Cool Wire: Nanostructure boosts superconductor

Superconductive wire remains a wannabe technology for many applications. Although some ceramic wires can compete with conventional copper for use in power lines, they don’t meet requirements for widespread use in industrial devices containing wire coils, such as transformers and motors.

Now, a ceramic-wire prototype has performed so well in superconductivity tests that it could win against copper across the gamut of expected uses, its inventors claim. Whether the wire could be made abundantly and cheaply remains uncertain.

Pin-length strips of the wire, a narrow, layered ribbon including a nickel-alloy base and a superconductive ceramic film, attained record currents in magnetic fields like those in coils, report researchers at Oak Ridge (Tenn.) National Laboratory.

“This is a first demonstration that, in a single superconducting wire, you can have such performance,” says Amit Goyal.

Goyal and his colleagues describe the new wire in the March 31 Science.

“We think this is a very important result. It’s a world-record result,” comments Alexis P. Malozemoff of the wire-manufacturing company American Superconductor in Westborough, Mass.

It’s a “proof of principle” but not an advance that could be incorporated directly into American Superconductor’s manufacturing approach, Malozemoff adds.

On the other hand, Goyal says, companies in Japan and Germany are pursuing a wire-making process compatible with the Oak Ridge advance.

Since the early 1990s, many researchers have made wires from ceramic materials, known as high-temperature superconductors, that carry electricity without resistance (SN: 11/30/02, p. 350: Available to subscribers at Resistancefree wire takes long jump). Although the materials superconduct only at less than about 135 kelvins, that’s balmy compared with the temperatures near absolute zero required by some other superconductors.

American Superconductor and other companies already produce such a wire, but it contains silver, making it pricey. Furthermore, it’s superconductive in high magnetic fields only when it’s cooled to about 30 K. A magnetic field tends to disrupt superconductivity when eddies of electric current created by the magnetic field move along with the main current.

In wires such as the Oak Ridge prototype, structural irregularities in the ceramic coating can preserve superconductivity by holding the eddies in place.

Using a laser in a vacuum, the Oak Ridge team vaporized a mixture of powders of the superconductive compound yttrium barium copper oxide (YBCO) and of barium zirconate, which doesn’t superconduct. As the vapor condensed on the ribbon, it formed a film of YBCO containing nanometer-scale disks of barium zirconate.

The Oak Ridge team reports that the barium zirconate disks stack up in orderly columns that span the film. The wires’ exceptional performance stems from how well such columns pin down the eddies, the group concludes.

Stephen R. Foltyn of the Los Alamos (N.M.) National Laboratory says that the Oak Ridge samples’ performance is similar to that of the wires that he and his colleagues previously made of YBCO with randomly scattered barium zirconate particles.

David C. Larbalestier of the University of Wisconsin–Madison says that the Oak Ridge wire is an advance because its superconducting layer is thicker than that of earlier prototypes. Although he doubts that the vaporization method is compatible with manufacturing, he says, “the new result … shows the technology has real legs.”

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