Current news about superconductors.
One of the things that can destroysuperconductivity is too much current. At the proper temperature a superconductor will convey electric current without resistance so long as the current density does not pass a limit that is an intrinsic characteristic of each individual material. For technological usefulness, a superconductor should be able to sustain sizable currents without switching back to normal conductivity or insulation. IBM now announces that scientists at its Thomas J. Watson Research Center in Yorktown Heights, N.Y., have managed to pass a current of 100,000 amperes per square centimeter through one of the new high-temperature superconductors.
This is a current density that opensup all kinds of technological possibilities, especially since it occurred at the temperature of liquid nitrogen, 77 kelvins. Superconductors now in technological use require refrigeration by liquid helium to 4 K. This current density is 100 times that previously reported for these materials, but in that case the people who had passed a current of 1,100 amperes per square centimeter through a sample of the material did not claim to have found any limit; at that current the contacts, which were normal conductors, burned out.
Specialists in the field had generallyexpected that these materials, which are ceramic compounds of copper oxide with rare-earth elements, would have the capacity to stand very high currents. However, most expected this achievement to take longer than it did, as it required the preparation of a single crystal of a pure form of the material on a substrate appropriate for the test. The crystal in this case is 1 micron thick (equal to 1/100 of a human hair) by 1 inch in diameter. IBM researchers have also made other crystals several millimeters thick.
At the same time, IBM studies haveshown that these materials are anisotropic conductors. Household conductors--copper, for example--are isotropic, bulk or three-dimensional conductors; that is, they conduct electricity equally well in all directions. However, substances more exotic than common metals often conduct electricity preferentially in one direction or better in some directions than others. According to the IBM announcement, these high-temperature superconductors carry current 30 times more readily in one particular direction than they do in others. This anisotropic quality will be important to theorists trying to deduce the mechanism responsible for superconductivity in the materials, and also in the design of devices that will use them for practical purposes.
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|Author:||Thomsen, Dietrick E.|
|Date:||May 16, 1987|
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