Work on the simulation was led by Wei Cai, an Associate Professor of mechanical engineering at Stanford, who holds a courtesy appointment in materials science and engineering. The Stanford scientists wanted to understand the manner in which van der Waals forces influence the growth of CNTs.
These forces are named for the Dutch physicist who first described the weak attractions that exist between molecules attractions that could not be explained by other known forces such as the chemical bonds that result when atoms share electrons.
Cai said that whereas van der Waals forces may not be critical in other types of structures, carbon nanotubes are so thin -- a mere atom or so thick in diameter -- that these minute forces could fundamentally affect them.
That is in fact what the simulation showed. Imagine a CNT attempting to grow straight, only to be bent to one side by the van der Waal attraction of another CNT crossing near its top, and perhaps bent to the other side by a different CNT that nears its bottom.
Taken together, the experimental results and computer simulation reinforce the findings that longer, less entangled CNTs would offer the best mixture of the desired characteristics strength, flexibility and heat transfer. But due to the van der Waals forces operating on these atom-thick carbon tubes, engineers are going to have to accept some bending and irregularity as they strive to create workable, though less than ideal, structures for dissipating heat.
"When you hear about nanotechnology it's usually about the superlatives, the strongest this, the thinnest," Goodson said. "But we think is the answers will lie in finding the right combinations of properties, something that's strong and conducts heat like a metal, but can flex and bend as well."
|Contact: Tom Abate|
Stanford School of Engineering