Theoretical modeling and calculations of molecular flow through carbon nanotubes and Sigma5 (310)[001] zirconium dioxide grain boundaries

This thesis will discuss two theoretical studies. The first is molecular flow and separation of molecular mixtures inside carbon nanotubes while the second is an investigation of the structure and stability of 36° Σ5 (310)[001] ZrO₂ grain boundaries. Molecular dynamics simulations with many-body, atomistic, empirical models and geometry optimization with density functional theory were used in this study.; For molecular fluids through carbon nanotubes, both one-component and binary systems were investigated at room temperature. Through this work, we systematically studied the mechanisms that control molecular flow inside carbon nanotubes. As a result, complete theoretical diffusive models were established for organic molecules that provide insight into how nanotube membranes may be used in ultrafiltration applications. Several factors are predicted to affect molecular flow over time. In the case of dynamic flow, the size (diameter) of the nanotubes, the molecular density of the molecular fluids, the type of molecules, and the rigidity of the nanotube walls are all shown to be important. In the case of diffusive flow, the size (diameter) of the nanotubes, molecular size and molecular shape are found to be important. Furthermore, the study shows that the type of atomic termination at the nanotube edge can have a significant effect on the diffusion of some molecules but almost no effect on others. Pore-pore correlations in bundles of carbon nanotubes are found to cause the diffusion velocities and coefficients/mobilities to decrease. For binary molecular fluids, the separation trends and diffusion behavior of binary molecular mixtures through carbon nanotubes and bundles were investigated. Molecular structure and the nanotube diameter are predicted to have large effects on the separation and diffusion behavior of the mixtures. The study also shows that the helical structure of the nanotube has no effect on the diffusion behavior of molecular mixtures. Different terminations of the carbon nanotubes also have little effect on the separation trends. In nanotube bundles, the diffusion behavior and coefficients of binary molecular systems are significantly different from the diffusion in individual nanotubes.; In the second part of the thesis, density functional theory was employed to investigate the Σ5 (310)[001] ZrO₂ grain boundary. Five pure grain boundary models were constructed based on experimental data for this grain boundary and the model with the lowest energy was identified based on calculated interfacial energies. Then the lowest energy model was relaxed and the structure was compared with experimental Z-contrast image data. The study also included investigation of the stability of Y3+ doped ZrO₂ grain boundaries to identify a possible doping pattern. This was accomplished by calculating substitution energies, segregation energies. The calculations indicate that doping along the boundaries will stabilize the grain boundary structure, but that doping the bulk ZrO₂ will stabilize the grain boundary structure. Free volume calculations indicate that the grain boundaries are a more active area than the bulk which is why the Y3+ prefers to dope the large free space positions.