Nanotubes guide phonons with ease
Aug 14, 2007 1 comment
Tiny nanotubes made of carbon or boron nitride can act as phonon waveguides and retain their excellent heat transmission properties even when they are severely distorted, report physicists in the US. The researchers believe that the discovery could lead to the development of communications systems that use phonons to transmit information along nanotubes - in much the same way as light is used to carry data through optical fibres. (Phys. Rev. Lett. 99 045901).
Heat can be transported through a solid by phonons, which are quantized sound waves. Some physicists believe that phonons could be used to transmit information along a fibre if a suitable material could be found. However, any fibre capable of carrying phonons would have to be just tens of nanometres in diameter. While it is very difficult to make such fibres from most materials, carbon and boron nitride nanotubes can be made at such thicknesses.
Now, Alex Zettl and colleagues at the University of California at Berkeley have shown that such nanotubes are exceptionally good at transporting phonons – even when the nanotubes are severely deformed. The researchers fixed individual carbon and boron nitride nanotubes between a suspended heat source and sink that created a temperature gradient across the nanotube (see Phonon waveguide). The individual nanotubes had diameters ranging from 10 to 40 nm and were several micrometres in length.
This method, pioneered by Berkeley’s Arunava Majumdar, allowed the researchers to measure how much heat was transferred through a tube. The apparatus was also attached to an electron microscope so that the inner and outer diameters of the nanotube could be measured while it was being bent using a piezo-electric manipulator.
Previous work showed that deforming a multiwalled nanotube produces ripple-like structures about 10 nm in size on the nanotube’s inner radius. The researchers had expected these ripples to scatter phonons and so degrade the nanotube's thermal conductivity. To their surprise, they found that the thermal conductivity does not change at all even when the nanotubes are drastically bent (see Bent nanotubes).
Team member Chih-Wei Chang told physicsworld.com that the result could be important for overcoming the problems of heat dissipation in microelectronic devices. "Our findings would have immediate applications for heat management since nanotubes exhibit high thermal conductivity (around one order of magnitude higher than that of silicon) and are robust against mechanical deformations too," he said.
More importantly, the work may lead to the use of phonons as information carriers. The team has already made solid-state thermal rectifiers, tuneable thermal links and phonon waveguides using nanotubes. It is now building more "phononic" devices that are analogous to optical devices used in electronics and photonics.
About the author
Belle Dumé is contributing editor at nanotechweb.org