Imagine an ultra-lightweight material that could lend incredible strength to a spacecraft and at the same time provide an array of other functions. For example, this material could serve as a superconductor that carries electricity with virtually no resistance. It could also collect and channel the sunlight’s energy to both propel the spacecraft and heat its interior to keep the astronauts comfortable.
With applications like this in mind, the National Aeronautics and Space Administration is providing ORNL with funding to find ways to line up single-walled carbon nanotubes (SWNTs)hollow tubes whose carbon atoms are arranged in a hexagonal configuration. Each of these tubes could be as long as a hair is wide, but their widths are 1/10,000th that of a hair.
Aligning SWNTs is much more difficult than growing them, according to David Geohegan, a researcher in ORNL’s Solid State Division (SSD). He and the University of Tennessee’s Alex Puretzky have been producing SWNTs by laser ablation for several years, using cameras and other diagnostic tools to study how they form. They use a laser beam to instantly vaporize a target consisting of nickel-cobalt and carbon inside an oven filled with argon gas at a temperature of 1200°C. The SWNTs grow microns long in just a second’s time, emerging from tiny metal nanoparticles that digest carbon clusters as they float inside the oven.
Geohegan prefers SWNTs to multi-walled nanotubes because of their structural perfection, which gives them their unique electronic and optical properties, as well as their incredible strength. “The dream we are pursuing is a high-strength, lightweight composite material,” Geohegan says. “Since carbon nanotubes have 100 times the tensile strength of steel with one-sixth the weight, we would like to use them to strengthen composite materials. Carbon nanotubes could reinforce a polymer, carbon, or metal matrix.”
One method that will be tried at ORNL to line up SWNTs is gel electrophoresis, the standard technique for DNA sequencing. The tubes dispersed in a specific solution can be charged and then dragged through the gel, using an electric field.
Geohegan is also interested in characterizing defect sites in SWNTs because they influence the chemical, electrical, optical, and mechanical properties. Studies show that perfect nanotubes do not strongly bond to a polymer material in which they are embedded. When the polymer matrix is bent, the tubes pull out.
To make a reliable composite, covalent bonds must be present between the SWNTs and the polymer matrix. Some defect sites on SWNTs contain functional groups, such as carboxylic acids (COOH), which can be used to covalently bond polymers to the nanotubes. Another approach to the nanotube-polymer problem is described in the “Carbon Nanotubes and Chemistry” sidebar.
ORNL researchers are trying out different types of particle beamsranging from ions to neutronsto create defects. Using Raman spectroscopy, they are characterizing the nature and number of defects on various SWNT samples in search of the right defect concentration.
“To fabricate a carbon-carbon composite, we deposited a thin layer of nanotubes on a silicon substrate and then used our pulsed-laser deposition technique to encapsulate this nanotube layer into an amorphous diamond-like carbon film,” Geohegan says. “As a result, we produced a wear-resistant amorphous diamond coating containing an electrically conducting net of single-walled nanotubes.”
A third type of nanotube composite that might be made would have a metal matrix. Craig Blue of ORNL’s Metals and Ceramics Division and Geohegan have a grant from the Defense Advanced Research Projects Agency to produce SWNTs by chemical-vapor deposition; align the nanotubes into a matte; and infiltrate it with powders of molybdenum, titanium, or tungstenmetals that react with carbon. This material would then be rapidly heated using infrared radiation from the plasma arc lamp at the Infrared Processing Center, a Department of Energy user facility at ORNL. The goal here would be to create a nanotube-metal matrix that might be used to make extremely strong structural materials for aircraft and spacecraft and for long power-transmission lines and suspension bridges.
Little carbon tubes could make a big difference in structural materials.
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