https://www.um.edu.mt/library/oar/handle/123456789/36094 2026-04-10T18:39:25Z 2026-04-10T18:39:25Z https://www.um.edu.mt/library/oar/handle/123456789/136794 2025-07-23T12:56:04Z 2018-01-01T00:00:00Z

Title: On folded graphene and its properties : a preliminary study Abstract: Graphene is one of the newly discovered forms of carbon and exists as a single sheet, made from sp2-hybridised carbon atoms, which is flexible enough to be able to adopt a variety of conformations. This material, or its variants, may exhibit wide-ranging properties, including a negative Poisson’s ratio (auxeticity). This property may be imparted to graphene through a process which involves modification through the use of patterned or randomly-placed defects which guide graphene to adopt particular three-dimensional conformations that are amenable to exhibiting negative Poisson’s ratios. This work re-examines and extends recent work which has shown how graphene with defects inserted along equally spaced lines may exhibit giant negative Poisson’s ratios as a result of the corrugated-sheet-like conformation it adopts, which defect lines act as crease lines. It is shown, through a combination of static force-field based simulations using the polymer consistent force-field (PCFF) and more computationally intensive NPT molecular dynamics simulations using the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) force-field that the corrugated forms of graphene which had previously been reported are neither the most stable nor the most auxetic forms that these systems may adopt. In fact, it is shown that unless constrained, the low-density corrugated forms of graphene are likely to fold and adopt much denser forms with graphite-like features that impart added stability, which may or may not be auxetic. It was also shown that other stable variants of the less dense form of the corrugated graphene may exist which may exhibit an even higher extent of auxeticity than what was reported so far. Other anomalous properties, such as negative tangential stiffness or zero Poisson’s ratios for some of these forms are also identified and discussed. Description: B.SC.(HONS)

2018-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/121342 2024-04-26T04:42:08Z 2018-01-01T00:00:00Z

Title: Influence of tuna penning activities on soft bottom macrofaunal assemblages Abstract: Aquaculture is an important food-producing industry that has often been criticised because of its potential adverse influence on water quality and benthic habitats present in the vicinity of a fish farm. A lucrative sector of the aquaculture industry is Atlantic Bluefin Tuna (ABT) ranching. The main source of pollution of the benthic environment at tuna farms is the uneaten feed-fish which accumulates on the seabed below the tuna pens, but the potential influence of ABT farming is expected to differ from those of other fish farm types such as those rearing sea bass and sea bream, because of the use of feed-fish instead of formulated feed and the large size of the farmed fish. Furthermore, differences in the characteristics of the tuna farms and of the receiving environment may result in varying levels of impact, if present. The present study investigated the influence of tuna penning activities on macrofaunal assemblages of the soft sediment habitat present in the vicinity of the fish pens. Grab samples for sediment physico-chemical attributes; namely mean sediment grain size (MSGS), and percent organic carbon content (POCC) and percent organic nitrogen content (PONC) in the sediment; and for macrofaunal studies, were collected from three tuna farms located c. 1 km off the northeastern to southeastern coast of Malta, at incremental distances from the sea cages (i.e., c. 0 m, 100 m, 1 km, and 2 km away) before initiation of the farming activities, and thereafter at six-monthly or annual intervals, over a period of ten years. The following study aspects were considered: (i) influence of the northeastern farm during its first year of operation on benthic habitat; (ii) use of polychaete, mollusc, amphipod and decapod taxocenes as indicators of the influence of ABT penning on macrobenthic assemblages; (iii) differences in the magnitude and spatial extent of influence of the three ABT farms that differed in size, stocking density, and location, on benthic habitat; (iv) spatial pattern in attributes of the macrofaunal assemblages present in the vicinity of a farm with incremental distance from the tuna pens; (v) suitability of benthic biotic indices (BBIs) AMBI, BENTIX, BOPA, BOPA-Fish farming (BOPA-FF) and M-AMBI, for monitoring the environmental impact of tuna farming; and (vi) temporal patterns in macrofaunal assemblages in the vicinity of three tuna farms over a ten-year period. Results from the study of the northeastern farm during its first year of operation indicated significantly elevated sediment POCC and PONC, and (albeit not significantly) higher abundance of capitellid polychaetes in the vicinity of the tuna cages, where uneaten feed-fish had accumulated on the seabed. The changes in benthic habitat were conspicuous in autumn towards the end of the tuna penning season, but some benthic recovery was observed after the fallow period. Of the considered taxoenes, polychaetes and amphipods appeared to be good benthic biotic indicators of the impact of tuna penning on macroinvertebrate assemblages. Results from the third study aspect indicated a higher magnitude of influence at the northeastern farm - the largest farm in terms of holding capacity - compared with the two southeastern farms, but a wider spatial extent of impact (1-2 km) was evident at one of the southeastern tuna farms. The spatial pattern in benthic macrofaunal assemblages was characterised by a high impact area directly below the cages, while a significant peak in diversity 100 m away from the cages was observed at only one of the investigated tuna farms. Of the considered BBIs, the BOPA-FF and M-AMBI indices appeared more sensitive to the environmental influence of tuna penning, but variation in Ecological Quality Status (EQS) assignment among BBIs showed the importance of including multivariate data analyses that are traditionally used in aquaculture environmental impact monitoring studies. Results from the sixth study aspect showed that the benthic EQS changed from ‘Bad’ and ‘Poor’ to ‘Good’/‘High’ categorisations at the northeastern farm after the first years of operation, but ‘Moderate’ EQS at the two southeastern farms towards the end of the study period was indicative of a ‘press’ disturbance. It was concluded that the seasonal nature of ABT penning and often offshore location of the farms, together with reduction of feed wastage, can mitigate the potential adverse benthic influence of these activities, while multiple tuna farms located close to one another result in added loading on the marine environment, hence highlighting the importance of good spatial planning for coastal aquaculture activities. The high spatio-temporal variation in the influence of tuna penning on benthic macrofaunal assemblages in the vicinity of a farm showed the importance of including multiple impacted and reference areas, as well as replicated sampling times in environmental monitoring of tuna farms. The overall findings are discussed in light of: (i) current knowledge on the influence of aquaculture, in particular ABT ranching, on soft bottom macrofauna present in the vicinity of the activity; and (ii) implications for environmental monitoring and mitigation strategies of tuna penning activities in the Mediterranean, and, in a more local context, the Maltese Islands. Finally, proposals are made for potential further research on aspects of the environmental effects of tuna penning. Description: PH.D.

2018-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/92915 2022-08-22T08:19:44Z 2018-01-01T00:00:00Z

Title: Nonlinear optomechanics Abstract: Developed in the 20th century, the branch of physics known as quantum mechanics is the science of studying the behaviour of matter and light on the atomic and sub atomic scale. Optomechanics is the discipline in quantum mechanics which studies the interaction of electromagnetic radiation in the form of photons with mechanical systems through the radiation pressure force. The optomechanical interaction is an intrinsically nonlinear one, and it can therefore cause classical states to develop nonclassical features. In this dissertation we explore the emergence of this nonclassical behaviour as a function of the system parameters. Many mathematical properties may be used to determine whether a state is in fact displaying classical or non-classical behaviour. Non-classicality may be generated for the light, mechanical or the light and mechanical field simultaneously. The properties of the Wigner quasi-probability distribution function were made use of throughout this dissertation in order to pinpoint any hints of non-classicality, specifically the negativity of the Wigner function. Interesting results were the analysis of the degree of non classicality and the analysis of the maximum negativities of the Wigner functions under specific conditions. Description: B.SC.(HONS)MATHS&PHYSICS

2018-01-01T00:00:00Z https://www.um.edu.mt/library/oar/handle/123456789/92483 2022-03-28T12:35:37Z 2018-01-01T00:00:00Z

Title: Charging a quantum battery : investigation of the quantum advantage provided by entanglement Abstract: A quantum system can be used to store energy in its energy levels, thereby a.ding as a battery. The main aim of this dissertation is to investigate the effect of quantum entanglement on the power generated during the charging of a quantum system for three different cases: closed, open and approximated classical systems. The first case illustrates the advantage provided by quantum entanglement, where the time evolution decreases by a factor of 1/N for N qubits and hence the power produced increases by the same factor. In the other two cases, the degree of entanglement and consequently the amount of power generated was reduced. This work shows that entanglement provides a quantum advantage in power generation over its classical counterparts. Description: B.SC.(HONS)PHYSICS

2018-01-01T00:00:00Z