Simulation “Bumps” Nanotubes
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They based the simulation on a classic physics “ball and spring” model — connecting the atoms with a network of massless springs. Like ripples in a pond, the movement of one atom affects the movement of its neighbors, and its neighbors’ neighbors, on through the tube.
Physicists use “normal” vibration modes to describe how such atomic systems oscillate. “Any type of vibration can be characterized as a combination of these very basic types of vibration,” Wood says. “We were looking at inputting the types of vibrations to see how much is transmitted through the length of the tube.” Thermal conduction depends on these properties.
“It turns out nanotubes, because they’re tubes, have some peculiar normal modes,” Wood says. In some cases, a compression wave travels the length of the tube. In others, the nanotube may pinch in certain spots, or breathe outward, expanding along its radius. And in one basic vibration, the ends are fixed and the middle of the tube moves up and down, like a guitar string.
In the simulation of imperfect nanotubes, heavier isotopes randomly replace carbon-12 atoms. The key question is how those imperfections will change the normal modes. “The jury is still out,” Wood says. During his summer at LBNL, he just had time to help finish the simulation for pristine nanotubes. The program now can simulate impure nanotubes, but there hasn’t been time to run it, Moore says. “The next step would be to do a bunch of nanotube sizes and impurity concentrations,” he adds. He hopes to eventually publish a paper on the research.
An example of a mixed radial and longitudinal distortion of a (10,10) armchair nanotube, viewed along the primary axis.
Click on the image for a larger version.
Wood’s biggest contribution, Moore says, was creating visualizations — short computer animations — of the nanotube vibrations. “An important step to understand what’s going on is to take that data and put it in visual form,” Moore says, and Wood brought expertise in that area.
Visualization also is a key component in Wood’s doctoral research at MIT, where he uses computers to simulate the properties and behaviors of materials. He focuses on ion transport through a class of materials called superionics. “They’re solids that have liquid-like diffusive properties,” he says. Ions — atoms that carry a negative or positive electrical charge as a result of gaining or losing electrons — move through these solids as if they were liquids at the atomic level. Superionic materials are important for alternative energy, particularly fuel cells and storage media for the hydrogen they use.