Rods enhance superconductor performance.
Superconductors are a case in point: The right kinds of defects, strategically incorporated into their crystal structure, can actually increase the electric current they can carry without resistance.
Peidong Yang and Charles M. Lieber of Harvard University have found a new way to introduce beneficial defects into superconductors. The chemists incorporate nanometer-scale rods of magnesium oxide into one type of superconducting material, they report in the Sept. 27 Science. The presence of the nanorods allows 10 times as much current to flow through the superconductor, Lieber says.
Theorists still do not fully understand why certain materials act as superconductors, let alone why defects should enhance their performance. The nanorods may boost current-carrying capacity by reducing the obstructive effect that magnetic fields have on superconductivity (SN: 2/10/90, p. 95).
The Harvard researchers have improved upon previous methods of adding defects.
One such approach has been to bombard superconductors with heavy ions, such as lead or gold, at very high energies. The ions tunnel through the material, knocking atoms out of place as they go.
Ions, however, can't penetrate materials to a depth of more than a tenth of a millimeter, says Masaki Suenaga, a metallurgist at Brookhaven National Laboratory in Upton, N.Y. This limitation would be a problem in manufacturing thicker superconducting wires. Besides, making large quantities of superconductor in a particle accelerator just isn't practical. "It's scientifically very interesting," he says, "but I don't have any hope for commercial applications of heavy ion radiation."
Another method of adding imperfections uses high-energy protons as the blasting agent. The protons induce nuclear fission in the superconductor's bismuth atoms, says Lieber. "Fission fragments go bombing out of the material and create defect tracks." Although protons penetrate much farther than ions, they tend to make the material radioactive.
The drawbacks to ion and proton irradiation prompted researchers to find other ways of incorporating defects. One group tried adding carbon nanotubes to superconductors in the manufacturing stage, but the nanotubes reacted chemically with the superconducting material.
Magnesium oxide, however, seemed like a good choice for the nanorods, Lieber says. "People grow [superconductor] crystals in magnesium oxide containers, so they know that it's inert and won't introduce impurities into the crystals."
Previous work had also showed that magnesium oxide "whiskers" improve the mechanical properties of superconductors, but those rods were much larger-micrometers in diameter-and tended to impair current-carrying capability.
"The main technical hurdle was developing a synthetic approach to making magnesium oxide whiskers with nanometer-scale diameters," Lieber says. After they overcame that obstacle, the scientists incorporated nanorods into the superconductor in two different ways. They either grew "a forest of little whiskers" in a fixed orientation on a surface and deposited superconductor around them or mixed a few nanorods into melted superconductor and allowed the material to crystallize.
The second technique worked because, Lieber says, "It turns out that these rods actually self-organize within this superconductor matrix."
The group used a superconductor known as BSCCO-2212 in the experiments; the name represents the proportions of bismuth, strontium, calcium, and copper in the material. However, Suenaga says he'd like to see nanorods added to another form, BSCCO-2223.
This variation interests many researchers because it remains a superconductor all the way up to a temperature of about 110 kelvins. On the other hand, BSCCO-2212 is easier to make, Suenaga says, which could facilitate large-scale synthesis.
Lieber says the next step is to reduce the size of the nanorods in order to increase their density. Demonstrating that the process can be scaled up to industrial production is important too, Suenaga says. "If they can make a tape out of it and actually test it, that would be very interesting."
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|Date:||Sep 28, 1996|
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