Recent progress of 122-type iron-based superconducting wires and tapes
Authors: Xu Guang-Xian  , Huang He  , Zhang Xian-Ping  , Huang Shang-Yu  , Ma Yan-Wei  ,   Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China;,   School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

With high transition temperature Tc (~38 K), high upper critical field Hc2 ( 100 T), superior transport Jc (~106 A/cm2) and extremely small anisotropy (1.5-2.0), the 122-type iron-based superconductors show great promise in high-field applications such as next-generation high energy physics accelerator and high-field magnetic resonance imaging (MRI). Power-in-tube (PIT) method is widely adopted to fabricate the iron-based superconducting wires and tapes due to low cost and easiness of large-scale fabrication. In the past few years, substantial efforts have been made to improve the transport performances of 122-type iron-based superconducting wires and tapes by ex-situ PIT technique. In this review, the recent progress of 122-type iron-based superconducting wires and tapes is presented. Firstly, we focus on the techniques for fabricating high-performance 122-type wires and tapes. We also discuss the key factors affecting the final performances of wires and tapes during the PIT process, including the preparation of high-quality precursor, the effect of chemical doping, the improvement of core density and grain connection. Recently, due to the improving of degree of c-axis texture and connectivity of grains, the transport Jc value of 122/Ag tapes reached 1.5105 A/cm2 at 4.2 K and 10 T, which exceeds the practical level of 105 A/cm2 and demonstrates their promise in high-field applications. Then, the progress of practical application of 122-type wires and tapes is summarized. In order to reduce the fabrication cost and improve the mechanical strengths of superconducting wires and tapes, an additional outer sheath such as Fe, Cu and stainless steel was used in combination with Ag. Besides, a favourable transport Jc was also obtained in the Cu-, or Fe-sheathed 122 tapes. For round wires, the highest Jc value reached 3.8104 A/cm2 in Cu/Ag composite sheathed wires at 4.2 K and 10 T, obtained by the hot-isostatic-press technology. From the viewpoint of practicality, the fabrication of multifilamentary wires and tapes is an indispensable step. The 7-, 19-and 114-filament 122 wires and tapes were successfully fabricated by the PIT method, and these multifilamentary tapes exhibited weak field dependence of Jc. Based on the experience of high-performance short samples and multifilamentary wires process, the scalable rolling process has been used to produce the first 115-m-long 7-filament Sr1-xKxFe2As2/Ag superconducting tape, confirming the great potential for large-scale manufacture. Moreover, the mechanical property, anisotropy and superconducting joint of 122 tapes are also studied. Finally, a perspective for the future development of 122-type wires and tapes in practical applications is given.