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David Bradley ISSUE #65
May 2007
Mark Schulz
The Long and the Short of It

A new composite material that acts as a catalyst to speed up chemical reactions has been developed to create arrays of the world's longest carbon nanotubes. These microscopic hollow fibers have intriguing mechanical, electrical, and optical properties. Researchers hope to exploit them as novel drug-delivery agents, as structural components in automotive and aerospace, as well as in the next generation of microelectronics.

Image courtesy of University Of Cincinnati

Carbon nanotubes were originally spun off from research into buckyballs, or fullerenes, the soccerball-shaped all-carbon molecules. They resemble extended buckyballs and can exist as single-walled versions or multiwalled nanotubes in which a nanotube nestles within nanotube. Numerous research groups have found ways to make these multiwalled nanotubes, but one property has remained elusive—length. Making nanotubes that are just a few billionths of a meter in diameter but several millimeters long has been achieved before, but Vesselin Shanov and Mark Schulz and their colleagues at the University of Cincinnati have made the longest yet. Moreover, they have produced long aligned carbon nanotube arrays rather than tangled heaps of the compounds.


 
One important feature of the technological development of carbon nanotube arrays is that they might be spun into fibers. Theoretical calculations on such materials suggest that they would be much stronger and lighter than any existing carbon or other fibers and be electrically conductive. They could therefore be used in lightweight, high-strength wires and connectors to replace copper wire in applications where reducing weight significantly would benefit performance and efficiency. The team has spent several years developing the technique for producing long carbon nanotubes. Success came with the development of a novel catalyst that allows their chemical vapor deposition (CVD) technique to produce aligned nanotubes of around 18 millimeters in length on a wafer-like layer of oxidized silicon. The synthesis is carried out in a highly reactive atmosphere of hydrogen hydrocarbon water vapor and argon gas at 750 Celsius.
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"We were able to grow the arrays up to 18 mm," Shanov enthuses. "We also produced a uniform carpet of 12 mm carbon nanotube arrays on a 10 cm wafer, which moves the invention into the field of scaled-up manufacturing for industrial application." The team has filed a patent application in the US.

J Phys Chem B. vol 110, 23920-23925; http://dx.doi.org/10.1021/jp057171g