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For selected YAG:Ce sample the temperature dependent absorbtion and emission spectra were measured and used to simulate white LED behavior. Various CIE-xy dependencies were observed. Sample surface scaterring parameters were calibrated using a custom-made automated goniospektrometer, which provided measured data with fine accuracy. Finally, the measured data were used to construct a Geant4 simulation to optimize two light devices for output intensity and target CIE-xy coordinates. Powered by TCPDF (www.tcpdf.org)

The production of cereals, especially maize, which is a staple food in sub-Saharan Africa, is challenged by pests and diseases. In addition, climate change will exacerbate the magnitude of these challenges and agriculture in general. Lepidopteran stemborers are major pests of cereal crops in sub-Saharan Africa. Two indigenous noctuids, Busseola fusca and Sesamia calamistis and one exotic crambid, Chilo partellus, occur as single and mixed-species communities infesting cereal crops in East and southern Africa. The composition of these communities however varies with altitude and over seasons. Interactions between the species especially when in combination, can affect the level of damage to cereal crops. The objective of this study was to describe the intraspecific and interspecific interactions between the three stemborer species when they make use of the same resources. This study involved field surveys in maize fields in major agro-ecological zones and also different experiments under greenhouse and laboratory conditions to describe the interactions between the species as well as the effect of temperature, species density and duration of the period of competition on outcomes of these interactions. Results showed that stemborer communities are composed of single and mixed species of B. fusca, S. calamistis and C. partellus, which varied with the different agro-ecological zones and also along altitudinal gradients. Temperature was the most important abiotic factor that influenced the composition of stemborer communities. However, infestation patterns varied with season. This study also showed that female moths of the three species did not avoid oviposition on plants that were previously infested by stemborers. Busseola fusca showed a high preference for heterospecific-infested plants, while C. partellus preferred conspecific-infested plants. Sesamia calamistis did not show a significant preference for one plant over the other. This study further showed that both intra- and interspecific competition characterise communities of these stemborer species which ultilise the same resources. Interspecific competition was stronger between the noctuids and the crambid than between the two noctuid species. Temperature had a significant influence on the competitive outcomes between the three species. Finally the study showed that densitydependent effects and the duration of the interactions are also important factors which influence the level of competitive outcomes between the species. Climate change is likely to influence the interactions and composition of stemborer communities. / PhD (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

The production of cereals, especially maize, which is a staple food in sub-Saharan Africa, is challenged by pests and diseases. In addition, climate change will exacerbate the magnitude of these challenges and agriculture in general. Lepidopteran stemborers are major pests of cereal crops in sub-Saharan Africa. Two indigenous noctuids, Busseola fusca and Sesamia calamistis and one exotic crambid, Chilo partellus, occur as single and mixed-species communities infesting cereal crops in East and southern Africa. The composition of these communities however varies with altitude and over seasons. Interactions between the species especially when in combination, can affect the level of damage to cereal crops. The objective of this study was to describe the intraspecific and interspecific interactions between the three stemborer species when they make use of the same resources. This study involved field surveys in maize fields in major agro-ecological zones and also different experiments under greenhouse and laboratory conditions to describe the interactions between the species as well as the effect of temperature, species density and duration of the period of competition on outcomes of these interactions. Results showed that stemborer communities are composed of single and mixed species of B. fusca, S. calamistis and C. partellus, which varied with the different agro-ecological zones and also along altitudinal gradients. Temperature was the most important abiotic factor that influenced the composition of stemborer communities. However, infestation patterns varied with season. This study also showed that female moths of the three species did not avoid oviposition on plants that were previously infested by stemborers. Busseola fusca showed a high preference for heterospecific-infested plants, while C. partellus preferred conspecific-infested plants. Sesamia calamistis did not show a significant preference for one plant over the other. This study further showed that both intra- and interspecific competition characterise communities of these stemborer species which ultilise the same resources. Interspecific competition was stronger between the noctuids and the crambid than between the two noctuid species. Temperature had a significant influence on the competitive outcomes between the three species. Finally the study showed that densitydependent effects and the duration of the interactions are also important factors which influence the level of competitive outcomes between the species. Climate change is likely to influence the interactions and composition of stemborer communities. / PhD (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

This thesis focuses on a number of aspects governing the transformation of gibbsite, via intermediate phases, to a-alumina. These aspects include the size and morphology of the gibbsite grains, the influence of additions of foreign elements, the effect of a mechanical treatment of the gibbsite prior to calcination, and combinations of these factors. The materials were characterisedb y scanninge lectron microscopy, X-ray diffraction and surfacea rea measurementsF. or someo f the calcined materials an attempt was made to sinter the powders to a dense body to investigate if any of the treatments during calcination had an effect on this process. The literature review covers the current state of understanding of the production of bulk alumina powder by the Bayer process and the phase changes seen on calcination of precursors to the stable a-alumina phase. A detailed description of the phase changes is given and the various routes and conditions necessary for the transformations to occur are considered. The transformations are examined in relation to the morphology of the crystals and the variables controlling the phase transformation route are discussed. Calcination in air showed that the size of the gibbsite grain governs the calcination route taken to reach a-alumina. The standard gibbsites used in this work show a mixed calcination sequence transforming both via the boehmite phase, followed by the y, 8 and 0 phases, and via the x and K phases. The formation of boehmite is attributed to retention of water vapour within the grainDifferences in morphology of the starting materials showed that for the range of materials seen, the morphology of the grain is less important than its size. The super fine material confirmed that a small grain size transforms via the non-boehmite route only, with the other gibbsites taking intermediate routes as for the standard gibbsites. Of the additions made prior to calcination, aluminium fluoride was found to reduce the transformation temperature to a-alumina by approximately 300°C. Other additions had little effect on the transformation temperature although a reduction in grain size was seen with aluminium chloride. It was found that good mixing of the alumina fluoride was essential to obtain reliable and reproducible results. This is due to the small amounts of additive that are needed and the sensitivity of the process to concentration variations. Mineralisation of a range of gibbsites showed that the presence of sodium in the starting material was crucial in reducing the calcination temperature. This led to the conclusion that the sodium and fluoride react to form a liquid phase. The presence of a liquid phase increases the mobility of the aluminium and oxygen atoms resulting in a reduction of the transformation temperature. Fluoride additions to the gibbsites with different morphologies showed that the presence of sodium was the governing factor in reduction of the transformation temperature. Milling of the starting materials showed that there was a small reduction in the transformation temperature between some of the phases. The energy involved in milling leads to activation of the gibbsite. This activation takes the form of a reduction in the grain size and in a reduction of the crystallinity seen in the XRD patternFluoride additions during the calcination of sapphire with a standard gibbsite powder showed preferential grain growth. It was possible to initiate growth of small plate-like crystals on the polished surface of a piece of sapphire parallel to the basal plane. Crystal growth was also seen in scratches on a polished surface perpendicular to the basal plane

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