Carbon
Nanotubes and Chemistry
It looks like soot, but it’s really a tangled mass of black carbon nanotubes mixed with metal and amorphous carbon particles. In transmission-electron-microscopy images, it resembles cooked spaghetti and meatballs. When Phil Britt of ORNL’s Chemical Sciences Division receives this laser-ablation product from Dave Geohegan, his group purifies the raw material by first using nitric acid treatment. The material is then heated in air from 450°C to 550°C, to oxidize the remaining carbonaceous impurities. The nanotubes are then treated with hydrochloric acid to remove the remaining nickel-cobalt catalyst particles added during laser ablation, to induce the growth of single-walled carbon nanotubes.
“When we get the laser ablation product, 15% of its weight is metal,” Britt says. “We have succeeded in purifying the product so that we have 99% carbon nanotubes and less than 1% metal by weight.”
The major challenge in this purification process is to avoid destroying the tubes. The oxidation process in the furnace used to burn out the amorphous carbon might destroy 80% of the nanotubes.
“We found that carbon nanotubes are amazingly stable,” Britt says. “We can heat them to 600°C in air before they get oxidized and form carbon dioxide. Most organic material is destroyed at 400°C in air.”
Britt and Geohegan are working on the problem of creating a nanotube-polymer composite that could be used for spacecraft, for example. They and their colleagues in ORNL’s Solid State Division are trying to determine how to incorporate nanotubes in thermoplastic polymers, such as polymethyl methacrylate and polystyrene. This project is receiving internal funding from ORNL’s Laboratory Directed Research and Development Program.
“We want to break up the bundles of carbon nanotubes and disperse them into the polymer,” Britt says. “Thermoplastic polymer melts and flows when heated. We will try to make it flow so as to manipulate and align the carbon nanotubes in the polymer matrix before it cools.”
Britt thinks multiwalled nanotubes (each being a tube inside a tube inside a tube) might be integrated into the polymer composite more easily than single-walled tubes. “Multiwalled nanotubes have many ends, or handles, to interact with a polymer matrix,” he says. “They may allow us to add functionality so that we can covalently bond the tubes to the polymer matrix.”
Mark Dadmun, a University of Tennessee (UT) polymer expert; a UT graduate student; and a postdoctoral scientist are working with Britt on this project. It is hoped that the nanotube and polymer material they are working with will be at least as compatible as the researchers themselves.
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