Simulation “Bumps” Nanotubes
Massachusetts Institute of Technology
Lawrence Berkeley National Laboratory
Story by Thomas R. O’Donnell
Sometimes, things just need a nudge — including the minuscule carbon straws called nanotubes.
Brandon Wood, a DOE CSGF fellow and materials science doctoral student at Massachusetts Institute of Technology, did the prodding in a computer simulation he helped develop during his summer 2005 practicum at Lawrence Berkeley National Laboratory (LBNL). The tests explored how a type of impurity affects nanotubes’ heat conductivity. Nanotubes, about 10,000 times thinner than a human hair, are carbon atoms joined into a sheet of hexagons that’s rolled into a tube. Keeping computer chips cool could be one use for them, says LBNL researcher Joel Moore. “Chips have to dissipate a lot of heat in a small space,” says Moore, who supervised Wood’s practicum. “You want to put something there that will really pull the heat away. Nanotubes are good for that because they have the highest thermoconductivity we know of — especially if they’re pure nanotubes” made entirely of one carbon isotope, like carbon-12.
Nanotubes made of one isotope are more expensive to make — and nanotubes aren’t cheap to start with. High-purity ones sell for more than $200 per gram, compared to about $25 per gram for gold. It’s more common for the tubes to contain a mix of isotopes with different weights, like carbon-13 or carbon-14.
The “torsional” mode of a (4,4) armchair nanotube. This mode represents a twisting motion around the primary axis of the tube.
Click on the image for a larger version.
Wood’s job at LBNL was to help create computer simulations of how these random isotopic mass defects scatter phonons, the primary mechanism for conducting thermal energy, or heat. The research has broad applications, Wood says, because the program he helped devise can simulate an entire class of defects — not just isotopic ones. High-performance computers, like those at the National Energy Research Scientific Computing Center at LBNL, are necessary for such simulations. “You can do the pristine nanotube case just by using equations, but if you want to do randomized order on a potentially infinite system, you need to use computers,” Wood says.
Wood collaborated with Padraig Murphy, a physics graduate student at the University of California – Berkeley, to create the simulation. It uses vibration as a model for thermal conductivity, because heat at the atomic scale is vibration. The higher the temperature, the faster atoms move.
“We artificially create an input wave — bump [the nanotube] from one side and watch how this propagates through the system” of atoms, Wood says. Moore’s group was running such simulations on two-dimensional models — a flat sheet of carbon atoms. Wood and Murphy turned that into a three-dimensional tube.