Supercondutividade em ligas de Ta1-xZrx

No presente estudo, amostras policristalinas ricas em Ta e com estequiometrias Ta1-xZrx; x < 0.15; foram preparadas através da mistura apropriada dos elementos metálicos, os quais foram fundidos em forno a arco elétrico sobre uma placa de cobre refrigerada a água e sob atmosfera de argônio de alta pureza. Os padrões de difração de raios-X das ligas, como fundidas (as cast) e tratadas termicamente a 850 °C por 24 h, revelaram a ocorrência de uma estrutura cristalina cúbica de corpo centrada bcc, tipo W, e parâmetros de rede que aumentam suavemente com o aumento do teor de Zr nas ligas. Medidas de susceptibilidade magnética dc, conduzidas nas condições de resfriamento da amostra em campo zero (ZFC) e do resfriamento com o campo magnético aplicado (FC), indicaram que supercondutividade volumétrica é observada abaixo de ~ 5.8, 6.9, 7.0 K em amostras com x = 0.05, 0.08, e 0.10, respectivamente. Essas temperaturas críticas supercondutoras são bastante superiores àquela observada no Ta elementar ~ 4.45 K. Medidas de resistividade elétrica na presença de campos magnéticos aplicados de até 9 T confirmaram a temperatura crítica supercondutora das amostras estudadas. O campo crítico superior Hc2 e o comprimento de coerência E foram estimados a partir dos dados de magnetorresistência. Os valores estimados de Hc2 foram de ~ 0.46, 1.78, 3.85 e 3.97 T, resultando em valores de E ~ 26.0, 13.6, 9.2 e 9.1 nm para as ligas as cast com x = 0.00, 0.05, 0.08 e 0.10, respectivamente. A partir dos dados experimentais do calor específico Cp das ligas, magnitudes estimadas do salto em Cp nas vizinhanças das transições supercondutoras indicaram valores maiores que o previsto pela teoria BCS. Utilizando as equações analíticas derivadas da teoria do acoplamento forte da supercondutividade foi então proposto que o aumento da temperatura de transição supercondutora nas ligas devido a substituição parcial do Ta por Zr está intimamente relacionado ao aumento do acoplamento elétron-fônon, visto que a densidade de estados eletrônicos no nível de Fermi foi estimada ser essencialmente constante através da série Ta1-xZrx com x < 0.10.

Estudo da geometria da aresta de corte de ferramentas aplicadas ao torneamento de superligas à base de níquel com alta velocidade de corte

Pesquisadores e indústrias de todo o mundo estão firmemente comprometidos com o propósito de fazer o processo de usinagem ser precisamente veloz e produtivo. A forte concorrência mundial gerou a procura por processos de usinagem econômicos, com grande capacidade de produção de cavacos e que produzam peças com elevada qualidade. Dentre as novas tecnologias que começaram a ser empregadas, e deve tornar-se o caminho certo a ser trilhado na busca da competitividade em curto espaço de tempo, está a tecnologia de usinagem com altas velocidades (HSM de High Speed Machining). A tecnologia HSM surge como componente essencial na otimização dos esforços para manutenção e aumento da competitividade global das empresas. Durante os últimos anos a usinagem com alta velocidade tem ganhado grande importância, sendo dada uma maior atenção ao desenvolvimento e à disponibilização no mercado de máquinas-ferramentas com rotações muito elevadas (20.000 - 100.000 rpm). O processo de usinagem com alta velocidade está sendo usado não apenas para ligas de alumínio e cobre, mas também para materiais de difícil usinabilidade, como os aços temperados e superligas à base de níquel. Porém, quando se trata de materiais de difícil corte, têm-se observado poucas publicações, principalmente no processo de torneamento. Mas, antes que a tecnologia HSM possa ser empregada de uma forma econômica, todos os componentes envolvidos no processo de usinagem, incluindo a máquina, o eixo-árvore, a ferramenta e o pessoal, precisam estar afinados com as peculiaridades deste novo processo. No que diz respeito às máquinas-ferramenta, isto significa que elas têm que satisfazer requisitos particulares de segurança. As ferramentas, devido à otimização de suas geometrias, substratos e revestimentos, contribuem para o sucesso deste processo. O presente trabalho objetiva estudar o comportamento de diversas geometrias ) de insertos de cerâmica (Al2O3 + SiCw e Al2O3 + TIC) e PCBN com duas concentrações de CBN na forma padrão, assim como modificações na geometria das arestas de corte empregadas em torneamento com alta velocidade em superligas à base de níquel (Inconel 718 e Waspaloy). Os materiais foram tratados termicamente para dureza de 44 e 40 HRC respectivamente, e usinados sob condição de corte a seco e com utilização da técnica de mínima quantidade de lubrificante (minimal quantity lubricant - MQL) visando atender requisitos ambientais. As superligas à base de níquel são conhecidas como materiais de difícil usinabilidade devido à alta dureza, alta resistência mecânica em alta temperatura, afinidade para reagir com materiais da ferramenta e baixa condutividade térmica. A usinagem de superligas afeta negativamente a integridade da peça. Por essa razão, cuidados especiais devem ser tomados para assegurar a vida da ferramenta e a integridade superficial de componentes usinados por intermédio de controle dos principais parâmetros de usinagem. Experimentos foram realizados sob diversas condições de corte e geometrias de ferramentas para avaliação dos parâmetros: força de corte, temperatura, emissão acústica e integridade superficial (rugosidade superficial, tensão residual, microdureza e microestrutura) e mecanismos de desgaste. Mediante os resultados apresentados, recomenda-se à geometria de melhor desempenho nos parâmetros citados e confirma-se a eficiência da técnica MQL. Dentre as ferramentas e geometrias testadas, a que apresentou melhor desempenho foi a ferramenta cerâmica CC650 seguida da ferramenta cerâmica CC670 ambas com formato redondo e geometria 2 (chanfro em T de 0,15 x 15º com raio de aresta de 0,03 mm).

Precursores funcionalizantes de poros à base de alginato para obtenção de cerâmicas bioativas

A complexidade de desenvolver novas tecnologias para aplicações em reconstituição óssea se deve à necessidade de combinar várias propriedades químicas e físicas para que o material proporcione o desempenho almejado. Particularmente, em aplicações que visam osteogênese, os enxertos sintéticos devem ser bioativos, possuir porosidade com volume, geometria e interconectividade de poros controlados, além de ter boas propriedades mecânicas, dentro de limites relativamente rígidos. Por essa razão, o recobrimento de materiais bioinertes com cerâmicas bioativas se tornou o foco da presente pesquisa. O objetivo desse estudo foi desenvolver um novo método de produção de enxertos cerâmicos com macroporosidade funcionalizada, onde a formação e o revestimento dos poros são realizados em uma única etapa. Foi realizado o estudo de recobrimento com vidro bioativo e fosfato de cálcio. Para isso, agentes porogênicos na forma de grânulos (de 600 μm a 2 mm de diâmetro) foram sintetizados pelo método da gelificação de uma solução aquosa de alginato de sódio gotejada em solução de nitrato de cálcio (0,5 M), com incorporação de outros elementos para a formação de biovidro ou fosfato de cálcio. Esses grânulos foram conglomerados a um vidro ou alumina em pó, formando um compósito, que foi tratado termicamente para sinterização e formação de poros. No caso da matriz vítrea, a sinterização ocorreu com cristalização simultânea e concorrente. As cerâmicas resultantes foram caracterizadas por microscopia óptica e eletrônica de varredura, sendo possível observar a formação de macroporos aproximadamente esféricos (de 600 μm a 2 mm de diâmetro) revestidos internamente por uma camada de material com possível composição bioativa.

Non-covalent immobilization of RhDuphos on carbon nanotubes and carbon xerogels

The immobilization of the chiral complex RhDuphos, by electrostatic or π–π (adsorption) interactions, on carbon nanotubes and carbon xerogels is investigated. To promote such interactions, the supports were either oxidized or heat treated to create carboxylic type surface groups or an apolar surface, respectively. The catalysts were tested in the hydrogenation of methyl 2-acetamidoacrylate. The prepared hybrid catalysts are less active than the homogeneous RhDuphos, but most of them show a high enantioselectivity and the one prepared with the oxidized carbon xerogel is also reusable, being able to give a high substrate conversion, keeping as well a high enantioselectivity. The anchorage by electrostatic interactions is more interesting than the anchorage by π–π interactions, as the π–π adsorption method produces a modification of the metal complex structure leading to an active hybrid catalyst but without enantioselectivity. The creation of carboxylic groups on the support surface has led to some hindering of the complex leaching.

Water characteristics in cooked beef as influenced by ageing and high-pressure treatment -an NMR micro imaging study

The water characteristics in cooked pressure-heat treated (45 degreesC for 45 min prior to pressurisation at 150 MPa for 30 min) and non-pressurised, cooked (control) samples of beef Longissimus aged for 1, 3, 8 or 16 days were studied by nuclear magnetic resonance microscopy. A multi-echo sequence was used to obtain T2 images, and independent of ageing period, the T2 values were found to be lower in pressure-heat treated meat revealing alterations in water characteristics of pressure-treated, cooked meat compared with cooked meat. With increasing ageing duration, the T2 values in both pressure-treated, cooked and cooked meat decreased indicating that the water became more tightly trapped in the protein network. In addition, independent of length of ageing period the relationship between cooking loss in the cooked meat and transverse relaxation differed between non-pressurised and pressure-treated meat. which reveals that the mechanisms changing the water properties in beef during ageing are different from those occuring during pressure-heat treatment of meat. (C) 2003 Elsevier Ltd. All rights reserved.

Simulation study of ammonia adsorption on graphitized carbon black

GCMC simulations are applied to the adsorption of sub-critical ammonia on graphitized carbon black at 240 K. The carbon black was modelled both with and without carbonyl functional groups. Large differences are seen between the amount adsorbed for different carbonyl configurations at low pressure (P < 10kPa). Once a single layer is formed on the carbon black, the adsorption behaviour is similar between the model surfaces with and without functional groups. Simulation isotherms are qualitatively similar to the few experimental isotherms available in the literature for ammonia on highly graphitized carbon black. The mode of adsorption up to monolayer coverage is exhaustively shown to be two-dimensional clustering using various techniques. A comparison between experiment and simulation isosteric heats shows that a surface without functional groups cannot reproduce the experimental isosteric heats of adsorption, even comparing with the experimental results of carbon black heat treated at 3373 K. The addition of carbonyls produces isosteric heats with similar features to those in the literature if the separation between the carbonyls is small.

Mitochondrial dysmorphology in the neuroepithelium of rat embryos following a single dose of maternal hyperthermia during gestation

Hyperthermia is teratogenic to human and animal embryos and induces mainly anomalies of the nervous system. However, the teratogenic mechanism is poorly understood. Mammalian embryos are known to switch from anaerobic to aerobic metabolism around the time of neural tube closure. This critical event might be sensitive to hyperthermia. The objective of the present study was to evaluate the ultrastructural changes of the mitochondria of the neuroepithelium (NE) of rat embryos following maternal exposure to hyperthermia. Pregnant rats were heat stressed for an hour on gestation day (GD) 9 and embryos were examined by electron microscopy on GD 10. NE presented extensive apoptosis. Intercellular junctions were weakened and copious cellular debris projected into the ventricle. The mitochondria were of diverse size and shape. Most of them were swollen and had short cristae and electron dense matrix. Hydropic changes were also observed in numerous mitochondria. Lipid-laden mitochondria were found in the apical portions of neuroblasts. The mesenchyme (ME) of heat-treated embryos showed paucity of cells and only as frequent apoptosis as the controls. Their mitochondria also showed changes similar to those of the NE. Additionally extensive lipid accumulation was observed in and in the vicinity of mitochondria, often surrounded by short strands of endoplasmic reticulum. Whereas mitochondrial pathology was associated with profound apoptosis in the NE, growth restriction and lipid accumulation accompanied mitochondrial changes in the ME. The results of this study indicate that the embryonic response to maternal heat shock is tissue-specific and morphologically distinct in this species.

Melamine phosphate as a component of intumescent coatings

Melamine orthophosphate has been shown to exhibit variations in its chemical constitution, and crystal shape and size, dependent upon the method of production. These crystal types have been incorporated with epoxy resin to produce intumescent coatings, which have been tested on a small scale fire testing device, designed and calibrated within this project. The factors influencing performance in three fire test regimes are the percentage loading of melamine phosphate, its chemical constitution, crystal size and shape, thermal degradation, and state of agglomeration . and dispersion in the coating, determined by the method of incorporation into the coating. When melamine phosphate is heat treated at 210ºC, a process designed to reduce its solubility, the performance of coatings produced with such material is profoundly affected, depending mainly on crystal size and shape alone. Consideration of heat transfer across the chars produced has allowed a quantitative evaluation of the thermal resistance of chars throughout a test. An optimum production route for melamine phosphate has been suggested, taking into account the requirements for weatherability of coatings as well as performance in a fire.

The relationship between fracture toughness and microstructure in titanium alloys

Linear Elastic Fracture Mechanics has been used to study the microstructural factors controlling the strength and toughness of two alpha-beta, titanium alloys. Fracture toughness was found to be independent of orientation for alloy Ti/6A1/4-V, but orientation dependent for IMI 700, bend and tension specimens giving similar toughness values. Increasing the solution temperature led to the usual inverse relationship between strength and toughness, with toughness becoming a minimum as the beta transus was approached. The production of a double heat treated microstructure led to a 100% increase in toughness in the high strength alloy and a 20% increase in alloy Ti/6A1/4V, with little decrease in strength. The double heat treated microstruoture was produced by cooling from the beta field into the alpha beta field, followed. by conventional solution treatment and ageing. Forging above the beta transus led to an increase in toughness over alpha beta forging in the high strength alloy, but had little effect on the toughness of Ti/6A1/4V. Light and electron microscopy showed that the increased toughness resulted from the alpha phase being changed from mainly continuous to a discontinuous platelet form in a transformed beta matrix. Void formation occurred at the alpha-beta interface and crack propagation was via the interface or across the platelet depending on which process required the least energy. Varying the solution treatment temperature produced a varying interplatelet spacing and platelet thickness. The finest interplatelet spacing was associated with the highest toughness, since a higher applied stress was required to give the necessary stress concentration to initiate void formation. The thickest alpha platelet size gave the highest toughness which could be interpreted in terms of Krafftt's "process zone size" and the critical crack tip displacement criterion by Hahn and Rosenfield from an analysis by Goodier and Field.

Control of soundness and mechanical properties in aluminium-silicon-magnesium alloys

Previously, specifications for mechanical properties of casting alloys were based on separately cast test bars. This practice provided consistently reproducible results; thus, any change in conditions was reflected in changes in the mechanical properties of the test coupons. These test specimens, however, did not necessarily reflect the actual mechanical properties of the castings they were supposed to represent'. Factors such as section thickness and casting configuration affect the solidification rate and soundness of the casting thereby raising or lowering its mechanical properties in comparison with separately cast test specimens. In the work now reported, casting shapes were developed to investigate the variations of section thickness, chemical analysis and heat treatment on the mechanical properties of a high strength Aluminium alloy under varying chilling conditions. In addition, an insight was sought into the behaviour of chills under more practical conditions. Finally, it was demonstrated that additional information could be derived from the radiographs which form an essential part of the quality control of premium quality castings. As a result of the work, it is now possible to select analysis and chilling conditions to optimize the as cast and the heat treated mechanical properties of Aluminum 7% Silicon 0.3% Magnesium alloy.

Tin oxide based composites derived using electrostatic spray deposition technique as anodes for lithium-ion batteries

Recent advances in the electric & hybrid electric vehicles and rapid developments in the electronic devices have increased the demand for high power and high energy density lithium ion batteries. Graphite (theoretical specific capacity: 372 mAh/g) used in commercial anodes cannot meet these demands. Amorphous SnO2 anodes (theoretical specific capacity: 781 mAh/g) have been proposed as alternative anode materials. But these materials have poor conductivity, undergo a large volume change during charging and discharging, large irreversible capacity loss leading to poor cycle performances. To solve the issues related to SnO2 anodes, we propose to synthesize porous SnO2 composites using electrostatic spray deposition technique. First, porous SnO2/CNT composites were fabricated and the effects of the deposition temperature (200, 250, 300 °C) & CNT content (10, 20, 30, 40 wt %) on the electrochemical performance of the anodes were studied. Compared to pure SnO2 and pure CNT, the composite materials as anodes showed better discharge capacity and cyclability. 30 wt% CNT content and 250 °C deposition temperature were found to be the optimal conditions with regard to energy capacity whereas the sample with 20% CNT deposited at 250 °C exhibited good capacity retention. This can be ascribed to the porous nature of the anodes and the improvement in the conductivity by the addition of CNT. Electrochemical impedance spectroscopy studies were carried out to study in detail the change in the surface film resistance with cycling. By fitting EIS data to an equivalent circuit model, the values of the circuit components, which represent surface film resistance, were obtained. The higher the CNT content in the composite, lower the change in surface film resistance at certain voltage upon cycling. The surface resistance increased with the depth of discharge and decreased slightly at fully lithiated state. Graphene was also added to improve the performance of pure SnO2 anodes. The composites heated at 280 °C showed better energy capacity and energy density. The specific capacities of as deposited and post heat-treated samples were 534 and 737 mAh/g after 70 cycles. At the 70th cycle, the energy density of the composites at 195 °C and 280 °C were 1240 and 1760 Wh/kg, respectively, which are much higher than the commercially used graphite electrodes (37.2–74.4 Wh/kg). Both SnO2/CNTand SnO2/grapheme based composites with improved energy densities and capacities than pure SnO2 can make a significant impact on the development of new batteries for electric vehicles and portable electronics applications.

In-Vivo Corrosion and Fretting of Modular TI-6AL-4V/CO-CR-MO Hip Prostheses: The Influence of Microstructure and Design Parameters

The purpose of this study was to evaluate the incidence of corrosion and fretting in 48 retrieved titanium-6aluminum-4vanadium and/or cobalt-chromium-molybdenum modular total hip prosthesis with respect to alloy material microstructure and design parameters. The results revealed vastly different performance results for the wide array of microstructures examined. Severe corrosion/fretting was seen in 100% of as-cast, 24% of low carbon wrought, 9% of high carbon wrought and 5% of solution heat treated cobalt-chrome. Severe corrosion/fretting was observed in 60% of Ti-6Al-4V components. Design features which allow for fluid entry and stagnation, amplification of contact pressure and/or increased micromotion were also shown to play a role. 75% of prosthesis with high femoral head-trunnion offset exhibited poor performance compared to 15% with a low offset. Large femoral heads (>32mm) did not exhibit poor corrosion or fretting. Implantation time was not sufficient to cause poor performance; 54% of prosthesis with greater than 10 years in-vivo demonstrated none or mild corrosion/fretting.

Estudo de duas pseudoligas de Nb-Cu obtidas pela rota da metalurgia do pó

The Nb-Cu pseudoalloys present themselves as potential substitutes for the alloys from a well known system and already commercially applied, as the W-Cu alloys, used in applications such as heat sinks, electrical contacts and coils for the generation of high magnetic fields. Because it is an immiscible system, where there is mutual insolubility and low wettability of the liquid Cu on the Nb surface, the processing route used in this work was the Powder Metallurgy. Two Nb alloys were used, with additions of 10% and 20% in weight of Cu, and times of 20, 30 and 40 hours for the high energy milling of the starting powders. The milling evolution of the powders is presented through the characterization techniques, such as the LASER diffraction for particle size, XRD, SEM, EDS, DSC, dilatometry, TEM and chemical analysis. After the milling, portions of the loads were submitted to the annealing heat treatment. The process used for the samples consolidation was the hot pressing, which has been applied both on some milled powders samples, as on the annealed powders. Subsequent heat treatments were performed in the samples at temperatures of 1000ºC (solid phase) and 1100ºC (in the Cu liquid phase). All sets of consolidated samples, and also the two sets of the heat treated, were analyzed by XRD, SEM, EDS, density and Vickers microhardness. Moreover, other Nb powder samples with 10% and 20% in weight of Cu obtained by simple mechanical mixing, were consolidated, thermally treated and characterized with the same techniques applied to the others, and the results were compared among themselves. Despite the difficulty of consolidation and densification of the two pseudoalloys of the Nb-Cu system of this study, on the route that passes through the HEM, samples were obtained with densities around 90% of the theoretical density. And, on the processing route of which were only mixed, the values reached up to 97%. Therefore, in this work are also emphasized the processes that made possible these results.

Síntese e caracterizações estrutural e magnética das ferritas de cobalto-manganês (Co1-xMnxFe2O4 E Co1,2Fe1,8-xMnxO4)

The cobalt-manganese ferrites (Co1¡xMnxFe2O4 and Co1,2Fe1,8¡xMnxO4) has a mixed structure of spinel type and it has been regarded as one of candidates for petitive wide variety of applications in devices from ultrasonic generation and detection, sensors, transformers, as well as in medical industry. Ferrites cobalt-manganese nanostructured were produced via mechanical alloying with subsequent heat treatment and were characterized by X-ray diffraction, X-ray fluorescence, scanning electron microscopy and magnetization. Samples of Co1¡xMnxFe2O4 and Co1,2Fe1,8¡xMnxO4 were obtained from the precursor powders Fe3O4, Co3O4 and Mn3O4 which were stoichiometrically mixed and ground by 10h and heat treated at 900°C for 2h. The diffraction confirmed the formation of the pure nanocrystalline phases to series Co1,2Fe1,8¡xMnxO4 with an average diameter of about 94nm. It was found that the lattice parameter increases with the substitution of Fe3Å by Mn3Å. The x-ray fluorescence revealed that the portions of metals in samples were close to the nominal stoichiometric compositions. The microstructural features observed in micrographs showed that the particles formed show very different morphology and particle size. The magnetic hysteresis measurements performed at low temperature showed that the saturation magnetization and remanence increased as the concentration of manganese, while the coercive field decreased. The anisotropy constant (Ke f ), was estimated from the data adjustments the law of approaching saturation. It was found that the anisotropy decreases substantially with the substitution of Fe by Mn.

Avaliação tribológica de novas modificações superficiais para cilindros de laminação a frio

The present work analyzed the tribological behavior of coatings/surface modifications traditionally used in cold rolling mill rolls and new coatings/surface modificationswith potential to replace the carcinogenic hard chrome. The study started with identification of wear mechanisms occurring in real coldrollingmill rolls. Due the high cost and dimensions of the rolls, thereplication technique was used. Replicas were obtained from 4 different rolling millBrazilian companies before and after a normal rolling campaign. Initial sliding tests were conducted using spherical and cylindrical counter bodies in order to verifywhichtribological conditions allowed to reproduce the wear mechanisms found in the replicas. These tests indicated the use of reciprocating sliding tests with cylindrical counter bodies (line contact), normal load of 100 N, and test times of and 1 h and 5 h. Different surface modifications were carried out on samples produced from a fragment of a rolling mill roll. The specimens were heat treated and ground on both sides. After, some specimens were surface textured by electrical discharge texturing (EDT). For both groups (ground and EDT), subsequent treatments of chromium plating, electroless NiP coating and plasma nitriding were carried out. The results of the reciprocating tests showed that specimens with electroless NiP coating presented the lowest friction coefficients, while plasma nitrided specimens showed the highest. In general, previous surface texturing before the coating/surface modification increased the wear of the counter bodies. Oneexceptionwas for EDT with subsequent electroless NiP coating, which presented the lowest counter bodies wear rate. The samples withelectroless NiP coating promoted a tribolayer consisting of Nickel, Phosphorus and Oxygen on both the specimens andthecounter bodies, which was apparently responsible for the reduction of friction coefficient and wear rate. The increase of the test time reduced the wear rate of the samples, apparently due the stability of the tribolayers formed, except for the nitrided samples. For the textured specimens, NiP coating showed the best performance in maintaining the surface topography of the specimens after the sliding tests.