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Growing a fibrous superconductor.

Growing a fibrous superconductor

One challenge in working with the recently discovered high-temperature superconductors is finding a way to produce these brittle ceramic materials as wires. Researchers at Stanford University have now taken one step toward that goal by using a laser-based technique to grow thin fibers capable of carrying a large current.

The Stanford group applies a technique known as laser-heated pedestal growth, conventionally used for rapidly growing small-diameter single crystals. Using appropriate amounts of finely ground oxides or carbonates of bismuth, calcium, strontium and copper, the researchers process the ingredients into small rectangular rods. A tightly focused laser beam first melts the top of a rod. A second, narrower rod acting as a seed crystal is dipped into the molten material, then slowly withdrawn at a rate of 1.5 to 50 millimeters per hour to produce a fiber. As the fiber lengthens, the laser beam melts more of the original rod to sustain this growth.

The resulting fibers are usually between 0.25 and 1 millimeter in diameter and up to 40 millimeters long. Theoretically, there is no limit on the length of fibers that can be grown using this technique, says Robert S. Feigelson, who led the work. Preliminary measurements show that typical fibers can withstand a pulsed current of at least 60,000 amperes per square centimeter at 68 kelvins before the fibers lose their superconducting properties.

With their laser technique, the Stanford researchers say they have a simple, controlled method for producing high-quality fibers. By changing the material's composition, the speed at which the seed rod is withdrawn and other conditions, they can study how various parameters affect the fiber's properties. The researchers describe their advance in the June 17 SCIENCE.
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Author:Peterson, Ivars
Publication:Science News
Date:Jun 25, 1988
Words:287
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