Filament Microstructure and Superconducting Properties of MgB2 Wires With Thin Filaments

Technical 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 mum 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/cm² at 3 Tesla for undoped and 100 kA/cm² at 6 Tesla for SiC doped wires with only 164 mum outside diameter and 65 mum filament diameter.