The fabrication of iron-based superconductors with high transport critical current density (J c ) and low cost is a crucial determinant of whether they can be used for practical applications. In this paper, Cu/Ag composite sheathed Sr 0.6 K 0.4 Fe 2 As 2 (Sr122) tapes and Ba 0.6 K 0.4 Fe 2 As 2 (Ba122) round wires were fabricated via an ex situ powder-in-tube method and heat-treated by the hot pressing and hot isostatic pressing process respectively. In order to thoroughly reveal the application potential of Cu/Ag composite sheathed '122' iron pnictide superconductors, transport J c of tapes and wires in high fields at temperatures up to 25 K was measured. High transport J c of 4.4×10 4 A cm −2 at 4.2 K and 3.6×10 3 A cm −2 at 20 K in 10 T was achieved in Cu/Ag composite sheathed Sr122 tapes. Transport J c of Ba122 wires is 9.4×10 3 A cm −2 at 4.2 K and 1.9×10 3 A cm −2 at 20 K in 10 T. These results demonstrate the great potential of Cu/Ag composite sheathed '122' iron pnictide superconducting tapes and wires for high-field applications at intermediate temperatures around 20 K, which can be easily obtained by using cryocoolers.

Superconducting joints are essential for iron-based superconductor's applications in future. In this study, a process for fabricating superconducting joints between Sr1-xKxFe2As2 (Sr-122) tapes is developed for the first time. The Ag sheath was peeled off from one side of each sample. The exposed superconducting parts of the two tapes were joined and wrapped again with Ag foil. The diffusion bonding of the iron-based superconducting joint was achieved by hot-pressing process in Argon atmosphere. The superconducting properties, microstructures and the elements distribution of the joint regions had been investigated. The pressure and pressing times were optimized in order to enhance the transport current of the joints. At 4.2 K and 10 T, a transport critical current Ic of 57 A for the joint was obtained, which is approximately 63.3% of the current capacity of the tapes themselves. Furthermore, the joint resistances dV/dI were estimated from the V-I curve of the joints and the calculated joint resistances values are below 10 -9 Ω. These results demonstrate that the hot pressing was useful for fabricating the superconducting joint samples.

Ag-sheathed CaKFe4As4 superconducting tapes have been fabricated via the ex-situ powder-in-tube method. Thermal and X-ray diffraction analyses suggest that the CaKFe4As4 phase is unstable at high temperatures. It decomposes into the CaAgAs phase which reacts strongly with the silver sheath. We therefore sintered the tape at 500 °C and obtain a transport critical current density Jc(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-xKxFe2As2 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 Jc. However, this system is still a promising candidate for superconducting wires and tapes if we further optimize the post-annealing process to achieve better grain connectivity.

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 Ba0.6K0.4Fe2As2 (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 (Jc) up to 5.8 × 104 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.

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 .