A superconducting tape ... mass produced?

A flexible superconducting tape that carries more than 1 million amperes of electric current per square centimeter of material has proven itself in the laboratory, scientists report. Stephen R. Foltyn, a materials scientist at the Los Alamos (N.M.) National Laboratory, and his colleagues describe making bendable strips that superconduct at 75 kelvins, the temperature of liquid nitrogen.

For a superconductor to carry so much electricity in such a small area -- a density of current 100 times greater than similar compounds have carried -- sets a record for electric transmission at that temperature, the scientists state. By contrast, the number 12 copper wire used in most homes carries only 800 amperes of current per square centimeter.

The researchers produce the tape as a three-layer sandwich by depositing yttrium-barium-copper oxide, a superconducting ceramic, onto nickel alloy coated with cubic zirconia. The resulting tape bends so easily that even after repeated flexing it shows no signs of cracks. That property is unusual for a metal coated with a ceramic, the scientists say.

To layer the superconducting ceramics on the nickel, the researchers used a technique called ion-beam-assisted deposition, says Paul Arendt, a materials scientist at Los Alamos. The method uses two beams of charged particles. One beam deposits atoms of the superconducting material, while the second one aligns the crystals as they form. The tape's ability to stay intact after a tight winding may facilitate the production of tiny, powerful magnets.

Such micromagnets could lead to small magnetic resonance imaging machines for examining parts of the body, such as a knee or elbow, rather than the whole-body scanners used today, the scientists say. Manufacturing highly efficient electric motors or portable devices for removing contaminants in soil at toxic waste sites might also become possible with flexible superconducting tape, they add.

...mass-produced?

For any type of superconducting wire or tape to become practical, a manufacturer has to produce it efficiently on a large scale.

In an effort to solve this problem, Amit Goyal, a materials scientist at the Oak Ridge (Tenn.) National Laboratory, and his colleagues have developed a system for producing high-temperature superconducting wires in bulk.

The key lies in depositing the superconductor onto a carefully chosen substrate so that the crystal grains fall into alignment. Researchers call such alignment "biaxial texture," a quality that strongly affects how much current a superconductor can carry.

The new method, or "rolling-assisted biaxial texture" process, enables a manufacturer to produce superconducting strips quickly and in large

quantities. "Since it's a bulk process, in theory someone can produce any length or width," Goyal says. "There's no limit."

So far, the team has made flat, 1-inch-wide wires with yttrium-barium-copper oxide deposited on them. Tests show that the superconductor does in fact have biaxial texture. If it did not, the material would not superconduct effectively in the presence of a magnetic field.

The technique, say the researchers, should work for other types of superconductors as well.