Breaking the Mold - ORNL Review Vol. 38, No. 3, 2005

Breaking the Mold

The potential of nanotube-polymer composites made at ORNL is attracting growing interest.

Natural bio-composites fascinate Ilia Ivanov, a University of Tennessee research assistant professor who makes light, strong, multifunctional composites of polymers and carbon nanotubes. "As we develop composites, a good model is the beetle, which surprisingly has a high level of complexity and multifunctionality," he says. "The beetle's exoskeleton is strong and lightweight yet has built-in thermal management, self-repair, and sensory capabilities.

"Today's advanced man-made composites, on the other hand, are generally heavy materials with a single function. Because of their properties, carbon nanotubes and other nanomaterials could break this paradigm, bringing unique multifunctionality to everyday composite materials, while making them stronger and lighter."

Over the past few years, with funding from the Department of Energy, National Aeronautics and Space Administration, Defense Applied Research Projects Agency, and ORNL's internal Laboratory Directed Research and Development Program, Ivanov—working with Dave Geohegan, Phil Britt, and many other collaborators across ORNL and UT—has explored methods of fabricating and testing the multifunctionality of nanotube composites. "Testing the properties of the small amounts of material we make has been difficult," Ivanov says.

"ORNL's George Pharr and Andrei Rar helped us examine the mechanical properties of aligned nanotube composites. We found that even though nanotubes occupy a tiny fraction of the polymer's volume, the resulting nanocomposite becomes stiffer in the direction in which the tubes align. We hope to apply this finding to the design of novel, directionally enhanced structural materials."

Compared with the pure polymer, the nanotube-polymer composite demonstrates much better thermal stability, suggesting that the nanocomposite could survive harsh environments. Furthermore, this material conducts heat almost anisotropically in the direction of nanotube alignment. According to measurements performed by Hsin Wang and his ORNL colleagues, the thermal diffusivity of nanotubes in this composite is near that of copper and aluminum, indicating that this material could serve as a thermal interface for such disparate applications as cooling computer chips and emergency personnel and astronauts in protective suits.

To understand how heat is transported in nanomaterials, Ivanov will be using a novel fixture designed by Mike Watson at NASA's Ames Laboratory. The device will enable thermal transport measurements on a single nanotube, nanowire, nanotube bundle, or tiny bit of a nanocomposite material.

Ivanov and colleagues have shown that the exposed ends of nanotubes protruding from a nanocomposite surface can serve as electrodes for the detection of small concentrations of redox-active species, chemicals that can donate or borrow electrons.

The ORNL team and UT's Bin Hu found that nanotubes can be perfect electron-and-hole conductors when illuminated, suggesting that nanotube composites could make brighter organic light-emitting devices using lower voltages, as well as photovoltaic cells that produce higher electrical currents from sunlight.

Because of their excellent electrical conductivity and high aspect ratio, nanotubes are perfect candidates for making a coating designed to conduct electricity. Such a transparent, antistatic coating could be useful in the canopies over airline pilots' cockpits, where electrical charges build up.

Ivanov showed that conductive nanotube membranes made in Geohegan's lab can be deposited on a metal, glass, or plastic surface. A nonconductive polymer can be made conductive if coated with a nanotube membrane.

"Our conductive nanotube membrane coating is much more flexible than the indium titanium oxide coating on today's computer monitors and television screens," Ivanov says. "These new membranes might someday replace the oxide coatings, the cost of which has doubled in the past few years."

Battelle, a member of the UT-Battelle partnership that has managed ORNL for DOE since 2000, is interested in making transparent, electrically conductive nanotube composites. Because of the excellent properties of ORNL nanocomposites, Battelle and ORNL are negotiating a cooperative research and development agreement.

"Understanding interactions at the interface of a nanomaterial and the matrix holds the key to very exciting science and promising, novel applications," says Ivanov, who is excited about the expanding interactions with researchers from outside organizations.

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