122 type pnictide superconductors are of particular interest for high-field applications because of their large upper critical fields H c2 (> 100 T) and low anisotropy γ (<2). Successful magnet applications require fabrication of polycrystalline superconducting wires that exhibit large critical current density J c , which is limited by poor grain coupling and weak-link behavior at grain boundaries.Here we report our recent achievement in the developing Sr 0.6 K 0.4 Fe 2 As 2 tapes with transport J c up to 0.1 MA/cm 2 at 10 T and 4.2 K. This value is by far the highest ever recorded for iron based superconducting wires and has surpassed the threshold for practical application. The synergy effects of enhanced grain connectivity, alleviation of the weak-link behavior at grain boundaries, and the strong intrinsic pinning characteristics led to the superior J c performance exhibited in our samples. This advanced J c result opens up the possibility for iron-pnictide superconducting wires to win the race in high-field magnet applications.

The high upper critical field and low anisotropy of iron-based superconductors make them being particularly attractive for high-field applications. However, the current carrying capability needs to be enhanced by overcoming the weak-link effect between misaligned grains inside wire and tape conductors. Here we demonstrate a high transport critical current density (Jc) reaching 1.5  10 5 A/cm 2 (Ic = 437 A) at 4.2 K and 10 T in Ba0.6K0.4Fe2As2 (Ba-122) tapes prepared by a combination of conventional powder-in-tube method and optimized hot-press technique. The transport Jc measured 2 at 4.2 K under high magnetic fields of 27 T is still on the level of 5.5  10 4 A/cm 2 , which is much higher than those of low-temperature superconductors. This is the first report of hot-pressed Ba-122 superconducting tapes and these Jc values are by far the highest ever reported for iron-based superconducting wires and tapes. These highperformance tapes exhibit high degree of c-axis texture of Ba-122 grains and low anisotropy of transport Jc, showing great potential for construction of high-field superconducting magnets.

High-performance Sr0.6K0.4Fe2As2 (Sr-122) tapes have been successfully fabricated using hot pressing (HP) process. The effect of HP temperatures (850–925°C) on the c-axis texture, resistivity, Vickers micro-hardness, microstructure and critical current properties has been systematically studied. Taking advantage of high degree of c-axis texture, well grain connectivity and large concentration of strong-pinning defects, we are able to obtain an excellent Jc of 1.2 × 105 A/cm2 at 4.2 K and 10 T for Sr-122 tapes. More importantly, the field dependence of Jc turns out to be very weak, such that in 14 T the Jc still remains ~ 1.0 × 105 A/cm2. These Jc values are the highest ever reported so far for iron-pnictide wires and tapes, achieving the level desired for practical applications. Our results clearly strengthen the position of iron-pnictide conductors as a competitor to the conventional and MgB2 superconductors for high field applications.

Iron-based superconductors (IBSs), discovered in 2008, formed the second high-Tc superconductor family after cuprate superconductors, and over the past decade have been the subject of extensive research into their physical nature and application potential. With their attractions of very high upper critical fields and small electromagnetic anisotropy, tremendous advances have been made in wire research and development (R&D) to explore the potential of IBSs for high-field applications. In recent years, rapid progress was made on the critical current density (Jc) of the 122-type IBS wires based on a powder-in-tube technique. Encouraging breakthroughs were made, including a high transport Jc exceeding the practical level of 105 A cm−2 (at 4.2 K, 10 T) and the first 100 meter-class wire. This review covers the state-of-the-art techniques and their mechanism in realizing high transport Jc with respect to the grain connectivity, grain texture and flux pinning for IBS wires and tapes, as well as the temperature and field angle dependence of critical currents. The mechanical properties, AC losses and magneto-thermal stability of IBS wires are investigated, and further improvements in IBS conductors for large-scale applications are proposed. In addition to long wire fabrication, this review also highlights some remarkable advances relevant to practical applications, including scalable process optimization, copper sheaths, multifilamentary fabrication, and superconducting joints. Finally, a summary and outlook for R&D for IBS wires are presented.

Abstract:We report the realization of grain alignment in Sn-added Sr 1-x K x Fe 2 As 2 superconducting tapes with Fe sheath prepared by ex-situ powder-in-tube method. At 4.2 K, high transport critical current densities J c of 2.5×10 4 A/cm 2 (I c = 180 A) in self-field and 3.5×10 3 A/cm 2 (I c = 25.5 A) in 10 T have been measured. These values are the highest ever reported so far for Fe-based superconducting wires and tapes. We believe the superior J c in our tape samples are due to well textured grains and strengthened intergrain coupling achieved by Sn addition. Our results demonstrated an encouraging prospect for application of iron based superconductors.* Author to whom correspondence should be addressed; E-mail: ywma@mail.iee.ac.cn 2 The discovery of superconductivity in LaFeAsO 1-x F x and related compounds, with a higher transition temperature, T c of ~55 K, has triggered great research interests from both theoretical and applied aspects [1][2][3][4][5][6][7]. In addition to T c , these iron based superconductors were reported to have a very high upper critical field, B c2 , and low B c2 anisotropy, making them potential candidates for a wide array of future applications [8][9][10][11]. The early results indicate that the global critical current is limited by intergrain currents over the grain boundaries in polycrystalline bulk and wires [12][13][14][15][16][17][18]. Recent experiments revealed that high-angle grain boundary largely deteriorates critical current density J c in Ba(Fe 1-x Co x ) 2 As 2 bicrystals [19,20]. These results suggest that the discovered iron based superconductors are exhibiting weak link grain boundary behavior similar to high-T c cuprate superconductors. An effective method to overcome the weak link problem is to engineering textured grains in iron based superconductors to minimize deterioration of the critical current density across high-angle grain boundaries.Recently Co-doped Ba122 coated conductors have been grown by several groups utilizing the existing YBCO coated conductor technology and have reached a self-field J c over 1 MA/cm 2 [21][22][23]. However, the technology has the shortcoming of low production rate, complexity and high equipment cost. Furthermore, it is hard to be applied to the volatile elements in iron based superconductors such as alkali metals doped 122 phase and F doped 1111 phase. Cold deformation process is a well-developed technique used to enhance the degree of grain alignment and critical current density of Bi2223 superconductors [24,25]. It is unsurprising that this technique may also be suitable for iron based superconductors too. Besides grain alignment, adding metallic elements is another effective way to improve the grain connectivity of the iron based superconductors. For example, we have reported that the superconducting properties of the 122 phase iron based superconductor can be significantly increased by Ag or Pb addition [26,27]. Recently Togano et al [28] reported further improvement in transport critical current in the Ba122 wires with A...

Improving transport current has been the primary topic for practical application of superconducting wires and tapes. However, the porous nature of powder-in-tube (PIT) processed iron-based tapes is one of the important reasons for low critical current density (Jc) values. In this work, the superconducting core density of ex-situ Sr0.6K0.4Fe2As2 + Sn tapes, prepared from optimized precursors, was significantly improved by employing a simple hot pressing as an alternative route for final sintering. The resulting samples exhibited optimal critical temperature (Tc), sharp resistive transition, small resistivity and high Vickers hardness (Hv) value. Consequently, the transport Jc reached excellent values of 5.1 × 104 A/cm2 in 10 T and 4.3 × 104 A/cm2 in 14 T at 4.2 K, respectively. Our tapes also exhibited high upper critical field Hc2 and almost field-independent Jc. These results clearly demonstrate that PIT pnictide wire conductors are very promising for high-field magnet applications.

From the application point of view, large critical current densities Jc (H) for superconducting wires are required, preferably for magnetic fields higher than 5 T. Here we show that strong c-axis textured Sr1−xKxFe2As2 tapes with nearly isotropic transport Jc were fabricated by an ex-situ powder-in-tube (PIT) process. At 4.2 K, the Jc values show extremely weak magnetic field dependence and reach high values of 1.7×104 A/cm2 at 10 T and 1.4×104 A/cm2 at 14 T, respectively, these values are by far the highest ever reported for iron based wires and approach the Jc level desired for practical applications. Transmission electron microscopy investigations revealed that amorphous oxide layers at grain boundaries were significantly reduced by Sn addition which resulted in greatly improved intergranular connectivity. Our results demonstrated the strong potential of using iron based superconductors for high field applications.

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