Defect engineering of phase transition materials to create optical metasurfaces

Active, widely tunable optical materials have enabled rapid advances in photonics and optoelectronics, especially in the emerging field of meta-devices. Of the tunable optical materials, one of the most prolifically studied is vanadium dioxide (VO2), which undergoes a reversible insulator-metal-transition (IMT) as the temperature reaches a critical temperature of approximately 67°C due to strong electron correlations. Effective optical properties of ultra-thin VO2 films arise as a consequence of the natural occurring phase coexistence within the temperature range of the IMT. In the first part of this thesis the application of ultra-thin VO2 films for optical metasurfaces in the mid-infrared spectral region is theoretically and experimentally investigated. The second part of this thesis is concerned with local ion beam modification of ultra-thin VO2 films. Energetic ion beams are widely used to modify the electronic and structural properties of solids by introducing impurity atoms into the crystal lattice. Commonly, the inevitable formation of irradiation damage during ion bombardment is described as disadvantageously for ion beam doping and subsequent post-implantation annealing procedures are required. Since the electronic structure of strongly electron correlated materials, such as VO2, is very sensitive to small amounts of lattice defects, ion beam induced damage formation combined with lithographic patterning is introduced and comprehensively studied in this thesis as a method to locally adjust the phase transitions of these materials. Using this robust technique, optical metasurfaces, including tunable absorbers with artificially induced phase coexistence and tunable polarizers based on thermally triggered dichroism in ultra-thin VO2 films are demonstrated.Aktiv veränderliche optische Materialien ermöglichen rasante Fortschritte in der Photonik und Optoelektronik, insbesondere im aufstrebenden Gebiet der Metaoberflächen. Eines der am besten untersuchten veränderlichen optischen Materialien ist Vanadiumdioxid (VO2). Aufgrund starker Elektronenkorrelation durchläuft dieses Material einen reversiblen Isolator-Metall-Übergang (IMT), wenn die Temperatur eine kritische Temperatur von ungefähr 67 °C überschreitet. Effektive optische Eigenschaften werden durch die natürlich vorkommende Phasenkoexistenz in VO2-Schichten innerhalb des Temperaturbereichs des Phasenübergangs ermöglicht. Im ersten Teil dieser Arbeit wird die Anwendung extrem dünner VO2-Schichten für optische Metaoberflächen im mittleren Infrarot theoretisch und experimentell untersucht. Der zweite Teil dieser Arbeit befasst sich mit der lokalen Modifikation extrem dünner VO2-Schichten. Energetische Ionenstrahlen werden häufig verwendet, um die elektronischen und strukturellen Eigenschaften von Festkörpern zu modifizieren, indem Dotieratome in das Kristallgitter eingebracht werden. Die während des Ionenbeschusses auftretende Bildung von Bestrahlungsschäden ist sehr nachteilig, was üblicherweise eine thermische Nachbehandlung zur Reduktion der Gitterschädigung erforderlich macht. Die elektronische Struktur stark elektronenkorrelierter Materialien, wie z.B. VO2, reagiert sehr empfindlich auf kleinste Gitterdefektkonzentrationen. Daher wird in dieser Arbeit die Kombination ionenstrahlinduzierter Gitterschädigung und lithographische Strukturierung als Methode zur lokalen Anpassung der Phasenübergänge elektronenkorrelierter Materialien eingeführt. Mit Hilfe dieser Technik werden optische Metaoberflächen, einschließlich einstellbarer Absorber mit künstlich induzierter Phasenkoexistenz und veränderliche Polarisatoren in extrem dünnen VO2-Schichten, demonstriert

Alternating current source to voltage source converter

Abstract: A direct alternating current source to voltage source converter, which does not employ energy storage components, is presented. Conversion is done by a current steering mechanism in the form of a current transformer with two secondary windings. The direct conversion concept, basic circuit, simulation and experimental results of a reduced scale model are presented

Nonlinearities in stock return prediction: A Blended Approach

Our prior research indicates that there are periods within which nonlinear stock selection models outperform their linear counterparts in the South African equity market. In order to explore the nonlinearities in stock return prediction, we propose a blended stock selection technique that has the potential of diversifying the risk of inaccurate forecasts of the linear and nonlinear models. The proposed technique has an objective of optimizing the Qian and Hua (2003) information ratio, which constitutes to the maximization of the forecasting accuracy per unit of forecasting volatility. The blended stock selection model is found to outperform the respective linear and nonlinear models in an out-of-sample fractile analysis on a risk-adjusted basis for South African stocks over the period from 2002 to 2007.Department of HE and Training approved lis

Determination of the full deformation tensor by multi-Bragg fast scanning nano X-ray diffraction

International audienceThis work showcases a method to map the full deformation tensor in a single micro-sized crystal. It is shown that measuring the position of two Bragg reflections in reciprocal space is sufficient to obtain the full deformation tensor, if the condition of incompressibility of the material is imposed. This method is used to reveal the surface tension induced deformation at the edges of an as-grown single-crystal VO 2 microwire. All components of the deformation tensor of the microwire were measured down to an absolute value of 10 À4 in an 8 Â 14 mm projected area of the wire. With a beam-defined spatial resolution of 150 Â 150 nm, the measurement time was merely 2.5 h

Comorbidities between tuberculosis and common mental disorders: A scoping review of epidemiological patterns and person-centred care interventions from low-to-middle income and BRICS countries

Background : There is increasing evidence that the substantial global burden of disease for tuberculosis unfolds in concert with dimensions of common mental disorders. Person-centred care holds much promise to ameliorate these comorbidities in low-tomiddle income countries and emerging economies. Towards this end, this paper aimed to review 1) the nature and extent of tuberculosis and common mental disorder comorbidity and 2) person-centred tuberculosis care in low-to-middle income countries and emerging economies. Main text : A scoping review was conducted of English-language studies published from 2000-2019 in peer-reviewed and grey literature, using established guidelines, for each of the study objectives. Four broad tuberculosis/mental disorder comorbidities were described in the literature, namely alcohol use and tuberculosis, depression and tuberculosis, anxiety and tuberculosis, and general mental health and tuberculosis. Rates of comorbidity varied widely across countries for depression, anxiety, alcohol use and general mental health. Alcohol use and tuberculosis were significantly related, especially in the context of poverty. The initial tuberculosis diagnostic episode had substantial socio-psychological effects on service users. While men tended to report higher rates of alcohol use and treatment default, women in general had worse mental health outcomes. Older age and a history of mental illness were also associated with pronounced tuberculosis and mental disorder comorbidity. Person-centred tuberculosis care interventions were almost absent, with only one study from Nepal identified. Conclusions : There is an emerging body of evidence describing the nature and extent of tuberculosis and mental disorders comorbidity in low-to-middle income countries. Despite the potential of person-centred interventions, evidence is limited. This review highlights a pronounced need to address psychosocial comorbidities with TB in LMICs, where models of person-centred TB care in routine care platforms may yield promising outcomes

Impact of University engagement on environmental resilience of urban spaces in Asia and Africa. SUEUAA Thematic Paper Series, TPS 103/19

By 2030 most of the globes inhabitants will be living with cities. It is therefore important that cities are resilient to sustenance and energy challenge in the face of increasing demands from population expansion and increasing environmental variability and uncertainty brought about by intensifying climate change. The Strengthening Urban Engagement of Universities in Asia and Africa (SUEUAA) project aims to enhance university engagement capacity in this context and optimise engagement strategy to maximise impact in urban areas. We use the term ‘environmental resilience’ to encapsulate both energy and sustenance challenges. We believe these must be tackled in a holistic manner as there are intimate links between the production and usage of energy, food and water and consequent generation and disposal of waste. This paper focuses on the experiences and vision of SUEUAA partners in Johannesburg (South Africa), Dar es Salaam (Tanzania), Duhok (Iraq), Sanandaj (Iran), and Manila (Philippines), so it encapsulates views and solutions for engagement strategies on several environmental challenges facing cities in very different climatic situations

Impact of University engagement on environmental resilience of urban spaces in Asia and Africa. SUEUAA Thematic Paper Series, TPS 103/19

By 2030 most of the globes inhabitants will be living with cities. It is therefore important that cities are resilient to sustenance and energy challenge in the face of increasing demands from population expansion and increasing environmental variability and uncertainty brought about by intensifying climate change. The Strengthening Urban Engagement of Universities in Asia and Africa (SUEUAA) project aims to enhance university engagement capacity in this context and optimise engagement strategy to maximise impact in urban areas. We use the term ‘environmental resilience’ to encapsulate both energy and sustenance challenges. We believe these must be tackled in a holistic manner as there are intimate links between the production and usage of energy, food and water and consequent generation and disposal of waste. This paper focuses on the experiences and vision of SUEUAA partners in Johannesburg (South Africa), Dar es Salaam (Tanzania), Duhok (Iraq), Sanandaj (Iran), and Manila (Philippines), so it encapsulates views and solutions for engagement strategies on several environmental challenges facing cities in very different climatic situations

After Mazibuko: exploring the responses of communities excluded from South Africa’s water experiment

Despite a constitutional right to water, challenges remain for access to sufficient water in South Africa. This article considers the degree to which current legal provisions perpetuate approaches, which are antithetical to genuinely eco-socio-sustainable water access. Water in South Africa has largely been re-cast as a commodity, exposed to market rules, proving problematic for many and giving rise to various responses, including litigation. In the seminal case of Mazibuko the Constitutional Court failed to provide robust protection to the right to water, providing impetus for the formation of “commons” strategies for water allocation. Indeed “commoning” is beginning to represent not only an emerging conceptual strand in urban resource allocation, but also a dynamic, contemporary, eco-sensitive, socio-cultural phenomenon, driving innovative, interactive and inclusive forms of planning and social engagement. Against the backdrop of unequal water access, commoning offers glimpses of an empowering and enfranchising subaltern paradigm

Impact of University engagement on environmental resilience of urban spaces in Asia and Africa

By 2030 most of the globes inhabitants will be living with cities. It is therefore important that cities are resilient to sustenance and energy challenge in the face of increasing demands from population expansion and increasing environmental variability and uncertainty brought about by intensifying climate change. The Strengthening Urban Engagement of Universities in Asia and Africa (SUEUAA) project aims to enhance university engagement capacity in this context and optimise engagement strategy to maximise impact in urban areas. We use the term 'environmental resilience' to encapsulate both energy and sustenance challenges. We believe these must be tackled in a holistic manner as there are intimate links between the production and usage of energy, food and water and consequent generation and disposal of waste. This paper focuses on the experiences and vision of SUEUAA partners in Johannesburg (South Africa), Dar es Salaam (Tanzania), Duhok (Iraq), Sanandaj (Iran), and Manila (Philippines), so it encapsulates views and solutions for engagement strategies on several environmental challenges facing cities in very different climatic situations

High-Throughput Discovery of Hf Promotion on the Formation of Hcp Co and Fischer-Tropsch Activity

A proxy-based high-throughput experimental approach was used to explore the stability and activity of Co-based Fischer Tropsch Synthesis catalysts with different promoters on various supports. The protocol is based on the estimation by XRD of active phase, Co, particle size and relative amounts of crystalline phases, Co to support. Sequential libraries samples enabled exploration of four Co loadings with five different promoters on six support materials. Catalysts stable to aging in syngas, displaying minimal change of particle size or relative area, were evaluated for their activity under industrial conditions. This procedure identified SiC as support for stable catalysts and a combination of Ru and Hf to promote the formation hcp Co. Unsupported bulk samples of Co with appropriate amounts of Ru and Hf revealed that the formation of hcp Co is independent of the support. The hcp Co containing catalyst presented the highest catalytic activity and C5+ selectivity amongst the samples tested in this study confirming the effectiveness of the proxy-based high-throughput method.</jats:p