Absorbing technique

Sports equipment, space research and car engines could benefit from the damping power of carbon nanotube additives. Stuart Nathan reports.

For several years researchers have been investigating carbon nanotubes in search of the properties they can give composites. Strength, semiconducting ability and ability to carry extra materials have all been examined, but a team from Rensselaer Polytechnic Institute has now found another possibility — vibration damping.

The finding could be used across a wide range of applications such as space research, sporting equipment, car engines, and audio equipment, according to lead researcher Nikhil Koratkar. The technique could be particularly useful because nanofibres tend to add strength as well, unlike traditional damping additives which are soft.

The key to the property, Koratkar and colleagues explain, is the way polymers slip against nanotubes in a composite. The ‘slip-stick’ friction at the interface between polymer and nanotube — where the shear force applied builds up until it reaches a certain value, at which point the materials ‘slip’ against each other — allows the material to absorb the energy of the vibration.

According to Koratkar, the damping power of carbon nanotube additives is a dramatic improvement over conventional viscoelastic damping polymers. ‘These are usually tapes that are bonded externally to the vibrating structure,’ he said. ‘They frequently peel off, and are also heavy. Our concept is to inject the damping directly into the structure by infiltrating nanotube fillers into the matrix resin.’

Not only is this built-in damping structurally more robust, it is also mechanically more efficient. ‘While traditional damping polymers are very soft, nanotubes offer the promise of enhancing mechanical properties such as strength and stiffness, in addition to structural damping,’ said Koratkar.

Another advantage of nanotube damping is the way the substances perform at different temperatures. Traditional viscoelastic polymers only work at relatively low temperatures; as they get hotter, performance declines rapidly. But nanotubes have an extremely high thermal conductivity and are stable at high temperature. in fact, Koratkar’s team has found that the hotter a nanotube-polycarbonate composite gets, the better the slip-stick mechanism works.

This, the team believes, is because raising the temperature makes the carbon-chain backbones of the polymer part of the matrix more flexible and reduces the force exerted by the polymer on the nanotubes.

This makes the polymer-nanotube system very attractive for space applications, said Koratkar. As spacecraft orbit the Earth, they experience very large swings in temperature, suggesting a need for a temperature-resistant damping mechanism, built into the material itself. ‘Our new materials provide excellent damping at high temperatures, suggesting that these nanocomposites show great potential for a variety of applications in aircraft, spacecraft, satellites, cars, and even sensors for missile systems —basically any structure that is exposed to vibration,’ said Koratkar.

Closer to the ground, the systems could have potential in the high-value area of premium audio and sporting goods. Incorporating the nanotube-damped composites into the diaphragms of bass speakers could dramatically reduce the buzzing noises associated with very low-frequency signals. And building the material into high-end tennis rackets and golf clubs could give players an extra edge. ‘Manufacturers want tennis racket and golf club shafts to be light and stiff, but without the annoying sting that comes from a bad shot,’ said Koratkar.

The next stage of the research is to try to expand the types of polymer that can be loaded with the vibration-damping nanotubes from polycarbonate to epoxy matrices, which are more widely used in real-world applications, said Koratkar.