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 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 Polytechnic Institute and coauthor 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 at Rensselaer 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.
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.
The foams are just the latest in a long line of nanotube-based materials that have been produced through collaborations with Ajayan’s lab, all of which have exhibited tantalizing properties. Ajayan and researchers from the University of Hawaii at Manoa recently developed tiny brushes with bristles made from carbon nanotubes, which could be used for tasks that range from cleaning microscopic surfaces to serving as electrical contacts. And in collaboration with scientists from the University of Akron, Ajayan and his team created artificial gecko feet with 200 times the sticking power of the real thing.
Funding for this research was provided by the Focus Center-New York, which is part of the Interconnect Focus Center.
Nanotechnology at Rensselaer
In September 2001, the National Science Foundation selected Rensselaer as one of the six original sites nationwide for a new Nanoscale Science and Engineering Center (NSEC). As part of the U.S. National Nanotechnology Initiative, the program is housed within the Rensselaer Nanotechnology Center and forms a partnership between Rensselaer, the University of Illinois at Urbana-Champaign, and Los Alamos National Laboratory. The mission of Rensselaer’s Center for Directed Assembly of Nanostructures is to integrate research, education, and technology dissemination, and to serve as a national resource for fundamental knowledge and applications in directed assembly of nanostructures. The five other original NSECs are located at Harvard University, Columbia University, Cornell University, Northwestern University, and Rice University.