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Nanotubes Take Flight

07 August 2009

In newly published research, Hauge's Rice University team describes a method for making Odako, bundles of single-walled carbon nanotubes (SWNT) named for the traditional Japanese kites they resemble.

It may lead to a way to produce meter-long strands of nanotubes, which by themselves are no wider than a piece of DNA.

Last year, Hauge and colleagues found they could make compact bundles of nanotubes starting with the same machinery the U.S. Treasury uses to embed paper money with unique markings that make the currency difficult to counterfeit.

Hauge and his team used this printing process to deposit elements onto a long Mylar roll. The top layer consisted of tiny iron particles that cause nanotubes to grow under proper conditions. Under that was a layer of aluminium oxide, and on the bottom was a release layer the team could activate with a solvent to loosen the aluminium oxide and iron, which was ground into flakes with a mortar and pestle.

In a mesh cage placed into a furnace, the flakes would lift off and “fly” in the flowing chemical vapour while arrays of nanotubes grew vertically in tight, forest-like formations atop the iron particles. After heating, when viewed under a microscope the bundles of tubes looked like the pile of a carpet.

In this case, the lines are nanotubes, hollow cylinders of pure carbon. Individually, they're thousands of times smaller than a living cell, but Hauge's new method creates bundles of SWNTs that are sometimes measured in centimetres, and, he said, the process could eventually yield tubes of unlimited length.

In the latest research, the team replaced the Mylar with pure carbon. In this setup, the growing nanotubes lift up the iron and aluminium oxide from which they’re sprouting, while the other ends of the tubes stay firmly attached to the carbon. As the bundle of tubes grows higher, the catalyst becomes like a kite, flying in the hydrogen and acetylene breeze that flows through the production chamber.

The team discovered that Odako grow not only on flat layers of Grafoil, a flexible graphite material, but also on carbon fibre, even when that fibre is woven into a material. Photos show the Odako follow the rounded form of the fibres even while growing to great lengths, though the researchers note shorter may be better for the manufacture of composite materials. The SWNTs remain bound to the carbon fibre while the free ends act like hooks in Velcro, grabbing onto the epoxy used in composites.

They noted Odako growth may be possible on such other materials as quartz fibres and a variety of metals.

Hauge and his team are now looking for a sustainable catalyst to enable the production of continuous threads of material.

“If we could get these growing so they never stop -- so that, at some point, you pull one end out of the furnace while the other end is still inside growing -- then you should be able to grow meter-long material and start weaving it. You’ve got to somehow make that catalyst stay alive forever,” he said.

“That’s a very difficult thing to do, but it’s not an impossible task.”

The image illustrates microscopic bundles of Odako grown at Rice University. The single-walled nanotubes lift iron and aluminium oxide ""kites"" as they grow, whilst remaining firmly rooted in a carbon base.






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