/library/oar/handle/123456789/97954 2025-11-04T22:18:26Z /library/oar/handle/123456789/137631 Title: Auxetic behaviour in high pressure polymorphs of CO2 and H2O : a computational chemistry study Abstract: In this work, a detailed study of the mechanical properties of the single crystal and polycrystalline aggregate of high-pressure polymorphs of H2O and CO2 will be carried out, paying particular attention to Poisson’s ratio of these systems. Using first principles density functional theory (DFT) calculations, this work will show for the first time that the high-pressure polymorphs ice VIII, ice X, CO2-V and CO2-II have the potential to exhibit a negative Poisson’s ratio with the auxetic behaviour of these systems increasing with increasing hydrostatic pressure. To adequately study these systems using DFT simulations, detailed convergence and benchmarking studies will be carried out. It will be shown that contrary to the single crystal, the Poisson’s ratio of the polycrystalline aggregate of these systems exhibits a positive Poisson’s ratio which increases with increasing hydrostatic pressure. The deformation mechanism for ice X, ice VIII and CO2-V will be studied through the application of stress, with the proposed mechanism being consolidated through the use of spectroscopy. In the case of ice X and ice VIII, the auxetic behaviour will be rationalised by studying the deformation of two orthogonally interpenetrating rhombi on application of a stress. In the case of CO2-V, the auxeticity will be explained from a 2D perspective by the relative rotation of semi-rigid projected squares which both rotate and deform on application of a stress. It will also be shown that these 2D squares are projections of 3D CO4 tetrahedra which rotate relative to each other whilst stretching and deforming. In the case of CO2-II, it will be shown that the application of a stress results in a non-continuous change in the structural parameters studied. Thus, a novel approach will be developed in this thesis, where the auxetic behaviour of the system will be rationalised by studying the variation of the Raman active and infrared active modes with varying hydrostatic pressure and comparing any shifts observed in these modes with shifts in the Poisson’s ratio. Description: Ph.D.(Melit.) 2022-01-01T00:00:00Z /library/oar/handle/123456789/136323 Title: Analysis of enhanced observations made by a SeaExplorer glider deployed in the central Mediterranean Abstract: As the Earth’s atmosphere continues to heat up and change the environment in unpredictable ways, it is necessary to not only have tools that are able to accurately predict future trends, but also to have an exact baseline of the studied properties to be able to accurately measure how these parameters may vary. The aim of this study is to verify existing oceanographic models and satellites in the Mediterranean Sea through the use of primary data collection on temperature, salinity, and chlorophyll-a through the use of an ocean glider. This ocean glider was deployed on a summer mission and a winter mission from areas along the Maltese Islands headed towards the northern coast of Africa, diving to depths of up to 700 metres. Ocean gliders are a branch of Underwater Autonomous Vehicles used for oceanographic measurements on a wide range of properties. During this time, satellite and model products were downloaded from an online service known as Copernicus Marine Environmental Monitoring Service which is widely relied upon across the world for policy making, climate monitoring, forecast predictions, and a realm of other services. All three datasets were uploaded to Matlab where the program could loop through to find the nearest model and satellite measurement in time and space to the nearest glider data point. A series of statistical analyses were performed to test the accuracy of the model and satellite products in comparison to the measured glider data. The accuracy between Missions 1 and 2 were also tested. These measurements included error bar plots, Mann-Whitney tests, correlations and Wilcoxon Signed Rank tests. A unique component of this study was the measure of correlation under certain conditions, this was done to determine if there were certain times or locations that the model or satellite products may be less accurate. These specific conditions measured were the correlation with the duration of the mission, with depth, and with the presence of backscattering. The key findings of this study included the importance of recalibration of the ocean glider perhaps earlier than what was previously predicted. Additionally, the reliability of using an ocean glider rather than model or satellite data when measuring any of the three parameters within the first 200 metres from the surface was realized. Finally, it was found that 95.5 percent of chlorophyll-a data points measured by the ocean glider were greater than the data points measured by the model, indicating there is likely an error with the model used for the chlorophyll-a product. Description: M.Sc.(Melit.) 2022-01-01T00:00:00Z /library/oar/handle/123456789/119999 Title: Water-soluble fluorescent 4-amino-N-aryl-1,8- naphthalimide logic gates for saccharides and metal cations Abstract: Twelve fluorescent logic gates were designed and synthesised based on a “fluorophore-spacer-receptor” model (for compounds 5-10) and a “receptor1-spacer- fluorophore-spacer-receptor2” model (for compounds 1-4, 11 and 12). The 12 compounds were built around a naphthalimide fluorophore, having aryl moieties at the imide position, including phenyl (1 and 5), 1,2-dimethoxyphenyl (2 and 6), benzo-15- crown-5 (3), benzo-18-crown-6 (4 and 7), ortho-phenylboronic acid pinacol ester (8 and 11), ortho-phenylboronic acid (9), and meta-phenylboronic acid (10 and 12). Substituents at the 4-position of the naphthalimide included piperazine (1-4, 11 and 12), azetidine (5-7), and chloro (8-10). The compounds were studied by means of UV-visible absorption spectroscopy and steady-state fluorescence spectroscopy. Compounds 1-12 were studied in terms of proton concentration and solvent polarity. Compounds 3, 4, and 7 were also studied in the presence of cations, including sodium and barium(II), whilst compounds 8-12 were studied in the presence of three monosaccharides; glucose, fructose, and galactose. Compounds 1-7 were studied in water, water/methanol mixtures, and methanol, whilst compounds 8-12 were studied in phosphate-buffered saline (PBS), PBS/methanol mixtures, and methanol. Compounds 1, 11, and 12 function as one-input proton-driven YES gates, displaying stronger fluorescence in methanol. Compounds 2, 5 and 6 function as one-input proton-driven PASS 1 gates which are brighter in methanol. Compound 7 functions as a two-input proton- and cation-driven PASS 1 gate, being brightest also in methanol. Compound 3 behaves as a three-input AND gate with respect to protons, sodium cations, and solvent polarity. Compound 4 behaves as a three-input AND-INHIBIT-OR combinatorial gate with respect to protons, barium(II) cations, and solvent polarity. Compound 8 behaves as a three-input OR1,2-INHIBIT3 gate with respect to protons, saccharides, and solvent polarity, compound 9 behaves as a three-input AND gate with respect to protons, fructose, and solvent polarity, and compound 10 functions as a two-input proton-enabled solvent polarity-disabled INHIBIT gate. Solvent polarity was shown to be a very important factor when considering the fluorescence responses and logic of 1-12. This discovery opens up the possibility of revisiting existing compounds and modifying their logic behaviour by simply tuning the solvent polarity in which they are studied, thus allowing for more potential applications. Description: Ph.D.(Melit.) 2022-01-01T00:00:00Z /library/oar/handle/123456789/113431 Title: Using game theory to solve a competitive supply chain network model with green investments and labour Abstract: In light of the recent severe Supply Chain (SC) disruptions that have occurred across multiple industries around the globe, three essential themes have emerged in SC management: the well-being of employees, the sustainability of SC practices and the competition between SCs for limited resources. Moreover, these three themes are linked, with an increasing number of employees demanding sustainability from the firms they work with, as well as an increasing competition amongst firms to find talent. Taking into consideration the above, this dissertation aims to create a SC network model that incorporates together SC competition, labour productivity and green investing. Each competing firm within the network seeks to maximise its profit by determining the optimal flow of products between manufacturing facilities, distribution centres, storage facilities and the eventual sales to demand markets, as well as how best to invest in green initiatives across the SC according to a predetermined budget. Factors such as the introduction of a carbon tax on emissions, consumer preferences and employee engagement are also included in the model. Game Theory is used to formulate a Nash Equilibrium solution to the model, such that no firm would be able to individually improve its profits by deviating from this solution. Numerical examples for various scenarios inspired by the Maltese dairy industry are solved using a Variational Inequality reformulation and the Extragradient algorithm, and sensitivity analysis is also carried out. The results highlight the importance of the three aforementioned themes on SC profitability and the close relationship between them. Description: B.Sc. (Hons)(Melit.) 2022-01-01T00:00:00Z