The high upper critical field and low anisotropy of iron-based superconductors (IBS) make them particularly attractive for high-field applications, especially for the construction of next-generation nuclear magnetic resonance spectrometers, particle accelerators and high-field magnets. However, for practical use it is essential to make IBS materials into wire and tape conductors with sufficient current carrying capability, which is limited by misaligned grains inside the conductors. Here, based on a simple and low-cost powder-in-tube (PIT) method, we demonstrate a high transport critical current density ( J _c ) reaching 1.5 × 10 ⁵ A cm ⁻² ( I _c = 437 A) at 4.2 K and 10 T in Ba _0.6 K _0.4 Fe ₂ As ₂ (Ba-122) tapes by texturing the grain orientation with optimized hot-press technique. The transport J _c measured at 4.2 K under high magnetic fields of 27 T is still on the level of 5.5 × 10 ⁴ A cm ^−2. Moreover, at 20 K and 5 T the transport J _c is also as high as 5.4 × 10 ⁴ A cm ⁻² , showing a promising application potential in moderate temperature range which can be reached by liquid hydrogen or cryogenic cooling. All these J _c values are the highest ever reported for IBS wires and tapes. The high-performance PIT Ba-122 tapes in this work suggest IBS to be a strong potential competitor of cuprate superconductors for the race of high-field applications in the future.

A 30 mm inner diameter iron-based superconductor (IBS) single pancake coil (SPC) was firstly fabricated and tested under a 24 T background field. This SPC was successfully made using the seven-filamentary Ba _1−x K _x Fe ₂ As ₂ (Ba122) tape by the wind-and-react method. This IBS coil shows the highest I _c value at a magnetic field reported so far. For example, the transport critical current of this Ba122 SPC achieved 35 A at 4.2 K and 10 T, which is about half of that of a short sample. This indicates that the non-insulation winding process together with the stainless steel tape is suitable for an IBS. Even more encouraging is the fact that the I _c of this SPC is still as high as 26 A under a 24 T background field, which is still about 40% of that at zero external magnetic field. These results clearly demonstrate that IBSs are very promising for high-field magnet applications.

Iron-based superconductors have become a promising candidate for high-field applications due to their high transition temperature, ultra-high upper critical field, and low anisotropy. For practical applications, it is important to improve the transport J _c and lower the cost of superconductors. In this paper, with groove rolling and hot isostatic pressing (HIP) process, the transport J _c of the Cu/Ag composite sheathed (Ba, K)Fe ₂ As ₂ powder-in-tube round wires was increased to 4.7 × 10 ⁴ A cm ⁻² at 10 T and 4.2 K. The utilization of HIP and groove rolling process improved the mass density of the wire. Moreover, a certain axial texture was introduced through groove rolling. It is suggested that the combination of groove rolling and HIP processes are of great help to improve the microstructure of the wires, thereby obtaining a higher transport J _c .

Understanding the mechanisms that limit the current transport in granular iron-based superconductors is necessary to unlock their potential for magnet applications. This paper reports the transport critical current surface I _C ( T,H ) of powder-in-tube Ag/Ba _1−x K _x Fe ₂ As ₂ tapes densified by flat rolling and uniaxial hot pressing. We investigate the amplitude of the isothermal- I _C ( H )-curve magnetic hysteresis, which is a marker for the electrical connectivity between superconducting grains. This allows us to demonstrate that, in spite of an improved connectivity, the percolative path through weakly linked grains continues to play a role in the I _C properties of hot-pressed conductors. The limitations to the current transport due to weak links depend on the orientation of the magnetic field and are more severe when H is on the tape plane. This finding provides a justification to the puzzling evidence that I _C ( H ) can be smaller for magnetic fields applied on the tape plane and not perpendicular to it. Finally, we report an unexpected increase of I _C upon augmenting the magnetic field from 0 to ∼2 T. SQUID measurements reveal a superparamagnetic-like magnetic background that takes place in the same range of magnetic fields. A microscopic study of the conductor-core elemental composition provides evidences that these features can be both related to the presence of Fe-rich non-stoichiometric regions in the superconducting core. On a more general note, this study points out that further advances in the in-field current transport properties of powder-in-tube Ag/Ba _1−x K _x Fe ₂ As ₂ conductors are possible and rely to a large extent on solutions to improve the inter-grain connectivity and the quality of the superconducting phase.

Iron-based superconductors (IBSs) with ultrahigh upper critical fields and low anisotropies have attracted significant attention in terms of the novel mechanism of superconductivity and high-field applications. A major concern for practical research is the fabrication of long wires with enhanced critical current density and low cost. In this paper, Cu/Ag composited sheathed Ba _0.6 K _0.4 Fe ₂ As ₂ (Ba122) tapes were fabricated through a hot isostatic pressing method, which is feasible for long-wire manufacturing. The Cu/Ag composite sheath can lower the tape cost by reducing the use of expensive Ag. A high-transport critical current density ( J _c ) up to 5.8 × 10 ⁴ A cm ⁻² under 10 T at 4.2 K was achieved in our tapes. Evidence has shown that these tapes have pure Ba122 phase, homogeneous element distribution, orientated grains and good grain connectivity. Our work shows that low-cost Cu/Ag-sheathed IBS tapes have great promise for practical applications in the future.

Ag-sheathed CaKFe ₄ As ₄ superconducting tapes have been fabricated via the ex situ powder-in-tube method. X-ray diffraction analyses suggest that the CaKFe ₄ As ₄ phase reacts strongly with the silver sheath at temperature above 500 °C. We therefore anneal the tape at 500 °C and obtain a transport critical current density J _c (4.2 K, 0 T) ∼ 2.7 × 10 ⁴ A cm ⁻² . The pinning potential derived from magnetoresistance measurements is one order of magnitude lower than that of the (Ba/Sr) _{1− x} K _x Fe ₂ As ₂ tapes. Combining with the scanning electron microscopy and magneto-optical imaging results, we suggest that bad connectivity between superconducting grains caused by the low sintering temperature is the main factor responsible for the low J _c . However, this system is still a promising candidate for superconducting wires and tapes if we find a compatible sheath material and further optimize the post-annealing process to achieve better grain connectivity.