"These nanocomposites show great potential for a variety of applications in aircraft, spacecraft, automobiles, and even sensors for missile systems -- basically any structure that is exposed to vibration," said researcher Nikhil Koratkar, a mechanical engineer at Rensselaer Polytechnic Institute in Troy, N.Y.
Carbon nanotubes are up to 100 times stronger than steel at only a sixth the weight, and have attracted much research as potential additives to improve the strength of composite materials. Koratkar and his colleagues have instead however investigated whether nanotube fillers could help dampen vibrations, bleeding off energy that might otherwise damage fragile parts.
"Frictional sliding of the nanoscale fiber within the matrix has the potential to cause significant dissipation of energy," Koratkar said. Since the nanotubes are so tiny, they altogether possess an incredible amount of surface area for the volume they occupy, giving them a great capacity to dissipate energy.
"The main challenge is to disperse the nanotubes uniformly and to prevent the bundling of nanotubes so that the full impact of the surface area increase can be taken advantage of," Koratkar said. He and his colleagues reported their findings in the February 8 issue of the journal Nano Letters.
State-of-the-art damping techniques employ tapes stuck onto vibrating structures. These damping tapes can prove heavy and bulky. Moreover, they perform fairly poorly at high temperatures, capable of simply peeling off at roughly 90 degrees C, Koratkar explained. The researchers found nanotube fillers actually got better at vibration damping as temperatures rose. Higher temperatures make it easier for the nanotubes to slide within the composites and thus dissipate vibration. "Our new materials provide excellent damping at high temperatures," he said.
While damping tapes are better than nanotube additives when both are working at their optimal temperature regimes, damping tapes "are bulky and heavy and are externally mounted to the structure," Koratkar said. The nanotubes, on the other hand, are laced directly into the composite, and enhance damping while taking up just one or two percent of the weight of the composite.
Koratkar said nanotube additives could find use in automotive suspensions and engine mounts as well as aircraft skins and fuselage panels. In terms of how vibration dampening could help in space, "Satellites contain gossamer structures such as robotic arms and antennas that telescope out and can be several tens of feet in length. These structures are thin walled and highly flexible and therefore prone to vibration and stability problems during normal operational use," Koratkar said.
Koratkar added that carbon nanotube fillers could find use in the diaphragms of audio speakers to help improve sound quality by reducing the buzz linked with high bass levels. The sporting goods market might also prove a promising outlet, particularly for golf clubs and tennis rackets. "Manufacturers want tennis rackets and golf club shafts to be light and stiff, but without the annoying sting that comes from a bad shot," Koratkar said. "We are working with a couple of sports manufacturers and also a loudspeaker company in the U.K. A steel manufacturer for automotive applications is also very interested as is a hand tools manufacturer."
In future, the researchers will focus on incorporating nanotube fillers into epoxy matrices, which are more widely used within industry. "It is also important to ensure that the nanotube fillers do not have a negative influence on the fatigue life and structural integrity on the composite," Koratkar said.
"Their findings are very interesting and promising in vibration damping applications," mechanical engineer Nader Jalili at Clemson University in South Carolina said. He suggested future work should also explore how sensitive the vibration damping seen in these composites is to changes in temperature and determine what maximum temperature the composites might function at.
Vesselin Shanov, a chemical and materials engineer at the University of Cincinnati, noted future directions for vibration dampening nanotube research could explore increasing the amount of single-walled nanotubes in the composites, as well as exploring other carbon materials, such as multi-walled carbon nanotubes. Shanov's colleague Yeo-Heung Yun, a mechanical engineer, also noted their lab was growing arrays of oriented carbon nanotubes, which might also find use in such composites.