Carbide whiskers shrink to nanometer size.

The tiny fibers of silicon carbide used to strengthen airplane wings and mountain bike frames just became a lot tinier. Researchers have created silicon carbide rods less than 30 nanometers in diameter, about one-thousandth the size of those used in high-performance materials today.

Fibers suspended in a ceramic or a metal reinforce the material, creating a strong, lightweight composite. Using a larger number of thinner fibers, which are less likely to contain weakening defects in their crystal structures, provides greater strength and surface area to bond with the ceramic or metal.

In addition to silicon carbide, the researchers produced titanium, niobium, iron, and boron carbide rods, reports Charles M. Lieber of Harvard University in the June 29 Nature. Each compound showed useful electrical or mechanical properties.

Eventually, carbide nanorods--rods on the nanometer scale--may serve as sensors, magnetic recording heads, or a component of superconducting materials that could raise their current-carrying capacity, Lieber says. They could also be used as atom-size wires in computer chips or as probes in microscopy, adds Daniel T. Colbert of Rice University in Houston.

Lieber formed the nanorods by exposing carbon nanotubes--basically, elongated buckyballs--to oxide or halide vapor of silicon or a metal. The hollow tubes transformed into solid carbide rods of about the same size. This technique has been used before, but only to make much larger carbide fibers.

"In a way, it was a logical extension," Lieber says. "Silicon oxide is known to react with fibers, but the carbide forms on the surface. It's difficult to penetrate them."

Carbon nanotubes come in two "flavors," says Colbert. One resembles a tube of graphite capped on the ends by two hemispheres of a buckyball, while the other, slightly larger form consists of 10 to 20 concentric cylinders of carbon. Lieber used the latter as a template for the carbide nanorods.

The key to making such small fibers was starting with nanotubes, says Richard E. Smalley of Rice, a codiscoverer of buckyballs who currently works with carbon nanotubes.

"To make a nanometer structure . . . that will survive in the real world, you must construct a surface where the atoms are happy," Smalley says. "We're beginning to learn about nanotubes and how to use them as a . . . template for other elements. Carbon is teaching them how to do it."

Lieber's carbide nanorods were between 2 and 30 nm in diameter and over 1,000 nm long. Although many of the rods were smooth and straight, some of them had unusual shapes. Lieber observed sawtooth titanium carbide nanorods and niobium carbide ones with a helical structure.

The properties and shapes of the nanorods open up many possible applications. Lieber found the niobium carbide nanorods to be superconducting and the iron carbide ones ferromagnetic. Hongjie Dai, a physical chemist who works with Lieber, speculates that passing a current through a corkscrew-shaped nanorod would produce "a very interesting magnetic field profile."

Lieber and his colleagues are continuing to characterize the properties of the nanorods, not an easy task considering the difficulty of handling such small structures. They plan to continue studying possible applications and making nanorods out of different carbides. "In principle, you can make all the carbides that exist," Dai says.