Influence of Ni and Cu contamination on the superconducting properties ofMgB2 filaments

Abstract

Technical MgB2 wires usually have a sheath composite consisting of different metals. For the inner sheathwith direct contact to the superconducting filament, chemically inert Nb maybe used as a reaction barrier and thermal stabilization is provided by a highlyconductive metal like Cu. A mechanical reinforcement can be achieved by theaddition of stainless steel. In order to illuminate the influence of defects in thereaction barrier, monofilament in situ wires with direct contact between theMgB2 filament and frequently applied reactive sheath metals like Cu, Ni or Monel are studied. Reactionsof Mg and B with a Cu-containing sheath lead to Cu-based by-products penetrating the wholefilament. Reactions with Ni-containing sheaths lead to Ni-based by-products which tend toremain at the filament–sheath interface. Cu and/or Ni contamination of the filament lowers theMgB2-forming temperature due to the eutectic reaction between Mg, Niand Cu. Thus, for the samples heat-treated at low temperaturesJC and(partly) TC are increased compared to stainless-steel-sheathed wires. At high heattreatment temperatures uncontaminated filaments lead to the highestJC values. From the point of view of broken reaction barriers in real wires, the contaminationof the filament with Cu and/or Ni does not necessarily constrain the superconductivity; itmay even improve the properties of the wire, depending on the desired application.