Ceramic superconductors disappoint

Predictions revisited

Why ceramic superconductors are disappointing

Helping to explain one of the biggest let-downs of 20th Century technology is a report from a US-European team centred at the University of Florida. It was released yesterday online by Nature Physics. Any scientifically minded person over 35 may remember the huge excitement about ceramic superconductors in 1987. For example, Time magazine called them “a discovery that could change the world” and continued:

That discovery, most scientists believe, could lead to incredible savings in energy; trains that speed across the countryside at hundreds of miles per hour on a cushion of magnetism; practical electric cars; powerful, yet smaller computers and particle accelerators; safer reactors operating on nuclear fusion rather than fission and a host of other rewards still undreamed of. There might even be benefits for the Strategic Defense Initiative, which could draw on efficient, superconductor power sources for its space-based weapons.

Most scientists believe.” Now where have we heard that before?

Writing in Scientific Europe (1990), Ian Corbett of the UK’s Rutherford Appleton Lab summarized the ceramics story, but much more cautiously.

Sensational events are rare in materials science, but superconducting ceramics attracted media attention of a kind normally reserved for football cup finals. In a heady year that followed their discovery in 1986, even respected newspapers implied that our lives were going to be revolutionized overnight, and that an instant fortune awaited anyone who was quick off the mark in exploiting the new superconductors commercially. Such exaggerated expectations were soon damped down by the reality of the technical problems still be overcome.

The scientific breakthrough was real enough. The phenomenon of superconductivity, in which a material loses all resistance to the flow of an electric current, was previously known only in certain metals and alloys, and at temperatures below -250 oC, close to absolute zero. A wholly new class of superconductors opens the way to higher operating temperatures and presumably to wider applications in the electrical and electronic ind ustries.

Georg Bednorz (left) and Alex Muller. Image AIP

In the autumn of 1987 one of the fastest Nobel Prizes ever was awarded to Georg Bednorz and Alex Muller of IBM’s Research Laboratories at Ruschlikon near Zurich – just a year after the first publication, in Zeitschrift für Physik, of their observation of apparent superconductivity in a ceramic material, lanthanum-barium-copper oxide, and at a somewhat higher temperature than any previously authenticated in other materials.

The promise of new applications remains, but unless another unexpected discovery changes the prospects, the road ahead appears at the time of writing to be long and stony.

After two decades, and 100,000 scientific papers on the subject, the applications of ceramic superconductors remain very limited. Small electromagnets for research purposes provide the main example. The difficulty is that the magical ceramics simply refuse to carry a substantial electric current at zero resistance.

The lack of a proper theory explaining why the higher temperature superconductivity occurs at all hasn’t helped matters. But now the Florida team at least gives us a theory for why the ceramics will carry only small currents.

Calculated pattern of supercurrents at a grain boundary in a ceramic superconductor. Diagram: P.J. Hirschfeld, UF

The problem arises at boundaries between the ceramic grains, which cause a big reduction in the critical current at which superconductivity fails. A misalignment of the rows of atoms across the boundary leads to a build-up of electric charge in the narrow angles where the rows meet, which then act like dams resisting the flow of current.


Team leader Peter Hirschfeld of Florida says “Nobody understood why it was such a strong effect, or why the current was so limited by these grain boundaries. And that is what we have explained in this paper.” Although the theoretical model doesn’t suggest how to break the dams, it does at least indicate that alignments at grain boundaries need attention.

Siegfried Graser, the lead author of the Nature Physics paper, is at the University of Augsburg in Germany, but did most of his research while in Hirschfeld’s group in Florida. Their co-authors are at Augsburg and Copenhagen.

By the way, I was quite brief and deadpan about the ceramic superconductors in Magic Universe, so no updating needed there.

References

Time magazine 11 May 1987 http://www.time.com/time/printout/0,8816,964303,00.html

Ian Corbett, “Resistance zero!” in N. Calder (editor) Scientific Europe, Nature and Technology, Maastricht, 1990

S. Graser, P. J. Hirschfeld, T. Kopp, R. Gutser, B. M. Andersen & J. Mannhart, “How grain boundaries limit supercurrents in high-temperature superconductors,” Nature Physics, Published online: 27 June 2010 | doi:10.1038/nphys1687 Preprint available at http://xxx.lanl.gov/PS_cache/arxiv/pdf/0912/0912.4191v1.pdf

U. Florida press release http://www.eurekalert.org/pub_releases/2010-06/uof-pew062410.php


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