Dec 302010

Scientists have discovered that tiny sonic booms cause carbon nanotubes to shear off from their graphene host material, by pressing in on them from either end (Image: Kim Lab/Brown University)

Single-walled carbon nanotubes are an essential component of many innovations in the field of nanotechnology, with particular potential in the fields of electronics, optics, and automotive technology.

Until recently, however, one of the processes for synthesizing them had not fully been understood. More precisely, no one was sure exactly what caused the nanotubes to break, or how to better control the process for the creation of higher-quality tubes. Now, researchers from Rhode Island’s Brown University and the Korea Institute of Science and Technology (KIST) think they have it figured out – it all comes down to tiny sonic booms pressing in on the tubes from either end.

In the production process that was studied, single-walled carbon nanotubes are immersed in a solution (usually water), which results in jumbled bundles of nanotubes that resembles a plate of spaghetti. Those bundles are then subjected to high-intensity sound waves that create cavities (or partial vacuums) in the solution. The bubbles arising from those cavities expand and collapse with such violence that the heat at each bubble’s core can reach 5,000 degrees Kelvin – close to the temperature of the surface of the sun. When the bubbles collapse, each bubble radius shrinks at its maximum acceleration 100 billion times greater than gravity. Something in the process causes the nanotubes to break at seemingly random points, meaning scientists then have to use sieves to sort the tubes by length.

It had initially been thought that the high temperatures caused the tubes to tear. German scientists then put forward the theory that sonic boomlets, caused by the rapidly-collapsing bubbles, caused the nanotubes to be pulled apart – “like a rope tugged so violently at each end that it eventually rips.”

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