https://www.um.edu.mt/library/oar/handle/123456789/137488 Fri, 17 Apr 2026 21:49:46 GMT 2026-04-17T21:49:46Z https://www.um.edu.mt/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.) Sat, 01 Jan 2022 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/137631 2022-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/137599 Title: An investigation of smart, inclusion-based mechanical metamaterials Abstract: Mechanical metamaterials have captured the attention of researchers worldwide owing to their exceptional mechanical properties. They are poised to emerge as a pivotal category of materials that will shape the next generation of innovative materials. Research on auxetic materials, a class of mechanical metamaterials, has been ongoing for more than four decades, with recent years witnessing a surge in interest in the design of 3D auxetic metamaterials. Another class of metamaterials that has shown great potential is active metamaterials, which are materials that can have their geometry and/or mechanical properties tuned post-fabrication in response to external stimuli. This thesis delved into the realm of mechanical metamaterials, focusing on these two distinct categories. It introduced a novel design approach for creating 3D auxetic structures and conducted an in-depth exploration of magneto-mechanical active metamaterials. The 3D auxetic systems were intentionally designed to lay the foundation for potential future research, envisioning their transformation into active magneto-mechanical metamaterials. The mechanical properties of the 3D auxetic structures produced in this thesis were investigated through numerical simulations validated by experimental tests. It was demonstrated that a system, created through equally sized voids with a constant cross-sectional area into a solid material at specific locations in various planes, could exhibit a negative Poisson's ratio of approximately -0.5 in multiple directions. This behaviour was observed over a significant range of aperture angles for the cross-sectional areas, especially when the voids were positioned close to each other. A scalable inclusion-based active magneto-mechanical metamaterial consisting of magnetic inclusions embedded within a non-magnetic matrix was also produced. The proposed structure, based on an accordion-like structure, was shown to respond to the magnitude and direction of an external magnetic field by tuning its geometry. The basic unit was then used to create a number of active systems including the auxetic re-entrant honeycomb and egg-rack structures. Finally, iron nanoparticles inclusions were used instead of permanent magnets and successfully produced a magneto elastomer. Through numerical simulations validated by experimental prototypes, the response to an external magnetic field was investigated. The numerical model showed good agreement with the experimental tests and following this the effect of nanoparticle concentration and other geometric parameters were investigated. Description: Ph.D.(Melit.) Sun, 01 Jan 2023 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/137599 2023-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/137497 Title: Modelling of folded graphene and related systems Abstract: Graphene, a quasi-planar monolayer of sp2 -bonded carbon atoms known for its exceptional physical properties, is highly amenable to out-of-plane deformation. Recent studies have revealed that the creation of folded, pleated-like domains imparts novel characteristics to this material whilst permitting some of its existing properties to be effectively controlled through straining action via regulation of the emergent folding parameters. Despite the considerable influence that strain can have on the material properties of folded graphene, the literature pertaining to the nano-mechanical unfolding of folded, graphene-type systems remains scarce. In this work, molecular dynamics simulations were performed on three novel forms of folded graphene using an ad hoc protocol executable within LAMMPS to study their mechanical response to uniaxial tensile deformation. Patterned line defects were shown to constrain multiply folded graphene to a quasi-periodic, highly ordered morphology that gave rise to instances of pronounced negative tangent modulus – coincidentally with each fold opening – upon the application of uniaxial stress. The severe lack of periodicity observed in the corresponding profiles of the pristine folded systems was attributed to the absence of defect lines which permitted folds to be more mobile and at times merge, effectively reducing the frequency of fold openings. These structural differences were explained, for the first time, via a macroscale model based on the mechanics of paper folding. Overall, this study attests to the potential for defect-type fold lines to guide the unfolding process of folded graphene, and provides valuable insight into the different mechanisms involved in the unfolding of specific forms of folded graphene. Description: M.Sc.(Melit.) Fri, 01 Jan 2021 00:00:00 GMT https://www.um.edu.mt/library/oar/handle/123456789/137497 2021-01-01T00:00:00Z