Superconducting properties and intergranular coupling of high-strength Sr1-xKxFe2As2/Ag/Nb/Cu composite conductors

Abstract

The mechanical properties of composite metal sheaths have large difference, resulting in the easy breakage of superconducting wires. Here, a roll-die drawing method is proposed to fabricate 19-filamentary Sr1-xKxFe2As2/Ag/Nb/Cu composite tapes with long length and uniform shape. It is found that there is no reaction layer between Ag matrix and superconducting core; Nb sheath provides high mechanical strength and prevents the diffusion of K element; and outer Cu sheath has good mechanical ductility. Consequently, the magnetic critical current density Jc of multi-core Sr1-xKxFe2As2 tape reaches up to 4.7 × 104 A/cm2 at 5 K and 5 T. Meanwhile, the magnetic Jc still remains 1.1 × 104 A/cm2 even at 20 K and 5 T. These remarkable magnetic Jc values are probably attributed to both high mass density of superconducting core and high magnetic onset Tc (∼35.2 K). While, the transport Jc of multi-core tape is greatly decreased into about 2.0 × 103 A/cm2 at 4.2 K and 10 T. Besides the Tp1 peak associated with the intragranular critical current, an additional broad Tp2 peak in the imaginary χ''(T) parts of AC susceptibility indicates the weak intergranular coupling in this tape. These results can provide a prime reference for the structure design and coupling performance of multi-core pnictide conductors.