"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," said Pulickel Ajayan of Rensselaer Polytechnic Institute.
Ajayan and colleagues created arrays of vertically aligned multiwalled carbon nanotubes by chemical vapour deposition using ferrocene and xylene precursors. The nanotubes formed an open-cell foam system, with a porosity of around 87%.
In general, the more flexible the foam, the less strong it is. For example, increasing the void area in a typical foam increases its compressibility but rapidly lowers its strength. But carbon nanotube foams are unusual in that they are both extremely strong - with a compressive strength of 12-15 MPa - and very flexible. In contrast, typical low-density flexible foams, such as latex rubber and polyurethane, generally have a compressive strength of about 20-30 kPa.
"The scanning electron microscope study revealed that the nanotubes formed regular buckles along their axis," Anyuan Cao of the University of Hawaii at Manoa told nanotechweb.org. "The buckles can fold completely to allow a volume compression as large as 85% and then unfold to free length upon load removal. The most fascinating thing is that all the nanotubes unanimously buckle at the same wavelength towards the same direction - they do not buckle randomly."
The nanotube foams recovered rapidly when compression loading was removed, expanding at a rate of at least 2000 µn;m/s. And they had good fatigue resistance, suffering less than 15% deformation after thousands of cycles.
"One result that puzzled us for a while was the different buckling behaviour at the top and bottom of the film," said Cao. "The top part of the nanotubes remained nearly straight after many compression cycles, while the bottom part developed heavily folded buckles. To clarify this phenomenon, we dipped as-grown nanotube films into acetone and found that they always curled toward the bottom side (due to more shrinkage of this side) after acetone evaporates, indicating that the bottom part of film is less dense (and so less strong)."
According to Cao, the nanotube foams could have applications as cushioning pads, energy absorbing coatings, and damping layers. "Nanotubes with buckles can construct electromechanical devices such as actuators to move small objects," he said. "The high compressibility of nanotube films also make them serve as low-density open-cell foams in situations where properties such as low weight, high strength and fatigue resistance are desired."
Now the scientists plan to explore the electrical properties of compressed nanotubes, for example by monitoring the electrical conductivity change when nanotubes buckle. They'll also aim to tailor the mechanical properties of the nanotubes for different application requirements by controlling the buckling wavelength - which determines the strength of buckled tubes - or using aligned single-walled nanotubes.
The researchers reported their work in Science.