A multi-scale electro-thermo-mechanical analysis of single walled carbon nanotubes
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Carbon nanotubes are formed by folding a graphene sheet. They have gained a lot of attention during the last decade due to their extra ordinary mechanical, thermal and electrical properties. Molecular dynamics simulations have been used extensively for studying the mechanical properties of carbon nanotubes. In this thesis, a quantum mechanics and molecular dynamics level multi-scale modeling and analysis of single walled carbon nanotubes is presented. This dissertation reports many findings based on these simulations such as some parameters that affect the correctness of the results obtained by molecular dynamics simulation like the boundary conditions and the displacement increment. The effects of the strain rate and the length of the nanotube on the mechanical properties of carbon nanotubes under uniaxial tension are also reported. A simplification for calculating the virial stresses with multibody potential is derived to use for calculating the stresses in carbon nanotubes and compared with the stresses calculated using continuum mechanics engineering stresses. Simulation of unraveling of carbon nanotubes during field emission is simulated using Molecular dynamics simulations. The force required to start the unraveling in carbon nanotubes with different chiralities is reported as well as the maximum force that can be sustained by the atomic chain. Due to the nonlinearity in the current-voltage relation of carbon nanotubes, the traditional Joule’s law for calculating joule heating in carbon nanotubes can not be used. In this thesis, the joule heating and the electron-induced wind forces per unit length of carbon nanotubes are calculated using a quantum mechanical formulation based on the energy and momentum exchange between the electrons and the phonons. Two approaches were used in the calculations; the first one is based on formulating an integral form that makes use of the relaxation time approximation into the modified Fermi-Dirac distribution for the electron occupation probability. The other approach uses the Ensemble Monte Carlo simulations and tracks the energy and the phonon exchange during the simulation time. The results are used to calculate the effective charge number in carbon nanotubes at different temperatures. The methods proposed in this thesis for calculating the joule heating and the effective charge number can be used for any nanoscale material, and can be extended to include effects like phonon-phonon interaction and hot phonon effects.