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Rensselaer Alumni Magazine Winter 2005-06
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ATRENSSELAER

NANOTECHNOLOGY

Nanotubes Form “Super Springs”

Buckled Carbon Nanotubes

Buckled carbon nanotubes under compression. Photo by Anyuan Cao

Carbon nanotubes have enticed researchers since their discovery in 1991, offering an impressive combination of high strength and low weight. Now a new study suggests that they also act like “super-compressible” springs, opening the door to foamlike materials for just about any application where strength and flexibility are needed, from disposable coffee cups to the exterior of the space shuttle.

The research, which is reported in the Nov. 25 issue of the journal Science, shows that films of aligned multiwalled carbon nanotubes can act like a layer of mattress springs, flexing and rebounding in response to a force. But unlike a mattress, which can sag and lose its springiness, these nanotube foams maintain their resilience even after thousands of compression cycles.

In foams that exist today, strength and flexibility are opposing properties: as one goes up, the other must go down. With carbon nanotubes, no such tradeoff exists.

“Carbon nanotubes display an exceptional combination of strength, flexibility, and low density, making them attractive and interesting materials for producing strong, ultra-light foam-like structures,” says Pulickel Ajayan, the Henry Burlage Professor of Materials Science and Engineering at Rensselaer and coauthor of the paper.

Carbon nanotubes are made from graphitelike carbon, where the atoms are arranged like a rolled-up tube of chicken wire. Ajayan and a team of researchers at the University of Hawaii at Manoa and the University of Florida subjected films of vertically aligned nanotubes to a battery of tests, demonstrating their impressive strength and resilience.

“These nanotubes can be squeezed to less than 15 percent of their normal lengths by buckling and folding themselves like springs,” says lead author Anyuan Cao, who did much of the work as a postdoctoral researcher in Ajayan’s lab and is now assistant professor of mechanical engineering at the University of Hawaii at Manoa. “After every cycle of compression, the nanotubes unfold and recover, producing a strong cushioning effect.”

The thickness of the nanotube foams decreased slightly after several hundred cycles, but then quickly stabilized and remained constant, even up to 10,000 cycles. When compared with conventional foams designed to sustain large strains, nanotube foams recovered very quickly and exhibited higher compressive strength, according to the researchers. Throughout the entire experiments, the foams did not fracture, tear, or collapse.

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