The Mixed Glass Former Effect- Modeling of the Structure and Ionic Hopping Transport

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Title: The Mixed Glass Former Effect- Modeling of the Structure and Ionic Hopping Transport
Authors: Schuch, Michael
Thesis advisor: Prof. Dr. Philipp Maass
Thesis referee: Prof. Dr. Helmut Eckert
Abstract: The origin of the Mixed Glass Former Effect (MGFE) is studied, which manifests itself in a non-monotonic behavior of the activation energy for long-range ion transport as a function of the mixing ratio of two glass formers. Two theoretical models are developed, the mixed barrier model and the network unit trap model, which consider different possible mechanisms for the occurrence of the MGFE. The mixed barrier model is based on the assumption that energy barriers are reduced for ionic jumps in regions of mixed composition. By employing percolation theory it is shown that this mechanism can successfully account for the behavior of the activation energy in various ion conducting mixed glass former glasses. The network unit trap model is based on the fact that a variety of network forming units, the so-called Q(n) species, can be associated with one glass former. Using a thermodynamic approach, the change of the concentration of these units in dependence of ionic concentration and the glass former mixing ratio is successfully predicted for alkali borate, phosphate and borophosphate glasses. In a second step, the charge distribution of the various units is considered and related to it, the binding energies to alkali ions. This gives rise to a modeling of the ionic transport in an energy landscape that changes in a defined manner with the glass former mixing ratio. Kinetic Monte Carlo simulations for alkali borophosphate glasses, which serve as a representative system for the MGFE in the literature, demonstrate that this approach succeeds to predict the behavior of the activation energy. In a further part of the thesis, Reverse Monte Carlo (RMC) simulations for the atomic structure of sodium borophosphate glasses are carried out with X-ray and neutron diffraction data as further input from experiments. Three-dimensional structures could be successfully generated that are in agreement with all experimental and theoretical constraints. Volume fractions of the ionic conduction pathways determined from these structures, however, do not show a substantial relationship to the activation energy, as earlier proposed in the iterature for alkali borate and alkali phosphate glasses.
Subject Keywords: Ionentransport; Gläser; Ion tranport; glasses; hopping; Reverse Monte Carlo
Issue Date: 11-Oct-2013
Type of publication: Dissertation oder Habilitation [doctoralThesis]
Appears in Collections:FB04 - E-Dissertationen

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