Practical Magnesium Diboride (MgB2) Superconductor

Abstract

AbstractEven though the \(T_{\text{c}}\) of MgB2 is 39 K only, much lower than the HTSs, yet it has been exploited for the manufacture of wires and cables for a variety of applications within a short span of time. It has already positioned itself as a practical superconductor to be used at 20 K and thus superior to Nb-Ti and Nb3Sn which need liquid helium for operation. Long lengths of multifilamentary MgB2 wires and tapes are being commercially manufactured with increasing \(J_{\text{c}}\) and Bc2. The cross section of the wire contains MgB2 filaments in a Nb, Ta or Ti diffusion barrier, Cu-Ni, SS, Ni or Fe metal casing and Cu core and/or outside for stability. Most popular techniques for manufacturing MgB2 multifilamentary wires/cables are powder-in-tube (PIT) technique, the internal magnesium diffusion (IMD), a hybrid method combining the IMD and PIT techniques and the thin-film-coating technology. High \(J_{\text{c}}\) values have been reported for IMD processed multifilamentary MgB2 wires, typically 9.9 × 104 A/cm2 (4.2 K, 10 T) and 3.3 × 105 A/cm2 (20 K, 1 T) without impurity doping. IMD-processed MgB2 wires, doped with 5 mol% C using coronene (C24H12) as C source, had \(J_{\text{c}}\) = 1.0 × 105 A/cm2 (4.2 K, 10 T) and high Hirr = 25 T. Among many thin film deposition techniques, the hybrid physical–chemical vapour deposition (HPCVD) technique has turned out to be the best technique to achieve high \(J_{\text{c}}\). An HPCVD epitaxial 1.7-µm-thick film of MgB2 on sapphire substrate achieved a \(J_{\text{c}}\) = 1.5 × 106 A/cm2 (30 K, 0 T) and a \(T_{\text{c}}\) = 40.5 K. HPCVD film deposited on Hastelloy tape yielded a \(J_{\text{c}}\) = 1.9 × 106 A/cm2 (20 K, 0 T). The material is being exploited for the development of high-current Rutherford cable and the cable-in-conduit (CIC) conductor. The fast development of this superconductor for magnet application can be attributed to a variety of reasons. The material is cheap, the wire does not need expensive silver like the HTSs, has small anisotropy of Hc2 and \(J_{\text{c}}\) and the high \(T_{\text{c}}\) enabling the MgB2 devices to operate at 20 K. These devices are therefore conduction cooled by the closed-cycle refrigerators (CCRs). The ASG superconductor is already marketing a conduction-cooled MRI scanner with the brand name ‘Paramed MROpen System’ using MgB2 magnets.