A Limitless Potential
ORNL is producing carbon nanotubes 100,000 times smaller than a human hair that could impact much of the world's economy.
Researchers believe carbon nanotubes may prove to be the most promising nanoscale materials for multifunctional applications. These hollow tubes of carbon often have multiple, concentric layers of carbon sheets, like rings of a tree. A single-wall carbon nanotube (SWNT)—one sheet of carbon atoms rolled into a tube—has special properties resulting from a structure much more like that of a one-dimensional molecule than bulk graphite. A typical SWNT is a few microns long and approximately 100,000 times smaller than a human hair, with a diameter around one nanometer.
Despite being long and thin, nanotubes are extremely strong and difficult to break. An optimized composite made from SWNTs is predicted to be 100 times stronger than stainless steel, with only one-sixth the weight.
The electronic properties of nanotubes are equally impressive. Depending on how carbon atoms are arranged, a particular nanotube may be either metallic or semiconducting. Because metallic nanotubes have been shown to conduct electrical currents without loss, they are envisioned as candidates for next-generation wires and power transmission cables. However, roughly two-thirds of SWNTs synthesized today are semiconducting. Requiring little power to switch them "on and off," these nanotubes are already being developed as transistors for next-generation "molecular" electronics. Semiconducting SWNTs are electroluminescent, emitting light when injected with electrons and holes.
Nanotubes readily emit electrons under a remarkably low applied electric field. New field-emission displays and ultrathin X-ray sources based upon this effect are now available.
Experiments demonstrate that individual SWNTs conduct heat more efficiently than almost any other material, making nanotubes potential tools to cool everything from laptop computer chips to the leading edges of wings on hypersonic aircraft.
The sharpness and length of carbon nanotubes make them ideal for probing surfaces, cells, and even molecules. One finding suggests that nanotubes combined with catalytic metal nanoparticles can store enough hydrogen to make lightweight, hydrogen-powered vehicles practical.
Because of the seemingly unlimited applications of carbon nanotubes, the race is on worldwide to find out how to manufacture them economically in large quantities. ORNL researchers are exploring applications of carbon nanotubes (see next article), but their chief focus has been the basic science of how nanotubes grow, how to sustain their growth, and how to grow them with high levels of purification.
For more than seven years, David Geohegan and Alex Puretzky, both of ORNL's Condensed Matter Sciences Division, have made nanotubes by creation of a high-temperature plasma with a laser. Using time-resolved optical probes of this laser ablation process to learn how SWNTs grow, they detected a miniature "big bang" when laser light vaporizes a carbon target impregnated with catalytic nickel and cobalt powders. About a millisecond after a hot plasma is formed at 3500°C, both the carbon and metal condense into tiny clusters and particles from which only SWNTs grow at rates of 1-5 microns/second while floating in hot argon within a 1200°C furnace. This relatively high growth rate lasts for only about a second. Growth then mysteriously stops.
At ORNL laser ablation produces 1 gram a day of carbon nanotubes. ORNL's new Advanced Laser Processing and Synthesis Facility promises to make SWNTs up to 30 times faster. Phil Britt's team in ORNL's Chemical Sciences Division has developed methods to purify laser-grown nanotubes by removing the unconverted amorphous carbon and almost all the metal catalyst.
Researchers are growing three-millimeter-tall "forests" of aligned carbon nanotubes through a process of chemical vapor deposition, in which hydrocarbon gas in a hot furnace is introduced to metal catalyst nanoparticles. The researchers use lasers and movie cameras to measure the lengths of trillions of nanotubes while they grow, hopefully putting the nanotube forests on the path toward multifunctional applications.
Although measured in an almost infinitesimal small scale, the scientific and economic potential of these nanotubes appears to be literally without limits.
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