12/05/91
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STANFORD -- Since the 1986 breakthrough when two IBM physicists created a superconductor that worked at warmer-than- ultracool temperatures, scientists have been hunting fruitlessly for the ultimate superconductor: one that works at room temperature.
Some physicists argue that room-temperature superconductors are as illusory as the Fountain of Youth. Others keep looking, though they are only slightly more optimistic.
Then, there's Theodore Geballe, professor emeritus at Stanford University's department of applied physics, who firmly believes the lack of success so far only means that scientists have not yet looked in the right place.
"I'm still hopeful," he said in a preview of his Dec. 4 acceptance speech for the 1991 Von Hippel Award, the highest honor of the Materials Research Society.
"Looking for superconductivity at room temperature is like looking for extraterrestrial life. There is plenty of space out there in which it might occur. We just have to know which galaxies to visit," Geballe said.
"There are too many possibilities to make random searching for superconductors a wise strategy. We should rely upon the information that already exists to provide clues for finding the star compounds."
A room-temperature superconductor would make energy use ultra- efficient, Geballe said. One spectacular property of superconductors is their lack of electrical resistance, which makes them almost ideal for producing and using electrical energy. Another is their remarkable sensitivity to magnetic fields, which makes it possible to produce superconducting junctions that excel as sensitive detectors and as elements for the next generation of computers.
Superconducting prototypes of motors, generators, transmission lines and ship propulsion units already have been built. Supercooled superconductors cause a prototype train in Japan to float on a test track several kilometers long.
Superconductors also have entered the marketplace, in the powerful magnets used in magnetic resonance imaging.
But there is a catch with current superconductors: The warmest temperature at which they can function is a chilly minus 234 degrees Fahrenheit. For widespread use, such as conducting electricity in homes or offices, superconductors will need to work at room temperature - about 68 degrees F.
Geballe believes a technological breakthrough is possible that would bring superconductors out of laboratories and into society's mainstream.
Superconductivity results when all the electrons in a compound form pairs, producing a low energy, superconducting state. In the paired state the electrical current can travel unhindered, scattered neither by vibrations nor imperfections in the conducting material.
"There are at least three classes of materials with promise for supporting superconductivity at much higher temperatures than are now possible," Geballe said.
Artificial structures in the form of thin films can be grown layer by layer to optimize the electron-electron interactions that give rise to superconductivity. A few laboratories in Europe, Asia and the United States, including at Stanford, already have grown films that conduct electricity without loss at minus 297 F in directions both parallel and perpendicular to the film. They have built "superlattices" by sandwiching layers of yttrium, barium, copper and oxygen with layers of related atoms. The highly ordered atoms in these lattices promote sophisticated patterns of superconductivity.
Molecular crystals are a second promising class of materials. One of these, buckminsterfullerene, is made up of 60 carbon atoms that fit together like Buckminster Fuller's geodesic domes or the panels of a soccer ball. AT&T Bell Labs discovered earlier this year that these "buckyballs" become superconducting when exposed to vapors of such alkali metals as potassium.
In a third class of materials, organic charge transfer salts, conductivity takes place along chains of organic molecules. So far in this relatively new field, superconductivity occurs only at temperatures slightly above liquid helium, minus 452 F. Superconductivity in these organic chains also competes with other forces, such as magnetism, which - to the consternation of scientists - suppresses the superconductivity.
Low-temperature superconductors were discovered in 1911 by Dutch physicist Heike Kamerlingh Onnes, who started a current flowing in a closed loop cooled to minus 452 F with liquid helium and isolated from electricity and magnetism. Calculations showed that if it were kept cold, such a current could be kept running for the life of the universe.
Before 1985, scientists wanted to produce materials that could superconduct at the temperature of liquid nitrogen (minus 321 F) which was cheaper than liquid helium and more practical for commercial applications. Working with niobium alloys, scientists could not get anywhere near the temperature of liquid nitrogen, Geballe said.
However, in 1986, IBM scientists Georg Bednorz and Alex Muller broke the temperature barrier with a new class of superconducting materials called layered copper oxide perovskites. For this discovery Bednorz and Muller shared the Nobel Prize. Intensive international research raised the maximum temperature for superconduction to minus 234 F by 1988, and there it has remained. Astonishingly, the materials the IBM scientists used were ceramic metals. Ceramics previously had been thought to be insulators, or at best poor conductors.
Since then, "ceramics have been and are being explored thoroughly," Geballe said. "I don't think they'll make room-temperature superconductors, even though there have been a number of unconfirmed claims."
Nonetheless, Geballe said he is "bullish on the possibility that there will be room temperature superconductors."
EDITORS: To convert Fahrenheit temperatures to degrees Celsius, subtract 32 from Fahrenheit figure, multiply by 5 and divide by 9.:
To convert to degrees Kelvin, a unit used by superconductor scientists because it is the only scale in which temperature is directly proportional to kinetic energy, add 273 to the Celsius temperature.
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