Forming foam

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 foam-like 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 November 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 trade-off 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 Polytechnic Institute and co-author of the paper.

Carbon nanotubes are made from graphite-like 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. 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 stabilised 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.

And their intriguing properties do not end there. Nanotubes also are stable in the face of extreme chemical environments, high temperatures, and humidity, all of which adds up to a number of possible applications, from flexible electromechanical systems to coatings for absorbing energy.

Buckled carbon nanotubes under compression. Photo: Cao/RPI