High critical currents in iron-clad superconducting MgB2 wires

Abstract

Technically useful bulk superconductors must have high transport critical current densities, Jc, 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 (MgB2) presents a new possibility for significant bulk applications, but many critical issues relevant for practical wires remain unresolved. In particular, MgB2 is mechanically hard and brittle and therefore not amenable to drawing into the desired fine-wire geometry. Even the synthesis of moderately dense, bulk MgB2 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 MgB2 wires, and demonstrate a transport Jc 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 MgB2 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 MgB2 itself does not show the 'weak-link' effect characteristic of the high-Tc superconductors, contamination does result in weak-link-like behaviour.

The relevance of the self-field for the ‘peak effect’ in the transportJc(H) of iron-sheathed MgB2wires

“…The doping improved the value of J c and its field dependence, but there was no obvious influence of the doping on the general shape of the J c (H ). The detailed description of the preparation of these wires can be found in the literature [2,3,19,20].…”

“…However, chemical incompatibility of ferromagnetic materials with HTS was a major problem, impeding the practical implementation of this concept. After the discovery of superconductivity in MgB 2 , it became clear that the best J c for MgB 2 wires can be obtained with Fe as the sheath material [2][3][4][5][6][7]. Because of this, the concept of magnetic shielding as a way of reducing ac loss and improving J c (H ) was reconsidered for MgB 2 .…”

The effect of nanoscale Fe doping on the superconducting properties of MgB2

“…This effect could be more dramatic if the Fe additives are at nano-scale. Jin et al have studied the effect of Fe addition on T c and the J c -H characteristics of MgB 2 [2]. They found that 5wt% Fe addition had no effect on T c and a negative effect on J c .…”

Transport properties and exponential n-values of Fe/MgB2 tapes with various MgB2 particle sizes

“…Many efforts have already been made concerning the fabrication of superconducting MgB 2 wires and tapes by both in-situ and ex-situ methods [2]. In particular it has already been shown that Fe [3,4] and steel are the most suitable sheath materials to produce high J c , powder-in-tube MgB 2 wires and tapes. It has been found that reducing the starting powder grain size leads to an increase of the critical current density J c and of the irreversibility field.…”