Edwin Flikkema is a lecturer at the Department of Physics at Aberystwyth University in Wales in the United Kingdom. Born in the Netherlands, he studied Theoretical Physics at the University of Groningen. He obtained a PhD on the subject of Monte Carlo simulation in polymer physics from the same university in 2002. He has worked as a postdoc at the Delft University of Technology, the University of Barcelona and the University of Cambridge. Since 2008 he is working as a lecturer at Aberystwyth University, specialising in computational materials science: structure prediction of atomic clusters, simulation of glass-forming materials and foams.
Molecular dynamics simulation of alkali silicates and MOFs
Edwin Flikkema1*, Wenlin Chen2 & Neville Greaves1
1Department of Physics, Aberystwyth University, Penglias, Aberystwyth, SY23 3BZ, UK
2Department of Geoscience, Virginia Tech, United States
Alkali-silicates form an important class of materials, many of which are well known as glass forming materials. Pure silica can be in a glassy state with its atomic structure described as a Continuous Random Network. When alkali species such as Sodium and Potassium are added, these act as network modifiers: the excess oxygen that is introduced alongside the alkali ions partially breaks down the silica network. The resulting structure is described by the Modified Random Network model. Pores are thought to be formed, going through the material, where the alkali ions reside, making the alkali ions much more mobile compared to the silica framework.
Molecular Dynamics simulation has been used to study the structure and dynamics of alkali-silicates in the super-cooled and glassy state. Specifically, sodium-disilicate, potassium-disilicate and mixtures thereof have been simulated. The disordered structure is arrived at by cooling from the liquid state. This methodology has been validated by comparing the resulting radial distribution functions to experiment. Advanced visualisation has been used to map the accessible free volume of the alkali ions.
An important part of this project is the study of the dynamics of the alkali-ions as they move through the silica network. Experimentally it is known that in glasses with a mixture of alkali species the ion dynamics is slower compared to the pure compounds. One of the main aims of this project is to see if this Mixed Alkali Effect can be replicated in Molecular Dynamics simulation. Diffusion constants for the alkali species have been obtained and Arrhenius plots of these versus temperature indeed show a slowing down of ion dynamics in the mixed case. Ion dynamics has been studied in more detail using intermediate scattering functions. Dynamic heterogeneity has been assessed using Pareto plots.
A related project is concerned with simulating the amorphisation of zeolites and Metal Organic Frameworks (MOFs) under pressure. Here the collapse of all silica Sodalite is compared to that of the topologically similar MOF ZIF-8. Both classical and ab-initio Molecular Dynamics results have been obtained, showing similar behaviour under pressure. These results show evidence of the existence of Low Density Amorphous and High Density Amorphous phases. This methodology is being used to study amorphisation transitions in atomic detail.