Filament microstructure and superconducting properties of MgB2 wires with thin filaments

Technical MgB2 conductors for magnet applications require a highly conductive sheath component and multifilament geometry to ensure sufficient thermal stability. As the filament size of such wires is in the order of 100 mu m and below, impurity phases can easily lead to a decrease of the current carrying capability. Not only the quality of precursor powders plays an important role in achieving homogeneous filaments, but also reaction products emerging during heat treatments. With decreasing filament diameter the influence of impurity phases on the current carrying capability becomes more and more pronounced. In this paper we present filament microstructures and superconducting properties of mono- and multifilamentary MgB2, wires with thin filaments. In SiC doped multifilament wires with Fe sheath and without Nb barrier a thick reaction layer emerged during heat treatments and changed the composition of the filament close to the sheath. These conductors showed a strong degradation of J(c) (B) with decreasing filament diameter. We show that a Nb barrier can easily prevent reaction layers at the filament sheath interface. For undoped and SiC doped monofilament wires with Nb barrier, Cu stabilizer and stainless steel (SS) reinforcement, J(c) values even increased with decreasing filament diameter. We obtained J(c) as high as 387 kA /cm(2) at 3 Tesla for undoped and 100 kA/cm(2) at 6 Tesla for SiC doped wires with only 164 mu m outside diameter and 65 mu m filament diameter.