High critical currents in iron-clad superconducting MgB2 wires

Technically useful bulk superconductors must have high transport critical c urrent densities, Jc, at operating temperatures. They also require a normal metal cladding to provide parallel electrical conduction, thermal stabiliz ation, and mechanical protection of the generally brittle superconductor co res. The recent discovery of superconductivity at 39 K in magnesium diborid e (MgB2)(1) presents a new possibility for significant bulk applications(2- 5), but many critical issues relevant for practical wires remain unresolved . In particular, MgB2 is mechanically hard and brittle and therefore not am enable to drawing into the desired fine-wire geometry. Even the synthesis o f moderately dense, bulk MgB2 attaining 39 K superconductivity is a challen ge because of the volatility and reactivity of magnesium. Here we report th e successful fabrication of dense, metal-clad superconducting MgB2 wires, a nd demonstrate a transport Jc in excess of 85,000 A cm(-2) at 4.2 K. Our ir on-clad fabrication technique takes place at ambient pressure, yet produces dense MgB2 with little loss of stoichiometry. While searching for a suitab le cladding material, we found that other materials dramatically reduced th e critical current, showing that although MgB2 itself does not show the 'we ak-link' effect characteristic of the high-Tc superconductors, contaminatio n does result in weak-link-like behaviour.