Konstantinou, K; (2017) Computational modelling of structural, dynamical and electronic properties of multicomponent silicate glasses. Doctoral thesis , UCL (University College London).

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Abstract

Amorphous silica doped with alkali, alkaline-earth and boron ions is an important class of materials used in many technological applications. Computer simulations are able to provide atomic pictures of the glass structures and detailed insight into compositional$-$ atomic structure relationships. In this thesis several glasses of current interest for nuclear waste immobilisation and microelectronics are studied, using classical and \textit{ab initio} methods. A method to calculate the glass transition temperature in alkali borosilicate glasses, with pre-defined molar composition, was implemented and validated through the comparison with the available experimental data. The addition of alkali and boron oxides to pure SiO$_2$ glass resulted in a vast decrease in the glass transition temperature. A systematic study of the density, atomic and electronic structure for lithium sodium borosilicate glasses with varying SiO$_2$ / B$_2$O$_3$ ratio and fixed molar composition of the alkali oxides is reported. \textit{Ab initio} molecular dynamics simulations were performed in order to model the local atomic structure of molybdenum in nuclear waste glasses. Analysis of the structural motifs and the environment of the molybdenum atoms provided direct insight into the solubility of Mo in the glass structures. The results significantly extend the understanding of how the chemical nature and molar composition of the glass host affect the bonding in Mo-containing nuclear glasses and demonstrate that tailoring the glass composition to specific heavy metal constituents can facilitate incorporation of heavy metals at high concentrations. A series of mixed alkali/alkaline-earth silicate glasses was investigated in order to understand the impact of the nature of the cations on the mobility of the alkaline-earth ions within the glass network. The size of the alkaline-earth cation was found to affect the short-range order and the short-time dynamical behaviour. An asymmetrical linear behaviour in the activation energy for diffusion with increasing cation size difference was observed. The electronic properties of excited states in sodium silicate glass were investigated with \textit{ab initio} modelling. The existence of two different configurations for the excited state are predicted, which correspond to two discrete ranges of luminescence energies. The different short-range ordering of Na cations corresponds to a structural precursor responsible for the calculated values of the luminescence energy.

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