Wrapping carbon into superstrong tubes.
Although Clarke's space elevator remains firmly embedded in the realm of science fiction, his notion of fabricating extremely tough, microscopically thin, flawless fibers from carbon gained credibility with the discovery that sheets of carbon atoms can wrap themselves into microscopic tubes (SN: 11/16/91, p. 310). Produced in a high-voltage electric arc, the resulting nanotubes appear to have perfectly formed structures, consisting of networks of carbon atoms arranged in a hexagonal pattern.
"This is a new kind of crystal;' says chemist Richard E. Sinalley of Rice University in Houston. If it were possible to manufacture such tubes in meter lengths, "they would be the strongest fibers that we could ever make from anything."
Nanotubes normally form as a thick black deposit on the end of a carbon electrode. Close examination of this deposit reveals a fibrous interior and a hard outer shell of fused carbon.
The entire structure seems organized as a hierarchy of tubes, says Thomas W. Ebbesen of the NEC Fundamental Research Laboratories in Tsukuba, Japan. The visible fibers consist of bundles, which themselves consist of smaller bundles, and so on, down to the carbon nanotubes. Each nanotube, in turn, typically consists of two or more tubules nested within one another to form a layered assemblage. These multilayered tubes range from 1 to 20 nanometers wide and up to 3 microns long.
Smalley and his co-workers have now taken a tentative step toward growing such tubes as continuous fibers. Basing their approach on the assumption that high electric fields are largely responsible for keeping nanotubes growing in an orderly fashion, the researchers create a high voltage between a carbon electrode and a sharp, needle-like electrode. Then they introduce benzene vapor, or some other hydrocarbon, into the narrow gap between the electrodes.
Using a laser to heat the hydrocarbon molecules and gradually increasing the separation between the electrodes, the researchers create a carbon fiber just 50 nanometers wide and up to a centimeter long. The product, however, lacks the perfection of true nanotubes. Smaller structures branch off from the main fiber at various places along its length. Only tiny portions actually appear tubular.
"We're beginning, but it's still very crude;' Smalley says. "It's not the fullerene fiber of our dreams."
The presence of hydrogen from the hydrocarbons may account for some of the defects, but there's no guarantee that this particular approach will ultimately lead anywhere, Smalley admits. He described this work at last week's American Physical Society meeting, held in Seattle.
Meanwhile, Ebbesen and his colleagues are exploring ways of stuffing carbon nanotubes with metal atoms to create microscopically thin wires. In earlier work, NEC researchers had managed to introduce lead into nanotubes (SN: 1/30/93, p.69). Because nanotubes normally form with capped ends, lead atoms had somehow broken the seals and entered the tubes.
"We [now] know how to open nanotubes in large quantities in a very simple way," Ebbesen reports. That makes it possible to fill tubes with metals other than lead.
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|Title Annotation:||carbon nanotubes|
|Date:||Apr 3, 1993|
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