Alternative technologies for superconductor fabrication

Superconductor fabrication for large magnetic devices has largely been accomplished by continuous multifilamentary fibre methods based on extrusion of composite billets followed by wire drawing. This paper reviews other methods aimed at achieving the same superconducting properties, but with other advantages. The "in situ" technologies are really older than the mechanical methods, which in fact were developed as a result of questions raised by early "in situ" research. The technical possibilities of the "in situ" approach were shown by Tsuei, who used rapidly cooled Nb-Cu and Nb-Cu-Sn alloys which were subsequently drawn into wire. High current densities are attainable in these alloys due to percolation through a contiguous structure of superconducting phase. By careful microstructural control the critical current densities can approach in magnitude those of continuous fibre bronze-matrix composites. Similar microstructures may be developed from powder composites, either of mixed powders or liquid infiltrated powder or fibre structures. Controlled directional solidification results in highly regular, oriented multiphase structures which may be processed to form superconducting wire composites. All of these methods are capable of high current densities, and at least some offer superior mechanical properties. In general, the use of phase transformations or powder methods to produce composition modulations on a fine scale has resulted in a class of materials with potentially superior technical properties.