Critical current density and microstructure of iron sheathed multifilamentary Sr_(1-x)K_xFe_2As_2/Ag composite conductors

摘要

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 Sr_(0.6)K_(0.4)Fe_2As_2 (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 (J_c) 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 J_c can be significantly enhanced by flat rolling, as for the 7- and 19-filament conductors. The highest transport J_c for Sr-122 filaments in the 7-, 19-, and 114-filament Sr-122/Ag/Fe tapes reached 1.4 × 10~4, 8.4 × 10~3, and 6.3 × 10~3 A cm~(-2) (4.2K, 10T), respectively, showing a J_c degradation with the increase of filament number. This J_c degradation can be ascribed to the sausage effect for SR-122 filaments in longitudinal direction and the grain refinement in these very fine filaments.