Need nanotubes? Go fly a kite

RICE (US)—Researchers have discovered a method for growing bundles of single-walled carbon nanotubes. The “odako” structures—named for the traditional Japanese kites they resemble—may lead to a way to produce meter-long strands of nanotubes, which by themselves are no wider than a piece of DNA.

Rice University chemist Bob Hauge leads the team that identified the new technique. An odako grown at Rice shows single-walled nanotubes (SWNT) lifting an iron and aluminum oxide “kite” as they grow while remaining firmly rooted in a carbon base.

Hauge and his coauthors say the odako bundles are analogous to the gigantic kites that take many hands to fly, hence the many lines that trail from them. 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 centimeters, and he says the process could eventually yield tubes of unlimited length.

Large-scale production of nanotube threads and cables would be a godsend for engineers in almost every field. They could be used in lightweight, superefficient power-transmission lines for next-generation electrical grids, for example, and in ultra-strong and lightning-resistant versions of carbon-fiber materials found in airplanes. Hauge says the SWNT bundles also may prove useful in batteries, fuel cells, and microelectronics.

To understand how Hauge makes nanokites, it helps to have a little background on flying carpets.

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 create thin layers of iron and aluminum oxide on a Mylar roll. They then removed the layers and ground them into small flakes.

In a mesh cage placed into a furnace, the metallic flakes would lift off and “fly” in a flowing chemical vapor. As they flew, arrays of nanotubes grew vertically from the iron particles in tight, forest-like formations. When they finished cooking and were viewed under a microscope, the bundles looked remarkably like the pile of a carpet.

While other methods used to grow SWNTs had yielded a paltry 0.5 percent ratio of nanotubes to substrate materials, Hauge’s technique brought the yield up to an incredible 400 percent.

In the latest study, published in the journal Nano Research, the team replaced the Mylar with pure carbon. In this setup, the growing nanotubes literally raise the roof, lifting up the iron and aluminum oxide from which they’re sprouting while the other ends 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.

Hauge and his team hope to follow up their work on flying carpets and nanokites with the holy grail of nanotube growth: a catalyst that will not die, enabling furnaces that churn out 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,” he says.

Rice University news: www.media.rice.edu