| Molecular dynamics simulations were performed to study various carbon and hydrocarbon systems. The adaptive intermolecular reactive empirical bond order (AIREBO) potential was used to simulate friction in concentric sliding carbon nanotubes, reconstructed diamond surfaces, and reactions of small molecules inside carbon nanotubes that contained junctions. The potential was also refit to account for the zero-point energy in the covalent bond.;Low friction is an important property in many proposed applications for carbon nanotubes, such as nanopistons, nanosyringes, electromechanical switches, and high-frequency oscillators. In order to study this property, simulations were performed on two concentric nanotubes as they slid past each other. Both static and dynamic friction were examined, as well as possible contributing factors to the friction such as edge effects, defects, and tribochemical effects. It was found that the friction between two sliding nanotubes is near zero unless there is cross-linking between the tubes. These intertube bonds stretch and build up strain, which is released as stick-slip events. The simulations that included cross-linked bonds showed that the friction forces in this system are 40--100 nN, which agrees with the experimentally measured friction forces of 85--150 nN.;The covalent bonding interactions in the (AIREBO) potential were reparameterized to account for the zero-point energy of the covalent bond. Originally, this model was parameterized to reproduce the ground state bonding potential, but classical simulations with that potential overpredict the bond dissociation energy; in addition, expensive calculations are needed to account for the zero-point energy. With the revised potential, these corrections are no longer necessary. The carbon-hydrogen and carbon-carbon covalent bonding parameters have been refit using experimental values for structural, energetic, and elastic properties of small molecules, diamond and graphite. For molecules not included in the fitting procedure, the energetics are substantially improved without requiring explicit calculations for the zero-point energy. The new model was used to predict the relative energetics of (2 x 1) and (1 x 1) diamond surfaces at different hydrogen coverage. The crossover is predicted by the new potential to occur at approximately 8% of a monolayer, in good agreement with experiment and calculations with explicit treatment of zero-point energies. Unfortunately, the exact energy values did not match between the new potential and explicit zero-point energy corrections, but the energetics were improved from the original potential.;An application of the newly revised potential is modeling reactions of small molecules inside of carbon nanotubes. Linear as well as branched nanotubes were studied. The types of nanotubes considered include T-junctions, Y-junctions, and two linear tubes with different diameters that had been joined together. Small molecules were placed at the ends of the nanotubes and given velocities corresponding to different incident energies toward each other. The tubes themselves were treated with three different conditions: fixed spatial constraints, canonical ensemble thermostatting, or constraint-free dynamics. The products created from the reactions inside the tubes varied depending on incident energy, angle of collision, and tube treatment. |
