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Walker, Ebonee
Ph.D. in Interdisciplinary Materials Science, May 2012

Research Information

Dissertation Title
Influence of Phonon Modes on the Thermal Conductivity of Single-Wall, Double-Wall, and Functionalized Carbon Nanotubes

Ph.D. Thesis Advisor
Greg Walker

Ph.D. Committee
Deyu Li, Mechanical Engineering
Clare McCabe, Chemical and Biomolecular Engineering
Keivan Stassun, Physics
Norman Tolk, Physics
Richard Mu, Fisk University

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Abstract. Carbon nanotubes (CNTs) are modeled using the Tersoff-Brenner potential and their thermal conductivities were estimated using non-equilibrium molecular dynamics. Thermal conductivity for SWNTs and DWNTs were studied for lengths from 25 nm to 1 μm. Thermal conductivity increases continually with length due to the inclusion of more phonon modes with length. To investigate influences of individual phonon modes on thermal conductivity, double-wall carbon nanotubes (DWNTs) are used to model vibrational mode confinement in single-wall carbon nanotubes (SWNTs). Also, various concentrations of united atom models of phenyl groups and values for the Lennard-Jones parameter σ are used to model functionalization and the influence of bond strength. Thermal conductivity is largely influenced by longitudinal and flexural modes. Due to scattering from phonon-phonon interactions, the combination of the longitudinal and flexural modes results in a lower thermal conductivity than other combinations of phonon modes. The influence of suppressing the flexural mode is also observed in the thermal conductivity behavior of functionalized CNTs. When using the united atom model, larger percentages of functionalization result in decreasing flexural modes and, consequently, higher thermal conductivity. Similarly, smaller values of σ, which indicate a stronger bond, showed better thermal conductivity due to less flexural motion. The best performance, however, resulted from functionalized DWNTs, which have the additional wall to transport energy when the outside is treated; this result is useful for extending CNTs to thermal management applications.


 
Vanderbilt University