Magister Scientiae - MSc / It is well known that a lead pencil is made of graphite, a naturally form of carbon, this is important but not very exciting. The exciting part is that graphite contains stacked layers of graphene and each and every layer is one atom thick. Scientists believed that these graphene layers could not be isolated from graphite because they were thought to be thermodynamically unstable on their own and taking them out from the parent graphite crystal will lead them to collapse and not forming a layer. The question arose, how thin one could make graphite. Two scientists from University of Manchester answered this question by peeling layers from a graphite crystal by using sticky tape and then rubbing them onto a silicon dioxide surface. They managed to isolate just one atom thick layer from graphite for the first time using a method called micromechanical cleavage or scotch tape. In this thesis chemical method also known as Hummers method has been used to fabricate graphene oxide (GO) and reduced graphene oxide. GO was synthesized through the oxidation of graphite to graphene oxide in the presence of concentrated sulphuric acid, hydrochloric acid and potassium permanganate. A strong reducing agent known as hydrazine hydrate has also been used to reduce GO to rGO by removing oxygen functional groups, but unfortunately not all oxygen functional groups have been removed, that is why the final product is named rGO. GO and rGO solutions were then deposited on silicon substrates separately. Several characterization techniques in this work have been used to investigate the optical properties, the morphology, crystallography and vibrational properties of GO and rGO.

Strawberry powdery mildew, caused by Podosphaera aphanis, has the potential to cause over 20% yield loss, particularly where strawberries are grown under cover. A holistic approach to the control of strawberry powdery mildew (P. aphanis) is important, since the disease is never absent from the crop. The new disease assessment key was developed to assess strawberry powdery mildew (the old one is for assessing red blotches on leaves, See appendix 8). The results (Chapter 3) showed that the disease is present in the crop when new plants are bought in from a propagator, with 14% of strawberry crowns were infected by P. aphanis in July 2013 and 4% of the strawberry plants had symptoms of powdery mildew in pre-assessment of plants for the 2013 Si nutrient fertigation field experiment. Control measures used in one growing season reduced the disease carry-over, thus reducing the initial inoculum in the following season. The use of a late autumn fungicide spray and a fungicide spray before the plants were covered by fleece in spring reduced the number and maturity of overwintering chasmothecia, thus contributing to a reduction in initial inoculum. The use of silicon (Si) nutrient (foliar spray and root treatment) also suppressed strawberry powdery mildew development (Chapter 4). The results of Area Under Disease Progress Curve (AUDPC) and the rate of epidemic growth curve (r) in 2012 indicated that the high concentration of Si nutrient foliar spray inhibited (r = 0.002, AUDPC = 44) the epidemic build-up of P. aphanis better than the low concentration of Si nutrient (r = 0.012, AUDPC = 51) and untreated (r = 0.018, AUDPC = 70). The Si nutrient root treatment (AUDPC = 12.8) was better in inhibiting strawberry powdery mildew development than the Si nutrient foliar spray treatment. Moreover, the high concentration of Si nutrient foliar spray resulted in fewer chasmothecia compared to the untreated. Si nutrient foliar spray and root treatments increased the concentration of Si in the plants and produced physiological changes in the plants, including wax formation on the adaxial leaf surface, greater leaf thickness and cuticle layer and increased Brix0 value in plants, which all were associated with reduced disease incidence. The integrated use of all these control strategies suppresses disease development so that control is achieved with less use of conventional fungicides.

Forest rings are large features common in Ontario’s boreal forests that comprise circular topographic depressions in carbonate mineral soil that are filled with peat. This thesis documents differences in peat and soil chemistry along transects across the “Bean” and “Thorn North” rings, which are centered on accumulations of CH4 and H2S, respectively. Within the mineral soil, ring edges are characterized by strong negative anomalies in pH, ORP and carbonate, as well as positive anomalies of Al, Fe and Mn in the results of aqua regia and hydroxylamine-hydrochloride digestions. Within the peat, positive carbonate and pH anomalies are recorded. This antithetic relationship suggests vertical migration of carbonate species from clay to peat. An inverse relationship exists between ORP, versus redox inferred from aqua regia. Strong ORP lows occur where oxidized products show highest concentrations. This is interpreted to reflect the proliferation of autotrophic organisms occupying the strong redox gradient at the ring edge.

Passive reactive filters have the potential to provide effective phosphorus (P) removal from stormwater or agricultural drainage, or to act as an add-on P-removal technology for decentralized or small community wastewater treatment systems. Passive filters require minimal energy consumption and human maintenance. Direct-reduced iron (DRI), a steel-making intermediate, was investigated as a passive filter media for wastewaters phosphorus reduction. Phosphorus is a biologically active element that is in excess in many natural waterways due to intensive human activity. Eutrophication can occur when P concentrations exceed 0.02 mg/L in freshwater lakes and rivers. The harmful consequence of this phenomenon includes oxygen deprivation, fish death and cyanobacteria-produced toxins. There is a pressing need to limit phosphorus over-discharge into natural waterways. DRI is a novel media in the application of wastewater treatment and was characterized to have a porous structure with high metallic iron content. The phosphorus retaining mechanisms in batch and column studies suggest a combination of adsorption and surface crystal formation as the dominant removal mechanisms. Batch studies demonstrated increasing removal capacity with P concentration with a plateau observed at 21 mg P/g DRI relating to initial 3000 mg P/L. Media rejuvenation was investigated through chemical treatment with two iron solutions (Fe2(SO4)3, FeCl3) and two acidic solutions (H2SO4 and HCl) at varying molarity. P removal capacity could be fully recovered with 0.05 M Fe3+ or 0.4 N H+ (HCl/H2SO4), while a 37.6% P recovery was also achieved in an acidic solution at 1.2 N H+ (HCl/H2SO4). A column study utilizing three media sizes of DRI (3.5, 11, 19 mm) and one media size of activated alumina (AA) (7.5 mm) was conducted for 315 days using synthetic P solution varying from 2 to 10 mg/L and hydraulic retention times (HRTs) varying from 0.7 – 15 h. The results demonstrated that removal efficiency increased with HRT and decreased with increasing media size and concentration with minimum HRTs to maintain an 80% removal efficiency varying from 4.4 to 15 hrs for DRI and 3.9 hrs for AA for influent P concentrations of 10 mg/L and below. After 1 year of column operation, the DRI media had demonstrated a minimum removal capacity of 1.82 mg P/g DRI, which can be used as a conservative design parameter. A short duration column study (34 days) utilizing municipal lagoon effluent exhibited similar removal efficiencies to the synthetic column study under the same operational conditions. The 10 years lifespan DRI filter with 80% removal rate in the treatment of stormwater, municipal lagoon effluent, septic tank effluent and dairy wastewater application would have been estimated to have filter volumes of 0.24, 4.69, 15.3 and 36.2 m3, respectively.

This thesis focuses on how the properties of polymer thin films depend on the film thickness and molecular weight (Mw). Previous studies of polystyrene (PS) films coated on silica with Mw of 2.4 and 212 kg/mol found that the viscosity of the films decreases with decreasing film thickness. A two-layer model assuming a 3 nm thick mobile layer situated at the top surface of the film and hydrodynamically coupled to a bulklike inner layer is able to describe the viscosity of the films. To better understand the dynamics exhibited by the surface mobile layer, this work extends the measurement to various other Mw up to 2316 kg/mol. The result shows that the viscosity of the films also decreases with decreasing film thickness and can be described by the same two-layer model. But there are exceptional findings as well. Specifically, the viscosity of the high-Mw films (> 60 kg/mol) exhibit a Mw^0 dependence, distinctly different from the Mw^3.4 dependence exhibited by the viscosity of the bulk polymer. Moreover, the surface chains in the high-Mw films, as inferred from the two-layer model, are in an unphysically stretched state. These observations led to the conjecture that the viscosity reduction in the high-Mw films is due to a different mechanism from that in the lower Mw films which is directly tied to the surface mobile layer as in the two-layer model. To scrutinize this conjecture, viscosity measurement is extended to PS films doped with Dioctyl phthalate (DOP). A previous experiment showed that the influence of the surface on the overall dynamics in this system is likely to be much smaller than in the undoped films. The measurement results are examined to determine, among other things, whether the viscosity reduction relative to that in the undoped films is weaker in the low-Mw than in the high-Mw regime.

Electromagnetic levitation experiments provide a powerful tool that allows for the study of nucleation, solidification and growth in a containerless processing environment. Containerless processing allows for the study of reactive melts at elevated temperatures without chemical interactions or contamination from a container. Further, by removing the interface between the liquid and its container, this processing technique allows for greater access to the undercooled region for solidification studies. However, in these experiments it is important to understand the magnetohydrodynamic flow within the sample and the effects that this fluid flow has on the experiment. A recent solidification study found that aluminum-nickel alloy sample have an unusual response of the growth rate of the solid to changes in undercooling. This alloy experienced a decrease in the growth velocity as the initial undercooling deepened, instead of the expected increase in solidification velocity with deepening undercoolings. Current work is exploring several different theories to explain this phenomenon. Distinguishing among these theories requires a comprehensive understanding of the behavior of the internal fluid flow. Our project, USTIP, has done flow modeling to support this and multiple other collaborators on ISS-EML. The fluid flow models presented for the aluminum-nickel sample provide critical insights into the nature of the flow within the aluminum-nickel alloy experiments conducted in the ISS-EML facility. These models have found that for this sample the RNG k-ε model should be used with this sample at temperatures greater than 1800 K and the laminar flow model should be used at temperatures lower than 1600 K. Other work in the ISS-EML, has studied the thermophysical properties of liquid germanium and has found the most recent measurements using oscillating drop techniques to have a discrepancy from the expected property measurements taken terrestrially. Investigating this discrepancy required the quantification of the velocity and characterization of the internal fluid flow in the drop. The models have found that the flow within the sample maintains turbulent behavior throughout cooling. This thesis presents the analysis of the internal flow of four additional samples processed in the International Space Station Electromagnetic Levitation facility. These samples consist of the following alloys: Ti39.5Zr39.5Ni21, Cu50Zr50, Vitreloy 106, and Zr64Ni36. Our collaborators work required the internal flow to be characterized and quantified for their work on solidification. In addition to quantifying the velocity of the flow, the Reynolds number was calculated to characterize the flow during processing. Additionally, the shear-strain rate was calculated for the flow during processing up to the recalescence of the melt.

Reduced graphene oxide as outstanding candidate electrode material for supercapacitor has been investigated. This thesis includes two topics. One is that three kinds of reduced graphene oxides were prepared by hydrothermal reduction under different pH conditions. The pH values were found to have great influence on the reduction of graphene oxides. Acidic and neutral media yielded reduced graphene oxides with more oxygen-functional groups, lower specific surface areas but broader pore size distributions than those in basic medium. Variations induced by the pH changes resulted in great differences in the supercapacitor performance. The graphene produced in the basic solution presented mainly electric double layer behavior with specific capacitance of 185 F/g, while the other two showed additional pseudocapacitance behavior with specific capacitance of 225 F/g (acidic) and 230 F/g (neutral), all at a constant current density of 1A/g. The other one is that different reduced graphene oxides were prepared via solution based hydrazine reduction, low temperature thermal reduction, and hydrothermal reduction. The as- prepared samples were then investigated by UV-vis spectroscopy, X-ray diffraction, Raman spectroscopy, and Scanning electron microscope. The supercapacitor performances were also studied and the hydrothermally reduced graphene oxide exhibited the highest specific capacitance.

This diploma thesis deals with the creation of computational model of arterial aneurysm, which respects its real geometry and constitutive behaviour. Subsequently there is explored the risk of rupture, comparing the values of the highest main stress in aneurysm´s and intact aorta´s wall. This thesis also offers the comprehensive summary and mutual comparision of current constitutive (hyperelastic) models, which are for simplicity isochoric considered. The main task of this thesis is to formulate the method for finding of „unloaded“ = „outstressed“ = „primary“ = reduced geometry, which is generally unavailable. Models of aneurysm´s geometry, used so far, present already deformed configurations from blood pressure, axial prestrech and rezidual strain.

Dissertation (MMus (Performing Art))--University of Pretoria, 2002. / Music / unrestricted

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