Superconductor: WIRES AND CABLES: MATERIALS AND PROCESSES

The phenomenon of superconductivity was first observed in 1912 in the laboratory of Heike Kamerlingh Onnes, at the University of Leiden (Holland). At low temperatures certain materials (about 800 superconducting compounds have been recently tabulated by Poole and Farch, 2000) suddenly lose their resistance to the flow of electricity on cooling. The phenomenon remained a laboratory curiosity until 1954 when G. B. Yntema at the University of Illinois, made the first successful superconducting magnet and even at that point magnet performance fell well below that predicted by the properties of the superconductor. The slow progress can be partially attributed to the difficulty in obtaining the extremely low temperatures required, and the lack of understanding of how to create electrically and thermally stable wires from superconductors. But the primary limitation was that the superconducting phenomenon was not only limited to low temperatures but also to a restricted range of electrical current density and magnetic field and that the range of these properties was particularly limited in early superconductors. The maximum values of temperature, electrical current and magnetic field are interdependent and when plotted in 3 axes form a "critical surface" (Fig. 1). Those early superconductors, such as Pb, In and Hg are classified as Type I superconductors, these are superconductors in which magnetic flux is excluded from their bulk and the critical current density is limited to a surface layer of approximately one tenth of a micron. The maximum fields in which these superconductors can operate are usually less that 0.1 T (similar to the flux-density between the poles of a horseshoe permanent magnet). These limitations make Type I superconductors impractical for wire and cable applications. All "technical superconductors," that is superconductors that can be readily fabricated into wires and cables for high current applications, are Type II superconductors. In Type II superconductors, magnetic flux penetrates the bulk of the superconductor to form individual flux quanta (see Electrodynamics of Superconductors: Flux Properties P. H. Kes). In the most widely used technical superconductors, Nb-Ti and Nb3Sn, the upper critical fields are 13 T and 27 T respectively and current densities > 10 A/m2 can be carried (compared to ~ 10 A/m2 for domestic Cu wire). In Fig. 2 the critical current densities at 4.2 K are compared for strands or tapes that are fabricated in lengths of more than 100 m. The temperature range is still limited (the critical temperatures for Nb-Ti and Nb3Sn are 10 K and 18 K respectively) but for many applications the cost of insulation and refrigeration is more than offset by the reduced energy costs resulting from non-resistive current flow. Because the amount of energy lost in current flow through a superconducting magnet is extremely low, it can be operated without a power supply once the magnet has been charged. This mode of operation, termed "persistent" is very desirable for MRI applications and NMR applications because the current flow (and consequently the field) is very stable. Liquid He is now readily available and provides cooling to 4.2 K at atmospheric pressure and as low as 1.8 K at reduced pressure.