Signs of a new high in ceramic superconductivity.

Signs of a New High in Ceramic Superconductivity 

By systematically altering the proportions of several key elements in a ceramic material, a physicist at the Georgia Institute of Technology in Atlanta has observed signs of superconductivity at temperatures substantially higher than the boiling point of water. A variety of measurements on several different samples of copper oxide compounds suggest that these materials may lose all resistance to the flow of electrical current at temperatures as high as 500 kelvins.

"The compounds seem to be stable and reproducible,' Georgia Tech's Ahmet Erbil told SCIENCE NEWS. "We have made them several times in the lab. We have observed many properties of the material that are consistent with superconductivity.' Erbil reported his results this week in Boston at a meeting of the Materials Research Society.

Like many researchers, Erbil started with a mixture of compounds containing the elements yttrium, barium, copper and oxygen. After processing these powdered compounds into a ceramic, he noticed that certain samples seemed to show superconductivity near room temperature. Under an electron microscope, the material appeared to have a granular structure. Erbil identified five different grain types, each of which contained somewhat different proportions of the constituent elements.

"We reproduced each grain type by starting with the right composition,' Erbil said in a telephone interview. One of the compositions seemed to show properties characteristic of superconductivity at a high temperature. Analyzing those particular samples again showed various grain types. By successively duplicating the composition of components that appeared to show the greatest superconducting potential, Erbil step by step camp up with a material that has superconducting properties at 500 K.

Erbil's findings are so far the most convincing evidence that it may be possible to create superconducting materials capable of operating at room temperature --which would make possible a wide range of inexpensive electrical applications. Earlier reports of room-temperature superconductivity were not reproducible, and researchers failed to follow up systematically the clues offered by these tantalizing glimpses (SN: 7/4/87, p.4). Meanwhile, efforts to develop a theory to explain the findings continue (see p.359).

"The composition range we have is very different from previously reported composition ranges,' says Erbil. However, even the new copper oxide ceramic is granular and made up of a mixture of phases. The superconducting fraction is still probably small, he says. The material is also sensitive to moisture. Samples must be encapsulated to protect them from water vapor.

"The key question is whether or not the superconducting phenomenon exists at those high temperatures,' says Erbil. "We have many types of measurements, all pointing in the same direction--that it is indeed a superconductor. If it exists, then the next question is how the material can be improved for practical application.'

"At this point, I don't know whether this 500 K superconductor is real or not,' says Robert C. Dynes of AT&T Bell Laboratories in Murray Hill, N.J. "I've seen the data. Obviously, there are some questions that [Erbil] doesn't know the answers to, and so it's got to be aired out. But it's not nonsense.' He adds, "One never believes these things until they're reproduced.'

At the same meeting, a group of Bell Labs researchers reported devising a way to prepare flexible superconducting wires that can carry more than 100 times as much current as any similar ceramic material even in the presence of a large magnetic field. "We've broken through a critical current barrier,' says Dynes.

The wire, says Sungho Jin, leader of the Bell Labs team, was created by melting an yttrium-barium copper oxide ceramic, then letting it solidify. The change in grain structure allowed the material to conduct electrical current more efficiently.