A programme jointly launched by Bhabha Atomic Research Centre, Trombay and Reactor Research Centre, Kalpakkam to develop indigenously, niobium titanium alloys for superconducting magnets is described. Results of short sample tests on specimens with different treatment are presented. Future plans are also outlined.

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.

The field of superconductivity has witnessed tremendous excitement in recent years, starting with the discovery of what has come to be known as ‘high temperature superconductors’. This has led to extensive activity on many aspects concerning the mechanism of superconductivity, new materials and systems and their technological applications. Further impetus to research in this area has been provided by the discovery of superconductivity in doped fullerenes. The first identification of superconductivity in a quarternary borocarbide system Y-Ni-B-C which must be regarded as the foremost fundamental Indian contribution in recent times, has further stimulated interest in this field. Notwithstanding the new excitements, the conventional superconductors continue to be the workhorses for technological applications. This review selectively presents some of the aspects of the developments in the entire gamut of the known superconducting systems from the stand point of materials and their applications.

In the high temperature superconducting material yttrium barium copper oxide (YBCO) the basal plane oxygen plays a dominant role in determining the superconducting properties of the material. We have carried outin situ high temperature measurements of (a) electrical resistivity and (b) X-ray diffraction in vacuum as well as in helium atmospheres to probe the nature of ordering of oxygen atoms. While published phase diagram of temperature vs oxygen concentration in different partial pressures allow only a tetragonal phase for the annealing temperatures we have investigated, the experiments point to the coexistence of orthorhombic (OII) and tetragonal phases.

The structural instability in Ti rich compositions of Nb-Ti alloy system, which leads to various anomalies in the normal and superconducting state properties, has been extensively studied in bulk samples in the past. In this paper we report the formation of thin films of several compositions in the Ti rich region of this alloy system by RF magnetron sputtering and investigation of their electrical properties. Compositional analysis of two representative films was carried out by the RBS technique and the compositions agreed to within 2% of the targeted values. The anomalous variations in the electrical properties characteristic of the bulk, which can be ascribed to the structural instability related to the formation of athermal ω-phase, are also observed in thin films.