Superconducting materials for large scale applications

Since the 1960s, Nb-Ti (superconducting transition temperature T-c = 9 K) and Nb3Sn (T-c = 18 K) have been the materials of choice for virtually all superconducting magnets. However the prospects for the future changed dramatically in 1987 with the discovery of layered cuprate superconductors with T-c values that now extend up to about 135 K. Fabrication of useful conductors out of the cuprates has been difficult, but a first generation of silver-sheathed composite conductors based on (Bi, Pb)(2)Sr2Ca2Cu3O10 (T-c similar to 110 K) has already been commercialized. Recent progress on a second generation of biaxially aligned coated conductors using the less anisotropic YBa2Cu3O7 structure has been rapid, suggesting that it too might enter service in the near future. The discovery of superconductivity in MgB2 below 39 K in 2001 has brought yet another candidate material to the large-scale applications mix. Two distinct markets for superconductor wires exist-the more. classical low-temperature magnet applications such as particle accelerators, nuclear magnetic resonance and magnetic resonance imaging magnets, and plasma-containment magnets for fusion power and the newer and potentially much larger market for electric power equipment, such as motors, generators, synchronous condensers, power transmission cables, transformers, and fault-current limiters for the electric utility grid. We review key properties and recent progress in these materials and assess their prospects for further development and application.