High critical currents in iron-clad superconducting MgB2 wires

Technically useful bulk superconductors must have high transport critical current densities, J c , at operating temperatures. They also require a normal metal cladding to provide parallel electrical conduction, thermal stabilization, and mechanical protection of the generally brittle superconductor cores. The recent discovery of superconductivity at 39 K in magnesium diboride (MgB 2 ) 1 presents a new possibility for significant bulk applications 2 , 3 , 4 , 5 , but many critical issues relevant for practical wires remain unresolved. In particular, MgB 2 is mechanically hard and brittle and therefore not amenable to drawing into the desired fine-wire geometry. Even the synthesis of moderately dense, bulk MgB 2 attaining 39 K superconductivity is a challenge because of the volatility and reactivity of magnesium. Here we report the successful fabrication of dense, metal-clad superconducting MgB 2 wires, and demonstrate a transport J c in excess of 85,000 A cm -2 at 4.2 K. Our iron-clad fabrication technique takes place at ambient pressure, yet produces dense MgB 2 with little loss of stoichiometry. While searching for a suitable cladding material, we found that other materials dramatically reduced the critical current, showing that although MgB 2 itself does not show the ‘weak-link’ effect characteristic of the high- T c superconductors, contamination does result in weak-link-like behaviour.