Improved transport critical current and irreversibility fields in mono- and multifilamentary Fe/MgB₂tapes and wires using fine powders

Abstract

Mono- and multifilamentary Fe/MgB2 tapes and wires with high transport critical current densities have been prepared by the powder-in-tube technique using fine powders. The influence of the initial MgB2 grain size on critical current density, upper critical and irreversibility fields has been studied. After reducing the MgB2 grains to micrometer size by ball milling, the critical current density, Jc, was enhanced, while the upper critical field, μ0Hc2, remained unchanged. The anisotropy ratio between the upper critical fields parallel and perpendicular to the tape surface was determined to be 1.3, reflecting a deformation induced texture. A good agreement has been found between resistive and inductive Jc values, measured at various temperatures between 4.2 and 25 K. On monofilamentary tapes, Jc values close to 105 A cm−2 were measured at 25 K/1 T, while Jc values ≈106 A cm−2 were extrapolated for 4.2 K/0 T. Fe/MgB2 tapes exhibit high exponential n factors for the resistive transition: n values of 60 and 30 were found at 4 T and 6 T, respectively. Multifilamentary wires (with seven filaments) show slightly lower Jc values, 1.1 × 105 A cm−2 at 4.2 K/2 T. The improvement of thermal and mechanical stability of MgB2/Fe tapes and wires appears clearly as a challenge for future developments.