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)₂Sr₂Ca₂Cu₃O₁₀ (T_c∼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.