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Tribological study of carbon nanotube oscillators

by Ma, Chi-chiu

Abstract (Summary)
(Uncorrected OCR) Abstract of thesis entitled TRIBOLOGICAL STUDY OF CARBON NANOTUBE OSCILLATORS submitted by MA Chi Chiu for the degree of Doctor of Philosophy at The University of Hong Kong in July 2004 Using micro-canonical molecular dynamics, I investigate energy transfer from the orderly intertube translational oscillation to intratube vibrational modes for an isolated system of coaxial double-walled carbon nanotubes. The energy transfer is found to take place via two distinct types of collective motion of the nanotubes, i.e., an off-axial rocking motion of the inner tube for systems of short inner tubes, and a radial wavy motion of the outer tube for systems of long inner tubes. These collective modes may or may not occur in an individual oscillator system, depending primarily upon the initial system energy. The rocking motion and the wavy motion are both self-mitigating, leading to an eventual settlement into a quasi-equilibrium in which intertube translational oscillation is sustained over an extended period of time. Frictional forces between 10-17 to 10-14 N per atom are found for various dissipative mechanisms, and they exhibit no proportional dependence upon the normal force between the two surfaces in relative sliding, contrary to the conventional understanding resulted from tribological studies of macroscopic systems. Double-walled oscillators with one-end-capped outershells as well as three-walled oscillators have also been studied. Collisional reversals at capped ends of the outer tube are found to provide additional energy dissipation. An initial extrusion of the middle tube in three-walled oscillators can cause off-axial inner-tube instabilities, and subsequently, strong frictional effects. The effect of pre-simulation heating on the frictional behavior of the nanotube oscillators is also studied. At low initial temperature, the frictional force increases with extrusion length because systems energies increase with extrusion length to initiate wavy motion of outer tube to hinder the translational motion. On the contrary, frictional force becomes insensitive to the initial extrusion length if the initial temperature is high to produce huge thermal roughness of contacting surfaces that is disruptive of the collective motion, such as the wavy motion, in the very early stage of the simulation. On the other hand, in the absence of the wavy motion (small initial extrusion lengths), the frictional force increases monotonically with the pre-simulation heating temperature (up to 120 K), but such a picture fails when the wavy motion dominates the energy-transfer process (large initial extrusion lengths) as the pre-simulation heating tends to disrupt formation of collective modes such as the wavy motion. The particular DWNT oscillator, in which the outer (inner) tube has a length of 70 A (55 A), ceases to produce useful oscillatory behavior as pre-simulation temperature is increased to 180 K and above. Finally, a particular case of energy exchange between orderly translational degrees of freedom and vibrational modes is studied for a nanotube oscillator. Similar to that in the Fermi-Pasta-Ulam (FPU) model, energy exchange is found to be reversible, and the dissipated excess intertube van der Waals energy is found primarily stored in the low-frequency vibrational modes of the oscillator during oscillation hibernation.
Bibliographical Information:

Advisor:

School:The University of Hong Kong

School Location:China - Hong Kong SAR

Source Type:Master's Thesis

Keywords:carbon nanotubes oscillators electric tribology

ISBN:

Date of Publication:01/01/2005

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