UBC Theses and Dissertations
Molecular dynamics study of effects of geometric defects on the mechanical properties of graphene Mallika Arachchige, Nuwan Dewapriya
Graphene is a flat monolayer of carbon atoms arranged in a two-dimensional hexagonal lattice. It is the strongest material ever measured with strength exceeding more than hundred times of steel. However, the strength of graphene is critically influenced by temperature, vacancy defects (missing carbon atoms) and free edges. A systematic Molecular Dynamics (MD) simulation study is performed in this thesis to understand the effects of temperature, free edges, and vacancy defects on the mechanical properties of graphene. Results indicate that graphene has a positive coefficient of thermal expansion. However, the amplitude of intrinsic ripples (out-of-plane movement of carbon atoms) increases with increasing temperature, which reduces the net effect of thermal expansion. This is probably the reason for negative values of thermal expansion coefficient observed in some experiments. The MD simulations provide significant insights. At higher temperatures the sheets are observed to fail at lower strains due to high kinetic energy of atoms. Excess edge energy of a narrow graphene sheet is found to induce an initial strain at equilibrium configuration. Free edges have a greater influence on the mechanical properties of zigzag sheets compared to those of armchair sheets. Simulation of sheets with vacancy defects indicates that a single missing atom could reduce the strength by nearly 20%. It is also found that the calculated strength based on Griffith's theory falls below the results from MD simulations. The results obtained in this study are useful to the design and fabrication of graphene based nano-devices.
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