Thesis (MSc)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Sorbic acid and sulphur dioxide are often used together, as preservatives,in many foods. Sorbic acid is known to undergo degradation in aqueous solutions, and this degradation appears to be accelerated in the presence of sulphur dioxide. The interaction between sorbic acid and sulphur dioxide is the subject of this thesis. The work is subdivided into five chapters, as follows: The first chapter deals with the chemistry of sorbic acid acid and sulphur dioxide. The second chapter describes a study of the degradation products of sorbic acid, in aqueous systems, in the presence of sulphur dioxide and a possible mechanism for the occurrence of these products is proposed. Chapter three deals with the preparation and degradation of 6-[[13]C] sorbic acid in order to find evidence for, or against, the mechanism proposed in chapter two. It also gives details of syntheses attempted in order to obtain 6-[[13]C] sorbic acid. The interaction of sorbic acid and sulphur dioxide in real food systems is the subject of the fourth chapter. The food systems studied were mayonnaise, tomato puree, orange juice and cottage cheese. The effect of packaging on the rate of degradation of sorbic acid was also investigated. The final chapter deals with a microbiological study of two homologues of sorbic acid, 2,4-heptadienoic acid, 2,4-octadienoic acid. The fungicidal activity of these two compounds, towards selected fungi, was analysed. 4-0xobut-2-enoic acid, a degradation product of sorbic acid in aqueous systems, was also analysed as a possible fungistat.

Les stratégies de maintenance et leurs évaluations demeurent une préoccupationparticulièrement forte au sein des entreprises aujourd’hui. Les enjeux socio-économiquesdépendant de la compétitivité de chacune d’entre elles sont de plus en plus étroitement liés àl’activité et à la qualité des interventions de maintenance. Une suite d’évènements particulierspeut, éventuellement, informer l’expert d’une panne prochaine. Notre étude tente d’appréhender« cette signature » à l’aide d’un modèle de Markov caché. Nous proposons à l’expert deuxstratégies sur l’estimation du niveau de dégradation du système maintenu. La première stratégieconsiste à utiliser des lois de dégradation non paramétrique et semi-paramétrique. La deuxièmestratégie consiste à utiliser une approche markovienne. Toutes les propositions sont illustrées surdeux études de cas correspondant à deux situations réelles de travail (système continu dans lafabrication de pain et produits moulés en alliages d'aluminium dans le cadre d’un processusdiscontinu). / Today, maintenance strategies and their analyses remain a worrying problem forcompanies. Socio economic stakes depending on competitiveness of each strategy are even morelinked to activity and quality of maintenance interventions. A series of specific events caneventually warn the expert of an imminent breakdown. Our study aims at understanding this“signature” thanks to an hidden Markov model. On that purpose, two strategies on damage levelestimation of maintained system are proposed to expert. The first one consists in using of nonparametric and semi parametric degradation laws. The second one consists in using of anmarkovian approach. All proposals are illustrated on two case studies corresponding to two realindustrial situations (continuous system for food processing and moulded products in aluminumalloys in an discontinuous process).

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Orientador: Mario Fernando de Goes / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Odontologia de Piracicaba / Made available in DSpace on 2018-08-16T20:24:00Z (GMT). No. of bitstreams: 1 Camilotti_Veridiana_D.pdf: 56406151 bytes, checksum: 4536fa8f005b6a5209bc724b594efe59 (MD5) Previous issue date: 2010 / Resumo: O objetivo neste estudo foi avaliar in vitro a rugosidade e a morfologia de superfície de resinas compostas armazenadas durante 180 dias em soluções ácidas ou alcoólica presentes na dieta. Sete resinas compostas indicadas para restaurações de dentes anteriores e posteriores foram selecionadas para este estudo (Charisma/Kulzer, Opallis/FGM, Esthet-X/Dentsply, 4Season/Ivoclar, Z250/3M ESPE, Z350/3M ESPE e P90/3M ESPE). Foram confeccionados cinqüenta e um corpos-deprova de cada material com 4 mm de diâmetro e 2 mm de espessura com auxílio de uma matriz de elastômero. Após a fotoativação por 40 segundos (Elipar Free-Light/3M ESPE), os corpos-de-prova foram armazenados em água destilada por 24h a 370C para posterior realização do polimento superficial com discos Soft Lex/3M ESPE na granulação fina e extrafina. Em seguida, os corpos-de-prova foram secos com papel absorvente e a rugosidade de superfície foi mensurada por meio de rugosímetro. Os corpos-de-prova foram então divididos aleatoriamente em cinco grupos (n=7) de acordo com as soluções testadas: G1- Água destilada (pH 5,30); G2 - Vinho tinto/ Trapiche Syha Ind. Argentina (pH 3,68); 3 - Coca-cola®/ Ind. Coca-cola LTDA (pH 2,73); 4 - Suco de laranja/ Purity Cocamar (pH 3,58) e G5 - Solução tampão lactato (2,74). As soluções testadas foram trocadas diariamente. O teste de rugosidade de superfície foi repetido após 15, 90 e 180 dias de imersão. Os valores médios da rugosidade de superfície foram submetidos à análise LSmeans e teste de Tukey-Kramer, em nível de significância de 5%. Os resultados de rugosidade de superfície revelaram que as resinas compostas que apresentaram aumento de rugosidade estatisticamente significativo para todos os períodos e soluções foram Opallis, Esthet-X, 4Season e Charisma. Um corpo-de-prova de cada resina composta foi selecionado para análise morfológica de superfície após 24h, 15, 90 e 180 dias, com microscopia eletrônica de varredura (JEOL- JSM 5600, Tokyo, Japão), numa ampliação de 3000X para revelar as alterações da micro-estrutura de superfície. A análise morfológica de superfície mostrou que as resinas compostas Z350 e P90 foram as mais resistentes à ação das diferentes soluções para os períodos analisados. Enquanto que para as demais resinas compostas foi observada a extrusão das partículas de carga caracterizada por porosidades ou também seu desprendimento permanecendo expostas na superfície. Com base nos resultados pôde-se observar que a degradação superficial das resinas compostas é dependente de sua composição, tempo de armazenamento e do pH da solução. / Abstract: The aim of this study was to evaluate in vitro of superficial roughness and surface morphology of composite resins stored 180 days in acid and alcoholic solutions present in the diet. Seven composite resins indicated for the previous and posterior teeth restorations were selected for this study (Charisma / Kulzer, Opallis / FGM, Esthet-X/Dentsply, 4Season/Ivoclar, Z250/3M ESPE ESPE and Z350/3M P90/3M ESPE). Fifty one specimens were made proof-of-each material with 4 mm in diameter and 2 mm thick with elastomer matrix aid. After curing for 40 seconds, the specimens were stored in distilled water for 24h at 370C for subsequent accomplishment of the superficial polishing with disks Soft Lex/3M ESPE and in the thin and extra thin granulation. Then the specimens were proof-of-dry with paper towels and the superficial roughness was measured using a rugosimeter. Specimens were then randomly divided into five groups according to the tested solutions: G1- Distilled water (pH 5,30) G2 - Red Wine / Argentina Trapiche SYHA Ind. (pH 3,68); G3 - Coca-Cola ® / Coca-Cola Ind. LTDA (pH 2,73); G4 - Orange Juice / Purity Cocamar (pH 3,58) and G5 - lactate buffer (pH 2,74). The superficial roughness test was repeated after 15, 90 and 180 days of immersion. The average values of the superficial roughness were statistically analyzed by LSmeans test and Tukey-Kramer test at a significant level of 5%.The results showed that the composite resins that statistically significant increase in roughness for all periods and solutions were Opallis, Esthet-X, 4Season and Charisma. One specimen more representative of each compound resin was selected for morphological analysis of surface after 24, 15, 90 and 180 days. This analysis was made using scanning electron microscopy (JEOL JSM-5600, Tokyo, Japan) at a magnification of 3000X to expose changes in the superficial micro structure. The surface morphology showed that the composite resins Z350 and P90 were more resistant to the action of different solutions for the periods analyzed. The other composite resins were observed displacement of charged particles or porosities also characterized by its detachment staying loose on the surface. Based on the results it was observed that the surface degradation of compound resins is dependent of its composition, storage time and pH solution. / Doutorado / Materiais Dentarios / Doutor em Materiais Dentários

Thicket degradation within the Addo Elephant National Park (AENP), Eastern Cape, South Africa is a key issue of concern. The increasing elephant densities have over time threatened thicket vegetation, which is endemic to this area. The “elephant problem” within the Park has raised many concerns as to the vulnerability of thicket to elephant impacts. Remote sensing provides an environment in which temporal changes in thicket vegetation condition can be assessed. The main aim of this study was to assess temporal and spatial trends in thicket degradation within the AENP, using multi-temporal satellite imagery. Elephant induced thicket degradation was assessed using multi-temporal satellite imagery from between 1973 and 2010. Changes in thicket condition in relation to the AENP expansion were analysed using the Normalized Difference Vegetation Index (NDVI), post-classification and landscape metrics. The ‘per-pixel’ based classification approach was carried out and change detection of land cover classes was analysed by post-classification. Landscape spatial metrics were used in order to gain an understanding of vegetation fragmentation trends. Temporal changes in vegetation gradients in relation to water points and thicket condition within the botanical reserves were also assessed. Thicket condition was noted to deteriorate as the AENP expanded. An expansion of degraded vegetation away from the water points was identified during the study period. Thicket condition in botanical reserves 1 and 2 fluctuated over time, with a registered increase in intact thicket within botanical reserve 2, with botanical reserve 3 remaining constant. Landscape spatial metric analyses revealed evidence of increased vegetation fragmentation as new areas of the AENP were opened for elephant activity. A progressive decline in intact thicket and increase in degraded thicket were observed. Considering the current elephant densities, thicket degradation within AENP is set to continue, causing a continued threat to the thicket biome.

Oil wealth enriches Nigeria, but it has not improved the lives of the majority of the masses living in the oil-bearing areas of the Niger Delta. Niger Delta region has been exposed to environmental risks that have caused many to lose their means of livelihood, triggering high level of poverty in the region. This study examines the impact of oil-related environmental problems and how it has induced human displacement in the Niger Delta. Furthermore, it examined the efficacy of the Nigerian environmental policies as it related to the oil-bearing areas. The research found that oil activities have caused more harm than good in the Niger Delta. At present, the oil-bearing areas remain marginalized from the mainstream economic, social, and political activities in Nigeria. The Nigerian government’s top-down approach to the development of the oil-bearing areas has not been people-centered and participatory. The paper also made some viable recommendations.

The perovskite solar cells have attracted much attention recently due to their low fabrication cost and high power conversion efficiency. However, the instability of such devices remained a serious challenge, which is yet to be resolved despite many attempts. In this thesis, the moisture degradation mechanism has been uncovered. It was found that the perovskite structure can be reconstructed when annealed at around 90℃, which leads to a reversible device of a much higher device lifetime. Following the experimental explorations, modelling work has also been conducted to simulate the water diffusion during the degradation and recovery. It was found that the moisture diffusion was initiated at the surface imperfection, where the activation energy for diffusion can be reduced. Based on these discoveries, a fresh structural negotiation method has been proposed to obtain a stable FAPbI3 phase of a suitable bandgap for photovoltaics, which showed a reduced Gibbs free energy of 0.12eV compared with the δ phase. In addition, the activation energy for such phase transition was calculated to be 0.45eV, meaning that the discovered phase is protected by both thermodynamics and kinetics. All this opens an unprecedented avenue in perovskite research, which will hopefully be of intrinsic interest to the broad materials research community as well. / Thesis / Doctor of Philosophy (PhD)

In developing photovoltaic technology for commercial use, it is essential to prioritize low cost, high efficiency, and long-term stability, along with low toxicity and quick energy payback time. Organic-inorganic metal halide perovskites are among the most promising photovoltaic materials due to their exceptional efficiency and cost-effectiveness. However, their practical significance remains unclear due to their notoriously short device operation duration. Mixed-metal tin-lead perovskites are a class of perovskites that have captured researchers' attention due to their unique optoelectronic features, including a small bandgap, making them useful for various applications (tandems, NIR light detection, and imaging e.g.). Still, the low ambient stability of tin-lead perovskites obstructs their commercialization, necessitating extensive research into their underlying breakdown mechanisms. This study aims to better understand these mechanisms and the involvement of halide chemistry, specifically emphasizing the significant role of iodine in perovskite deterioration. Our findings reveal that tin-lead-based perovskites experience cyclic degradation, with iodine and SnI4 as key degradation products that harm the stability of the perovskite. Investigating the impact of iodine is critical because it is a common component of the perovskite material, and its presence has been shown to play a crucial role in the optoelectronic properties of the perovskite. However, the presence of iodine can also lead to the degradation of the perovskite material over time, reducing the efficiency and lifespan of the perovskite solar cells. Therefore, understanding the role of iodine in perovskite deterioration is essential to improve the stability and durability of tin-lead perovskites and bring them closer to commercialization. By gaining insight into the degradation mechanisms of tin-lead perovskites, we can develop effective strategies to mitigate their degradation, enhance their stability and lifespan, and unlock their full potential for use in various photovoltaic applications, contributing to a more sustainable and environmentally conscious future.

The insulation of the high voltage power cable will be placed under higher electrical stress by switching surges during the power system operation. The switching surge weakens the power cable insulation and it leads to failure of cables. It has a significant effect on the aging of the insulation material that affects the lifetime of the power cables. This research studies the electrical degradation of 15 kV XLPE and EPR power cables insulation by applying 100, 500, 1000 and 5000 switching impulses of 100 kV. The status of the polymer cable insulation aging is evaluated by the measurement of partial discharge level and ac breakdown voltage. Partial discharge measurements are taken throughout the aging process at specific intervals of applied impulses. The ac breakdown voltage is also measured at the end of the accelerated aging process.