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3 results on '"Sugden, I. J."'

1. Carboranes and their incorporation into siloxane polymers

Author

Sugden, I. J. and Bell, R. G.

Subjects
540
Abstract

This is a report detailing the computational investigation of copolymers containing carborane and siloxane monomers, to aid in the design an industrially relevant material for use in high neutron radiative environments. This includes determining the optimal carborane/siloxane ratio in the designed material with regards to macroscale physical properties; with experimentally determined values for pure siloxane phases reproduced, using classical methods. The investigation shows that increasing carborane content increases bulk modulus and decreases the thermal expansion coefficient, levelling off beyond 50% carborane content. It also includes the effect of including specific side groups to polymer strands in order to affect properties; for instance, it is seen that phenyl groups increase the flexibility of the polymer strands. Alongside this, the report includes the simulation of property “aging”; using classical crosslinking methods to model the effect of high energy ions travelling through the material following neutron capture events, and ab initio simulation of damage to the monomer. Unsurprisingly, crosslinking sees a reduction in flexibility, leading to an increase in bulk modulus and a decrease in the thermal expansion coefficient, whilst the changes in vibrational spectra as a result of neutron capture events are predicted: due to changes in bond strength and orbital structure, modes involved with cage elements move to a higher frequency, and B-H modes move to lower. Finally for the designed material, context is given by examining the current state of the art: solid boron carbide, and its remarkable resistance to radiation of several different forms, with experimental theories and mechanisms discussed. Carborane clusters are further examined in other technological areas: thermal rearrangements of single carboranes and metallo-carboranes, with the 40 to 145 kJ/mol (dependent on mechanism) difference between theoretical and experimental activation energies rationalised, and investigations of icosahedral boron cluster anions in lithium battery electrolytes, where a ≈17% improvement in lithium mobility is theorised.

Published
2014

3. Large-Scale Parameter Estimation for Crystal Structure Prediction. Part 1: Dataset, Methodology, and Implementation

Author

Bowskill, D. H., Tan, B. I., Keates, A., Sugden, I. J., Adjiman, C. S., and Pantelides, C. C.

Abstract

Crystal structure prediction (CSP) seeks to identify all thermodynamically accessible solid forms of a given compound and, crucially, to establish the relative thermodynamic stability between different polymorphs. The conventional hierarchical CSP workflow suggests that no single energy model can fulfill the needs of all stages in the workflow, and energy models across a spectrum of fidelities and computational costs are required. Hybrid ab initio/empirical force-field (HAIEFF) models have demonstrated a good balance of these two factors, but the force-field component presents a major bottleneck for model accuracy. Existing parameter estimation tools for fitting this empirical component are inefficient and have severe limitations on the manageable problem size. This, combined with a lack of reliable reference data for parameter fitting, has resulted in development in the force-field component of HAIEFF models having mostly stagnated. In this work, we address these barriers to progress. First, we introduce a curated database of 755 organic crystal structures, obtained using high quality, solid-state DFT-D calculations, which provide a complete set of geometry and energy data. Comparisons to various theoretical and experimental data sources indicate that this database provides suitable diversity for parameter fitting. In tandem, we also put forward a new parameter estimation algorithm implemented as the CrystalEstimator program. Our tests demonstrate that CrystalEstimator is capable of efficiently handling large-scale parameter estimation problems, simultaneously fitting as many as 62 model parameters based on data from 445 structures. This problem size far exceeds any previously reported works related to CSP force-field parametrization. These developments form a strong foundation for all future work involving parameter estimation of transferable or tailor-made force-fields for HAIEFF models. This ultimately opens the way for significant improvements in the accuracy achieved by the HAIEFF models.

Published
2024
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