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Researchers Boost Superconductor Ability to Carry Current

November 2, 1989

NEW YORK (AP) _ Researchers said today they have greatly improved the ability of a crystal of superconducting material to carry electrical power by an odd-sounding strategy: They made defects in its structure.

The result boosts hopes for making practical use of the recently discovered class of compounds called high-temperature superconductors, scientists said.

The study indicates that a solution is possible for one of two key obstacles to making the materials carry useful amounts of current, said co- author Robert B. van Dover.

Superconductors are materials that carry electricity without resistance. All those discovered so far must be chilled to work.

In recent years, scientists have created the high-temperature superconductors , so called because they require less chilling than previously known materials. Scientists hope these compounds will have more uses than the previous materials.

Researchers want to use these new materials in such applications as high- power magnets, floating high-speed trains and devices for storing electrical power.

The new study addressed a barrier to such uses: In a magnetic field, bulk samples of high-temperature superconductor have been able to carry only small levels of electrical current without losing superconducting ability.

The researchers bombarded a crystal of superconducting material with neutrons, tiny particles that make up parts of atoms. The crystal could then carry about 100 times the current that untreated crystals could without losing superconductivity, they found.

The neutron bombardment created structural defects within the crystal, the researchers said.

These defects hampered the movement of tiny areas of magnetic field within the crystal. Movement of such magnetic fields causes electrical resistance, spoiling the performance of the superconductor.

The performance of the bombarded crystal represents about a 50-fold improvement over what other researchers have attained with single crystals of bulk superconductor, van Dover said in a telephone interview.

It is comparable to the performance of extremely thin films of superconductin g material, said David Welch of Brookhaven National Laboratory on Long Island. Thin films, however, are not practical for making magnets and other relatively large-scale uses, he said.

The crystal study is presented in today’s issue of the British journal Nature by van Dover, Lynn Schneemeyer, E.M. Gyrogy and others at AT&T Bell Laboratories in Murray Hill, N.J., and scientists in Sweden.

The work does not address a second crucial obstacle for bulk superconductors: loss of current-carrying capacity that occurs between crystals in a sample. ″That problem is still out there,″ van Dover said.

In addition, he said, researchers would like to come up with a more practical method than neutron bombardment for creating improved superconductors, he said.

Nonetheless, the work is encouraging for efforts to overcome current- carrying limits within crystals. ″It certainly indicates it is possible,″ said Welch.

Brian Maple of the University of California, San Diego, called the study ″an important step″ toward solving the overall problem of low current capacity in bulk superconductors.

The experiments involved crystals containing the standard superconductor ingredients of yttrium, barium, copper and oxygen.

The neutron-irradiated crystal achieved a ″critical current density″ of about 600,000 amperes per square centimeter, when chilled to minus 321 degrees Fahrenheit and exposed to a magnetic field about 18,000 times that of Earth.

Other researchers had previously reported improving superconductor performance by creating structural defects, but they did not produce as dramatic an improvement, Maple said.