Rashidi, Abdul Ghaffar; (2021) Computational Investigation of Crystalline and Amorphous Borosilicates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

By utilising both Molecular Dynamics (MD) and Density Functional Theory (DFT), the project focuses on the structure and bonding in glassy and crystalline boron oxide and borates, particularly on the distribution of BO3 and BO4 groups though ring and structural analysis and on the bonding and structural properties of borates. There are two main areas of this research: the first section presents the results of the computational investigation of Boron Trioxide (B2O3) via MD. The second focuses on the alkali-infused crystalline borates to understand their network structure in detail. The MD component of this investigation concentrates on pure B2O3 including the crystalline, molten, and glassy structure. The atomic structure of this material is one that has been widely debated and was a key topic in this project. Characteristics are identified through various structural and mechanical properties and via ring analysis. The results of the initial MD simulations indicate a substantial proportion of BO3 groups present in the structure of pure B2O3 (87.5%). However, expanding the simulation cell size reveals an increase in the BO4 subgroup, indicating a possibility of an increase in boroxol ring formation, this finding demonstrates the need for large simulation cells which is now possible with contemporary computational resources. The thesis then focusses on the testing and identification of a force field to utilise in the MD studies. It explores the two available force fields, containing two- and threebody terms and shows that both can model the molten structure. However, as the Computational Investigation of Crystalline and Amorphous Borosilicates quenching of the simulation takes place, the two-body potential fails to adequately create the glassy phase; whereas the potential with the three-body term is able to achieve the glassy-B2O3 structures at a much quicker rate. Crystalline borates with a range Na2O/B2O3 ratios are next investigated using Density Functional Theory. Various crystals compositions are analysed, with a focus primarily on electronic and mechanical properties. We show that an increase in sodium content in these types of crystals has a direct effect on elasticity and mechanical strength. The results reveal interesting aspects of the bonding in these materials and give insight into the relationship between structure and composition.

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