Transport critical current density of highstrength Sr_1-xK_xFe₂As₂/Ag/Monel composite conductors

Iron-based superconductors are very promising candidates for high-field applications owning to their ultrahigh upper critical field (H_c2) and very small anisotropy. For practical application, wires and tapes with high mechanical strength are essential. In this work, using Monel alloy as an outer reinforcing sheath, 7-filament Sr_1-xK_xFe₂As₂/Ag/Monel composite tapes with various thicknesses were developed based on the powder-in-tube method. Through microhardness characterization, it is found that the transport critical current density (J_c) of the tapes has a heavy dependence on the mass density of the Sr-122 phase, which varies widely according to the coldwork parameters and heat treatment temperatures for the tapes. In large-scale applications, conductors are usually designed to work under compressive state for safety, and so the compressive strain dependence of transport J_c was investigated for our tapes, which shows almost no J_c degradation under a large compressive strain of 0.6%. The transport J_c for the rolled tapes can be further significantly enhanced to 3.6 × 10₄ A cm_-2 (4.2 K, 10 T) by a hot-press process. These results suggest the great potential of Monel/Ag composite sheath for developing high-strength and high J_c performance iron-based superconducting wires and tapes for high-field applications.