the long wire, and vary considerably along the length of the wire.…”

High critical current density in low-cost iron-based superconducting round wires annealed at ambient pressure

“…High isotropic pressure up to 200 MPa results in the radial contraction of the whole wire and an almost 100% dense superconducting filament. Since then, the HIP method has gained widespread acceptance as an essential approach for the post-annealing of both the round wires and the solenoid coils made from them [20][21][22][23]. Nevertheless, it was soon recognized that the core density is not the only necessity for high J c [24,25].…”

Fabrication of multi-filament(Ba,A)Fe2As2 (A: Na, K) HIP round wires and a small superconducting coil

“…In hot-isostatic pressed (HIP) tapes, a J c under 100 kOe also exceeded the practical level of 100 kAcm −2 [24]. On the other hand, the highest transport J c of round wires at 4.2 K under 100 kOe are 54 and 49 kAcm −2 in (Ba,Na)Fe 2 As 2 and (Ba,K)Fe 2 As 2 HIP wires, respectively [29,31]. Compared with tapes, in which higher texturing of superconducting grains in the core can be realized easily [21], texturing in wires is limited [26,27].…”

“…For further enhancement of the J c in round wires, which are more suitable for winding coils, an improvement in texturing, an increase in the density of the superconducting core, and a reduction in impurities are demanded. On the other hand, other aspects of tape and/or wire developments besides enhancement of the J c have been studied, such as multi-filament wire fabrication [32][33][34], fabrication of coils using long wires or tapes [29,31,[35][36][37][38][39], making joints between tapes [40][41][42], evaluation of the strain effect on the J c [7,43], and low cost fabrication using a Cu or Cu/Ag composite metal sheath [44][45][46]. In particular, fabrication of multi-filament wires and tapes is an indispensable process toward applications, to provide protection against magnetic flux jumps and thermal quenching [47], and to reduce the alternating current (AC) loss induced by the eddy currents in the metallic matrix and the inter-filament coupling currents [2].…”

“…Furthermore, the fabrications of 100 m class multi-filament tapes and superconducting coils using those tapes were one of the significant achievements that were made toward high-magnetic-field applications, although the generation of a magnetic field has not been reported [35]. Recently, we have reported the fabrication of superconducting magnets using 10 m class mono-core round wires and evaluation of the generated field [29,31,39]. As a next step, fabrication of superconducting coils using multi-filament round wires and their characterizations are required.…”