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Heat Transport in Nanotube Composites:

Carbon nanotubes (CNTs) possess thermal conductivities that are a thousand times better than that of polymers. However, when they are incorporated into composites, the gain in thermal conductivity is usually on the order of a few percent. Why?

Chemical engineers Dimitri Papavassiliou and Lloyd Lee were keenly interested in the problem, and wanted to simulate heat transport efficiently. I collaborated with them in developing an algorithm that looks at the motion of random walkers in random (see figure) and alligned arrays of CNTs. By injecting hot walkers on one side and removing them on the other we could relate their distribution to the thermal conductivity of the composite.

The trick was fixing the random walk algorithm so that heat was instantly distributed along the CNTs. It's quite subtle: mess it up and you violate the Second Law of Thermodynamics. We found that alligning the CNT's helps in thermal conductivity, and are using this to create "thermal switches."

The original problem was the poor performance in CNT composites. One explanation is the mismatch of their phonon modes. Initial experiments done by Brian Grady's group agree with our predictions. More to come soon!

Publications:

• “Random walks in nanotube composites: Improved algorithms and the role of thermal boundary resistance,” by H.M. Duong, D. V. Papavassiliou, L. L. Lee, and K.J. Mullen, App. Phys. Lett., 87, 013101, 2005.
• “Computational modeling of the thermal conductivity of single-walled carbon nanotube–polymer composites,'' Hai M Duong, Dimitrios V Papavassiliou,  Kieran J Mullen and Shigeo Maruyama, Nanotechnology 19, 065702 (2008).
• "Calculated Thermal Properties of Single-Walled Carbon Nanotube
  Suspensions,'' Hai M. Duong, Dimitrios V. Papavassiliou, Kieran J. Mullen, Brian L.Wardle, and Shigeo Maruyama, J. Phys. Chem. C, 2008, 112 (50).
• "Random walks in composites with high conductivity," K. Mullen and D. V. Papavassiliou, in preparation.