MPH / Department of Public Health

Offshore wind is rapidly growing and optimised grid connections are crucial for its success. Generally, costs and losses are higher for HVDC at short distances due to the converters, while HVAC costs and losses increase more rapidly with distance due to the ac cables. Hence, there is a breaking point over which HVDC becomes beneficial, which is important knowledge for grid connection design. Recent research and practice indicate increasing distances for the breaking point, enabled by the introduction of offshore reactive compensation substations (RCS) for HVAC. In the study, steady-state models of HVAC and modular-multilevel converter (MMC) based VSC HVDC systems up to 260 km have been simulated in the Matlab/Simulink based program EeFarm-II. For base case assumptions, the average loss breaking point is 80 km and the levelised cost breaking point is 229 km. The resulting breaking point with respect to levelised cost of energy (LCOE) is 205 km and with respect to net present value (NPV) 186 km, agreeing with the trend of increasing breaking points. Given the range of distances in literature, it is of interest to also investigate how the breaking point depends on assumptions on technical, practical and economic parameters. For the NPV breaking point: lifetime and interest rate have no impact, availability and cost of RCSs have low impact, electricity price has moderate impact, operation and maintenance (O&M) cost has high impact while investment cost and lead time have very high impact. This could be taken into consideration in offshore projects and in future research.

Particle-reinforced MMCs (pMMC) such as aluminum alloys reinforced with ceramic silicon carbide particles (AlSiC) require special cutting tools due to the high hardness and abrasive properties of the ceramic particles. Diamond coated cutting tools are ideal for machining this type of pMMC. Previous research studies focus on the machinability of pMMCs with low ceramic content. The aim of this research is to determine the optimal cutting parameters for machining AlSiC material containing high silicon carbide particle reinforcement (>25%). The optimal cutting parameters are determined by investigating the relationship between cutting forces, tool wear, burr formation, surface roughness, and material removal rate (MRR). Experimental milling tests are conducted using CVD diamond coated end mills and non-diamond tungsten carbide end mills. It was found that low tool rotation speeds, feed rates and depths of cut are necessary to achieve smoother surface finishes of R a < 1 μm. A high MRR to low tool wear and surface roughness ratio was obtainable at a tool rotation speed of 6500 r/min, feed rate of 762 mm/min and depth of cut of 3 mm. Results showed that a smooth surface roughness of the workpiece material was achieved with non-diamond tungsten carbide end mills, however, this was at the expense of extreme tool wear and high burr formation. The use of coolant caused a 50% increase in tool wear compared to the dry-cutting experiments which had lower cutting tool forces.

The ideal properties of a structural material are light weight with extensive strength and ductility. A composite with high strength and tailorable ductility was developed consisting of nanocrystalline AA5083, boron carbide and coarser grained AA5083. The microstructure was determined through optical microscopy and transmission electron microscopy. A technique was developed to determine the nitrogen concentration of an AA5083 composite from secondary ion mass spectrometry utilizing a nitrogen ionimplanted standard. Aluminum nitride and amorphous nitrogen-rich dispersoids were found in the nanocrystalline aluminum grain boundaries. Nitrogen concentration increased as a function of cryomilling time up to 72hours. A greater nitrogen concentration resulted in an enhanced thermal stability of the nanocrystalline aluminum phase and a resultant increase in hardness. The distribution of the nitrogen-rich dispersoids may be estimated considering their size and the concentration of nitrogen in the composite. Contributions to strength and ductility from the Orowan relation can be more accurately modeled with the quantified nitrogen concentration.

The development of metals that can overcome the strength-ductility-weight trade-off has been an ongoing challenge in engineering for many decades. A promising option for making such materials are Metal matrix composites (MMCs). MMCs contain dispersions of reinforcement in the form of fibers, particles, or platelets that significantly improve their thermal, electrical, or mechanical performance. This dissertation focuses on reinforcement with two-dimensional (2D) materials due to their unprecedented mechanical properties. For instance, compared to steel, the most well-studied 2D material, graphene, is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa). Examples of reinforcement by graphene have achieved increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. However, the superior mechanical properties of graphene are not fully transferred to the matrix in conventional MMCs, a phenomenon known as the "valley of death." In an effort to develop key insight into how the relationships between composite design, processing, structure, properties, and mechanics can be used to more effectively transfer the intrinsic mechanical properties of reinforcements to bulk composite materials, nanolayered composite systems made of Ni, Cu, and NiTi reinforced with graphene or 2D hexagonal boron nitride h-BN is studied using experimental techniques and molecular dynamics (MD) simulations. / Doctor of Philosophy / The design of new metals with concurrently improved strength and ductility has been an enduring goal in engineering for many decades. The utilization of components made with these new materials would reduce the weight of structures without sacrificing their performance. Such materials have the potential to revolutionize many industries, from electronics to aerospace. Traditional methods of improving the properties of metals by thermomechanical processing have approached a point where only minor performance improvements can be achieved. The development of Metal matrix composites (MMCs) is among the best approaches to achieving the strength-ductility goal. Metal matrix composites are a class of materials containing reinforcements of dissimilar materials that significantly improve their thermal conductivity, electrical conductivity, or mechanical performance. Reinforcements are typically in the form of dispersed fibers, particles, or platelets. The ideal reinforcement materials have superior mechanical properties compared to the metal matrix, a high surface area, and a strong interfacial bond with the matrix. Two-dimensional (2D) materials (materials made up of a single to a few layers of ordered atoms) are attractive for reinforcement in composite materials because they possess unprecedented intrinsic properties. The most well-studied 2D material, graphene, is made of a single layer of carbon atoms arranged in a hexagonal honeycomb pattern. It is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa) than steel. Examples of graphene reinforcing have shown increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. Despite their exceptional mechanical properties, the superior mechanical properties of graphene are not fully transferred to the matrix when incorporated into conventional metal matrix composites. This phenomenon, known as the "valley of death," refers to the loss of mechanical performance at different length scales. One cause of this phenomenon is the difficulty of evenly dispersing the reinforcements in the matrix using traditional fabrication techniques. Another is the presence of dislocations in the metal matrix, which cause very large local lattice strains in the graphene. This atomistic-scale deformation at the interface between the metal and the graphene can significantly weaken it, leading to failure at low strains before reaching its intrinsic failure stress and strain. This dissertation aims to provide insight into how the relationships between composites' design, processing, structure, properties, and mechanics can be used to transfer intrinsic mechanical properties of reinforcements to bulk composite materials more effectively. For this, nanolayered composite systems of Ni and Cu reinforced with graphene or 2D h-BN were studied using experimental techniques and molecular dynamics (MD) simulations to elucidate the underlying mechanisms behind the composites' material structure and mechanical behavior. Additionally, we explore the incorporation of graphene in a metallic matrix that does not deform through dislocations (or shear bands), such as the shape memory alloy nickel-titanium ( Nitinol or NiTi), to avoid low strain failure of the metal/graphene interface. This theoretical strengthening mechanism is investigated by designing and fabricating NiTi/graphene composites.

No description available.

Made available in DSpace on 2014-12-17T14:06:49Z (GMT). No. of bitstreams: 1 LeilianeAO.pdf: 1040153 bytes, checksum: 7b51c6820dc4a457d8c742b3fc25c179 (MD5) Previous issue date: 2008-08-22 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The present work shows a contribution to the studies of development and solid sinterization of a metallic matrix composite MMC that has as starter materials 316L stainless steel atomized with water, and two different Tantalum Carbide TaC powders, with averages crystallite sizes of 13.78 nm and 40.66 nm. Aiming the metallic matrix s density and hardness increase was added different nanometric sizes of TaC by dispersion. The 316L stainless steel is an alloy largely used because it s high resistance to corrosion property. Although, its application is limited by the low wear resistance, consequence of its low hardness. Besides this, it shows low sinterability and it cannot be hardened by thermal treatments traditional methods because of the austenitic structure, face centered cubic, stabilized mainly in nickel presence. Steel samples added with TaC 3% wt (each sample with different type of carbide), following a mechanical milling route using conventional mill for 24 hours. Each one of the resulted samples, as well as the pure steel sample, were compacted at 700 MPa, room temperature, without any addictive, uniaxial tension, using a 5 mm diameter cylindrical mold, and quantity calculated to obtain compacted final average height of 5 mm. Subsequently, were sintered in vacuum atmosphere, temperature of 1290?C, heating rate of 20?C/min, using different soaking times of 30 and 60 min and cooled at room temperature. The sintered samples were submitted to density and micro-hardness analysis. The TaC reforced samples showed higher density values and an expressive hardness increase. The complementary analysis in optical microscope, scanning electronic microscope and X ray diffractometer, showed that the TaC, processed form, contributed with the hardness increase, by densification, itself hardness and grains growth control at the metallic matrix, segregating itself to the grain boarders / O presente trabalho apresenta uma contribui??o ao estudo do desenvolvimento e sinteriza??o s?lida de um Comp?sito de Matriz Met?lica CMM que tem como materiais de partida um a?o inoxid?vel 316L atomizado a ?gua, e duas partidas diferentes de Carbeto de T?ntalo TaC, com tamanhos m?dios de cristalitos de 13,78 nm e 40,66 nm. Objetivando aumentar a densidade e dureza da matriz met?lica foi adicionado, por dispers?o diferentes part?culas nanom?tricas de TaC. O a?o inoxid?vel 316L ? uma liga largamente utilizada pela sua propriedade de alta resist?ncia ? corros?o. Contudo, sua aplica??o ? limitada pela baixa resist?ncia ao desgaste, conseq??ncia da sua baixa dureza. Al?m disso, apresenta baixa sinterabilidade e n?o pode ser endurecido pelos m?todos tradicionais de tratamentos t?rmicos, devido a sua estrutura austen?tica, c?bica de face centrada, estabilizada principalmente pela presen?a do N?quel. Amostras de a?os adicionadas com 3% em peso de TaC (cada amostra com carbetos de partidas diferentes), seguiram uma rota de moagem mec?nica em moinho convencional por 24 horas. Cada uma das amostras resultantes, assim como amostras do a?o puro foram compactados a 700 MPa, a frio, sem nenhum aditivo, uniaxialmente, em uma matriz cil?ndrica de 5 mm de di?metro, em quantidade calculada para ter uma altura m?dia final do compactado de 5 mm. Posteriormente, foram sinterizadas em forno a v?cuo, em temperatura de at? 1290? C com incremento de 20 ?C por minuto, sendo mantidas neste patamar por 30 ou 60 minutos e resfriadas ? temperatura ambiente. As amostras sinterizadas foram submetidas aos ensaios para a medi??o da densidade e da micro-dureza. As amostras contendo o refor?o de TaC apresentaram maiores valores de densidade e um aumento significativo na sua dureza. As an?lises complementares no microsc?pio ?tico, no microsc?pio eletr?nico de varredura e no difrat?metro de raios-X, mostram que o TaC, na forma processada, contribuiu com o aumento da dureza, pela densifica??o, pela sua pr?pria dureza e pelo controle do crescimento dos gr?os da matriz met?lica, segregando-se nos seus contornos

The UK Government has set an ambitious target to achieve 15% of final energy consumption from renewable sources by 2020. High Voltage Direct Current (HVDC) technology is an attractive solution for integrating offshore wind power farms farther from the coast. In the near future, more windfarms are likely to be connected to the UK grid using HVDC links. With the onset of this fairly new technology, new challenges are inevitable. This research is undertaken to help assist with these challenges by looking at possibilities of problems with respect to faster AC/DC interaction modes, especially, on the impact of inrush currents which occur during fault-recovery transients. In addition to that, possible mitigation strategies are also investigated. Initially, the relative merits of different transformer models are analysed with respect to inrush current transient studies. The most appropriate transformer model is selected and further validated using field measurement data. A detailed electro-magnetic-transient (EMT) model of a grid-connected MMC-HVDC system is prepared in PSCAD/EMTDC to capture the key dynamics of fault-recovery transformer inrush currents. It is shown that the transformer in an MMC system can evoke inrush currents during fault recovery, and cause transient interactions with the converter and the rest of the system, which should not be neglected. It is shown for the first time through a detailed dynamic analysis that if the current sensors of the inner-current control loops are placed at the converter-side of the transformer instead of the grid-side, the inrush currents will mainly flow from the grid and decay faster. This is suggested as a basic remedial action to protect the converter from inrush currents. Afterwards, analytical calculations of peak flux-linkage magnitude in each phase, following a voltage-sag recovery transient, are derived and verified. The effects of zero-sequence currents and fault resistance on the peak flux linkage magnitude are systematically explained. A zero-sequence-current suppression controller is also proposed. A detailed study is carried out to assess the key factors that affect the maximum peak flux-linkage and magnetisation-current magnitudes, especially with regard to fault specific factors such as fault inception angle, duration and fault-current attenuation. Subsequently, the relative merits of a prior-art inrush current mitigation strategy and its implementation challenges in a grid-connected MMC converter are analysed. It is shown that the feedforward based auxiliary flux-offset compensation scheme, as incorporated in the particular strategy, need to be modified with a feedback control technique, to alleviate the major drawbacks identified. Following that, eight different feedback based control schemes are devised, and a detailed dynamic and transient analysis is carried out to find the best control scheme. The relative merits of the identified control scheme and its implementation challenges in a MMC converter are also analysed. Finally, a detailed EMT model of an islanded MMC-HVDC system is implemented in PSCAD/EMTDC and the impacts of fault-recovery inrush currents are analysed. For that, initially, a MMC control scheme is devised in the synchronous reference frame and its controllers are systematically tuned. To obtain an improved performance, an equivalent control scheme is derived in the stationary reference frame with Proportional-Resonant controllers, and incorporated in the EMT model. Following that, two novel inrush current mitigation strategies are proposed, with the support of analytical equations, and verified.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-03-01T13:17:10Z No. of bitstreams: 1 isabelacostadasilva.pdf: 696658 bytes, checksum: 08de4dce4966b7e52cb5eaa5330a65c4 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2016-03-03T14:36:32Z (GMT) No. of bitstreams: 1 isabelacostadasilva.pdf: 696658 bytes, checksum: 08de4dce4966b7e52cb5eaa5330a65c4 (MD5) / Made available in DSpace on 2016-03-03T14:36:32Z (GMT). No. of bitstreams: 1 isabelacostadasilva.pdf: 696658 bytes, checksum: 08de4dce4966b7e52cb5eaa5330a65c4 (MD5) Previous issue date: 2013-11-01 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / O Movimento de Mulheres Camponesas (MMC) é um movimento social feminista e camponês, que tem empreendido lutas pelos direitos das mulheres e pela efetivação de um Projeto Popular para a Agricultura no Brasil. Esta dissertação de Mestrado tem por objetivo a análise das lutas e das concepções políticas deste movimento, bem como sua atuação no campo da Via Campesina. Para tal propósito, num primeiro momento, faremos apontamentos sobre os movimentos sociais e a questão agrária brasileira. Em um segundo momento, discorreremos sobre as bases da exploração-dominação das mulheres na sociedade atual, sobre a organização destas no movimento feminista e sobre a relação entre feminismo e socialismo, trazendo a contribuição teórica tanto de marxistas clássicos como das estudiosas de gênero da contemporaneidade para a compreensão dessas questões. No trabalho também apresentaremos os resultados da pesquisa de campo realizada junto ao MMC, que aborda temas como a auto-organização das mulheres, a compreensão do Movimento a respeito da realidade de dominação-exploração feminina, as suas principais reivindicações e lutas, além de sua contribuição política à Via Campesina - visto que o MMC é o único movimento autoorganizado de mulheres em seu interior. Por fim, serão apresentados alguns desafios postos aos movimentos de mulheres na atualidade. / The Movement of Peasant Women (MMC) is a feminist and peasant movement, wich has waged struggles for women's rights and the realization of a Popular Project for Agriculture in Brazil. This Master's thesis aims to analyze the struggles and political ideas of this movement and its activities at Via Campesina. For this purpose, at first, we will do notes on social movements and agrarian issue. In a second stage of the dissertation , we will discuss the basics of exploitation -domination of women in modern society, the organization of these in the feminist movement and the relationship between feminism and socialism, bringing the theoretical contribution both Marxists classics as scholars of gender to the contemporary understanding of these issues . That work also presents the results of field research about the MMC , which covers topics such as self - organization of women , understanding the movement about the reality of female exploitation , their main demands and struggles, and their political contribution to the Via Campesina - since the MMC is the only self-organized movement of women inside. Finally, we will present some challenges faced by women's movements today.

Les packagings d’électronique embarquée sont actuellement en alliages d’aluminium. A partir d’une étude de sélection des matériaux, complétée par une simulation numérique thermique,nous avons démontré qu’un matériau composite constitué d’une matrice aluminium et de fibres de carbone à forte conductivité thermique, représente un fort potentiel de gain de masse sur ces équipements. Cependant, le couplage de ces deux matériaux génère des problèmes d’élaboration en raison d’incompatibilités fortes parmi lesquelles un mouillage très faible du carbone par l’aluminium liquide et une réactivité chimique élevée qui conduit à la formation de carbures d’aluminium préjudiciables pour le matériau final. Deux voies d’élaboration distinctes ont été envisagées : Une voie liquide où l’utilisation d’un agent de mouillage (un sel fluoré) a permis d’obtenir la montée par capillarité du métal dans des mèches de fibres. Une voie solide basée sur une technique originale d’empilements de feuillets d’aluminium et de fibres de carbone avec le procédé de Spark Plasma Sintering (SPS). .La seconde technique s’est révélée prometteuse en permettant d’obtenir des échantillons multicouches sans porosités, un endommagement très limité des fibres et une architecture contrôlée.Notre étude a montré que la formation de carbures d’aluminium est limitée. De plus, une meilleure compréhension du SPS ou l’application d’un revêtement sur les fibres devraient permettre d’éviter la formation de ces carbures. Les tentatives de caractérisations mécanique et thermique effectuées sur ces échantillons donnent un premier aperçu de l’efficacité du renforcement de l’aluminium par les fibres de carbone. / Embedded electronic packagings are currently made of aluminum. A first study – basedupon a material selection method completed by numerical analysis – showed that a metal matrixcomposite made of aluminum and highly thermal conductive continuous carbon fibers represents ahigh potential upon weight savings for those equipments. Though, coupling these componentsrepresents numerous challenges due to their incompatibility such as a really low wetting of carbonliquidaluminum system and its unavoidable chemical reactivity that leads to the formation ofaluminum carbides that are harmful for the final material. Two manufacturing routes were considered: A liquid route using a wetting agent (fluorinated salts) led the metal to rise alongcarbon fibers by capillarity. A solid route based upon a novel technique of aluminum foils and carbon fibersstacking using the Spark Plasma Sintering (SPS) process.This second technique revealed to be very promising and allowed to obtain multilayer samples with noporosities, highly limited fiber damages and controlled composite architecture. Our study shows thataluminum carbides formation is limited. Moreover, a deeper comprehension of SPS process or thedeposit of fiber coatings would prevent this carbide formation. Attempts of mechanical and thermalcharacterization led upon such samples give a first overview of the efficiency of the aluminumreinforcement by carbon fibers.