Overview of High Temperature Superconductivity

The advent in 1987 of the new high temperature superconducting materials, such as YBaCuO (YBCO), was hailed as a major step forward in the development of superconductors. Some applications for conventional superconductors—such as high power magnets— were already developed and commercially viable, despite needing liquid helium cooling. Until 1986 the critical temperature had risen by only about 20°C since the first observations of superconductivity were made by Onnes in 1911 (1). Within weeks of the disclosure by Bednorz and Muller of a new class of superconducting material based on copper oxides containing barium and rare earth elements (2), critical temperatures as high as 100 K were being reported. For nearly two years a new high temperature superconducting material or variant was being reported weekly, but relatively little progress was made in understanding the phenomenon. In some respects, the classical superconductivity theory (3) is inadequate to describe the new superconductors, but even now there is no universally accepted replacement. Certain parts of the theory, including Cooper pairs of charge carriers, are retained by most newer theories.

The main drawback arises when attempting to explain the pairing interaction and how it can survive to much higher temperatures than the classical phonon coupling mechanism allows.

Frantic formulation, patenting and publicising of new combinations of materials with superconducting properties at temperatures above 77 K has now been replaced by an alternative approach driven by potential applications. Investigators were initially obsessed with raising the critical temperature ever higher, but it became clear that once critical temperatures substantially above 77 K had been reached, further increases would not be of great significance, unless room temperature could be achieved. Indeed, for most applications the most important parameter is the critical current—the maximum current that can be attained until the self-induced magnetic flux associated with this current causes the system to become unstable and collapse due to interactions between nearby superconducting regions. The low critical currents observed were the major barrier to commercialisation, and to a large extent this is still die case, although progress has been made. The same was true of the classical Type I superconductors, for example niobium, until Type II superconductors such as niobium-tin, were discovered.

The increase in critical current in Type II superconductors depends on the recognition of the phenomenon of flux pinning. Essentially, flux pinning ensures that the magnetic flux associated with localised high current densities of superconducting regions does not interact with the flux due to neighbouring regions in such a way as to exceed the critical magnetic flux density and thereby collapse the superconducting currents. A modern Type II superconductor is an inhomogeneous system by design, with the discontinuities serving to stabilise the system, and effectively prevent instabilities from escalating and destroying the system via a domino effect. Clearly, any region which becomes even momentarily non-superconducting will give rise to a Joule heating effect, which, if the heat is not conducted away rapidly, will result in other regions exceeding the critical temperature, losing their superconductivity, and precipitating further thermal runaway. Thus the thermal design of superconducting systems with, for example, copper sheaths around niobium-tin wire, was die key to establishing the very high critical currents now possible in conventional superconductors.

Successful commercialisation of many new materials has generally depended to a large extent on the recognition of the role of in-homogeneities. In the broadest possible sense we can regard any departure from perfect crystallinity, or a local departure from a single phase amorphous material, as an inhomogeneity. Deliberately introduced inhomogeneities have been used to enhance, and frequently stabilise, the properties of materials. Thus while extremely high purity levels are essential for growing single crystal materials, regions deliberately doped with controlled amounts of impurities are used to produce, for example, the pn junctions which are the basis of many electronic devices. The same is true of the mechanical properties: the work hardening of metals leads to the interaction and entanglement of dislocations such that further deformation becomes progressively more difficult. Similarly, the stabilisation of grain boundaries in materials, such as ZGS platinum, gives enhanced properties. Furthermore mechanical damage on the back of silicon wafers is used to “getter” unwanted impurities from the bulk material during quenching after high temperature diffusion processes.

The role of inhomogeneities in the new superconductors is also crucial to their operation, if only because they are Type II superconductors. Different thermal treatments of these materials give rise to widely differing microstructures and electrical properties. Inhomogeneities can be regarded as real and possibly essential parts of superconducting behaviour, rather than as unwanted artefacts. In fact the occurrence of the higher critical temperatures seems to be associated with “dirty” specimens (4).

Comparison of Semiconductor and Superconductor Development

Certain parallels can be drawn between the development of semiconductor materials, as a part of the electronics industry (5), and the new superconductors which may help to predict major applications for the latter, and identify technological goals that will have to be achieved before commercialisation can take place. The new superconductors are generally semiconductors above the critical temperature, and their discovery was also driven by the same requirement for high speed solid state switching.

Another reason for comparing semiconductors and superconductors is that the required performance of the latter is only likely to be attainable, at least for the forseeable future, using techniques which give very close control over material properties, such as crystallinity and chemical composition. Single crystal growth and thin film deposition are currently able to give higher performance characteristics than, for example, ceramic processing, and the former techniques are more likely to be used in the first commercial applications. The semiconductor industry is skilled in such matters and much information is available on die techniques for depositing thin films on single crystal substrates, and so on.

There are likely to be problems in processing superconductors but, except for applications needing bulk materials, the techniques used in the electronics industry are immediately relevant. This is also true of many of the analytical methods, such as secondary ion mass spectroscopy and X-ray scanning auger microanalysis, that are necessary to achieve the required control. Thus, there is not the same learning curve, which compensates for the greater complexity of chemical composition and microstructure of the materials. Fortunately it appears that chemical purity of itself is not as essential as it is with semiconductors.

Growth in Literature on Superconductors

Superconductivity was first reported in 1911 (1). Coincidentally, the first element Onnes investigated was platinum, which does not exhibit superconductivity. Now there are over 46,000 publications on superconductors. Commercial applications for the “old” superconductors are still growing, mostly for applications requiring high power magnets, but there has been a massive upsurge in effort since 1987 when the new high temperature superconductors were discovered.

Platinum in High Temperature Superconductors

At temperatures as low as two millionths of a degree Kelvin and magnetic fields as low as two milli Gauss, no superconductivity was observed in platinum, palladium and rhodium, according to a study in 1978 (6). However, a number of alloys of platinum have been found to be superconducting, such as UPt (7) and MoPt (8), but with one possible exception these have all been low temperature superconductors. The possible exception is CuPt, which a recent patent suggests is superconducting at temperatures as high as 200 K (9)! This work has not been confirmed, however, and no other reference to it has been found in the literature.

Over 100 papers and patents have been published specifically mentioning platinum in connection with high temperature superconductivity in either the title or the abstract, and in addition many more papers refer to platinum and its uses in the text. These can be divided into several main groups, in terms of: the final form of the superconducting materials, the techniques used for production and the application areas.

In this way, we can attempt to predict the application areas that may be commercially viable first, the form these superconducting materials will take, the materials processing technologies which will be most relevant and finally what the uses of the platinum are likely to be. It is assumed here that a high level of activity relates to a market need, and that such activity is likely to lead to the first commercial products.

Conclusions

High temperature superconductors show every promise of being a commercial success despite their technical complexity. Potential application areas, currently unsatisfied, abound. Academic institutions are striving to unravel the complexities of materials and mechanisms and major companies are investing substantially in research and development. Never have so many different technologies and analytical techniques been applied to the understanding and advancement of a class of materials. This is partly the result of the proliferation of the new techniques and analytical instrumentation available, and partly because of the wide range of potential applications.

As with semiconductor materials, thermal processing will be important and there are numerous opportunities here for platinum to assist in the achievement of the desired degree of process control and material stability. The exclusion of ferromagnetic materials during processing is particularly important, as is controlling oxygen migration.

High temperature superconductivity will have an impact on power applications, such as for transportation and energy storage devices (79). However, of the many potential applications for the new superconductors, magnetic and optical detectors, satellite communications, magnetic screening, and magnet/motor technology may be among the first to be commercialised. In terms of volume of platinum, the use for superconducting magnets is likely to be the dominant one, but critical current densities will need to be substantially increased. The use of thin films rather than wire in this application warrants serious study, since in thin films the critical currents can be much larger than in bulk material.

Electronic device applications, using thin films, will probably be commercialised sooner than high temperature superconducting magnets. Specialised detectors (such as antennas) and superconducting interconnects on silicon are emerging as likely applications in the short to medium term. In the long term, the high speed switching application may compete with advances in optical computing for the next generation of electronic devices.