Dispositifs optoélectroniques à base de semi-conducteurs organiques en couches minces

Les petites molécules de type p à bandes interdites étroites sont de plus en plus perçues comme des remplaçantes possibles aux polymères semi-conducteurs actuellement utilisés conjointement avec des dérivés de fullerènes de type n, dans les cellules photovoltaïques organiques (OPV). Par contre, ces petites molécules tendent à cristalliser facilement lors de leur application en couches minces et forment difficilement des films homogènes appropriés. Des dispositifs OPV de type hétérojonction de masse ont été réalisés en ajoutant différentes espèces de polymères semi-conducteurs ou isolants, agissant comme matrices permettant de rectifier les inhomogénéités des films actifs et d’augmenter les performances des cellules photovoltaïques. Des polymères aux masses molaires spécifiques ont été synthétisés par réaction de Wittig en contrôlant précisément les ratios molaires des monomères et de la base utilisée. L’effet de la variation des masses molaires en fonction des morphologies de films minces obtenus et des performances des diodes organiques électroluminescentes reliées, a également été étudié. La microscopie électronique en transmission (MET) ou à balayage (MEB) a été employée en complément de la microscopie à force atomique (AFM) pour suivre l’évolution de la morphologie des films organiques minces. Une nouvelle méthode rapide de préparation des films pour l’imagerie MET sur substrats de silicium est également présentée et comparée à d’autres méthodes d’extraction. Motivé par le prix élevé et la rareté des métaux utilisés dans les substrats d’oxyde d’indium dopé à l’étain (ITO), le développement d’une nouvelle méthode de recyclage eco-responsable des substrats utilisés dans ces études est également présenté.

Negócios internacionais: Estratégias de investimentos no contexto luso-brasileiro

Este ensaio descreve e analisa novos desafios e oportunidades no âmbito dos negócios internacionais no contexto Luso-Brasileiro. Inicialmente, o ensaio desafia a hegemonia do conhecimento acadêmico em negócios internacionais alcançado por conexões Anglo-Americanas para sustentar que a valorização das conexões Luso-Brasileiras em um mundo globalizado pode promover conexões mais sólidas com outros países e regiões. A seguir, o ensaio descreve e analisa dados referentes aos fluxos de entrada e saída de investimentos direto externo no contexto Luso-Brasileiro com propósito de destacar, dentre os desafios e oportunidades contemporâneos, a crescente internacionalização das empresas portuguesas e brasileiras nas últimas duas décadas. Como considerações finais, o ensaio sustenta que a intensificação das conexões Luso-Brasileiras em termos de fluxos de investimentos requer estudos interdisciplinares entre os âmbitos de economia política internacional, negócios internacionais e gestão internacional.

THE IMPACT OF UPPER-LEVEL PROCESSES ON THE INTENSITY AND STRUCTURAL CHANGES OF HURRICANE SANDY (2012)

The first part of this study examines the relative roles of frontogenesis and tropopause undulation in determining the intensity and structural changes of Hurricane Sandy (2012) using a high-resolution cloud-resolving model. A 138-h simulation reproduces Sandy’s four distinct development stages: (i) rapid intensification, (ii) weakening, (iii) steady maximum surface wind but with large continued sea-level pressure (SLP) falls, and (iv) re-intensification. Results show typical correlations between intensity changes, sea-surface temperature and vertical wind shear during the first two stages. The large SLP falls during the last two stages are mostly caused by Sandy’s moving northward into lower-tropopause regions associated with an eastward-propagating midlatitude trough, where the associated lower-stratospheric warm air wraps into the storm and its surrounding areas. The steady maximum surface wind occurs because of the widespread SLP falls with weak pressure gradients lacking significant inward advection of absolute angular momentum (AAM). Meanwhile, there is a continuous frontogenesis in the outer region during the last three stages. Cyclonic inward advection of AAM along each frontal rainband accounts for the continued expansion of the tropical-storm-force wind and structural changes, while deep convection in the eyewall and merging of the final two survived frontal rainbands generate a spiraling jet in Sandy’s northwestern quadrant, leading to its re-intensification prior to landfall. The physical, kinematic and dynamic aspects of an upper-level outflow layer and its possible impact on the re-intensification of Sandy are examined in the second part of this study. Above the outflow layer isentropes are tilted downward with radius as a result of the development of deep convection and an approaching upper-level trough, causing weak subsidence. Its maximum outward radial velocity is located above the cloud top, so the outflow channel experiences cloud-induced long-wave cooling. Because Sandy has two distinct convective regions (an eyewall and a frontal rainband), it has multiple outflow layers, with the eyewall’s outflow layer located above that of the frontal rainband. During the re-intensification stage, the eyewall’s outflow layer interacts with a jet stream ahead of the upper-level trough axis. Because of the presence of inertial instability on the anticyclonic side of the jet stream and symmetric instability in the inner region of the outflow layer, Sandy’s secondary circulation intensifies. Its re-intensification ceases when these instabilities disappear. The relationship between the intensity of the secondary circulation and dynamic instabilities of the outflow layer suggests that the re-intensification occurs in response to these instabilities. Additionally, it is verified that the long-wave cooling in the outflow layer helps induce symmetric instability by reducing static stability.

UV and visible photoconductivity of undoped diamond films: morphology and related electrical transport phenomena

UV and visible photoconductivity and electrical features of undoped diamond thin films grown by microwave plasma-assisted chemical vapour deposition (MP-CVD) on silicon and copper substrates are studied. The results are correlated with morphology properties analysed by atomic force microscopy (AFM) and micro-Raman. The photoconductivity presents several bands from 1.8 to 3.8 eV that are dependent on the substrate used to grow the samples in spite of some common bands observed. The J-V curve tin DC) in samples grown on Si has a rectifier behaviour (Schottky emission) in opposition to the samples grown on Cu that have no rectification (SCLC conduction). With these results we can conclude that diamond based optoelectronic devices behaviour is controlled by two kinds of structural defects localized in microcrystal and in its boundaries. A general structure model for the optoelectronic behaviour is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

A Thermal Management Solution for Compact Power Converters

Regulated Transformer Rectifier Units contain several power electronic boards to facilitate AC to DC power conversion. As these units become smaller, the number of devices on each board increases while their distance from each other decreases, making active cooling essential to maintaining reliable operation. Although it is widely accepted that liquid is a far superior heat transfer medium to air, the latter is still capable of yielding low device operating temperatures with proper heat sink and airflow design. The purpose of this study is to describe the models and methods used to design and build the thermal management system for one of the power electronic boards in a compact, high power regulated transformer rectifier unit. Maximum device temperature, available pressure drop and manufacturability were assessed when selecting the final design for testing. Once constructed, the thermal management system’s performance was experimentally verified at three different power levels.

A não cumulatividade do IPI na exportação de mercadorias nacionais sob a perspectiva da análise jurídica da política econômica : neutralidade da incidência tributária e efetiviade do creditamento

Dissertação (mestrado)—Universidade de Brasília, Faculdade de Direito, Programa de Pós-Graduação Stricto Sensu em Direito, 2016.

UV and visible photoconductivity of undoped diamond films: morphology and related electrical transport phenomena

UV and visible photoconductivity and electrical features of undoped diamond thin films grown by microwave plasma-assisted chemical vapour deposition (MP-CVD) on silicon and copper substrates are studied. The results are correlated with morphology properties analysed by atomic force microscopy (AFM) and micro-Raman. The photoconductivity presents several bands from 1.8 to 3.8 eV that are dependent on the substrate used to grow the samples in spite of some common bands observed. The J-V curve tin DC) in samples grown on Si has a rectifier behaviour (Schottky emission) in opposition to the samples grown on Cu that have no rectification (SCLC conduction). With these results we can conclude that diamond based optoelectronic devices behaviour is controlled by two kinds of structural defects localized in microcrystal and in its boundaries. A general structure model for the optoelectronic behaviour is discussed. (C) 2000 Elsevier Science S.A. All rights reserved.

Electrical AC behaviour of MPCVD diamond Schottky diodes

The present work reports some experimental results on the electrical AC behaviour of metal-undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 mum thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs, frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole-Cole plot. Taking into account the existence of traps in the depletion region, a best fit to the experimental results was obtained. The difference between the Fermi level and the band edge of 0.2-0.3 eV is in agreement with the activation energy found from the loss tangent analysis. The capacitance with applied voltage (Mott-Schottky plots) gives a defect density of 10(16) cm(-3) with contact potentials near 0.5 V and the profile of defect density obtained shows a major density (approx. 10(17) cm(-3)) in a layer with a thickness less than 50 nm from the junction, decreasing by one order of magnitude with increasing distance. Finally a structural model is proposed to explain the AC behaviour found. (C) 2001 Elsevier Science B.V. All rights reserved.

High Frequency Class DE Converter Using a Multilayer Coreless PCB Transformer

In modern power electronics equipment, it is desirable to design a low profile, high power density, and fast dynamic response converter. Increases in switching frequency reduce the size of the passive components such as transformers, inductors, and capacitors which results in compact size and less requirement for the energy storage. In addition, the fast dynamic response can be achieved by operating at high frequency. However, achieving high frequency operation while keeping the efficiency high, requires new advanced devices, higher performance magnetic components, and new circuit topology. These are required to absorb and utilize the parasitic components and also to mitigate the frequency dependent losses including switching loss, gating loss, and magnetic loss. Required performance improvements can be achieved through the use of Radio Frequency (RF) design techniques. To reduce switching losses, resonant converter topologies like resonant RF amplifiers (inverters) combined with a rectifier are the effective solution to maintain high efficiency at high switching frequencies through using the techniques such as device parasitic absorption, Zero Voltage Switching (ZVS), Zero Current Switching (ZCS), and a resonant gating. Gallium Nitride (GaN) device technologies are being broadly used in RF amplifiers due to their lower on- resistance and device capacitances compared with silicon (Si) devices. Therefore, this kind of semiconductor is well suited for high frequency power converters. The major problems involved with high frequency magnetics are skin and proximity effects, increased core and copper losses, unbalanced magnetic flux distribution generating localized hot spots, and reduced coupling coefficient. In order to eliminate the magnetic core losses which play a crucial role at higher operating frequencies, a coreless PCB transformer can be used. Compared to the conventional wire-wound transformer, a planar PCB transformer in which the windings are laid on the Printed Board Circuit (PCB) has a low profile structure, excellent thermal characteristics, and ease of manufacturing. Therefore, the work in this thesis demonstrates the design and analysis of an isolated low profile class DE resonant converter operating at 10 MHz switching frequency with a nominal output of 150 W. The power stage consists of a class DE inverter using GaN devices along with a sinusoidal gate drive circuit on the primary side and a class DE rectifier on the secondary side. For obtaining the stringent height converter, isolation is provided by a 10-layered coreless PCB transformer of 1:20 turn’s ratio. It is designed and optimized using 3D Finite Element Method (FEM) tools and radio frequency (RF) circuit design software. Simulation and experimental results are presented for a 10-layered coreless PCB transformer operating in 10 MHz.