Resistência à flexão e microestrutura de vidro-cerâmicas feldspáticas processadas por injeção em moldes à alta temperatura

Enquanto os métodos tradicionais de processamento de restaurações cerâmicas tornaram-se notórios por sua complexidade, as técnicas mais modernas vêm privilegiando a simplicidade de execução e a automação. Dentre estas, destaca-se a injeção em moldes, que recentemente, foi associada a métodos CAD-CAM. Estudos anteriores demonstraram a viabilidade de utilização de um vidro feldspático de baixa expansão térmica, Alpha (Vita Zahnfabrik), para injeção, porém, faltam informações quanto às propriedades mecânicas e a microestrutura deste material quando submetido à injeção. Os objetivos deste estudo são: produzir pastilhas vidrocerâmicas para injeção a quente a partir de Alpha e da mistura deste vidro com partículas de alumina e zircônia; avaliar a resistência à flexão dos materiais processados, e compará-la a um material compatível existente no mercado (PM9 - Vita Zahnfabrik); estudar a estrutura microscópica dos materiais e correlacioná-la com suas propriedades mecânicas; identificar por meio de difração de raios X a formação de fases cristalinas durante as diferentes etapas de processamento. A injeção aumentou a resistência do vidro Alpha devido à redução na quantidade e tamanho dos defeitos internos, principalmente porosidades. Apesar de ter sido observada nucleação de cristais nos dois materiais, durante o processamento, não foi possível determinar de que forma este fenômeno afetou as propriedades mecânicas dos materiais. Não foi detectada alteração no padrão de distribuição das fases cristalinas observadas em microscópio eletrônico de varredura antes e depois da injeção. Não foi verificada diferença estatística significante entre a resistência à flexão de Alpha injetado e PM9. A adição de partículas de alumina e zirconia ao vidro Alpha provocou redução da resistência, devido à formação de aglomerados durante a confecção das pastilhas e a incapacidade da injeção em dispersá-los. Tais aglomerados funcionaram como concentradores de tensões, enfraquecendo o material.

紫外激光诱导近化学计量比钽酸锂晶体铁电畴反转

对紫外激光诱导近化学计量比钽酸锂晶体铁电畴反转进行了实验研究。波长为351 nm的连续紫外激光被聚焦在近化学计量比钽酸锂晶体的-z表面,同时沿与晶体自发极化相反的方向施加均匀外电场。实验证实紫外激光辐照可以有效地降低晶体畴反转所需的矫顽电场,采用数字全息干涉测量技术检测证实在激光辐照区域实现局域畴反转。研究表明采用紫外激光诱导可以实现对近化学计量比钽酸锂晶体铁电畴反转的局域控制。提出了物理机理的理论分析,认为外电场和激光辐照场的共同作用在晶体内部产生高浓度、大尺寸的缺陷结构,缺陷一定程度上降低畴体成核和畴

Field and Laboratory Studies of Atmospheric Organic Aerosol

This thesis is the culmination of field and laboratory studies aimed at assessing processes that affect the composition and distribution of atmospheric organic aerosol. An emphasis is placed on measurements conducted using compact and high-resolution Aerodyne Aerosol Mass Spectrometers (AMS). The first three chapters summarize results from aircraft campaigns designed to evaluate anthropogenic and biogenic impacts on marine aerosol and clouds off the coast of California. Subsequent chapters describe laboratory studies intended to evaluate gas and particle-phase mechanisms of organic aerosol oxidation.

The 2013 Nucleation in California Experiment (NiCE) was a campaign designed to study environments impacted by nucleated and/or freshly formed aerosol particles. Terrestrial biogenic aerosol with > 85% organic mass was observed to reside in the free troposphere above marine stratocumulus. This biogenic organic aerosol (BOA) originated from the Northwestern United States and was transported to the marine atmosphere during periodic cloud-clearing events. Spectra recorded by a cloud condensation nuclei counter demonstrated that BOA is CCN active. BOA enhancements at latitudes north of San Francisco, CA coincided with enhanced cloud water concentrations of organic species such as acetate and formate.

Airborne measurements conducted during the 2011 Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) were aimed at evaluating the contribution of ship emissions to the properties of marine aerosol and clouds off the coast of central California. In one study, analysis of organic aerosol mass spectra during periods of enhanced shipping activity yielded unique tracers indicative of cloud-processed ship emissions (m/z 42 and 99). The variation of their organic fraction (f₄₂ and f₉₉) was found to coincide with periods of heavy (f₄₂ > 0.15; f₉₉ > 0.04), moderate (0.05 < f₄₂ < 0.15; 0.01 < f₉₉ < 0.04), and negligible (f₄₂ < 0.05; f₉₉ < 0.01) ship influence. Application of these conditions to all measurements conducted during E-PEACE demonstrated that a large fraction of cloud droplet (72%) and dry aerosol mass (12%) sampled in the California coastal study region was heavily or moderately influenced by ship emissions. Another study investigated the chemical and physical evolution of a controlled organic plume emitted from the R/V Point Sur. Under sunny conditions, nucleated particles composed of oxidized organic compounds contributed nearly an order of magnitude more cloud condensation nuclei (CCN) than less oxidized particles formed under cloudy conditions. The processing time necessary for particles to become CCN active was short ( < 1 hr) compared to the time needed for particles to become hygroscopic at sub-saturated humidity ( > 4 hr).

Laboratory chamber experiments were also conducted to evaluate particle-phase processes influencing aerosol phase and composition. In one study, ammonium sulfate seed was coated with a layer of secondary organic aerosol (SOA) from toluene oxidation followed by a layer of SOA from α-pinene oxidation. The system exhibited different evaporative properties than ammonium sulfate seed initially coated with α-pinene SOA followed by a layer of toluene SOA. This behavior is consistent with a shell-and-core model and suggests limited mixing among different SOA types. Another study investigated the reactive uptake of isoprene epoxy diols (IEPOX) onto non-acidified aerosol. It was demonstrated that particle acidity has limited influence on organic aerosol formation onto ammonium sulfate seed, and that the chemical system is limited by the availability of nucleophiles such as sulfate.

Flow tube experiments were conducted to examine the role of iron in the reactive uptake and chemical oxidation of glycolaldehyde. Aerosol particles doped with iron and hydrogen peroxide were mixed with gas-phase glycolaldehyde and photochemically aged in a custom-built flow reactor. Compared to particles free of iron, iron-doped aerosols significantly enhanced the oxygen to carbon (O/C) ratio of accumulated organic mass. The primary oxidation mechanism is suggested to be a combination of Fenton and photo-Fenton reactions which enhance particle-phase OH radical concentrations.

Observation and interpretation of tectonic strain release mechanisms

In four chapters various aspects of earthquake source are studied.

Chapter I

Surface displacements that followed the Parkfield, 1966, earthquakes were measured for two years with six small-scale geodetic networks straddling the fault trace. The logarithmic rate and the periodic nature of the creep displacement recorded on a strain meter made it possible to predict creep episodes on the San Andreas fault. Some individual earthquakes were related directly to surface displacement, while in general, slow creep and aftershock activity were found to occur independently. The Parkfield earthquake is interpreted as a buried dislocation.

Chapter II

The source parameters of earthquakes between magnitude 1 and 6 were studied using field observations, fault plane solutions, and surface wave and S-wave spectral analysis. The seismic moment, M_O, was found to be related to local magnitude, M_L, by log M_O = 1.7 M_L + 15.1. The source length vs magnitude relation for the San Andreas system found to be: M_L = 1.9 log L - 6.7. The surface wave envelope parameter AR gives the moment according to log M_O = log AR₃₀₀ + 30.1, and the stress drop, τ, was found to be related to the magnitude by τ = 0.54 M - 2.58. The relation between surface wave magnitude M_S and M_L is proposed to be M_S = 1.7 M_L - 4.1. It is proposed to estimate the relative stress level (and possibly the strength) of a source-region by the amplitude ratio of high-frequency to low-frequency waves. An apparent stress map for Southern California is presented.

Chapter III

Seismic triggering and seismic shaking are proposed as two closely related mechanisms of strain release which explain observations of the character of the P wave generated by the Alaskan earthquake of 1964, and distant fault slippage observed after the Borrego Mountain, California earthquake of 1968. The Alaska, 1964, earthquake is shown to be adequately described as a series of individual rupture events. The first of these events had a body wave magnitude of 6.6 and is considered to have initiated or triggered the whole sequence. The propagation velocity of the disturbance is estimated to be 3.5 km/sec. On the basis of circumstantial evidence it is proposed that the Borrego Mountain, 1968, earthquake caused release of tectonic strain along three active faults at distances of 45 to 75 km from the epicenter. It is suggested that this mechanism of strain release is best described as "seismic shaking."

Chapter IV

The changes of apparent stress with depth are studied in the South American deep seismic zone. For shallow earthquakes the apparent stress is 20 bars on the average, the same as for earthquakes in the Aleutians and on Oceanic Ridges. At depths between 50 and 150 km the apparent stresses are relatively high, approximately 380 bars, and around 600 km depth they are again near 20 bars. The seismic efficiency is estimated to be 0.1. This suggests that the true stress is obtained by multiplying the apparent stress by ten. The variation of apparent stress with depth is explained in terms of the hypothesis of ocean floor consumption.

Heteroepitaxy of Group IV and Group III-V semiconductor alloys for photovoltaic applications

Photovoltaic energy conversion represents a economically viable technology for realizing collection of the largest energy resource known to the Earth -- the sun. Energy conversion efficiency is the most leveraging factor in the price of energy derived from this process. This thesis focuses on two routes for high efficiency, low cost devices: first, to use Group IV semiconductor alloy wire array bottom cells and epitaxially grown Group III-V compound semiconductor alloy top cells in a tandem configuration, and second, GaP growth on planar Si for heterojunction and tandem cell applications.

Metal catalyzed vapor-liquid-solid grown microwire arrays are an intriguing alternative for wafer-free Si and SiGe materials which can be removed as flexible membranes. Selected area Cu-catalyzed vapor-liquid solid growth of SiGe microwires is achieved using chlorosilane and chlorogermane precursors. The composition can be tuned up to 12% Ge with a simultaneous decrease in the growth rate from 7 to 1 μm/min^-1. Significant changes to the morphology were observed, including tapering and faceting on the sidewalls and along the lengths of the wires. Characterization of axial and radial cross sections with transmission electron microscopy revealed no evidence of defects at facet corners and edges, and the tapering is shown to be due to in-situ removal of catalyst material during growth. X-ray diffraction and transmission electron microscopy reveal a Ge-rich crystal at the tip of the wires, strongly suggesting that the Ge incorporation is limited by the crystallization rate.

Tandem Ga_1-xIn_xP/Si microwire array solar cells are a route towards a high efficiency, low cost, flexible, wafer-free solar technology. Realizing tandem Group III-V compound semiconductor/Si wire array devices requires optimization of materials growth and device performance. GaP and Ga_1-xIn_xP layers were grown heteroepitaxially with metalorganic chemical vapor deposition on Si microwire array substrates. The layer morphology and crystalline quality have been studied with scanning electron microscopy and transmission electron microscopy, and they provide a baseline for the growth and characterization of a full device stack. Ultimately, the complexity of the substrates and the prevalence of defects resulted in material without detectable photoluminescence, unsuitable for optoelectronic applications.

Coupled full-field optical and device physics simulations of a Ga_0.51In_0.49P/Si wire array tandem are used to predict device performance. A 500 nm thick, highly doped "buffer" layer between the bottom cell and tunnel junction is assumed to harbor a high density of lattice mismatch and heteroepitaxial defects. Under simulated AM1.5G illumination, the device structure explored in this work has a simulated efficiency of 23.84% with realistic top cell SRH lifetimes and surface recombination velocities. The relative insensitivity to surface recombination is likely due to optical generation further away from the free surfaces and interfaces of the device structure.

Finally, GaP has been grown free of antiphase domains on Si (112) oriented substrates using metalorganic chemical vapor deposition. Low temperature pulsed nucleation is followed by high temperature continuous growth, yielding smooth, specular thin films. Atomic force microscopy topography mapping showed very smooth surfaces (4-6 Å RMS roughness) with small depressions in the surface. Thin films (~ 50 nm) were pseudomorphic, as confirmed by high resolution x-ray diffraction reciprocal space mapping, and 200 nm thick films showed full relaxation. Transmission electron microscopy showed no evidence of antiphase domain formation, but there is a population of microtwin and stacking fault defects.

Red laser-induced domain inversion in MgO-doped lithium niobate crystals

A laser beam at wavelength 647 nm is focused on a sample of 5 mol% MgO-doped lithium niobate crystal for domain inversion by a conventional external electric field. In this case, a reduction of 36% in the electric field required for domain nucleation (nucleation field) is observed. To the best of our knowledge, it is the longest wavelength reported for laser-induced domain inversion. This extends the spectrum of laser inducing, and the experimental results are helpful to understand the nucleation dynamics under laser illumination. The dependence of nucleation fields on intensities of laser beams is analysed in experiments.

A Continuum Model for Slip-Twinning Interactions in Magnesium and Magnesium Alloys

Due to their high specific strength and low density, magnesium and magnesium-based alloys have gained great technological importance in recent years. However, their underlying hexagonal crystal structure furnishes Mg and its alloys with a complex mechanical behavior because of their comparably smaller number of energetically favorable slip systems. Besides the commonly studied slip mechanism, another way to accomplish general deformation is through the additional mechanism of deformation-induced twinning. The main aim of this thesis research is to develop an efficient continuum model to understand and ultimately predict the material response resulting from the interaction between these two mechanisms.

The constitutive model we present is based on variational constitutive updates of plastic slips and twin volume fractions and accounts for the related lattice reorientation mechanisms. The model is applied to single- and polycrystalline pure magnesium. We outline the finite-deformation plasticity model combining basal, pyramidal, and prismatic dislocation activity as well as a convexification based approach for deformation twinning. A comparison with experimental data from single-crystal tension-compression experiments validates the model and serves for parameter identification. The extension to polycrystals via both Taylor-type modeling and finite element simulations shows a characteristic stress-strain response that agrees well with experimental observations for polycrystalline magnesium. The presented continuum model does not aim to represent the full details of individual twin-dislocation interactions, yet it is sufficiently efficient to allow for finite element simulations while qualitatively capturing the underlying microstructural deformation mechanisms.

Monica Alis Brick Lane: A tale of two cities or How Nazneen left her fate

O objetivo desta dissertação é analisar o romance Brick Lane (2003), da escritora Monica Ali, focalizando o processo de empoderamento da protagonista Nazneen. Usando os conceitos de lugar e não-lugar propostos por Marc Augé, pretendo examinar passagens do romance que caracterizem a sensação de não-pertencimento vivida pela personagem. Nazneen recorre às memórias de sua infância com a irmã Hasina em Bangladesh para tentar se distanciar do espaço físico de Brick Lane (a comunidade onde reside em Londres), que se constitui em um não-lugar, pois ali não é possível reconstruir sua identidade fragmentada pelo deslocamento de uma cultura para outra. Considerando aspectos históricos e culturais de Bangladesh, terra natal de Nazneen, este trabalho pretende discutir as experiências da protagonista e de outros sujeitos diaspóricos do romance, além de analisar o papel crucial que Hasina (e suas cartas) desempenham na narrativa. O atual contexto multicultural e cosmopolita da cidade de Londres também será investigado, com a discussão de situações que afetam os imigrantes e suas representações no romance. Esta dissertação também considera o caráter gendrado dos movimentos migratórios contemporâneos que possibilitam novas perspectivas acerca das consequências da diáspora. A análise de passagens selecionadas do romance ratifica o processo gradual de autonomia de Nazneen que, por fim, consegue se sentir em casa novamente

Phase mapping of domain kinetics in lithium niobate by digital holographic interferometry

The phase mapping of domain kinetics under the uniform steady-state electric field is achieved and investigated in the LiNbO3 crystals by digital holographic interferometry. We obtained the sequences of reconstructed three-dimensional and two-dimensional wave-field phase distributions during the electric poling in the congruent and near stoichiometric LiNbO3 crystals. The phase mapping of individual domain nucleation and growth in the two crystals are obtained. It is found that both longitudinal and lateral domain growths are not linear during the electric poling. The phase mapping of domain wall motions in the two crystals is also obtained. Both the phase relaxation and the pinning-depinning mechanism are observed during the domain wall motion. The residual phase distribution is observed after the high-speed domain wall motion. The corresponding analyses and discussions are proposed to explain the phenomena.

Verificação à fadiga de pontes rodoviárias mistas (aço-concreto)

As pontes rodoviárias mistas (aço-concreto) estão sujeitadas às ações dinâmicas variáveis, por exemplo, devido ao tráfego de veículos sobre a superfície irregular do pavimento. Estas ações dinâmicas podem gerar a nucleação de fraturas ou mesmo a sua propagação sobre a estrutura. A correta consideração desses aspectos objetivou o desenvolvimento de uma metodologia de análise, com a finalidade de avaliar os níveis dos esforços e tensões oriundos do tráfego dos veículos sobre a superfície irregular do pavimento e, bem como, proceder uma verificação à fadiga de obras de arte rodoviárias em aço e mistas (aço-concreto). Para tal, as técnicas para a contagem de ciclos de tensão e a aplicação das regras de dano acumulado foram analisadas através de curvas do tipo S-N, associadas a diversas normas de projeto. A ponte rodoviária mista (aço-concreto) investigada neste estudo é constituída por quatro vigas de aço longitudinais e por um tabuleiro de concreto armado. O modelo numérico-computacional, desenvolvido para a análise dinâmica da ponte, foi concebido com base em técnicas usuais de discretização através do método dos elementos finitos. Simulam-se as almas das vigas de aço e as lajes de concreto do tabuleiro através de elementos finitos de casca. As mesas dessas vigas, transversinas e os enrijecedores são modelados por elementos de viga tridimensionais. Os veículos são representados a partir de sistemas "massa-mola-amortecedor". O tráfego dessas viaturas é considerado mediante a simulação de comboios semi-infinitos, deslocando-se com velocidade constante sobre a ponte. As conclusões da presente investigação versam acerca da vida útil de serviço dos elementos estruturais de pontes mistas (aço-concreto).

A (her)story of ones own: fiction and autobiography in Julia Alvarezs novels

Escritoras migrantes frequentemente publicam romances autobiográficos que mesclam ficção com suas histórias pessoais. Essas escritoras usam suas experiências pessoais para discutir questões coletivas relacionadas aos diversos tipos de deslocamento associados ao processo diaspórico. As migrações em massa das ex-colônias para as metrópoles dos países desenvolvidos cresceram significantemente após a Segunda Guerra Mundial, gerando ao mesmo tempo contato mais próximos e conflitos entre culturas. Essa dissertação pretende analisar os romances autobiográficos How the García Girls Lost their Accents (1991) e Yo! (1997) da escritora dominicana-americana Julia Alvarez, A família Alvarez migrou para os Estados Unidos em 1960 devido a perseguição política. Em seus romances, a escritora lida com os traumas do deslocamento e com o processo de crescimento de meninas divididas entre valores culturais diferentes. Pretendo discutir como Alvarez, em sua prática autobiográfica, problematiza questões relacionadas à migração, como gênero, hibridismo cultural, memória lacunar e identidades fragmentadas. Também analiso como essas narrativas contestam as convenções formais tanto do gênero autobiográfico como da ficção, frisando o quanto o limite entre o real e o fictício, entre o privado e o político, é tênue

Eventos de vida produtores de estresse e acidentes de trabalho: Estudo Pró-Saúde, Rio de Janeiro - Brasil

A proposta desta tese consiste em um recorte de uma abordagem mais ampla da determinação dos acidentes de trabalho, e tem como objetivo geral investigar o perfil de acidentes de trabalho que acometem funcionários técnico-administrativos do quadro efetivo de uma universidade do Rio de Janeiro. Para o alcance do objetivo, esta tese está estruturada em dois artigos, e como tal pretende explorar o perfil sócio-demográfico e ocupacional de funcionários públicos na ocorrência de acidentes de trabalho (Artigo 1); e investigar a associação entre os eventos de vida produtores de estresse (EVPE) e a ocorrência de acidentes de trabalho (Artigo 2). Dados seccionais da fase 2 de um estudo de natureza prospectiva (Estudo Pró-Saúde) foram coletados entre 3572 funcionários. A história de acidentes de trabalho foi captada por meio de perguntas dicotômicas (sim vs. não) para cada um dos seguintes tipos de acidentes: perfuração com agulha; perfuração com outro objeto; corte; queimadura; choque elétrico; contusão ou distensão muscular; fratura, entorse ou luxação; e envenenamento ou intoxicação. O período de referência para aferição tanto dos EVPE quanto da ocorrência de acidente de trabalho correspondeu aos 12 meses anteriores a aplicação de questionário autopreenchível. No artigo 1 utilizou-se a técnica de análise de correspondência múltipla para delimitar agrupamentos de funcionários quanto ao perfil sócio-demográfico e ocupacional associado à ocorrência de acidente de trabalho, de acordo com as seguintes Característica : sexo, idade, escolaridade, renda per capita, ocupação, setor e local de trabalho. No artigo 2, a associação entre EVPE e acidentes de trabalho foi avaliada através de análise multivariada por meio de modelo lineares generalizados (logpoisson), sendo os resultados expressos através de razões de prevalência (RP) ajustadas e seus respectivos intervalos de 95% de confiança (IC95%). A prevalência total de acidentes no período de 12 meses foi de 25,6%. Dos tipos de acidentes referidos, o mais frequente foi a contusão ou fratura, com cerca de (10,2%) de relatos. Em seguida, aparecem as perfurações com agulha (6,5%). Os resultados da análise de correspondência revelam três grupos, destacando-se aquele formado pelos que sofreram perfuração com agulha com um perfil que abrange os auxiliares de enfermagem, trabalham no Hospital Universitário e setores adjacentes, especificamente em setores de terapia intensiva, emergência, cirurgia geral, clinica geral e ambulatório. Em relação à associação com EVPE, ter sido testemunha de agressão foi o evento mais fortemente associado com acidentes de trabalho (RP= 1,98, IC95%= 1,67; 2,34). Este estudo trouxe informações acerca da importância das características sócio-demográficas e de aspectos psicossociais na ocorrência dos acidentes de trabalho que podem ser úteis na elaboração de medidas para a prevenção desse importante problema de saúde pública.

Temperature dependence of thermal properties of Ag8In14Sb55Te23 phase-change memory materials

The dependence of thermal properties of Ag8In14Sb55Te23 phase-change memory materials in crystalline and amorphous states on temperature was measured and analyzed. The results show that in the crystalline state, the thermal properties monotonically decrease with the temperature and present obvious crystalline semiconductor characteristics. The heat capacity, thermal diffusivity, and thermal conductivity decrease from 0.35 J/g K, 1.85 mm(2)/s, and 4.0 W/m K at 300 K to 0.025 J/g K, 1.475 mm(2)/s, and 0.25 W/m K at 600 K, respectively. In the amorphous state, while the dependence of thermal properties on temperature does not present significant changes, the materials retain the glass-like thermal characteristics. Within the temperature range from 320 K to 440 K, the heat capacity fluctuates between 0.27 J/g K and 0.075 J/g K, the thermal diffusivity basically maintains at 0.525 mm(2)/s, and the thermal conductivity decreases from 1.02 W/m K at 320 K to 0.2 W/m K at 440 K. Whether in the crystalline or amorphous state, Ag8In14Sb55Te23 are more thermally active than Ge2Sb2Te5, that is, the Ag8In14Sb55Te23 composites bear stronger thermal conduction and diffusion than the Ge2Sb2Te5 phase-change memory materials.

New transparent Er3+-doped oxyfluoride tellurite glass ceramic with improved near infrared and up-conversion fluorescence properties

Transparent glass ceramics have been obtained by nucleation and growth of Y2Te6O15 or Er2Te5O13 cubic phase in a new Er3+-doped oxyfluoride tellurite glass. Effect of beat treatment on absorption spectra, luminescence and up-conversion properties in the oxyfluoride tellurite glass has been investigated. With heat treatment the ultraviolet absorption edge red shifted evidently for the oxyfluoride telluride glass. The near infrared emission that corresponds to Er3+:I-4(13/2)-> I-4(15/2) can be significantly enhanced after heat treatment. Under 980 nm LD pumping, red and green up-conversion intensity of Er3+ in the glass ceramic can be observed much stronger than that in the base glass. (C) 2006 Elsevier B.V. All rights reserved.