Résumé : En imagerie médicale, il est courant d’associer plusieurs modalités afin de tirer profit des renseignements complémentaires qu’elles fournissent. Par exemple, la tomographie d’émission par positrons (TEP) peut être combinée à l’imagerie par résonance magnétique (IRM) pour obtenir à la fois des renseignements sur les processus biologiques et sur l’anatomie du sujet. Le but de ce projet est d’explorer les synergies entre l’IRM et la TEP dans le cadre d’analyses pharmacocinétiques. Plus spécifiquement, d’exploiter la haute résolution spatiale et les renseignements sur la perfusion et la perméabilité vasculaire fournis par l’IRM dynamique avec agent de contraste afin de mieux évaluer ces mêmes paramètres pour un radiotraceur TEP injecté peu de temps après. L’évaluation précise des paramètres de perfusion du radiotraceur devrait permettre de mieux quantifier le métabolisme et de distinguer l’accumulation spécifique et non spécifique. Les travaux ont porté sur deux radiotraceurs de TEP (18F-fluorodésoxyglucose [FDG] et 18F-fluoroéthyle-tyrosine [FET]) ainsi que sur un agent de contraste d’IRM (acide gadopentétique [Gd DTPA]) dans un modèle de glioblastome chez le rat. Les images ont été acquises séquentiellement, en IRM, puis en TEP, et des prélèvements sanguins ont été effectués afin d’obtenir une fonction d’entrée artérielle (AIF) pour chaque molécule. Par la suite, les images obtenues avec chaque modalité ont été recalées et l’analyse pharmacocinétique a été effectuée par régions d’intérêt (ROI) et par voxel. Pour le FDG, un modèle irréversible à 3 compartiments (2 tissus) a été utilisé conformément à la littérature. Pour la FET, il a été déterminé qu’un modèle irréversible à 2 tissus pouvait être appliqué au cerveau et à la tumeur, alors qu’un modèle réversible à 2 tissus convenait aux muscles. La possibilité d’effectuer une conversion d’AIF (sanguine ou dérivée de l’image) entre le Gd DTPA et la FET, ou vice versa, a aussi été étudiée et s’est avérée faisable dans le cas des AIF sanguines obtenues à partir de l’artère caudale, comme c’est le cas pour le FDG. Finalement, l’analyse pharmacocinétique combinée IRM et TEP a relevé un lien entre la perfusion du Gd-DTPA et du FDG, ou de la FET, pour les muscles, mais elle a démontré des disparités importantes dans la tumeur. Ces résultats soulignent la complexité du microenvironnement tumoral (p. ex. coexistence de divers modes de transport pour une même molécule) et les nombreux défis rencontrées lors de sa caractérisation chez le petit animal. / Abstract : In medical imaging, different modalities are frequently combined in order to obtain complementary information. For example, positron emission tomography (PET) can be associated with magnetic resonance imaging (MRI) to derive both anatomical and biological information. This project explores the synergies between MRI and PET for pharmacokinetic modeling. Specifically, it exploits the high spatial resolution of MRI as well as the information about perfusion and vascular permeability derived from dynamic contrast-enhanced studies to better assess these parameters in a PET radiotracer injected shortly after the MRI examination. This more precise assessment of perfusion is thought to improve metabolism quantification for the radiotracer and to discriminate between its specific and non-specific accumulation. The present work focussed on 2 PET radiotracers, (18F-fluorodeoxyglucose [FDG] and 18F-fluoroethyltyrosine [FET]) as well as a MRI contrast agent (gadopentetic acid [Gd-DTPA]) applied to a rat glioblastoma model. Images were acquired using a sequential MRI-PET protocol and blood was drawn to derive the arterial input function (AIF) for each molecule. PET and MR images were subsequently registered and pharmacokinetic modeling was performed on regions of interest (ROI) or voxel-wise. For FDG, an irreversible 3 compartments (2-tissue) model was used in accordance to the literature. For FET, it was determined that an irreversible 2-tissue model is applicable for the brain and the tumor and a reversible 2-tissue model is preferred for the muscles. AIF (blood or image-derived) conversion between Gd-DTPA and FET, or vice versa, was also considered and proved feasible for the blood AIF derived from the caudal artery, similar to FDG. Finally, combined kinetic modeling for MRI and PET showed a relationship between the perfusion of FDG, or FET, and that of Gd-DTPA in muscle. Important disparities were noted for the tumor. These results illustrate the complexity of the tumor microenvironment (e.g. presence of various transport mechanisms for the same molecule) and the numerous challenges encountered during its characterization in small animals.

In the course of the PhD thesis large area homogeneous strictly monolayer graphene films were successfully synthesized with chemical vapor deposition over both Cu and Si (with surface oxide) substrates. These synthetic graphene films were characterized with thorough microscopic and spectrometric tools and also in terms of electrical device performance. Graphene growth with a simple chemo thermal route was also explored for understanding the growth mechanisms. The formation of homogeneous graphene film over Cu requires a clean substrate. For this reason, a study has been conducted to determine the extent to which various pre-treatments may be used to clean the substrate. Four type of pre-treatments on Cu substrates are investigated, including wiping with organic solvents, etching with ferric chloride solution, annealing in air for oxidation, and air annealing with post hydrogen reduction. Of all the pretreatments, air oxidation with post hydrogen annealing is found to be most efficient at cleaning surface contaminants and thus allowing for the formation of large area homogeneous strictly monolayer graphene film over Cu substrate. Chemical vapor deposition is the most generally used method for graphene mass production and integration. There is also interest in growing graphene directly from organic molecular adsorbents on a substrate. Few studies exist. These procedures require multiple step reactions, and the graphene quality is limited due to small grain sizes. Therefore, a significantly simple route has been demonstrated. This involves organic solvent molecules adsorbed on a Cu surface, which is then annealed in a hydrogen atmosphere in order to ensure direct formation of graphene on a clean Cu substrate. The influence of temperature, pressure and gas flow rate on the one-step chemo thermal synthesis route has been investigated systematically. The temperature-dependent study provides an insight into the growth kinetics, and supplies thermodynamic information such as the activation energy, Ea, for graphene synthesis from acetone, isopropanol and ethanol. Also, these studies highlight the role of hydrogen radicals for graphene formation. In addition, an improved understanding of the role of hydrogen is also provided in terms of graphene formation from adsorbed organic solvents (e.g., in comparison to conventional thermal chemical vapor deposition). Graphene synthesis with chemical vapor deposition directly over Si wafer with surface oxide (Si/SiOx ) has proven challenging in terms of large area and uniform layer number. The direct growth of graphene over Si/SiO x substrate becomes attractive because it is free of an undesirable transfer procedure, necessity for synthesis over metal substrate, which causes breakage, contamination and time consumption. To obtain homogeneous graphene growth, a local equilibrium chemical environment has been established with a facile confinement CVD approach, inwhich two Si wafers with their oxide faces in contact to form uniform monolayer graphene. A thorough examination of the material reveals it comprises facetted grains despite initially nucleating as round islands. Upon clustering these grains facet to minimize their energy, which leads to faceting in polygonal forms because the system tends to ideally form hexagons (the lowest energy form). This is much like the hexagonal cells in a beehive honeycomb which require the minimum wax. This process also results in a near minimal total grain boundary length per unit area. This fact, along with the high quality of the resultant graphene is reflected in its electrical performance which is highly comparable with graphene formed over other substrates, including Cu. In addition the graphene growth is self-terminating, which enables the wide parameter window for easy control. This chemical vapor deposition approach is easily scalable and will make graphene formation directly on Si wafers competitive against that from metal substrates which suffer from transfer. Moreover, this growth path shall be applicable for direct synthesis of other two dimensional materials and their Van der Waals hetero-structures. / Im Zuge dieser Doktorarbeit wurden großflächige und homogene Graphen-Monolagen mittels chemischer Gasphasenabscheidung auf Kupfer- (Cu) und Silizium-(Si) Substraten erfolgreich synthetisiert. Solche monolagigen Graphenschichten wurden mithilfe mikroskopischer und spektrometrischer Methoden gründlich charakterisiert. Außerdem wurde der Wachstumsmechanismus von Graphen anhand eines chemo-thermischen Verfahrens untersucht. Die Bildung von homogenen Graphenschichten auf Cu erfordert eine sehr saubere Substratoberfläche, weshalb verschiedene Substratvorbehandlungen und dessen Einfluss auf die Substratoberfläche angestellt wurden. Vier Vorbehandlungsarten von Cu-Substraten wurden untersucht: Abwischen mit organischen Lösungsmitteln, Atzen mit Eisen-(III)-Chloridlösung, Wärmebehandlung an Luft zur Erzeugung von Cu-Oxiden und Wärmebehandlung an Luft mit anschließender Wasserstoffreduktion. Von diesen Vorbehandlungen ist die zuletzt genannte Methode für die anschließende Abscheidung einer großflächigen Graphen-Mono-lage am effektivsten. Die chemische Gasphasenabscheidung ist die am meisten verwendete Methode zur Massenproduktion von Graphen. Es besteht aber auch Interesse an alternativen Methoden, die Graphen direkt aus organischen, auf einem Substrat adsorbierten Molekülen, synthetisieren konnen. Jedoch gibt es derzeit nur wenige Studien zu derartigen alternativen Methoden. Solche Prozessrouten erfordern mehrstufige Reaktionen, welche wiederrum die Qualität der erzeugten Graphenschicht limitieren, da nur kleine Korngrößen erreicht werden konnen. Daher wurde in dieser Arbeit ein deutlich einfacherer Weg entwickelt. Es handelt sich dabei um ein Verfahren, bei dem auf einer Cu-Substratoberfläche adsorbierte, organische Lösungsmittelmoleküle in einer Wasserstoffatmosphäre geglüht werden, um eine direkte Bildung von Graphen auf einem sauberen Cu-Substrat zu gewahrleisten.Der Einfluss von Temperatur, Druck und Gasfluss auf diesen einstufigen chemothermischen Syntheseweg wurde systematisch untersucht. Die temperaturabhängigen Untersuchungen liefern einen Einblick in die Wachstumskinetik und thermodynamische Größen, wie zum Beispiel die Aktivierungsenergie Ea, für die Synthese von Graphen aus Aceton, Isopropanol oder Ethanol. Diese Studien untersuchen außerdem die Rolle von Wasserstoffradikalen auf die Graphensynthese. Weiterhin wurde ein verbessertes Verständnis der Rolle von Wasserstoff auf die Graphen-synthese aus adsorbierten, organischen Lösungsmitteln erlangt (beispielsweise im Vergleich zur konventionellen thermischen Gasphasenabscheidung). Die direkte Graphensynthese mittels chemischer Gasphasenabscheidung auf Si-Substraten mit einer Oxidschicht (Si/SiOx ) ist extrem anspruchsvoll in Bezug auf die großflächige und einheitliche Abscheidung (Lagenanzahl) von Graphen-Monolagen. Das direkte Wachstum von Graphen auf Si/SiOx -Substrat ist interessant, da es frei von unerwünschten Übertragungsverfahren ist und kein Metall-substrat erfordert, welche die erzeugten Graphenschichten brechen lassen können. Um ein homogenes Graphenwachstum zu erzielen wurde durch den Kontakt zweier Si-Wafer, mit ihren Oxidflachen zueinander zeigend, eine lokale Umgebung im chemischen Gleichgewicht erzeugt. Diese Konfiguration der Si-Wafer ist nötig, um eine einheitliche Graphen-Monolage bilden zu können. Eine gründliche Untersuchung des abgeschiedenen Materials zeigt, dass trotz der anfänglichen Keimbildung von runden Inseln facettierte Körner erzeugt werden. Aufgrund der Bestrebung der Graphenkörner ihre (Oberflächen-) Energie zu minimieren, wird eine Facettierung der Körner in polygonaler Form erzeugt, was darin begründet liegt, dass das System idealerweise eine Anordnung von hexagonal geformten Körnern erzeugen würde (niedrigster Energiezustand). Der Prozess ist vergleichbar mit der sechseckigen Zellstruktur einer Bienenstockwabe, welche ein Minimum an Wachs erfordert. Dieser Prozess führt auch zu einer nahezu minimalen Gesamtkorn-grenzlänge pro Flächeneinheit. Diese Tatsache zusammen mit der hohen Qualität der resultierenden Graphenschicht spiegelt sich auch in dessen elektrischer Leistungsfähigkeit wider, die in hohem Maße mit der auf anderen Substraten gebildeten Graphenschichten (inklusive Cu-Substrate) vergleichbar ist. Darüber hinaus ist das Graphenwachstum selbstabschliessend, wodurch ein großes Parameterfenster für eine einfache und kontrollierte Synthese eröffnet wird. Dieser Ansatz zur chemischen Gasphasenabscheidung von Graphen auf Si- Substraten ist leicht skalierbar und gegenüber der Abscheidung auf Metallsubstraten konkurrenzfähig, da keine Substratübertragung notig ist. Darüber hinaus ist dieser Prozess auch für die direkte Synthese anderer zweidimensionalen Materialien und deren Van-der-Waals-Heterostrukturen anwendbar.

Student Number : 0107553H - MSc research report - School of Science Education - Faculty of Science / One of the most important challenges of the secondary school science teacher is to help learners to obtain, analyze and evaluate evidence using experimental and investigative methods. The understanding and application of concepts of evidence play a central role in setting up valid experiment and is usually ignored in the secondary school science curriculum. Recognizing the significance and value of an understanding of evidence in developing learners’ ability to carry out appropriate experimental work in science, the question arises of learners’ understanding of ‘concepts of evidence’ in experimental design. In this study the researcher endeavored to answer some of the questions concerning learners’ understanding of evidence: What understanding do learners entering the Further Education and Training (FET) band have of the concepts of validity in experimental design? What is the effect of an intervention on their understanding of concepts of validity? How well do learners show their ability to apply concepts of evidence to an assessment task? The study investigated the ability of a sample of grade 10 Physical Science learners, in one Gauteng Province secondary school to use and understand the concepts of evidence before and after an intervention, and their ability apply these ideas about evidence in an assessment sheet task. First, a survey of learners’ understanding of the concepts of validity was conducted using a written diagnostic test, consisting of three probes that had been previously used to diagnose learners’ understanding of validity in experimental design. Second, an evaluation of changes in such understanding due to a specific teaching intervention was completed using the same diagnostic test. Finally, the 2004 Grade 9 Common Tasks for Assessment (CTA) investigation task was administered to the same subjects in order to investigate whether or not their understanding of concepts of evidence would be apparent in this CTA investigation task. Several key findings emerged from the data. The major findings of this study were that the sample investigated began the study with a level of understanding equal to a group of Foundation University students tested in a previous study and that their understanding did not change much during the intervention. In fact, analysis of the results from the probes in the diagnostic test revealed that learners had little problem choosing the independent variable and controlling variables. However, the findings from the probes indicated that the intervention did not seem to have had a major effect since the difference in the number of responses between the pre-test and post- test was relatively small. Moreover, the results also revealed that it was possible to analyze learners’ responses using the existing model of learners’ understanding of validity, and that the majority of learners fell into Level C in the model which refers to the application of previous understanding in a new situation. The results from the CTA investigation task revealed that learners’ understanding of concepts of evidence could be demonstrated in new science context. In fact, the results revealed that learners showed ability to apply their understanding of concepts of evidence in experimental design to the assessment task. Moreover, the CTA results indicated that the understanding of identifying variables, constructing graphs; describing the relationship between variables in a graph; formulating the investigative questions were the most applied concepts of evidence by learners. Finally, the results of this study highlight the need for providing secondary science learners with opportunities to practice and develop concepts of evidence. In the light of these findings it is recommended that children need to be explicitly taught about the purpose of science investigations and introduced to important ideas about evidence if their ability to perform investigation tasks is to be improved.

Thesis (Ph.D. (Education))--North-West University, Potchefstroom Campus, 2008.

Amorphous Oxide Semiconductors (AOS) are widely being explored in the field of flexible and transparent electronics. In this thesis, solution processed zinc-tin oxide (ZTO) n-channel TFT based circuits are studied. Inverters, single stage amplifiers and ring oscillators are designed, fabricated and tested. 7-stage ring oscillators with output frequencies up to 106kHz and 5-stage ring oscillators with frequencies up to 75kHz are reported. A stable three stage op-amp with a buffered output is designed for a gain of 39.9dB with a unity gain frequency of 27.7kHz. A 7-stage ring oscillator with output frequency close to 1MHz is simulated and designed. The op-amp and the ring oscillator are ready to be fabricated and tested. An RPI model for a-Si, adapted to fit the ZTO device characteristics, is used for simulation. Development of a new model based on the physics behind charge transport in ZTO devices is explored. An expression for gate bias dependent mobility in ZTO devices is derived. / text

Graphene is widely considered to be a promising candidate for a new generation of electronics, but there are many outstanding fundamental issues that need to be addressed before this promise can be realized. This thesis focuses on the production and properties of graphene grown epitaxially on the carbon terminated face (C-face) of hexagonal silicon carbide leading to the construction of a novel graphene transistor structure. C-face epitaxial graphene multilayers are unique due to their rotational stacking that causes the individual layers to be electronically decoupled from each other. Well-formed C-face epitaxial graphene single layers have exceptionally high mobilities (exceeding 10,000 cm^2/Vs), which are significantly greater than those of Si-face graphene monolayers. This thesis investigates the growth and properties of C-face single layer graphene. A field effect transistor based on single layer graphene was fabricated and characterized for the first time. Aluminum oxide or boron nitride was used for the gate dielectric. Additionally, an all graphene/SiC Schottky barrier transistor on the C-face of SiC composed of 2DEG in SiC/Si2O3 interface and multilayer graphene contacts was demonstrated. A multiple growth scheme was adopted to achieve this unique structure.

When Outcomes-Based Education (OBE) reached the Further Education and Training (FET) band in 2006, teachers were confronted for the first time with a new curriculum that challenged them to implement a learner-centred approach to teaching, learning and assessment. Since assessment is seen as the heart of effective teaching and learning, this research was aimed at establishing how effective teachers of English First Additional Language (EFAL) in the FET band were in implementing Outcomes-Based Assessment (OBA). A thorough literary survey on OBE and OBA was conducted. This survey included a study of the principles and philosophical underpinnings of OBE and the clarification of the key concepts of OBE and OBA. Various documents on the teaching, learning and assessment of English Second Language from the Departments of Education in New Zealand, Canada and Australia were studied to gain an international perspective. This was followed by an analysis of South African policy documents on the teaching, learning and assessment of EFAL. Empirical research was conducted by means of a survey in which both qualitative and quantitative methods of data collection were used. The data analysis revealed that teachers of EFAL in the FET band experience problems with the practical implementation of the curriculum and the assessment thereof. Problems were mainly experienced with setting achievable outcomes, designing lessons, teaching material and learning activities, teaching and assessing in a learner-centred way, employing the most appropriate types and methods of assessment and using feedback to enhance learning. A model for assessment has been designed to fill the gap left between the theory of OBE and OBA as expressed in the NCS for EFAL and the successful, practical implementation thereof. It is anticipated that this model will contribute to the improvement of teaching, learning and assessment of EFAL in the FET band in public schools. / Thesis (Ph.D. (Education))--North-West University, Potchefstroom Campus, 2008.

Linearity and efficiency are both critical parameters for radio frequency transmitter applications. In theory, a Doherty amplifier is a linear amplifier that is significantly more efficient than comparable conventional linear amplifiers. It comprises two amplifiers, connected at their outputs by a quarter-wave transformer. The main amplifier is always on, while the peaking amplifier is off during low power levels. Load modulation of the main amplifier occurs when the peaking amplifier is on due to the quarter-wave transformer, ensuring the main amplifier never enters saturation. This results in an efficiency characteristic that increases with respect to input power at twice the normal rate at low power levels, and plateaus to a high value at high power levels. However, in much of the research that has been done to-date, less-than-ideal results have been achieved (although efficiency was better than a conventional amplifier). It was decided to investigate the cause of the discrepancy between theoretical and practical results, and devise a method to counteract the problem. It was discovered that the main cause of the discrepancy was non-ideal transistor gate-voltage to drain-current characteristics. The implementation of a gate bias control scheme based upon measured transistor transfer characteristics, and the desired main and peaking amplifier output currents, resulted in a robust method to ensure near-ideal results. A prototype amplifier was constructed to test the control scheme, and theoretical, simulated and measured results were well matched. The amplifier had a region of high efficiency in the high power levels (over 34% for the last 6 dB of input power), and the gain was nearly constant with respect to input power (between 4 and 5 dB over the dynamic range). Furthermore, it was decided to investigate the role harmonics play within the Doherty amplifier. A classical implementation shunts unwanted harmonics to ground within the main and peaking amplifiers. However, odd harmonics generated by the peaking amplifier can be used to operate the main amplifier like a class F amplifier. This means its supply voltage can be lowered, without the amplifier entering saturation, and the efficiency of the Doherty amplifier can be increased without a detrimental effect on the its linearity. A prototype amplifier was constructed to test this theory, and gave good results, with better efficiency than that of a conventional amplifier, and a constant gain with respect to input power (between 6.4 dB and 6.5 dB over the dynamic range).

Since its inception, the Outcomes-Based Education (OBE) curriculum in South Africa has caused much confusion and frustration among teachers, especially with regard to the implementation of Outcomes- Based Assessment (OBA). From the onset of the implementation of OBE in South Africa, teachers were confronted with unfamiliar challenges with regard to teaching, learning and assessment. Most teachers experienced these challenges as very stressful, because they were inadequately prepared for the didactical paradigm shift. This resulted in a general degree of negativity and resistance towards OBE amongst teachers. In particular, there seemed to be much confusion and frustration about the workload and administrative burden that OBA imposed on teachers. In the light of the afore-mentioned, the researcher wished to determine how the teachers from the North- West Province experience the OBA of Physical Sciences in the Further Education and Training (FET) Band. In order to achieve the aim and objectives of the research, a literature study as well as an empirical investigation was undertaken. The literature study focused on OBA, OBE, and the National Policy Document on the teaching, learning and assessment of Physical Sciences in the FET Band. For the purposes of the empirical investigation, quantitative and qualitative data were collected by means of a questionnaire that was distributed among a sample of teachers who taught Physical Sciences in the FET Band schools in the North-West Province. The data were quantitatively and qualitatively analyzed and on the basis of the findings the following conclusions were drawn: • Although most participants indicated that they received training in OBA and were in possession of the relevant National Curriculum Statement (NCS) documents, some of their responses indicated a lack of practical implementation skills. It also transpired that some of the participants experienced a lack of resources and inadequate support from subject advisors in implementing OBA. • Participants experienced the following obstacles with regard to the assessment of Physical Sciences in the FET Band: - inadequate training to conduct practical work; - a complicated and confusing NCS document that does not contain clear guidelines; - an overloaded curriculum; and - limited resources, time constraints, overcrowded classrooms and an overloaded curriculum make practical work and experimentation very difficult. On the basis of the findings emanating from the research, a model for the implementation of OBA of Physical Sciences in the FET Band was proposed to close the gap between the theory and assessment practice. / Thesis (PhD (Natural Science Education))--North-West University, Potchefstroom Campus, 2013.

The quality of education in especially the Further Education and Training phase (FET) is recognised as one of the most significant challenges facing the South African government. Low teacher morale, lack of parental involvement, learners' ignorance in taking responsibility for their own learning and other educational problems are all identified as contributing factors. This study was undertaken to investigate how critical thinking skills can contribute to promoting quality teaching and learning in secondary schools with specific reference to the FET phase. The study also sought to make recommendations that will assist teachers to integrate critical thinking skills into the teaching and learning experiences in the FET phase. The nature of quality teaching and learning, the nature of critical thinking skills and the correlation between the concepts to improve the quality of education were researched by means of a literature study. A quantitative research design followed, using questionnaires as research instrument. From the research conducted it was found that learners in the FET phase do not take responsibility for their learning and therefore many of these learners still do not perceive learning as important. However, effective learning could take place if learners are confident, independent, active participants and explore a variety of learning strategies. Teachers, on the other hand, should adopt particular forms of behaviour to foster certain types of learning. The ability to teach critical thinking skills to learners poses a great challenge as many teachers are not trained to do so. This has a great influence on the exposure of an integrated constructivist method of teaching and learning and therefore inflicts on the delivery of quality teaching and learning in the FET phase of secondary schools in South Africa. / MEd, North-West University, Vaal Triangle Campus, 2011