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Warm Superconductors

by John G. Cramer

Alternate View Column AV-22
Keywords: high temperature, superconductor, liquid nitrogen, barium yttrium copper oxide
Published in the October-1987 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 3/6/87 and is copyrighted © 1987, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.

    I'm sure you know the plot: a lost magical formula has eluded the wisest practitioners of the arcane art down through the ages. Now a cabal of young upstarts, with disdain for conventional wisdom and working against all odds, discovers the key ingredients that make the magic spell work, with spectacular results.

    Sounds familiar, right? But this isn't schlock fantasy, folks. It's low temperature physics. The discovery of "warm" superconductivity was announced in mid-February of this year by a little known research group led by Prof. Ching-Wu (Paul) Chu of the University of Houston. A report of the work was published in the March 2 issue of Physical Review Letters. It describes a new material, [(Y.6Ba.4)2CuO4], that at atmospheric pressure becomes superconducting between 80 and 93 degrees Kelvin (represented as 93 K, or 93 Celsius-degrees above absolute zero). The paper estimates that the material should retain its superconducting properties even in the presence of magnetic fields as large as 80 to 180 Tesla.

    What's so startling about this report? First, it offers the promise that superconductors can be made to operate above the temperature of boiling liquid nitrogen (77 K). Second, the spectacular magnetic properties of the new material suggest that it may soon be possible to construct superconducting electromagnets capable of producing extremely large magnetic fields. These possibilities have important implications for energy production, transmission, and storage, for surface transportation, for space propulsion, for new scientific tools, for medicine, and for gadgets of which no one has yet dreamed.

    After Chu's announcement, perhaps 100 research laboratories around the world jumped into the investigation of warm superconductors. A veritable tidal wave of new data has emerged. Reports now suggest that the onset of superconductivity may have been observed at 240 K. That is just -33o C or -27o F, a typical outdoor temperature in a Minnesota winter. It offers the promise that superconductivity can soon be achieved in a good home freezer.


    To show the significance of this breakthrough, let's do some questions and answers.

Q: What is superconductivity? Superconductivity is an peculiar state of solid matter in which electrical resistance completely vanishes. Electrical resistance is a measure of the energy lost when an electrical current flows in a conducting material like copper or carbon. Normally the moving electrons of electrical current flow have collisions along their way and lose some energy. The lost energy becomes heat. An electric kitchen range, for example, uses this effect to convert electrical energy into heat for cooking food.

The loss rate of electrical energy due to current flow in a resistive wire follows the Ohm's law power equation P=I2R, in which the electrical resistance R sets the rate of energy loss P due to a current flow I. In the best conductors, materials like silver and copper, the energy loss is minimized because R is small. And if R vanishes there is no energy loss at all.

Q: When was superconductivity discovered? Until 1911 it was believed that all conductors had electrical resistance, even at low temperatures. Then from the University of Leiden in the Netherlands came a remarkable announcement which shook the world of physics. Prof. Heike Kamerlingh Onnes of Leiden, the man who a few years earlier had been the first to liquefy helium at low temperatures, had discovered that when a thin column of mercury is cooled with liquid helium to below 4 K, its electrical resistance completely disappears. Onnes could find no evidence at all for even a trace of electrical resistance in the cooled sample. Later, as a dramatic demonstration of this effect, Onnes cooled a ring of lead, another superconducting material, to a few degrees Kelvin and set an electrical current flowing around it. He then transported the ring, still immersed in a bath of liquid helium, to London where a physics conference was in progress. He showed the startled physicists that the current flow he had initiated on the other side of the Channel was still to be flowing in the lead ring. At many laboratories similar rings were constructed, with currents flowing undiminished for decades with no observable current reduction or energy loss. It has been remarked that the zero resistance of a superconductor is the "zero-est zero" in physics.

Further investigation has shown that some 23 of the 92 natural elements in the periodic table become superconductors when cooled to temperatures sufficiently near to absolute zero. Among these superconducting elements are aluminum, zinc, niobium, tin, mercury, lead, and uranium. Curiously, the new [(Y.6Ba.4)2CuO4] material showing warm superconductivity is composed of elements (oxygen, copper, barium, and yttrium) that are not superconducting as individual materials.

Q: How does superconductivity work? From 1911 until 1957 the surprising disappearance of electrical resistance at low temperatures remained a mystery, despite the best efforts of some of the giants of theoretical physics to explain it. Finally in 1957 John Bardeen, Leon N. Cooper, and J. Robert Schrieffer, then of Bell Telephone Laboratories, produced a theory that finally explained what was happening in a superconductor. Their explanation, called the BCS theory of superconductivity, depends on subtle quantum mechanical effects acting in concert to completely eliminate electrical resistance. Conduction electrons in the material pair off to form "Cooper pairs" that move together along the conduction path. One Cooper pair follows closely in the wake of the pair preceding it along the path, each pair "drafting" the pair in front like race cars on a straightaway. This path-following reduces the energy loss to a value below the minimum permitted by quantum mechanics, with the result that there is no energy loss at all. Roughly in this way the frequent collisions and energy loss of normal conduction are eliminated and smooth superconductive current flow, electrical conduction without energy loss, becomes possible. For the new warm superconductors a more subtle mechanism seems to be involved that cannot be handled by the BCS theory as it stands. A new theory will probably be needed.

Q: How can superconductivity be used? While it took from 1911 to 1957 to gain some theoretical understanding of superconductivity, it required even longer for the first practical applications of superconductivity. The most interesting application of superconductors was to produce large magnetic fields, but it was found that at modest magnetic fields the superconducting properties disappeared. It took until the 1960's for this problem to be solved. Then the first superconducting magnets began to appear, bringing with them the possibility of large magnetic fields (up to 8-12 Tesla) for laboratory use. The superconducting SQUID devices for accurately detecting and measuring extremely small magnetic fields also came into wide laboratory use. High field superconducting magnets have become standard in the construction of large particle accelerators like those at CERN and FermiLab. And superconducting resonant cavities that efficiently store electrical energy and produce very large electric fields are now used in accelerators at many nuclear physics laboratories, including my own laboratory at the University of Washington. But outside physics laboratories the applications of superconductors have been slower in coming. Superconducting magnets are now coming into use in medicine because they are required by the new technique of NMR imaging. And there has been serious engineering studies of a superconducting "energy pipeline" that might bring electrical power to the East Coast.

Q: Why are warm superconductors important? All of the above applications are hampered by the very low temperatures required by conventional superconductors. Liquid helium must be available in large quantities, and the devices themselves must reside in large and elaborate "cryostats" (giant thermos bottles) that protect the ultracold materials from the normal room-temperature surroundings. The energy needed to maintain these very low temperatures removes much of the attraction of using these no energy loss materials. Clearly superconductors that operate at higher temperatures will be very useful. Finding such "warm" superconductors, preferably that remain superconducting above the 77 K boiling point of liquid nitrogen, has become a sort of "Holy Grail" of low temperature physics. And now the Grail has been found!

Q: What kind of materials are needed for warm superconductors? In the search for warm superconductors, the theoretical understanding provided by the BCS theory has proved a treacherous ally. That theory roughly indicated that 35 K might be the highest temperature at which superconductivity could exist. This prediction is now known to be wrong. The theory has also been used to give rough guidelines for which alloys and materials might be the best warm superconductor candidates. Unfortunately, these predictions steered physicists away from materials containing oxygen (because oxygen tends to gobble up two electrons that might otherwise be used for conduction). Therefore, it was thought, materials containing oxygen were a bad bet and they were ignored. It now appears that oxygen is a key ingredient in warm superconductivity. The [(Y.6Ba.4)2CuO4] material contains more oxygen atoms than anything else. The crystal structure of the material is not yet understood. The samples exhibiting superconductivity seem to be a mixture of two or more different crystal structures that are simultaneously present in the sintered material.

Q: How long will it be before we will have devices using warm superconductors? This is a very difficult question to answer. The [(Y.6Ba.4)2CuO4] material can be thought of as a kind of powdered rock that had been "sintered" or heated under pressure to fuse the power particles together. Chu's group used very small samples of this material, only 4 millimeters long and with a cross sectional area of 0.5 mm2. Practical applications cannot really begin until literally miles of warm superconducting cable can be made available. Such cable might be sheath of high purity copper enclosing tubes filled with powdered superconducting material. Perhaps this cable would be drawn, coils wound, and then heat and pressure applied to sinter the [(Y.6Ba.4)2CuO4] material in place. Development of fabrication techniques for such cable will require large investments in time, technical manpower, and development capital. Materials science groups at major federal laboratories are already moving into high gear to develop the needed technology. I would guess that practical applications of warm superconductors might appear in as little as three years, but they could also require a much longer time if major problems are encountered.

Q: Finally, what are the implications this new technology for science fiction? As a teenager I remember reading stories in John Campbell's Astounding featuring space ships that used vacuum tube electronics in their control systems and that were piloted by individuals who did calculations with slide rules and tables of logarithms. It all sounded quite plausible at the time, but it has all been long since obsoleted by the invention of the transistor and the development of microelectronics. This is a pitfall to be avoided. We would like to write stories that at best predict the future and at worst are not rendered quaint and mildly ludicrous by the inevitable march of scientific progress.


    In the event that the technology of warm superconductors becomes practical, it will profoundly change the way things are done. The familiar electrical transmission transmission towers that march across the countryside and foul radio reception with their corona static will be utterly gone, replaced by underground "energy pipelines" that move electrical energy across the continent with no measurable energy loss. Power generation facilities can be located farther from the consumers, possibly configured in large "power parks" containing multiple reactors or fusion plants (using warm superconducting magnets to contain their plasmas). Trains will glide frictionlessly over the countryside, levitated on cushions of superconducting magnets.

    Energy storage with high field superconducting magnets may compete with storage batteries and chemical fuels for the same turf. Magnetic fields have very large energy densities, and the energy stored goes up as the square of the field. A volume of space containing a 100 Tesla magnetic field would contain about as much energy as an equivalent volume of TNT. One can imagine a doughnut shaped superconducting magnet storage device that holds far more energy than the same volume of gasoline and is given a current boost perhaps once a month to top off its energy store.

    In any case, we have crossed a threshold. We now know how to make warm superconductors. We will learn to make them better. The world will never the same. But it will be better, too.


References:

Warm Superconductors:
A. L. Robinson, Science 235, 531 (1987);
M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, Physical Review Letters 58, 908 (1987).


SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 177 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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This page was created by John G. Cramer on 7/12/96.