The research within this thesis focuses on the behaviour of glass-forming systems as they are cooled towards the glass transition. In particular, the origins of fragile behaviour are examined. The study of glasses and supercooled liquids involves careful consideration of issues such as equilibrium, ergodicity and metastability. Simulations of the binary Lennard-Jones model glass former explore the diagnosis of broken ergodicity using an energy fluctuation metric and this approach is used to consider ergodic and short-time nonergodic behaviour of the diffusion constant. An underlying Arrhenius temperature dependence of the diffusion constant can be extracted from fragile, super-Arrhenius diffusion. This Arrhenius diffusion can be related to the true super-Arrhenius behaviour by a correction factor that depends on the average angle between atomic displacements in successive time intervals. This correction factor accounts for the fact that on average successive displacements are negatively correlated. This negative correlation can be linked directly with the higher apparent activation energy for diffusion in fragile glass formers at lower temperature. Using information from the potential energy surface, the process of an atom exiting its cage of nearest neighbours, a cage-break, can be explored. The assignment of such an event as a fundamental step for diffusive behaviour can be justified by accurate calculations of diffusion constants. Negative correlation is seen in the direct reversals of a large proportion of the cage-breaking events. Productive cage-breaks, i.e. cage-breaks that are not reversed, can provide a definition of superstructures (megabasins or metabasins) in the energy landscape.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598458 |
| Date | January 2008 |
| Creators | de Souza, V. K. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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