Un procédé de création de surfaces poreuses sur deux aciers inoxydables respectivement austénitique et ferritique a été étudié. Ce procédé est en deux étapes. Une couche d’oxyde, dont l’épaisseur et la composition sont contrôlées, est d’abord générée par de la vapeur d’eau à 1100 °C. Puis cette couche est ensuite réduite par du dihydrogène à haute température. Ces travaux ont pour objectifs de mieux comprendre les mécanismes de formation des pores ainsi que l’influence des divers paramètres de réaction, tant pour l’oxydation que pour la réduction.Une caractérisation complète des couches d’oxydes a été d’abord réalisée. Le principal paramètre influent est la structure de l’alliage, austénitique ou ferritique. Par la suite, les surfaces poreuses ont été étudiées afin d’établir, étape par étape, les mécanismes mis en jeu.Il a été établi que la non-stœchiométrie des oxydes de fer préalablement formés sur l’acier austénitique est à l’origine de la formation des pores sur la surface de cet acier.Un nouveau type de porosité a pu être obtenu par la formation préalable de couches d’oxydes riches en chrome formées sur l’acier ferritique. Les morphologies ainsi que les mécanismes sont ici totalement différents. Un processus de formation des pores spécifique a été proposé. / A process for pore creation on the surface of two stainless steels, respectively austenitic and ferritic, has been investigated. That process follows two steps. An oxide scale with controlled thickness and composition is firstly generated by water vapour exposition at 1100 °C. That layer is subsequently reduced at high temperature by dihydrogen. The present work aims to better understand the mechanisms of pore formation and the influence of various reaction parameters on both oxidation and reduction course.A comprehensive characterisation of the oxide layers has been first performed. The main parameter is the alloy structure, austenitic or ferritic. The porous surfaces have been thereafter studied to establish step by step the formation mechanisms involved.It was demonstrated that the non-stoichiometry of iron oxides formerly developed on the austenitic steel is behind the pore growth mechanism on that steel.A new kind of porosity could be obtained through the preliminary building of chromium-rich oxide scales on the ferritic steel. The morphologies are in that case completely different as well as the pore formation mechanism. A specific process for the pore growth has been proposed.

Alkali silica reaction (ASR) is detrimental to concrete. It is a time-dependent phenomenon, which can lead to strength loss, cracking, volume expansion, and premature failure of concrete structures. In essence, it is a particular chemical reaction involving alkali hydroxides and reactive form of silica present within the concrete mix. Geopolymer is a type of alkaline activated binder synthesized through polycondensation reaction of geopolymeric precursor and alkali polysilicates. In this thesis, three types of reactive aggregates with different chemical compositions were used. Systematic laboratory experiments and microstructural analysis were carried out for the geopolymer concrete and the OPC concrete made with the same aggregates. The result suggests that the extent of ASR reaction due to the presence of three reactive aggregates in geopolymer concrete is substantially lower than that in OPC based concrete, which is explained by the pore solution change and verified through their microstructural variations and FTIR images.

Process dependent microstructural effects in ferroelectric SrBi2Ta2O9 (SBT) thin films were characterized and distinguished from material dependent optical properties using a systematic multi-layer modeling technique. Variable angle spectroscopic ellipsometry (VASE) models were developed by sequentially testing Bruggeman effective-media approximation (EMA) layers designed to simulate microstructural effects such as surface roughness, porosity, secondary phases, and substrate interaction. Cross-sectional analysis by atomic force microscopy (AFM), transmission and scanning electron microscopy (TEM) and (SEM) guided and confirmed the structure of multi-layer models for films produced by pulsed laser deposition (PLD), metal-organic chemical vapor decomposition (MOCVD), and metal-organic deposition (MOD). VASE was used to estimated the volume percentage of second phase Bi2O3 in SBT thin films made by MOD. Since Bi₂O₃ was 10 orders of magnitude more conductive than SBT, second phase Bi₂O₃ produced elevated leakage currents. Equivalent circuits and percolation theory were applied to predict leakage current based on Bi₂O₃ content and connectivity. The complex role of excess Bi2O3 in the crystallization of SBT was reviewed from a processing perspective. VASE helped clarify the nature of the interaction between SBT films and Si substrates. When SBT was deposited by MOD and annealed on Si substrates, the measured capacitance was reduced from that of SBT on Pt due mainly to the formation of amorphous SiO₂ near the SBT/Si interface. VASE showed that the thickness and roughness of the SiO₂ reaction layer increased with annealing temperature, in agreement with TEM measurements. Unlike PZT, SBT crystallization was not controlled by substrate interaction. / Master of Science

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Yes / Sedimentation processes are fundamental to solids/liquid separation in water and wastewater treatment, and therefore a robust understanding of the settlement characteristics of mass fractal aggregates (flocs) formed in the flocculation stage is fundamental to optimized settlement tank design and operation. However, the use of settling as a technique to determine aggregates’ traits is limited by current understanding of permeability. In this paper, we combine experimental and numerical approaches to assess settling velocities of fractal aggregates. Using a non-intrusive in situ digital image-based method, three- and two-dimensional fractal dimensions were calculated for kaolin-based flocs. By considering shape and fractal dimension, the porosity, density and settling velocities of the flocs were calculated individually, and settling velocities compared with those of spheres of the same density using Stokes’ law. Shape analysis shows that the settling velocities for fractal aggregates may be greater or less than those for perfect spheres. For example, fractal aggregates with floc fractal dimension, Df ¼ 2.61, floc size, df > 320 μm and dp ¼ 7.5 μm settle with lower velocities than those predicted by Stokes’ law; whilst, for Df ¼ 2.33, all aggregates of df > 70 μm and dp ¼ 7.5 μm settled below the velocity calculated by Stokes’ law for spheres. Conversely, fractal settling velocities were higher than spheres for all the range of sizes, when Df of 2.83 was simulated. The ratio of fractal aggregate to sphere settling velocity (the former being obtained from fractal porosity and density considerations), varied from 0.16 to 4.11 for aggregates in the range of 10 and 1,000 μm, primary particle size of 7.5 μm and a three-dimensional fractal dimension between 2.33 and 2.83. However, the ratio decreases to the range of 0.04–2.92 when primary particle size changes to 1.0 μm for the same fractal dimensions. Using the floc analysis technique developed here, the results demonstrate the difference in settlement behaviour between the approach developed here and the traditional Stokes’ law approach using solid spheres. The technique and results demonstrate the improvements in understanding, and hence value to be derived, from an analysis based on fractal, rather than Euclidean, geometry when considering flocculation and subsequent clarification performance / Rodrigo B. Moruzzi is grateful to São Paulo Research Foundation (Fundação de Amparo à Pesquisa do Estado de São Paulo – FAPESP) Grant 2017/19195-7 for financial support and to CNPq for the fellowship Grant 301210/2018-7.

Silicon oxycarbide (SiOC) is a ceramic obtained through the heating of a polymer precursor, which undergoes partial decomposition to go from an organic polymer to an inorganic ceramic. The microstructure of SiOC is not uniform at the nanometer scale, and contains nanometer sized silicon dioxide, carbon, and silicon carbide. Porous SiOC has shown great promise in applications such as lithium ion batteries, gas separation, and thermal barriers. The microstructure, and thus the properties of the SiOC, is influenced by the initial polymer and the processing conditions. In this thesis, SiOC is fabricated using a base polysiloxane polymer using different gases during heating, different additives that change the initial polymer chemical composition or polymer shape, and polymers with different reactive groups. Porosity was introduced into the SiOC ceramics through either etching the SiOC with hydrofluoric acid, which removes the silicon dioxide and produces pores with diameters less than 20 nanometers, or through decomposition during heating of a certain polymer in a two polymer mixture, producing pores that are dozens of microns in diameter. The effects of the processing parameters on the porosity and pore size are used to understand the differences in the microstructure during pyrolysis. / Master of Science / Silicon oxycarbide (SiOC) is a ceramic obtained through the heating of a polymer precursor, which undergoes partial decomposition to go from an organic polymer to an inorganic ceramic. The microstructure of SiOC is not uniform at the nanometer scale, and contains nanometer sized silicon dioxide, carbon, and silicon carbide. Porous SiOC has shown great promise in applications such as lithium ion batteries, gas separation, and thermal barriers. The microstructure, and thus the properties of the SiOC, is influenced by the initial polymer and the processing conditions. In this thesis, SiOC is fabricated using a base polysiloxane polymer using different gases during heating, different additives that change the initial polymer chemical composition or polymer shape, and polymers with different reactive groups. Porosity was introduced into the SiOC ceramics through either etching the SiOC with hydrofluoric acid, which removes the silicon dioxide and produces pores with diameters less than 20 nanometers, or through decomposition during heating of a certain polymer in a two polymer mixture, producing pores that are dozens of microns in diameter. The effects of the processing parameters on the porosity and pore size are used to understand the differences in the microstructure during pyrolysis.