Critical current density and microstructure of iron sheathed multifilamentary Sr1−xKxFe2As2/Ag composite conductors

Abstract

Iron-based superconductors have been considered to be very promising in high-field applications for which multifilamentary wire and tape conductors with high mechanical strength are essential. In this work, 7-,19-, and 114-filament Sr0.6K0.4Fe2As2 (Sr-122) superconducting wires and tapes with silver as matrix and iron as outer reinforcing sheath were produced by the ex situ powder-in-tube method. The mass densities of Sr-122 phase in 7- and 19-filament conductors were investigated by microhardness characterization, which revealed a positive correlation between hardness and transport critical current density (Jc) in round wires and flat tapes with various thicknesses. For Sr-122/Ag/Fe 114-filament conductors, in which an average cross-sectional filament size smaller than 50 μm was achieved by drawing into round wires of 2.0 mm in diameter, the transport Jc can be significantly enhanced by flat rolling, as for the 7- and 19-filament conductors. The highest transport Jc for Sr-122 filaments in the 7-, 19-, and 114-filament Sr-122/Ag/Fe tapes reached 1.4 × 104, 8.4 × 103, and 6.3 × 103 A cm−2 (4.2 K, 10 T), respectively, showing a Jc degradation with the increase of filament number. This Jc degradation can be ascribed to the sausage effect for SR-122 filaments in longitudinal direction and the grain refinement in these very fine filaments.