Recently, multiferroics-defined as materials with coexistence of at least two of the ferroelectric, ferroelastic and ferromagnetic effects-have attracted enormous research activities. In this thesis, the structure and properties of multiferrioic BiFeO3-x%PbTiO3 and Fe-x%Ga crystalline solutions were investigated. First, the results show that modified BiFeO3-PbTiO3 based ceramics have significantly enhanced multiferroic properties, relative to BiFeO3 single crystals. The data reveal: (i) a dramatic increase in the induced polarization; and (ii) the establishment of a remnant magnetization by a breaking of the translational invariance of a long-period cycloidal spin structure, via substituent effects. In addition, temperature dependent magnetic permeability investigations of BiFeO3-xPbTiO3 crystalline solutions have shown that aliovalent La substitution results in a significant increase in the permeability. Second, room temperature high-resolution neutron and x-ray diffraction studies have been performed on Fe-x%Ga crystals for 12<x<25at%. It has been observed that the structures of both Fe-12%Ga and Fe-25%Ga are tetragonal; however, near the phase boundary between them, an averaged-cubic structure was identified. In addition, an unusual splitting along the transverse direction indicates that the crystals are structurally inhomogeneous. / Master of Science

Computer modeling using finite element analysis (FEA) was performed to examine the effects of constructing multilayered thick film inductors using an artificially modulated magnetic composite structure. It was found that selectively introducing regions of low permeability material increased both the inductance and the current carrying capacity compared to thick film inductors made with single material magnetic cores. Permeabilities of the composite cores ranged from 1 to 220. The frequency for the models ranged from 0 to 5.0 MHz. Experimental devices were constructed using thick film screen printing techniques and characterized to validate the models and to determine the effectiveness of the design modifications. Quantitative comparisons were made between inductors of single permeability cores with inductors produced with magnetic composite cores. It was found that significant (> 130%) increases could be gained in saturation current with only a 12% decrease in inductance. It was found that the key parameters affecting performance were 1) the placement of low permeability regions, 2) the extent of non-uniform flux distribution within the structure, and 3) the volume fraction of low permeability material. / Master of Science

Additive Manufacturing (AM, also known as 3D printing) can produce novel three-dimensional structures using low-loss dielectric materials. This enables the construction of dielectrics with complex shapes that enable innovative microwave applications such as resonators, filters, and metamaterial lenses. This thesis addresses the production and characterization of cellular structures of various designed densities created with a low loss ceramic material, alumina (aluminum oxide), via vat photopolymerization. The permittivity of these printed structures is variable over roughly an octave, with a range of relative permittivites from 1.78 to 3.60, controlled via part geometry. Two additional materials, ferrite and nickel, have been explored for inclusion within these dielectric structures to enable the production of multi-material electromagnetic structures with conductive, magnetic, and dielectric elements. / Master of Science / Additive Manufacturing (AM, also known as 3D printing) has unique manufacturing capabilities. 3D printing can create structures that cannot be produced using traditional manufacturing methods. For example, sponge like structures, with internal voids inaccessible from the outside of the structure, can be created out of a variety of materials. Such structures, known as cellular structures, can be used to create new advanced materials. Ceramic cellular structures can be produced using 3D printing. Ceramics possess many advantages over other materials for use in high frequency radio systems, such as those used for radar and communications. Notably, ceramics are known as low-loss materials, meaning that when electromagnetic waves travel through them they lose less energy than other materials. Cellular structures can be used to vary a material property known as the dielectric constant. Creating cellular structures with designed dielectric constants will enable the creation of new and useful electromagnetic structures. Measuring how this material property changes with the geometry of the cellular structures is important to enable their use. These measurements are described in this work. Additionally, other materials are printed into the ceramic structures. Ferrite, a magnetic material, is extruded as a paste from a nozzle into the ceramic structures. This material is also important for radio systems. Nickel, a good conductor, has also been embedded into the ceramic to provide the ability to create electrically conductive paths inside the part.

La ferrite allotriomorphe est une des morphologies de la ferrite dont la répartition spatiale influe fortement sur les propriétés mécaniques dans les aciers dual-phase. En fonction des traitements qu'ils subissent, la ferrite peut s'y répartir suivant les bandes de ségrégation en manganèse, issues de l'étape de solidification. Pour établir le rôle que joue le processus de croissance de la ferrite allotriomorphe sur la mise en place de la structure en bandes, nous avons développé un modèle de champ de phases possédant deux spécificités originales, imposées par le problème. D'une part, ce modèle est capable de reproduire les différents régimes cinétiques observés dans les alliages ternaires Fe-C-X, pilotés par la présence concomittante du carbone diffusant rapidement,et d'un élément substitutionnel X diffusant lentement. Nous avons ainsi mis en évidence la transition d'un régime initial rapide de paraéquilibre vers une croissance lente en orthoéquilibre, en bon accord avec des résultats expérimentaux de la littérature. D'autre part, notre modèle incorpore de manière économe la présence des joints de grains austénitiques, dont le rôle dans l'élimination des structures en bande est souligné par nos calculs. Nous observons ainsi qu'il existe un seuil d'intensité deségrégation en manganèse en dessous duquel le mouillage de la ferrite le long des joints de grain de plus grande énergie peut contrecarrer la croissance dans les bandes ségrégées négativement / The growth of allotriomorphic ferrite plays a major role in the formation of martensite bands in Dual-Phase steels. We have thus developed a phase field model to study the ferritic growth in different ternary Fe-C-X alloys, incorporating two necessary features. First, we have paid a particular attention to recover the different growth regimes due to the huge difference between the diffusion rates of Cand X substitutional species. Our calculations have exhibited a transition from fast paraequilibrium to slow orthoequilibrium in good agreement with experimental measurements in the literature. Second, austenite grain boundaries have been included in the model because they can conterbalance the manganese segregation bands, as shown in our calculations. Indeed, our results show that the bands can be broken bythe wetting of ferrite along the austenite grain boundaries, provided that the segregation is below a threshold value, and provided that the grain boundary energies are sufficiently high

Les aciers Dual Phase (DP) ferrito-martensitiques sont largement utilisés sous la forme de tôles minces dans la construction automobile en raison de leur excellent compromis résistance/ductilité et donc pour leur potentiel d’allègement. Ils sont élaborés par coulée continue, laminage à chaud et à froid suivis d’un recuit continu. Durant l’étape de chauffage et de maintien de ce recuit, la microstructure ferrito-perlitique déformée issue des étapes de laminage se transforme en microstructure ferrito-austénitique recristallisée. L’expérience montre que les cinétiques de recristallisation et de transformation ainsi que la distribution spatiale et morphologique des microstructures résultantes sont très sensibles aux vitesses de chauffage. Ce travail de thèse s’intéresse aux différents mécanismes expliquant cette sensibilité comme la maturation des carbures, la restauration, la recristallisation de la ferrite et la transformation austénitique et toutes leurs interactions. Ces mécanismes métallurgiques ont été caractérisés à différentes échelles et par des approches in situ sur un acier industriel puis modéliser par des approches à base physique pour guider une possible production. Après un premier chapitre dédié aux techniques expérimentales et de modélisations utilisées, le second chapitre de ce travail s’intéresse principalement à la caractérisation de la morphogénèse des microstructures ferrito-austénitique en microscopie électronique à balayage (MEB). Le troisième chapitre est une étude détaillée en Microscopie Electronique à Transmission (MET) et par modélisation thermocinétique (ThermoCalc, DICTRA) de la composition des carbures tout au long du processus, du laminage à chaud au recuit. Restauration et recristallisation sont étudiées au chapitre 4 principalement par des expériences in situ en Diffraction des Rayons X à Haute Energie (DRXHE) sur ligne de lumière synchrotron et modélisées par une approche originale à champs moyen. Enfin, le chapitre 5 propose une étude sous DICTRA pour comprendre les cinétiques de transformation austénitique en fonction des vitesses de chauffe. Cette approche est novatrice car elle prend en compte les carbures intergranulaires de la ferrite, a été conduite en conditions anisothermes et propose une analyse fine des modes de croissance de l’austénite associées au manganèse, élément clef de la composition de ces alliages. / Ferrite/Martensite Dual-Phase steels are largely used in the form of thin sheets in the automotive industry for their excellent balance between resistance and strength and thus for their lightening potential. They are elaborated by continuous casting, hot- and cold- rolling, followed by a continuous annealing. During the heating and the soaking stages of this latter process, the deformed ferrite/pearlite microstructure obtained after rolling evolves is transformed into a recrystallized ferrite-austenite microstructure. The experiments show that recrystallization and austenite transformation kinetics as well as the resulting spatial and morphological distribution of the phases are highly sensitive to the heating rate. This PhD thesis aims at understanding the different metallurgical mechanisms explaining this particular sensitivity as carbides ripening, recovery, recrystallization and austenite transformation and all their possible interactions. The mechanisms were characterized at different scales and by in situ technics on an industrial steel and model by physical based approaches in order to drive future production lines. After a first chapter dedicated to the experimental and modeling methods, the second chapter deals with the characterization of the morphogenesis of ferrite-austenite microstructures by Scanning Electron Microscopy (SEM). Chapter 3 is a study by Transmission Electron Microscopy (TEM) and by thermokinetic modeling (ThermoCalc, DICTRA) of the chemical composition of carbides along with manufacturing, from hot-rolling to annealing. Recovery and recrystallization are studied in chapter 4 by the means of in situ High Energy X-Ray Diffraction (HEXRD) experiments conducted on a synchrotron beamline and modeled by an original mean-field approach. Finally, chapter 5 proposes an analysis with DICTRA to understand austenite transformation kinetics as function of heating rates. The proposed approach is innovative as it accounts for intergranular carbides in the ferrite matrix, is conducted in non-isothermal conditions and propose a fine analysis of growth modes of austenite associated to manganese, a key alloying element of the studied steels.

With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys. / <p>QC 20130503</p>

This work increased the value of ferrite process (FP) in three directions: firstly, changed the inferiority of FP on cost by transforming the FP sludge into a catalyst; secondly, used ERFP and elutriation to promote the performance of FP; thirdly, developed Fenton oxidation as a pretreatment step for avoiding the interference from chelating agents. Six ferrite catalysts (MxFe(3-x)O4, M = Cu, Zn, Mn, Ni, Cr or Fe) formed from FP were tested. Experimental results indicate that the Cu-ferrite catalyst with a Cu/Fe ratio of 1/2.5 can completely convert CO to CO2 at an inlet CO concentration of 4000 ppm and a space velocity of 6000 hr-1 were held at 140¢J. The catalytic performance of Cu-ferrite did not reduce even when the concentration of O2 was just 1%. This work proves that the ferrite catalysts have good potential for catalyzing oxidation. For developing FP for effectively treating almost all heavy metal waste liquid, hence an extremely difficult treating target- simulated waste liquid was designed. It contains ten heavy metals - Cd, Pb, Cu, Cr, Zn, Ag, Hg, Ni, Sn and Mn, each at a concentration of 0.002 M. Although conventional FP could not be used to treat the simulated waste liquid completely, the enhanced FP, i.e. ERFP, could be used to satisfy regulatory limits. FeSO4 can be added in the extended stage of ERFP intermittently rather than continuously. The optimum operating parameters in the initial stage are pH = 9 , FeSO4 dosage = 0.2 mol/L, temperature = 90 oC, air supply rate = 3 L/min/L and reaction time = 40 min; in the extended stage, they are intermittent dosing, adding 10 mL 1M-FeSO4 solution per liter waste liquid every 5 min, pH = 9, temperature = 90 oC, air supply rate = 3 L/min/L and reaction time = 80min. Elutriation was conducted to reduce the cost of ERFP and ensure that the sludge met Toxicity Characteristic Leaching Procedure (TCLP) standards. An operating pH from 2.88 to 4 and an elutriation time of 6 h were recommended. Used Fenton oxidation to decompose chelating agent in waste liquid and then treated heavy metal by FP, this research showed that under proper operational conditions Fenton/ ERFP could completely solve the chelating agent interference problem. The best condition for decreasing EDTA using the Fenton method was: pH = 2, ferrous ion initial concentration [Fe2+]0 = 1¡Ñ10-2M, hydrogen peroxide addition rate = 5¡Ñ10-4 mol/min/L and reaction time = 10 min. Lastly, a lot of real waste liquids were treated satisfactorily by applying the results of this study.

Le travail présenté dans cette thèse s’inscrit dans la thématique « intégration de puissance ». Il porte sur l’étude des propriétés du matériau ferrite NiZnCu nanostructuré produit par SPS (frittage simple et réactif) et la réalisation de composants électromagnétiques monolithiques intégrés et leurs caractéristiques électriques. Les objectifs fixés au début de la thèse ont été atteints. Nous avons montré que des ferrites nanostructurés ayant des bonnes propriétés peuvent être obtenues par SPS. Pour un frittage simple, les principales caractéristiques structurales, diélectriques et magnétiques ont été déterminées pour différentes températures et temps de densification. Ces échantillons frittés montrent qu’après une décarburation et des valeurs élevées de résistivité électrique, des faibles valeurs de la permittivité diélectrique, et des valeurs élevées de permabilité magnétique initiale et du facteur de mérite peuvent être obtenues et controlées par le temps et la température de frittage. La synthèse in-situ par SPS montrent aussi des propriétés similaires à celles obtenues par frittage simple. Nous avons aussi montré, pour des oxydes de départ de taille nanométrique, que les conditions de broyage et la synthèse in-situ par SPS permettent de concevoir des ferrites ayant des propriétès intéressantes avec des pertes constantes jusqu’à 3 MHz à 15 mT et jusqu’à une température voisine à 60 °C. Les derniers obectifs ont été aussi atteints, l’ensemble des cofrittages du ferrites avec des matériaux conducteurs et diélectriques montrent qu’il est possible de réaliser des composants électromagnétiques monolithiques intégrés. Ces cofrittages montrent que le ferrite est compatible avec le cuivre, le titanate de barrium et des bandes coulées de diélectriques. Des composants inductifs et des transformateurs intégrés ont été réalisées. Les caractéristiques fréquentielles des composants inductifs montrent que l’insertion du diélectrique augmente les performances du composant en reduisant les pertes et l’impact des courants de polarisation. Pour les transformateurs réalisés avec diélectriques les résultats n’ont pas été satisfaisants. Seul un transformateur sans matériau diélectrique a pu être réalisé et testé et comme attendu, le coefficient de couplage entre le primaire et secondaire s’est avéré faible. / The work presented in this thesis is part of the theme «power integration». It covers the study of the nanostructured NiZnCu ferrite material sintered by SPS (simple and reactive sintering) and the realization of monolithic integrated components.The objectives set for the thesis have been achieved. We have shown that nanostructured ferrites with good properties can be obtained by SPS. For a simple sintering, the principal structural, dielectric and magnetic characteristics were determined for different temperatures and densification duration. These sintered samples after decarburization exhibit high electrical resistivity, low dielectric permittivity, high initial magnetic permeability and merit factor.These characteristics can be controlled by sintering time and temperature. The in-situ synthesis by SPS also shows similar properties than those obtained by simple sintering. We have also shown that the starting from oxides of nanometric size, the milling conditions and the in-situ synthesis by SPS can lead to design ferrites with interesting properties with constant losses up to 3 MHz, at 15 mT and up to a temperature of 60 °C.Other the last objectives were also achieved. The co-sintering of ferrites with conductives and dielectric materials show that is possible to produce monolithic integrated electromagnetic components. These experiments show that the co-sintering ferrite is compatible with copper, barrium titanate and dielectric green tape. Inductive components and transformers were designed and fabricated. The frequency characteristics of inductive components show that the insertion of a dielectric increases the performance of components. Indeed, with dielectric, losses are lower and there is less variation in series resistance and inductance with bias current. For transformers with a dielectric layer, the results aren't satisfactory because the dielectric between the primary and secondary melts, during the fabrication creating a short-circuit. The low values of the coupling coefficient and efficiency of the transformer fabricated without a dielectric were predictable because of the large leakage inductance associated with the presence of a magnetic layer between the two windings of the transformer.

Within this thesis, (ultra)thin NiFe2O4 (NFO) and CoFe2O4 (CFO) films are prepared via reactive molecular beam epitaxy (RMBE) on MgO(001) and SrTiO3(001) substrates and are characterized in terms of their structural, electronic, and magnetic properties. In a first step, the structural properties of ultrathin off-stoichiometric NixFe(3-x)O4 films (0<x<1.5) deposited via RMBE on MgO(001) are investigated in situ during film deposition by means of synchrotron radiation-based x-ray diffraction (XRD) and ex situ after film growth by high energy surface x-ray diffraction (HESXRD). In the second major step of this work, a more extensive study on the dependence of the cationic ratio in NixFe(3-x)O4 thin films (0<x<2.07) grown on MgO(001) is conducted. The film surface structure and chemical composition is characterized in situ by low energy electron diffraction (LEED) and laboratory-based soft x-ray photoelectron spectroscopy (XPS), respectively. Film thicknesses are determined via analysis of x-ray reflectivity (XRR) data, while the film structure is analyzed by XRD measurements. Further, chemical properties and the electronic structure of the NFO films with focus on the cationic valencies of Ni and Fe cations with varying x is investigated by means of (angle-resolved) hard x-ray photoelectron spectroscopy [(AR-)HAXPES]. Complementary x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) investigations are conducted to obtain information on the cationic site occupancies and on the element-specific magnetic moments. The latter are compared to magnetic properties characterized via superconducting quantum interference device (SQUID) magnetometry. In a third step, the type of substrate is changed to SrTiO3(001) to investigate the influence of a larger strain applied by the substrate to NFO films with varying thicknesses. Structural characterization at the surfaces and in the films is conducted by means of LEED, XRR, and (grazing incidence) XRD, whereas XPS and HAXPES provide information on the chemical composition and electronic structure in the near-surface region and in deeper subsurface layers, respectively. Magnetic properties are characterized by SQUID magnetometry. In a fourth step, an alternative pathway for the formation of ferrite thin films is demonstrated exemplarily for CoFe2O4 films on SrTiO3(001), which are formed by interdiffusion of Fe3O4/CoO bilayers. The interdiffusion process was monitored via XRR, soft XPS and AR-HAXPES to determine the bilayer/film structure, stoichiometry, and chemical properties. Analysis of complementary XAS measurements provides additional information on the occupancies of Fe and Co cations during interdiffusion. Final SQUID magnetometry measurements are performed to gain information on the magnetic properties before and after complete interdiffusion. Overall, within this thesis, it was demonstrated that NFO and CFO thin films can be prepared in high structural quality with sharp interfaces and surfaces, which is crucial for the applicability in the fields of spintronics and spincaloritronics.

Les systèmes embarqués dans les aéronefs comme dans les véhicules spatiaux doivent impérativement se conformer aux impératifs de masse et de volume mais aussi de coût. Les modules actifs pour antenne à balayage électronique constituent, dans ce cadre, un enjeu stratégique majeur en termes de masse, de volume et de fiabilité. Dans une antenne à balayage électronique actuelle, on peut trouver jusqu’à 1000 modules, chacun étant équipé d’un circulateur-isolateur afin de garantir ses performances. La technologie des circulateurs et des isolateurs à ferrite reste la plus performante en termes d’isolation et de pertes d’insertion. Elle est de plus totalement passive et ne demande aucune énergie extérieure pour son fonctionnement. Elle reste néanmoins coûteuse car la fabrication industrielle de ces dispositifs consiste à assembler mécaniquement de nombreuses pièces usinées précisément : céramiques magnétiques et diélectriques, aimants, conducteurs en cuivre et pièces en métal magnétique doux. De plus, le niveau d’intégration recherché nécessite de réduire les dimensions sans augmenter les pertes pour des niveaux de puissance qui peuvent être importants. S’inspirant des composants céramiques multicouches (condensateurs et inductances) ainsi que de la technologie LTCC (Low Temperature Cofired Ceramics), une nouvelle voie de fabrication de ces composants, en rupture avec les technologies d’assemblage traditionnelles, est abordée dans ces travaux de thèse. L’idée consiste à réaliser par cofrittage le cœur du composant qui est le plus délicat à ajuster et qui détermine le volume final. Les ferrites qui constituent actuellement le cœur des circulateurs sont principalement des grenats ferrimagnétiques fabriqués par des techniques céramiques classiques et frittés à haute température (> 1400°C). Pour les rendre compatibles de la technologie LTCC, il est nécessaire de diminuer leur température de frittage. Les températures visées doivent être inférieures à 1000°C pour pouvoir cofritter avec des parties métalliques en or et si possible être proches de 900°C pour espérer cofritter avec de l’argent. Dans ce contexte, l’objectif de ce travail de thèse était de mettre au point des grenats ferrimagnétiques pour des applications en hyperfréquences présentant des températures de frittage proche de 900°C. Ces ferrites ont alors été utilisés pour la réalisation de circulateurs hyperfréquences, composants indispensables dans les systèmes de Radars et de télécommunications. De plus, des études d’optimisation des propriétés magnétiques et diélectriques de ces ferrites ont aussi été réalisées pour adapter le ferrite aux conditions de fonctionnement (gamme de fréquence et niveau de puissance). / Embedded systems in aircraft must comply with the requirements of mass, volume and cost. The active modules of electronic scanning antenna are, in this context, a strategic challenge in terms of mass, volume and reliability. Today, there are up to 1000 modules per antenna, each one containing a circulator-isolator in order to guarantee its performances. The technology of ferrite circulators and isolators remains the most efficient in terms of isolation and insertion losses. It is also fully passive as no external energy is required to work. However this technology is expensive due to complex mechanical assembling of the different materials: magnetic and dielectric ceramics, magnets, conductors made of copper and soft metallic material. The integration of such devices also requires the reduction of dimensions without increasing losses for power levels that can be high. Based on by multilayer ceramic components (capacitors and inductors) as well as Low Temperature Cofired Ceramics (LTCC) technology, a new way of manufacturing these components, is investigated in this PhD work.. The idea is to be able to cofire the heart of the component which is the most difficult to adjust and also determines the final volume. The ferrites which currently constitute the core of the circulators are ferrimagnetic garnets synthetized by using a conventional ceramic process and sintered at high temperature (> 1400°C). To make them compatible with LTCC technology, it is essential to reduce their sintering temperature. The targeted temperatures must be less than 1000°C in order to cofire with gold metal parts and, if possible, close to 900°C for circulators with silver. In this context, the objective of this PhD work was to develop a ferrimagnetic garnet for microwave applications with sintering temperatures close to 900°C. This ferrite was then used for the preparation of microwave circulators which are essential components in radar and telecommunications systems. In addition, studies of optimization of the magnetic and dielectric properties have also been carried out to meet the operating requirements (frequency band and power level).