Resumo em Inglês:

Structure is at the heart of the materials science paradigm connecting processing with properties. In the hierarchy of structures that exist in materials microstructure offers the richest variety of structural arrangements. This variety is often conveniently accessible, e.g., simply by heat treatment or mechanical deformation. Exploration of the relation between properties and microstructure serves to establish a target range of microstructural states that will perform. In order to attain a target microstructure it is necessary to understand what microstructures are, and how they evolve in processing. This presentation focuses upon the set of tools that must be combined to achieve this control: 1. Geometry 2 Thermodynamics 3. Kinematics 4. Kinetics. The content of these tools is reviewed briefly and their uses illustrated in developing an understanding of how microstructures evolve. In this development an attempt is made to carry the description of each microstructural process as far as possible without making simplifying assumptions. The study of microstructures with this rigorous point of view was termed by F.N. Rhines, "microstructology".

Resumo em Inglês:

A systematic methodology for microstructural description can help the task of obtaining the processing x microstructure x properties x performance relationships. There are, however, some difficulties in performing this task, which are related mainly to the following three factors: the complexity of the interactions between microstructural features; difficulties in evaluating geometric parameters of microstructural features; and difficulties in relating these geometric parameters to process variables. To solve some of these problems, it is proposed a methodology that embodies the following features: takes into account the different possible types of approaches for the microstructural description problem; includes concepts and tools of Total Quality Management; is supported on techniques of system analysis; and makes use of computer modeling and simulation and statistical design of experiments tools. The methodology was applied on evaluating some topological parameters during sintering process and its results were compared with available experimental data.

Resumo em Inglês:

Understanding and predicting grain growth in Metallurgy is meaningful. Monte Carlo methods have been used in computer simulations in many different fields of knowledge. Grain growth simulation using this method is especially attractive as the statistical behavior of the atoms is properly reproduced; microstructural evolution depends only on the real topology of the grains and not on any kind of geometric simplification. Computer simulation has the advantage of allowing the user to visualize graphically the procedures, even dynamically and in three dimensions. Single-phase alloy grain growth simulation was carried out by calculating the free energy of each atom in the lattice (with its present crystallographic orientation) and comparing this value to another one calculated with a different random orientation. When the resulting free energy is lower or equal to the initial value, the new orientation replaces the former. The measure of time is the Monte Carlo Step (MCS), which involves a series of trials throughout the lattice. A very close relationship between experimental and theoretical values for the grain growth exponent (n) was observed.

Resumo em Inglês:

This approach assumes three functions independently acting on a set of microparticles. The first one, w1, concerns re-distribution of mass to decrease the surface energy. The second one, w2, concerns re-distribution of mass to increase the entropy of the microparticle set. The third one, w3, is a further re-distribution of mass that vanishes a microparticle. Once vanished, its mass is distributed among its neighbors. w1 and w3 release energy, whereas w2 absorbs energy. Part of the energy released should be available to sustain w2. The action frequency of w1, w2, and w3, the amount of mass exchanged in each iteraction, the fraction of released energy available to sustain w2, and the size of a vanishing microparticle can be varied. As the dynamical system formed by w1, w2, and w3 act on an initial microparticle set, it is observed an evolution resembling the Ostwald ripening concerning steady-state size distribution and microparticle growth.

Resumo em Inglês:

This work addresses a new image segmentation method founded on Digital Topology and Mathematical Morphology grounds. The ABA (attribute based absorptions) transform can be viewed as a region-growing method by flooding simulation working at the scale of the main structures of the image. In this method, the gray level image is treated as a relief flooded from all its local minima, which are progressively detected and merged as the flooding takes place. Each local minimum is exclusively associated to one catchment basin (CB). The CBs merging process is guided by their geometric parameters as depth, area and/or volume. This solution enables the direct segmentation of the original image without the need of a preprocessing step or the explicit marker extraction step, often required by other flooding simulation methods. Some examples of image segmentation, employing the ABA transform, are illustrated for uranium oxide samples. It is shown that the ABA transform presents very good segmentation results even in presence of noisy images. Moreover, it's use is often easier and faster when compared to similar image segmentation methods.

Resumo em Inglês:

Many chemical and physical methods, such as plasma, e-beam, sputtering, CVD and others, have been used to modify the structure of polymer surfaces by depositing thin inorganic films. Most of these techniques are based upon the use of high energy sources that ultimately can damage either chemically or physically polymer surfaces. Moreover, these methods are usually not versatile enough to allow the design of structurally and chemically tailored surfaces through the control of the distribution of chemical functionalities throughout the surface. In this work, inorganic species were introduced onto polymer substrates in a controlled manner by performing a sequence of chemical reactions at the surface. Sulfonation followed by silanization reactions were used to graft alkoxysilane species at the surface of poly(aryl sulfones). The heterogeneous chemical modification of poly(aryl sulfones) was monitored by FTIR-ATR (Attenuated Total Reflection - FTIR). Model compounds were used to study the chemical reactions occurring during the grafting procedure. The results showed that the developed procedure can allow a controlled introduction of inorganic species onto polymer surfaces. Furthermore, in order to prove that this procedure enables the deposition of specific chemical functionalities onto polymer surfaces that can be used to create chemically and structurally tailored surfaces, silicate films were deposited on previously silanated PAS bioactive glass composites. In vitro tests showed that the surface modified composite can enhance the rates of hydroxy-carbonate-apatite precipitation.

Resumo em Inglês:

The oxide layer formed on the surface of a grain oriented silicon steel was characterized by SEM and EDS. 3% Si steel substrates were coated by two types of slurries: one formed by MgO and water and other formed by MgO, water and SrSO4. The ceramic films were evaluated by SEM, EDS and X-ray diffraction. Depth profiles of Fe, Si and Mg were obtained by GDS. The magnetic core losses (at 1.7 Tesla, 60 Hz) of the coated steel samples were evaluated as well. The use of MgO containing strontium reduced the volume fraction of forsterite particles beneath the outermost ceramic layer. It was observed a reduced magnetic core loss with the use of the slurry with MgO containing strontium.

Resumo em Inglês:

Basic regularities of silicon nitride based materials microstructure formation and development in interrelation with processing conditions, type of sintering additives, and starting powders properties are discussed. Models of abnormal or exaggerated grain growth are critically reassessed. Results of several model experiments conducted in order to determine the most important factors directing the microstructure formation processes in RE-fluxed Si3N4 ceramics are reviewed. Existing data on the mechanisms governing the microstructure development of Si3N4-based ceramics are analyzed and several principles of microstructure tailoring are formulated.

Resumo em Inglês:

The aim of this work was to study the hydrothermal synthesis of Si and SiAlMCM-41 performed under both autogenic pressure and refluxing conditions. XRD data showed that the MCM-41 phase may be formed by both processes and that the synthesized material in the presence of Al and/or under reflux presents the hexagonally arrangement of less ordered mesopores. However, as verified by XRD and physisorption data, the order was improved with higher synthesis times. 29Si and 1H - 29Si C/P MAS NMR spectra showed that a great part of the Si atoms exists as silanol groups which originate resonance peaks at -110, -100 and -91 ppm. The presence of Al atoms may generate Si(3Si, Al) and Si(2Si, 2Al) environments which might be contributing to resonance peaks at -100 and -91 ppm. The 27Al MAS NMR spectrum of the as synthesized AlSiMCM-41 showed a resonance peak of tetrahedral framework aluminum close to 53 ppm and two others, one close to 14 ppm attributed to Al(H2O)6+3 species and the other a weak signal close to 32 ppm attributed to pentacoordinated Al. 27Al MAS NMR spectra of the calcined sample showed a peak at 0 ppm corresponding to an hexacoordinated extra-framework aluminum formed during calcination.

Resumo em Inglês:

The oxidation of NiAl, NiAl-20at.%Fe and NiAl-30at.%Fe at 1000-1100 °C in air has been studied. Pure NiAl shows excellent oxidation resistance due to the formation of an Al2O3 layer. NiAl-20Fe also shows good oxidation resistance due to the formation of an Al2O3 scale on a <FONT FACE="Symbol">b</font>-phase substrate. Moreover, some nodules consisting of mixed oxides of Fe and Ni grow over the ductile <FONT FACE="Symbol">g</font>-phase surface incorporated to the <FONT FACE="Symbol">b</font>-phase substrate. NiAl-30Fe alloy undergoes a much faster oxidation due to the formation of a non-protective Fe and Ni-rich scale, which is extremely susceptible to spallation. The addition of Fe to NiAl is detrimental to its oxidation resistance.

Resumo em Inglês:

Microstructural aspects of polycrystalline copper sheets subjected to complex strain paths were analysed in this work. Dislocation structures developed during the strain paths (rolling and tension) and the evolution of this microstructure during reloading have been studied. The active slip systems developed in each strain path were used to explain the microstructural evolution. The heterogeneous surface deformation observed on polished tensile specimens prestrained in rolling was also analysed. The structural aspects are related with the mechanical behaviour of the material, namely with the increase in yield stress in reloading, the work hardening evolution and the premature occurrence of plastic instability for some prestrain values.

Resumo em Inglês:

Two cast <FONT FACE="Symbol">g</font> titanium aluminides, Ti47Al and Ti50Al (at. %), were obtained by arc melting under an argon atmosphere. The as-cast microstructure modification by annealing under protective argon atmosphere and hot isostatic pressing (HIP) was analysed by optical and scanning electron microscopy with microanalysis facilities. As-cast structures presented strong microsegregations, being composed of primary <FONT FACE="Symbol">a</font> dendrites (with low Al content), which transformed into lamellar <FONT FACE="Symbol">a</font>2 + <FONT FACE="Symbol">g</font> during solid state cooling, and interdendritic <FONT FACE="Symbol">g</font> phase (with high Al content). Annealing in the <FONT FACE="Symbol">a</font> + <FONT FACE="Symbol">g</font> field was responsible for a partial reduction of microsegregation, a decreasing of the volume fraction of <FONT FACE="Symbol">a</font>2 + <FONT FACE="Symbol">g</font> lamellar dendrites, an increasing of <FONT FACE="Symbol">g</font> volume fraction and the occurrence of a small fraction of <FONT FACE="Symbol">a</font>2 particles and plates in some of the <FONT FACE="Symbol">g</font> grains. Annealing in the <FONT FACE="Symbol">a</font> field at 1400 °C for one hour was insufficient to eliminate the as-cast microstructure in the Ti50Al alloy and produced large equiaxed lamellar colonies of <FONT FACE="Symbol">a</font>2+<FONT FACE="Symbol">g</font> in the Ti47Al alloy. HIP in the <FONT FACE="Symbol">a</font> + <FONT FACE="Symbol">g</font> field (to avoid excessive grain growth) was responsible for microstructure modifications similar to the ones obtained by heat treating under similar soaking conditions.

Resumo em Inglês:

Received: August 15, 1998; Revised: March 30, 1999 In this work it was evaluated the influence of the type and concentration of the acid catalyst and the gelation temperature on the structure of pores of silica gels obtained with TEOS. The values of the major structural parameters related to the pores such as volume fraction of pore (VV), surface area per unit volume (SV), the average pore size (rP) and the connectivity of pores (GV), obtained in this work showed the possibility of tailoring the pore structure of silica gels by changing the processing variables. For the HF catalyzed gels the values of average pore radius varied from 6.8 nm to 21.6 nm, and the gelation time varied from 2 min to 140 min. For the gels obtained in this work the activation energy for gelation (E) was found to be 11.8 kcal/mole.

Resumo em Inglês:

The methodology of obtention of nanostructured glass, incorporated with lead compounds, obtained by sol-gel processing and the structural characterizations are presented. Specific surface area, porosity, pore morphology and pore size distributions of the gels were characterized with nitrogen sorption (BET) and mercury pycnometry. The pore size distributions of gels obtained using lead nitrate were more narrow than the ones observed for gels made with lead acetate. It was also observed the presence of a bimodal pore size distribution for gels produced using lead nitrate.

Resumo em Inglês:

The Al2O3-ZrO2 composite was studied by impedance spectroscopy, a non destructive technique that was found to be sensitive to the composite's microstructure. The observed decrease in the zirconia grain and grain boundary conductivities points to compression on zirconia grain by alumina matrix. This effect increased with decreased concentration of zirconia in the composite. Measurements were taken of composites above the percolation threshold for vacancy conduction along the zirconia grains. The effect of densification and grain growth on the composite's conduction was measured. The changes in the zirconia grain and grain boundary specific conductivities were found to be correlated.

Resumo em Inglês:

Yttria stabilized zirconia sintered by silicate glass phase with addition of ytterbium or gadolinium was investigated. The grain growth rate was found to be higher in the gadolinium doped samples. The glass phase of the ytterbium doped sample showed partial crystallization on cooling. Grain conductivity was found to be highest in samples with a half hour of sintering, having decreased due to stabilizing ion migration from the grains to the grain boundary glassy phase. The differences observed in the sintering behavior and properties of the sintered bodies are attributed to the different ionic radii of the Yb+3 and Gd+3 ions.

Resumo em Inglês:

Bifunctional catalysts formed by metals (nickel, platinum as well as an association of both metals) supported in an ultrastabilized Y molecular sieve (USY) were prepared. The samples were obtained using a competitive ion exchange method, followed by calcination and reduction of the transition metal under hydrogen atmosphere. The solids were characterized by ICP for their chemical composition, XRD, TEM and the catalytic properties evaluated in the isomerization of the n-hexane. It was observed that the NH4USY zeolite support presented better ion exchange efficiency in comparison to that of the zeolite HUSY, due to the decrease of the pH of the reaction medium in the last case. The TEM images of the catalysts, obtained in both bright and dark field, showed that the metal particles are dispersed almost inside the molecular sieve micro and mesopores. TEM images of the catalysts obtained in bright field showed that the average particle diameter were 64 and 108 Å for platinum and nickel respectively and about 70 Å when both metals are present. In the mentioned reaction, platinum catalysts were more active and stable than the nickel ones and the catalysts formed by both metals (0,2%Pt + 0,8% Ni/HUSY) presented the highest activity.

Resumo em Inglês:

Sintered nickel powder is proposed to be used as porous wicks in heat pipes and capillary pumps. In this work the manufacturing procedure for tubular wicks for capillary pump application is discussed. The porosity, mechanical resistance and roundness of tubular wicks made of carbonila powder, atomized powder and a powder mixture of both are analyzed. A powder mixture was selected as the best raw material. In this case, pore size in the range of 2 to 24 <FONT FACE="Symbol">m</font>m and porosity about 50% were measured. First tests carried out in the laboratory, using acetone as the working fluid, show capillary pumping pressures up to 4 kPa and heat fluxes of about 1 W/cm2 in a two-phase heat transfer loop.

Resumo em Inglês:

Off-stoichiometric NiZn ferrite was obtained by hydrothermal process and compacted in torus form under different pressures. Two samples A1 and A2 - cobalt doped (0.5 %) were sintered at 1573 K in air atmosphere during 3 h. The magnetic properties were studied by vibrating sample magnetometry, Mössbauer spectroscopy and complex impedanciometry. X-ray diffraction and Hg porosimetry were used in order to determine the average grain size and the type of packing in the samples. Both samples exhibited superparamagnetic behavior in the hysteresis loop. This effect does not agree with Mössbauer results, which were fitted using Normos, a commercial computer program. All samples parameters were compared.

Resumo em Inglês:

Granular systems composed by nanostructured magnetic materials embedded in a non-magnetic matrix present unique physical properties that depend crucially on their nanostructure. In this work, we have studied the structural and magnetic properties of NiZn-ferrite nanoparticles embedded in SiO2, a granular system synthesized by sol-gel processing. Samples with ferrite volumetric fraction x ranging from 6% to 78% were prepared, and characterized by X-ray diffraction, Mössbauer spectroscopy and vibrating sample magnetometry. Our results show the formation of pure stoichiometric NiZn-ferrite in the SiO2 matrix for x < 34%. Above these fraction, our samples presented also small amounts of Fe2O3. Mössbauer spectroscopy revealed the superparamagnetic behaviour of the ferrimagnetic NiZn-ferrite nanoparticles. The combination of different ferrite concentration and heat treatments allowed the obtaintion of samples with saturation magnetization between 1.3 and 68 emu/g and coercivity ranging from 0 to 123 Oe, value which is two orders of magnitude higher than the coercivity of bulk NiZn-ferrite.