RésuméLes matériaux polymères biporeux interviennent dans diverses applications en tant que scaffolds pour l’ingénierie tissulaire, ou matériaux modèles mimant des milieux poreux tels que les roches et les sols. Le rôle de chaque niveau de porosité sur les propriétés de transport de ces milieux demeure une question fondamentale. Dans ce contexte, une démarche alliant conception et caractérisation physico-chimique de matériaux polymères à deux niveaux de porosité a été développée. Des réseaux biporeux modèles à base de poly(méthacrylate de 2-hydroxyethyle) ont ainsi été conçus en utilisant deux types de gabarits comme porogènes : des particules de NaCl (frittées ou non ) générant le premier niveau de porosité et un solvant générant le deuxième. Une caractérisation structurale et morphologique a été réalisée par microscopie électronique à balayage (MEB) et porosimétrie à intrusion de mercure (MIP) afin d’étudier l’influence des agents porogènes sur la structure. Un réseau présentant deux distributions de tailles de pores comprises entre 10 nm et 10 µm et de 100 µm de diamètre ont été observées. Pour décrire plus finement la microstructure, notamment la forme réelle des pores et l’interconnectivité des réseaux, des analyses par microtomographie à rayons X et au synchrotron ont été réalisées. L’optimisation des paramètres expérimentaux (taille du voxel, énergies mises en jeu) a permis d’obtenir des images de haute résolution. Certaines coupes ont été sélectionnées pour la simulation de l’écoulement d’un fluide dans un milieu biporeux bi- ou tridimensionnel. Les milieux poreux étudiés comportant au moins trois échelles, à savoir les échelles caractéristiques de deux niveaux de porosité et l’échelle macroscopique, une démarche par double changement d’échelle a été élaborée. Les approches envisagées reposent sur la transformée de Fourier rapide (FFT). L’utilisation de l’équation de Brinkman a permis de combiner les équations de Stokes et Darcy et de déterminer une perméabilité macroscopique / Biporous polymeric materials are widely used in many applications as scaffolds for tissue engineering or model materials that mimick porous medium as rocks and soils. The role of each porosity level on the transport properties of such porous frameworks is crucial. This study highlights the design and thorough physico-chemical characterization of model polymeric materials exhibiting two levels of porosity. Model biporous poly (2-hydroxyethyl methacrylate) (PHEMA) materials were prepared by using two macroporogenic agent, i.e. NaCl particles, and a solvent. To this purpose, sieved NaCl particles of different size ranges were used, either sintered or non-fused, in conjunction with a porogenic solvent. The resulting biporous polymeric materials were finely characterized in terms of porosity by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). A porous network with pore sizes ranging from 10 nm to 10 µm was obtained, while larger pores had an average diameter of about 100 µm. A careful X-ray computed and synchrotron microtomography analysis of the 3-D microstructure and pores interconnectivity of porous materials were also performed. The optimization of experimental parameters (voxel size, energies involved) allowed to obtain high-resolution images. Some slices were then selected to compute a fluid flow through biporous 2-D and 3-D porous media. Such networks having at least three scales, namely the characteristic scales of the two porosity levels and the macroscopic scale, a double upscaling approach has been developed. The methodology was based on the Fast Fourier Transform (FFT). The use of the Brinkman equation combined the Stokes and Darcy equations and allowed the computation of a macroscopic permeability

Une large hystérèse centrée autour de la température ambiante constitue l'un des objectifs principaux de la recherche sur les matériaux commutables fonctionnels. Dans le domaine très étudié de la conversion de spin, un tel comportement apparaît très rarement. Un nouveau composé, le complexe [Fe(PM-PeA)2(NCSe)2] présentant une large hystérèse autour de la température ambiante a été synthétisé, sous forme de monocristal et de poudre. Ce composé a été la base de deux axes de recherche. Le premier concerne l’étude multi-échelles des relations structure-propriétés en combinant la diffraction des rayons X sur poudre et sur monocristal, à température variable. Un focus tout particulier a été fait sur l’échelle microstructurale, très peu explorée à ce jour. Pour la première fois,la taille des domaines cohérents et le taux de microdéformations, ont été quantifiés pour un composé moléculaire discret à conversion de spin. Le deuxième axe concerne l’investigation de la transition de spin induite par la pression. L’étude in situ par diffraction des rayons X sur monocristal a permis une caractérisation complète de la structure cristalline des deux états de spin, sous pression. Par la suite, un suivi fin de la transition de spin, révélant une piezo-hystérèse, a été fait grâce à la diffraction des rayons X sur poudre, sous pression in situ à l’aide du rayonnement synchrotron. Les expériences couplant pression et température ont donné accès à des informations cruciales, telles que la variation des modules d’élasticité avec la température, les effets de la température sur la pression de transition, le caractère coopératif de la transition et la largeur de la piezo-hystérèse. / Spin crossover (SCO) compounds with a large hysteresis centered around room temperature (RT) are being constantly pursued although such behavior is very rare and most often noticed in coordination networks. In this context, a new molecular discrete compound, the complex [Fe(PM-PeA)2(NCSe)2], showing a large SCO hysteresis spanning RT has been synthesized in both singlecrystal and powder forms. Then, two research lines emerged. The first one concerns the multi-scales study of the structure-properties relationships, combining single-crystal and powder X-ray diffraction at variable temperature. A particular focus has been made on the microstructural scale, almostunexplored until now. For the first time, the coherent domain sizes and the micro-deformation rate has been quantified for a molecular discrete SCO compound. The second part of this work investigates in detail the pressure-induced SCO. The in situ single-crystal X-ray diffraction studies has been carried out to perform a complete characterization of the crystal structure under pressure. Thereafter, an accurate high-pressure X-ray diffraction measurement on the powder, with a synchrotron radiation,provides a fine track of the pressure-induced SCO and showed a piezo-hysteresis. Experiments coupling pressure and temperature brought crucial information pertaining to variation of bulk moduli withtemperature, the piezo-hysteresis width, the temperature dependence of the pressure transition and pressure-induced SCO abruptness, are provided in detail for the first time.

La congélation des produits alimentaires est un procédé qui permet d'augmenter leur durée de conservation. Ceci est possible grâce à l'abaissement de la température mais surtout à la solidification de l'eau réduisant la disponibilité de l'eau nécessaire au développement des micro-organismes. Cependant, si les produits ne sont pas emballés ou si l'emballage n'est pas adhérent à leur surface pendant la congélation, un transfert d'eau a également lieu simultanément à la libération de chaleur. Ce transfert d'eau se traduit par la déshydratation des produits qui implique une perte de masse. Celle-ci a un impact direct sur la masse de produit congelé qui pourra être vendue et donc un coût économique pour les industriels.La cristallisation de l'eau est également un paramètre clé de la congélation. En effet, les caractéristiques des cristaux de glace formés pendant la congélation (nombre, taille et forme) peuvent provoquer des modifications de structure des aliments et des dommages irréversibles. Ceux-ci se traduisent par la modification des propriétés organoleptiques et nutritionnelles des produits une fois décongelés et une baisse de leur qualité.Que ce soit pour la déshydratation ou la cristallisation, les conditions opératoires de congélation (température, vitesse d'écoulement d'air et hygrométrie) ainsi que certaines caractéristiques des produits (température initiale, surface, épaisseur) ont une influence importante. L’objectif de la thèse est d’étudier et savoir estimer la perte en eau et les caractéristiques des cristaux formés dans un aliment non poreux en fonction des conditions opératoires afin de choisir une technologie de congélation adaptée.Le travail a été axé sur l'étude des transferts de chaleur et de matière (eau) à la surface et à l'intérieur du produit lors de la congélation à deux niveaux d'échelle : macroscopique pour la déshydratation et microscopique pour la cristallisation. Cette approche multi-échelles s’est appuyée à la fois sur l’expérimentation et sur la modélisation.Pour ce qui est de la déshydratation, un dispositif expérimental a été conçu à l’échelle du produit pour mesurer la variation de la masse et de la température de l’aliment au cours de sa congélation (conditions mécaniques ou cryogéniques).Parallèlement, un modèle (1D) de prédiction de la déshydratation et de la durée de congélation en fonction du produit (homogène et non poreux) et des conditions opératoires de congélation a été développé. Cette étude a été réalisée pour un produit modèle, le gel de méthyl-cellulose (Tylose®) et pour de la viande hachée de bœuf (5 % de matière grasse).Pour l’étude de la cristallisation, un modèle à l’échelle des cristaux (2D) a pour vocation de représenter les phénomènes liés au changement de phase de l’eau (apparition et croissance des cristaux) en fonction des conditions opératoires de congélation. Il simule la cristallisation de l’eau dans des solutions notamment en prenant en compte la diffusion des molécules de solutés dans la solution cryo-concentrée ainsi que la libération et la dissipation de la chaleur latente de solidification de l'eau.Des observations de la cristallisation au sein d'échantillons de viande hachée de bœuf ont été faites par micro-tomographie à rayon X et cryo-microscopie électronique à balayage sur des échantillons congelés mais aussi par microscopie électronique à balayage et microscopie optique sur des échantillons lyophilisés (après congélation en conditions mécaniques ou cryogéniques). / Freezing extends food shelf life by lowering the temperature and mainly thanks to water solidification which decreases water availability for micro-organism growth. Nevertheless, if the food is not packaged or if the packaging is not adherent to its surface, a water transfer occurs simultaneously with the heat transfer during freezing. This water transfer leads to product dehydration which means weight loss. Food dehydration during freezing has consequences on the product weight and so has an economic cost for industrials.Water crystallization is also an important parameter of the freezing process. Indeed, ice crystal characteristics (number, size and shape) may induce food structure changes and irreversible damages. These modifications can alter organoleptic properties and food quality after thawing.Freezing operating conditions (temperature, flow velocity, hygrometry) and some food characteristics (initial temperature, surface, thickness) have an influence on both dehydration and water crystallization.The objective of this thesis is to study and to be able to estimate the water loss and crystal characteristics for non-porous food according to the freezing operating conditions in order to select the best freezing technology.The work was focused on the multi-scale study of heat and mass (water) transfers during freezing, at the product surface and throughout the product : on a macroscopic point of view for dehydration and on a microscopic point of view for crystallization. Each study is built on an experimental and a modelling work.For dehydration, an experimental device was developed to measure weight loss and product temperature kinetics during freezing (mechanical and cryogenic freezing conditions).Moreover, a predictive model (1D) was established for estimation of dehydration and freezing time according to the product and the freezing conditions. Dehydration was studied with a model material (methylcellulose gel-Tylose®) and with minced beef (5 % fat).For crystallization, a model at crystal scale (2D) aims to simulate phenomena related to the water phase change (nucleation and crystal growth) according to the freezing operating conditions. It takes into account water crystallization in solution considering the diffusion of the solute in the cryo-concentrated solution, the release and dissipation of the latent heat of water solidification.Crystallization observations were done in frozen samples of minced beef by X-ray micro-tomography and by cryo-scanning electron microscopy. Some visualizations were also carried out on freeze-dried samples by scanning electron microscopy and optical microscopy (after mechanical or cryogenic freezing).

En réponse aux exigences de la réglementation REACH qui prévoit que les substances à base de chrome hexavalent devront être soumises à autorisation dès 2017, les industriels du secteur aéronautique mènent depuis plusieurs années des activités de recherche et développement de nouveaux procédés de traitement de surface moins néfastes pour l’environnement et la santé. Le travail prévu dans le programme de recherche NEPAL (NouvellEs Protections des ALuminiums) s’inscrit dans cette dynamique. Les traitements mettant en oeuvre des composés à base de Cr(VI) sont destinés à disparaître dans un proche avenir ; ainsi de nouvelles formulations ont été développées parmi lesquelles des procédés de conversion à base de chrome trivalent. Le projet de thèse vise à renforcer la robustesse de ces procédés en apportant des éléments de compréhension pour ce qui concerne la tenue à la corrosion des alliages d’aluminium revêtus de couches de conversion au chrome trivalent en relation avec leur état microstructural. Les travaux de thèse ont été développés sur une nuance d’alliage d’aluminium 2024, parmi celles les plus utilisées dans le secteur aéronautique. Plusieurs microstructures ont été considérées dans le but de modifier la distribution en éléments d’alliages majoritaires, le cuivre et le magnésium. La caractérisation fine de la microstructure et l’évaluation de la réactivité des échantillons dans les différents bains de traitement de conversion ont permis de mettre en évidence l’influence de la répartition du cuivre dans l’alliage, élément néfaste pour la croissance des couches de conversion lorsqu’il se trouve sous forme de précipités fins. Finalement, l’étude de deux tôles d’alliage d’aluminium d’épaisseurs différentes met en évidence l’influence de nouveaux paramètres microstructuraux tels que les joints de grains ou la précipitation grossière. Ce travail a été réalisé dans le cadre du projet FUI NEPAL. Le CIRIMAT a été financièrement soutenu par le Ministère de l’Économie et de l’Industrie français (BPI-France), la Région Occitanie Pyrénées-Méditerranée et l’Union Européenne (FEDER/ERDF). / In reply to REACH regulation, which stipulates that hexavalent chromium substance shall besubject to authorization in 2017, manufacturers of aeronautic industries have performed, forseveral years, research and development of new surface treatments less harmful for theenvironment and health. The work planned in NEPAL (NEw Protections for ALminium) researchprogram is part of this large program. The chromate-containing treatments, using Cr(VI)compounds, will disappear in the near future; new formulations have been developed includingtrivalent chromium conversion processes. The thesis project aims to reinforce robustness of theseprocesses by providing data useful for understanding the corrosion resistance of aluminium alloyscoated with trivalent chromium coatings, in relation to their microstructural state. The thesis workwas developed on a 2024 aluminium alloy, among those most used in the aeronautics sector.Several microstructures were considered in order to modify the distribution of major alloyselements, copper and magnesium. The fine characterization of the microstructure and theevaluation of the reactivity of the samples in the different conversion treatment baths highlight theinfluence of the copper distribution in the alloy, this alloying element being detrimental to thegrowth of the conversion coating when it is in the form of fine precipitates. Finally, the study of twosheets of aluminium alloy with different thicknesses reveals the influence of new microstructuralparameters such as grain boundaries or coarse precipitation. This work was performed in theframework of the NEPAL FUI project. CIRIMAT was financially supported by the French Ministry ofEconomy and industry (BPI-France), the Région Occitanie Pyrénées-Méditerranée and theEuropean Union (FEDER/ERDF).

Les métaux micro ou nanoporeux sont très attrayants notamment pour leur grande surface spécifique. Le désalliage dans un bain de métal liquide permet une dissolution sélective d'une espèce chimique (l'élément soluble) à partir d'un alliage d'origine (le précurseur) composé de l'élément soluble et d'un élément cible (qui deviendra nano/micro poreux) non soluble dans le bain de métal liquide. Quand le précurseur est plongé dans le bain de métal liquide, à son contact, l'élément soluble va se dissoudre dans le bain tandis que l'élément cible va en parallèle se réorganiser spontanément afin de former une structure poreuse. Quand l'échantillon est retiré du bain, il est sous la forme d'une structure bi-continue composée de deux phases : l'une étant la structure poreuse composée de l'élément cible et l'autre est une phase dans laquelle est présente l'élément du bain avec l'élément sacrificiel en solution solide. Cette phase peut être dissoute par une attaque chimique afin d’obtenir le métal nano/micro poreux. Les objectifs principaux de cette thèse sont l'élaboration et la caractérisation microstructurale et mécanique de 3 différents types de matériaux par désalliage dans un bain de métal liquide : des composites métal-métal (FeCr-Mg), des métaux poreux (FeCr) et des composites métal-polymère (FeCr-matrice époxy). Le dernier objectif est l'évaluation des possibilités d'utiliser la technique de désalliage dans un bain de métal liquide dans un contexte industriel. L'étude de la microstructure est basée sur des observations 3D faites par tomographie aux rayons X et des analyses 2D réalisées en microscopie électronique (SEM, EDX, EBSD). Pour mieux comprendre le désalliage, le procédé a été suivi in situ en tomographie aux rayons X et diffraction. Enfin, les propriétés mécaniques ont été évaluées par nanoindentation et compression. / Nanoporous metals have attracted considerable attention for their excellent functional properties. The first developed technique used to prepare such nanoporous noble metals is dealloying in aqueous solution. Porous structures with less noble metals such as Ti or Fe are highly desired for various applications including energy-harvesting devices. The less noble metals, unstable in aqueous solution, are oxidized immediately when they contact water at a given potential so aqueous dealloying is only possible for noble metals. To overcome this limitation, a new dealloying method using a metallic melt instead of aqueous solution was developed. Liquid metal dealloying is a selective dissolution phenomenon of a mono-phase alloy solid precursor: one component (referred as soluble component) being soluble in the metallic melt while the other (referred as targeted component) is not. When the solid precursor contacts the metallic melt, only atoms of the soluble component dissolve into the melt inducing a spontaneously organized bi-continuous structure (targeted+sacrificial phases), at a microstructure level. This sacrificial phase can finally be removed by chemical etching to obtain the final nanoporous materials. Because this is a water-free process, it has enabled the preparation of nanoporous structures in less noble metals such as Ti, Si, Fe, Nb, Co and Cr. The objectives of this study are the fabrication and the microstructure and mechanical characterization of 3 different types of materials by dealloying process : (i) metal/metal composites (FeCr-Mg), (ii) porous metal (FeCr) (iii) metal/polymer composites (FeCr-epoxy resin). The last objective is the evaluation of the possibilities to apply liquid metal dealloying in an industrial context. The microstructure study was based on 3D observation by X-ray tomography and 2D analysis with electron microscopy (SEM, SEM-EDX, SEM-EBSD). To have a better understanding of the dealloying, the process was followed in situ by X-ray tomography and X-ray diffraction. Finally the mechanical properties were evaluated by nanoindentation and compression.

Les mousses métalliques synthétisées par plasma électrolytique et leur mise en forme développées au CEA présentent certains problèmes de tenue mécanique, d’homogénéité en structure et en densité, et leur porosité n’est pas encore maîtrisée. L’objectif de cette thèse est de comprendre et d’apporter des solutions à ces difficultés rencontrées lors des différentes étapes de la synthèse. Ainsi, la thèse porte sur l’étude du système dans son ensemble pour maîtriser à la fois le procédé et la structure des mousses formées.La compréhension du plasma et de son implication dans la formation de mousses métalliques passe par celle de trois phénomènes principaux : la formation de la bulle de gaz, la formation et la propagation des décharges électriques et le transfert de charges, qui conditionnent chacun la structure finale des mousses. En premier lieu, la formation et l’évolution de la bulle gazeuse ont été observées et des modifications du procédé ont permis de maîtriser cette bulle. Ensuite, l’étude des propriétés et caractéristiques physiques des mousses d’or a été réalisée en synthétisant des mousses à partir de paramètres expérimentaux variables, puis leurs propriétés physiques et structurelles ont été mesurées. L’ensemble des expériences réalisées a permis d’établir un modèle phénoménologique qui rend compte des différents mécanismes impliqués dans la formation et la croissance des brins de mousse. Différentes expérimentations menées directement sur les décharges puis sur le système dans sa globalité ont vérifié partiellement ces hypothèses. / Some troubles are noticed on the metallic foams synthesized by an electrolytic plasma process and on the way they are manufactured. Those foams present a poor mechanical resistance, their structure is lacking of homogeneity and their porosity is far from being controlled. This PhD work aims at understanding and bringing solutions to those problems for each step of the process. Thus the whole process is studied in order to control both the experimental process and the foams structure.The plasma comprehension and especially its contribution in the metallic foams formation is driven through three main phenomena: the gaseous bubble formation, the sparks formation and propagation, and the charge transfer. Each one is playing a role on the final foam structure. First, the formation of the gaseous bubble and its evolution were observed, and then the process was modified to control the bubble. Second, foams were synthesized with different process parameters and their characteristics were studied in order to highlight their influences. All those experiments led to the creation of a phenomenological model which goal is to explain the different mechanisms involved in the foam strands growth. Several experiments were carried out, some on the sparks, others on the whole system, and the hypotheses were partially proved.

The deformation mechanisms and contact response of TiN-based thin films deposited onto a soft substrate using a physical vapour deposition (PVD) technique is still an area of both technological importance and considerable discussion. These coatings are commonly applied to various kinds of steel cutting tools, creating surfaces with enhanced tribological properties. However, no extensive systematic study of the deformation mechanisms in these thin film systems has been performed to date. In the present study, the effect of the coating microstructure, indenter geometry, coating thickness and substrate hardness on the deformation mechanisms in both TiN and TiAlN coatings of varying thickness deposited onto ductile steel substrates has been investigated using a combination of nanoindentation and microstructural analysis, including focused ion beam (FIB) milling and transmission electron microscopy (TEM). Different modes of cracking, such as columnar and transverse cracking, as well as shear steps at the coating/substrate interface, were observed. The microstructure of the TiN coatings was found to be very important in controlling their modes of deformation. Thicker coatings were seen to contain more equiaxed grains, so less columnar shearing occurred and inclined cracks were found to be a more dominant fracture type in the thicker coating. Also, it was found that soft substrates absorbed most of the energy from indentation by plastic deformation. It was found that both the TiN and TiAlN/TiN dual-layer coatings exhibited broadly similar mechanisms of deformation. The epitaxial interface between the TiAlN and TiN in the dual-layer coating did not appear to affect the deformation behaviour. As a further investigation of the overall deformation behaviour for the coating/substrate systems studied, a DualBeam FIB was used to generate three dimensional images of the indented regions which provided additional information on the crack morphology. For the first time, a systematic study of the deformation behaviour of TiN and TiAlN coatings upon indentation has been carried out. FIB milling was demonstrated to be a highly appropriate technique for characterization of the deformation behaviour of these coatings, allowing detailed, high resolution microstructural investigations to be performed in both two and three dimensions.

<p>Nano-powders of BaTiO₃, SrTiO₃, Ba_0.6Sr_0.4TiO₃, a mixture of the composition (BaTiO₃)_0.6(SrTiO₃)_0.4 with particle sizes in the range of 60 to 80 nm, and Bi₄Ti₃O₁₂ with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO₃)_0.6(SrTiO₃)_0.4 mixture is 125 ^oC, ~75 ^oC for Bi₄Ti₃O₁₂, ~25^oC for BaTiO₃ and SrTiO₃, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba_0.6Sr_0.4TiO₃. During the densification of the (BaTiO₃)_0.6(SrTiO₃)_0.4 mixture the reaction 0.6BaTiO₃+0.4SrTiO₃ → Ba_0.6Sr_0.4TiO₃ takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures.</p><p>The dielectric properties of selected samples of (Ba, Sr)TiO₃ ceramics have been studied. The results are correlated with the microstructural features of these samples, <i>e.g.</i> to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region.</p><p>The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi₄Ti₃O₁₂ (BIT) and CaBi₂Nb₂O₉ (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism.</p><p>A new processing strategy for obtaining highly textured ceramics is described. It is based on a<i> directional dynamic ripening mechanism</i> <i>induced by superplastic deformation</i>. The new strategy makes it possible to produce a <i>textured</i> microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties.</p><p>The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi₄Ti₃O₁₂ ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi₄Ti₃O₁₂ ceramics. Textured CaBi₂Nb₂O₉ ceramics exhibited a very high Curie temperature, <i>d</i><i>33</i>-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.</p>

Nano-powders of BaTiO3, SrTiO3, Ba0.6Sr0.4TiO3, a mixture of the composition (BaTiO3)0.6(SrTiO3)0.4 with particle sizes in the range of 60 to 80 nm, and Bi4Ti3O12 with an average particle size of 100 nm were consolidated by spark plasma sintering (SPS). The kinetics of reaction, densification and grain growth were studied. An experimental procedure is outlined that allows the determination of a “kinetic window” within which dense nano-sized compacts can be prepared. It is shown that the sintering behaviour of the five powders varies somewhat, but is generally speaking fairly similar. However, the types of grain growth behaviour of these powders are quite different, exemplified by the observation that the kinetic window for the (BaTiO3)0.6(SrTiO3)0.4 mixture is 125 oC, ~75 oC for Bi4Ti3O12, ~25oC for BaTiO3 and SrTiO3, while it is hard to observe an apparent kinetic window for obtaining nano-sized compacts of Ba0.6Sr0.4TiO3. During the densification of the (BaTiO3)0.6(SrTiO3)0.4 mixture the reaction 0.6BaTiO3+0.4SrTiO3 → Ba0.6Sr0.4TiO3 takes place, and this reaction is suggested to have a self-pinning effect on the grain growth, which in turn explains why this powder has a large kinetic window. Notably, SPS offers a unique opportunity to more preciously investigate and monitor the sintering kinetics of nano-powders, and it allows preparation of ceramics with tailored microstructures. The dielectric properties of selected samples of (Ba, Sr)TiO3 ceramics have been studied. The results are correlated with the microstructural features of these samples, e.g. to the grain sizes present in the compacts. The ceramic with nano-sized microstructure exhibits a diffuse transition in permittivity and reduced dielectric losses in the vicinity of the Curie temperature, whereas the more coarse-grained compacts exhibit normal dielectric properties in the ferroelectric region. The morphology evolution, with increasing sintering temperature, of bismuth layer-structured ferroelectric ceramics such as Bi4Ti3O12 (BIT) and CaBi2Nb2O9 (CBNO) was investigated. The subsequent isothermal sintering experiments revealed that the nano-sized particles of the BIT precursor powder grew into elongated plate-like grains within a few minutes, via a dynamic ripening mechanism. A new processing strategy for obtaining highly textured ceramics is described. It is based on a directional dynamic ripening mechanism induced by superplastic deformation. The new strategy makes it possible to produce a textured microstructure within minutes, and it allows production of textured ferroelectric ceramics with tailored morphology and improved physical properties. The ferroelectric, dielectric, and piezoelectric properties of the textured bismuth layer-structured ferroelectric ceramics have been studied, and it was revealed that all textured samples exhibited anisotropic properties and improved performance. The highly textured Bi4Ti3O12 ceramic exhibited ferroelectric properties equal to or better than those of corresponding single crystals, and much better than those previously reported for grain-orientated Bi4Ti3O12 ceramics. Textured CaBi2Nb2O9 ceramics exhibited a very high Curie temperature, d33-values nearly three times larger than those of conventionally sintered materials, and a high thermal depoling temperature indicating that it is a very promising material for high-temperature piezoelectric applications.

In the last decade, a significant amount of research has been performed to characterize the microstructure of unsheared and sheared triaxial sand specimens to advance the understanding of the engineering behavior of soils. However, most of the research has been limited to two-dimensional (2-D) image analysis of section planes that resulted in loss of information regarding the skeleton of the soil (pore structure) and other attributes of the three-dimensional (3-D) microstructure. In this research, the 3-D microstructures of triaxial test specimens were, for the first time, characterized. A serial sectioning technique was developed for obtaining 3-D microstructure from 2-D sections of triaxial test specimens. The mosaic technique was used to get high-resolution large field of view images. Various 3-D characterization parameters were used to study the microstructures of the specimens. To study the preparation method induced variation in soil microstructure, two specimens prepared with air pluviation and moist tamping methods were preserved with epoxy impregnation. A coupon was cut from the center of each specimen, and following a serial sectioning and image capture process, the 3-D structure was reconstructed. To study the evolution of structure during shearing tests, two additional specimens prepared to the same initial conditions with the same methods were subjected to axial compression loading under constant confining pressure up to an axial strain level of 14%. After shearing, the structure of these specimens were also preserved and analyzed following the same procedures as the unsheared specimens. The evolution of the pore structures was investigated accordingly. It was found that generally, moist tamped specimens were initially less uniform but had a more isotropic structure than air pluviated specimens. The standard deviations of 2-D local void ratio and 3-D pore size in dilated regions of sheared air pluviated and moist-tamped specimens were found to be smaller than those of as-consolidated specimens at a given void ratio. Tortuosity decreased with increasing pore size. It was also evident that the soil structures evolved differently depending on the initial structure. Comparison between 2-D and 3-D results indicated that it is not sufficient to use 2-D section information for characterizing some microstructural features.