The family of carbon cages known as fullerenes has already shown a vast range of mechanical, chemical, magnetic and electrical properties, including high-temperature superconductivity. Now a Japanese scientist has found yet another attribute of these clusters: they apparently form tubes that are thinner, more perfect in their molecular structure, more resistant to chemical attack and almost certainly stronger than any other fiber.
Sumio Iijima, a scientist at NEC Corporation, made the discovery this past May while examining sooty deposits on a carbon-arc electrode he had used to generate fullerenes. There he found tiny, tubular fibers, up to a micron long, tiled in hexagonal arrays that tightly bind the carbon atoms and that terminate in faceted, conical caps. His first paper describing the tubes was scheduled to be published in Nature in early November.
Because the family is named for its 60-atom archetype, buckminsterfullerene, or buckyball, Iijima's fiber was quickly dubbed buckytube. "It has a very interesting structure - to me much more interesting than buckyballs," Iijima says.
"First, it is helical--such structures have never been found before in inorganic material. Second, it is cylindrical-a very unusual crystal lattice." He later corrected himself, noting that Robert L. Whetten of the University of California at Los Angeles had proved that the fullerene C76 is helical. (Whetten says he thinks many or all of the larger fullerenes are helical.)
Iijima deduced the structure of his tubes in part from images produced by the 500,000-fold magnifications of a high-resolution transmission electron microscope, in part from electron diffraction patterns. The images showed cross sections of two or more tubes 0.34 nanometer apart - a distance roughly equal to the gap between layers of carbon atoms in graphite and to the radius of a buckyball.
The diffraction patterns indicate a crystalline structure of hexagonal carbon rings, such as are found in graphite sheets, but with a twist. The tubes are formed of sheets of hexagons whose Rows look as if they had been curled up and pasted to those of a nearby row, creating a spiral.
Because the tubes enclose channels as small as two nanometers wide, they hold the possibility of weird physical properties. "It could be a quantum pipe," Iijima speculates, referring to a hypothetical structure through which electrons might "tunnel" effortlessly. "They could be superconducting. Who knows?"
The possible mechanical strength of buckytubes is a more immediate boon. "It could be the strongest fiber that exists, maybe the strongest that can exist," lijima says. Its strength flows from the nature of carbon-carbon bonds, on the one hand, and the nearly flawless structure of the tubular crystals, on the other. "A carbon fiber is the strongest thing in nature, very stiff for its weight," says Mildred S. Dresselhaus of the Massachusetts Institute of Technology. "Buckyfibers have very few defects and so in that sense are better than graphite."
Dresselhaus in fact predicted the tubes before their existence became generally known, while discussing hypothetical molecules with Richard E. Smalley of Rice University, a fullerene pioneer. Smalley has since propounded the theory that buckytubes will heal their open ends when broken. He cites Leonhard Euler, the 18th-century Swiss mathematician, who proved that a hexagonal sheet of any size can close into a polyhedron only if it adds exactly 12 pentagons to the mix. Smalley draws a fascinating conclusion: if buckytubes are indeed closed - and hence true fullerenes - they might tend to "heal" themselves. A broken buckytube should tie up dangling bonds by producing new pentagons at the open end, thus closing it.
More interesting than how the fibers close is how they start growing in the first place. "The fibers have to be nucleated," Dresselhaus says. "They would start from clusters, but instead of becoming balls they grow like fibers. " If such growth could be extended, fine-tuned and scaled to macroscopic yields, chemists would be able to cook up inch-long buckytubes having any number of layers.
Iijima says he is trying to optimize for length and for yield, but he just laughs when asked how he hopes to do it and what results he expects to obtain. He says he also wants to test the self-healing theory by administering a tube-busting jolt of electricity and seeing what happens at the break points.
Iijima's findings have not yet been reproduced. Yet word of his discovery only began to percolate through the international fullerene fraternity in mid October, after Iijima gave a presentation at a conference held in Richmond, Va. Whetten says that if buckytubes are confirmed, researchers would begin reconfiguring their labs "in about a month." The preliminary nature of lijima's results did not stop many of the 200 or so participants at the Richmond conference from suggesting off-the-cuff applications. A physicist imagined arrays of parallel buckytubes functioning as a gamma-ray window, enabling the high-energy radiation to propagate through their pores while discouraging the diffusion of gases, such as air.
Engineers speculated about carbon fibers that would outperform graphite as a matrix for carbon-carbon composites, extremely strong and light materials used in high-performance aircraft. But graphite fibers are vulnerable to the slightest scratch, which opens the broken ones to oxidafion. A buckytube composite capable of healing itself would retain its strength.
Science lags ever less behind fiction. David Jones, writing under the pen name Daedalus, imagined fullerenes 19 years before they were detected. In Nature this past June, he dreamed of superstrong "graphitic foam" made of carbon tubes - weeks after Iijima had discovered buckytubes.
by Philip E. Ross
Scientific American
December 1991