Glass Reflections
Cambridge 7th to 9th September

Presenting Author:
Edwin Flikkema

article posted 10 Mar 2015

Edwin Flikkema

Edwin Flikkema is a lecturer at the Department of Physics at Aberystwyth University in Wales (United Kingdom). He works in the general field of computational materials science. He is fairly new to the field of glass science. His other interests include the modelling of polymers, nano-particles, zeolites and foams

Ion dynamics in alkali-silicates: a Molecular Dynamics study
Edwin Flikkema*, Wenlin Chen, Zhongfu Zhou, Neville Greaves
Department of Physics, Aberystwyth University, Penglais Campus,
Aberystwyth SY23 3BZ, United Kingdom

Silicates form an important class of glass-forming materials. Their structure can be described as a network of silicon and oxygen atoms. This presentation focuses on sodium-potassium-disilicates (i.e mixtures of Na2O-(SiO2)2 and K2O-(SiO2)2). The introduction of alkali ions together with oxygen in the form of Na2O or K2O leads to the formation of non-bridging oxygens, partially breaking the silicon-oxygen network. Often, a static network is formed with pores in it, through which the alkali ions are relatively free to move.

The Mixed Alkali Effect (MAE) is an experimentally found phenomenon where the electrical conductivity as a function of alkali composition (Na to K ratio) is non-linear and exhibits a minimum. One of the goals of this computational study is to replicate the MAE in simulations and to be able to investigate it in atomic detail.

Sodium-potassium-disilicates with various Na to K ratios have been studied computationally using Molecular Dynamics. Initial atomic configurations are based on annealing from a high temperature molten phase. Simulations were performed at a wide range of temperatures.

Static properties such as the radial distribution function and Q species distribution were calculated from the simulations and compared to experiment.

The focus of this study is on dynamic properties, mainly on the dynamics of the alkali ions. The mean square displacement of the ions as a function of time has been studied and diffusion constants have been obtained. The intermediate structure factor has been calculated and this has been analysed by modelling it as a stretched exponential.

Dynamic heterogeneity of the alkali ions is analysed. Advanced visualisation is used to map out the free volume that is accessible by the alkali ions and to track the motion of the ions in order to gain a deeper understanding of the MAE.