Current trends in the development and applications of superconducting materials

The discovery of the phenomenon of superconductivity by Kamerlingh Onnes in 1911 was the first indication of the possibility of electrical conduction without any associated Joule loss. The technological application of the property (which was essentially manifested at liquid helium temperatures) had to await the development of stable superconducting materials capable of withstanding high currents and large magnetic fields. Although many materials — elements, alloys, ternary chalcogenides, and recently oxides — have been found to be superconducting, only a few of them have received attention for significant applications. This is based on three important parameters namelyT _c , the transition temperature,H _c2, the upper critical field andJ _c , the critical current density.T _c andH _c2 are considered intrinsic to the material, whileJ _c is influenced by the microstructure, and has to be optimised during fabrication of the material in the useful form. On these considerations, Nb-Ti, Nb₃Sn and V₃Ga have emerged as proven materials for significant applications while PbMo₆S₈ is still under development. Despite the fact that all these materials have to be used only at liquid helium temperatures on account of their lowT _c , major developments have taken place in harnessing particularly the niobium alloys to produce superconducting magnets. Towards the end of 1986, a break-through has been achieved in the direction of raising theT _c . Many ceramic oxides, notably Y₁Ba₂Cu₃O₇, have exhibitedT _c in the vicinity of 100 K. These materials have also been shown to have highH _c2, about 180 Tesla. Attempts are now being made to realise a highJ _c . It is too early to say whether such materials can be fabricated in suitable forms capable of carrying high currents. Among the major areas in which superconducting materials have so far been used, mention should be made of superconducting magnets for high energy particle accelerators, magnetohydrodynamic power generation, magnetic resonance imaging, and fusion research programmes. In other potential applications such as motors and magnetically levitated transportation, economic break-even has not been achieved, mostly on account of the need to use liquid helium. The discovery of the high temperature superconductors capable of operating at liquid nitrogen temperatures thus promises a revolution in electrical technology. The paper reviews the development and applications of superconducting materials, with reference to work being done in India. Talk delivered at the International Conference on Recent Advances in Materials and Processes, Varanasi, November–December 1987.