Les matériaux magnéto-électro-élastiques (MEE) sont l'association des matériaux piézo-électrique et piézo-magnétique qui présentent un couplage "magnéto-électrique". L'objectif de cette thèse est d'étudier d'une part, le comportement effectif de ces composites et d'autre part, la propagation des ondes planes dans le milieu homogène équivalent. Dans la première partie de la thèse, nous avons modélisé le comportement effectif des composites MEE à partir de la méthode de la moyenne. Nous avons d'abord établi les équations générales, ensuite nous avons traité le cas où chaque phase est supposée isotrope transverse suivant un axe et polarisée suivant la même direction. La loi de comportement obtenue, ainsi que les tenseurs effectifs du milieu homogène équivalent ont été établis. Les propriétés élastiques effectives sont influencées par les propriétés électriques et magnétiques et réciproquement. Nous nous sommes intéressés en particulier à deux types de composites : les stratifiés et les fibres longues. L'influence de la géométrie des constituants ainsi que la proportion des phases sur le comportement effectif ont été étudiées. La deuxième partie du travail a porté sur l'étude de la propagation d'ondes planes dans les stratifiés. En utilisant la méthode d'homogénéisation périodique, nous avons obtenu les équations de la dynamique dans le milieu homogène équivalent. La longueur d'onde considérée est supposée grande devant la périodicité spatiale. Une méthode de résolution numérique a été développée afin d'obtenir les courbes de dispersion. Nous avons obtenu ces courbes en fonction des proportions des phases constitutives. Bien que les trois propriétés: élastique, électrique, magnétique, contribuent au comportement oscillatoire global, l'onde conserve essentiellement une nature élastique. / The objective of this thesis is estimating fundamental properties and studying the propagation of waves in the equivalent homogeneous medium based on the periodic magneto-electro-elastic (MEE) composite. These artificial MEE media are realized by means of combining piezo-electric and piezo-magnetic materials, and featurea direct "magneto-electric" coupling. In the first part, we modeled the effective behavior of these composites applying the averaging method. First, we derived the general equations, then we treated a particular case a uni axial medium comprised of alternating layers of piezo-electric and piezo-magnetic phases and polarized in the direction normal to the surfaces of these layers. The law of behavior was obtained as well as the effective tensors of the equivalent homogeneous medium. The effective elastic properties are influenced by the magnetic and electrical properties and inversely. We were particularly interested in two types of materials: multi-layers (planar symmetry) and long fibers (cylindrical symmetry). The influence of the geometry of these components on the effective behavior was revealed. The second part of the study focuses on the propagation of plane waves in the case of periodic multilayer structures. Using the periodic homogenization method, we obtained the effective tensors and the equations of propagation of elasto-electro-magnetic plane waves. The wavelength is supposed much larger than the spatial period of the investigated structure, hence the quasi-static approximation for the equations of electromagnetic could be used. The chosen method has allowed estimation of the wave frequency as a function of the wave number, the corresponding dispersion curves were plotted for a wide range of proportions of the constituent materials. Although all the three properties, mechanical, electrical and magnetic contribute to the global oscillatory behavior, the wave is essentially elastic.

Master of Engineering (Research) / Multiple antenna systems are an appealing candidate for emerging fourth-generation wireless networks due to its potential to exploit space diversity for increasing conveyed throughput without wasting bandwidth and power resources. Particularly, layered space-time architecture (LST) proposed by Foschini, is a technique to achieve a significant fraction of the theoretical capacity with a reasonable implementation complexity. There has been a great deal of challenges in the detection of space-time signal; especially to design a low-complexity detector, which can efficiently remove multi-layer interference and approach the interference free bound. The application of iterative principle to joint detection and decoding has been a promising approach. It has been shown that, the iterative receiver with parallel interference canceller (PIC) has a low linear complexity and near interference free performance. Furthermore, it is widely accepted that the performance of digital communication systems can be considerably improved once the channel state information (CSI) is used to optimize the transmit signal. In this thesis, the problem of the design of a power allocation strategy in LST architecture to simultaneously optimize coding, diversity and weighting gains is addressed. A more practical scenario is also considered by assuming imperfect CSI at the receiver. The effect of channel estimation errors in LST architecture with an iterative PIC receiver is investigated. It is shown that imperfect channel estimation at an LST receiver results in erroneous decision statistics at the very first iteration and this error propagates to the subsequent iterations, which ultimately leads to severe degradation of the overall performance. We design a transmit power allocation policy to take into account the imperfection in the channel estimation process. The transmit power of various layers is optimized through minimization of the average bit error rate (BER) of the LST architecture with a low complexity iterative PIC detector. At the receiver, the PIC detector performs both interference regeneration and cancellation simultaneously for all layers. A convolutional code is used as the constituent code. The iterative decoding principle is applied to pass the a posteriori probability estimates between the detector and decoders. The decoder is based on the maximum a posteriori (MAP) algorithms. A closed-form optimal solution for power allocation in terms of the minimum BER is obtained. In order to validate the effectiveness of the proposed schemes, substantial simulation results are provided.

Dans cette thèse, nous étudions les trois sujets suivants sur les systèmes de multiples entrées multiples sorties (MIMO) avec retour limité: Distorsion bout-à-bout: La première partie de la thèse présente l'impact conjoint des les nombres d'antenne, le ratio de bande passante de la source au canal, la corrélation spatiale et diversité dans le temps sur l'espérance de la distorsion optimale bout-à-bout dans les MIMO systèmes sans panne. En particulier, repose sur l'expression analytique pour tout les ratios du signal au bruit (SNR), l'expression asymptotique de l'espérance de la distorsion optimal bout-à-bout au SNR élevé est dérivé, composé de l'exposant de distorsion optimale et le facteur de distorsion optimale. Les résultats des simulations montrent que, à un SNR élevé pratique, l'analyse sur les impacts de l'exposant de distorsion optimale et le facteur de distorsion optimale explique le comportement de la distorsion optimale bout-à-bout. Les résultats présentés dans cette partie pourraient être les objectifs de performance pour les systèmes qui transmettent les sources analogique et en outre les directives sur la conception du système. Retour analogique du canal: Dans la seconde partie de cette thèse, nous proposons d'appliquer le codage bloc d'espace-temps orthogonal (OSTBC) sur retour analogique linéaire du canal. Considérant l'information du canal MIMO est une sorte de source vectorielle analogique, par rapport au retour du canal quantifié, le retour analogique linéaire a les avantages comme sans panne, l'adaptation automatique au canal et peu complexe. Il est prouvé que la méthode de transmission analogique linéaire OSTBC peutatteindre le borne de filtre adapté (MFB) sur SNR reçu. Par rapport en méthode analogique linéaire du codage bloc d'espace-temps circulant, la méthode linéaire analogique OSTBC obtient de meilleurs résultats à l'égard de SNR reçu et erreur quadratique moyenne (MSE) ; Par rapport en méthode de la quantification vectorielle aléatoire, les résultats des simulations montrent que sous une contrainte stricte de latence, leur performances sont proches à l'égard de l'erreur moyenne au sens et la méthode linéaire analogique OSTBC exécute encore mieux à l'égard erreur quadratique moyenne. Nous étudions également l'incidence de appliquer le retour linéaire analogique OSTBC au beamforming en liaison descendante pour multiutilisateurs MIMO et nous montrons que le retour linéaire analogique OSTBC peut rendre l'approche du système des performances optimales dans un temps de latence court. Multiplexage par couche: Dans la troisime partie de cette thèse, en considérant les systèmes avec des blocs courts, une nouvelle stratégie de multi-plexage par couches est proposée d'adapter un canal incertain par le codage couche-temps Walsh, l'annuleur d'interférence successif et HARQ signalisation. Comme l'illustrent les résultats de la simulation, en raison de ses performances proches mais beaucoup moins complexe, cette stratégie serait un bon substitut µa la stratégie de modulation QAM adaptive qui est largement utilise.

Investigating the structure-property relationships of solid state materials can help improve many of the materials we use each day in life. It can also lead to the discovery of materials with interesting and unforeseen properties. In this work the structure property relationships of newly discovered layered oxypnictide phases are presented and discussed. There has generally been worldwide interest in layered oxypnictide materials following the discovery of superconductivity up to 55 K for iron arsenides such as LnFeAsO[subscript 1-x]F[subscript x] (where Ln = Lanthanoid). This work presents efforts to understand the structure and physical property changes which occur to LnFeAsO materials when Fe is replaced with Rh or Ir and when As is replaced with Sb. As part of this work the solid solution between LaFeAsO and LaRhAsO was examined and superconductivity is observed for low Rh content with a maximum critical temperature of 16 K. LnRhAsO and LnIrAsO compositions are found to be metallic; however Ce based compositions display a resistivity temperature dependence which is typical of Kondo lattice materials. At low temperatures a sudden drop in resistivity occurs for both CeRhAsO and CeIrAsO compositions and this drop coincides with an antiferromagnetic transition. The Kondo scattering temperatures and magnetic transition temperatures observed for these materials can be rationalized by considering the expected difference in N(E[subscript F])J parameters between them, where N(E[subscript F]) is the density of states at the Fermi level and J represents the exchange interaction between the Ce 4f¹ electrons and the conduction electrons. In addition to studying these 4d and 5d substituted systems the LaFeSbO compositional system was investigated. While LaFeSbO has not been successfully synthesized the transition metal free layered oxypnictide composition La₂SbO₂ was discovered and its structural and physical properties have been examined along with the properties of La₂BiO₂. Density functional theory was used to calculate the heats of formation for competing phases within the LaFeSbO system, in order to better understand the stability of LaFeSbO and why it has not yet been observed. The materials La₂SbO₂ and La₂BiO₂ were investigated for the presence of oxygen vacancies using powder neutron diffraction. Structure refinement reveals that there is significant disorder within the a-b plane for Sb compositions. / Graduation date: 2012

Ab Initio calculations suggest that partially lithiated layered LixMnO₂ transforms to spinel in a two-stage process. In the first stage, a significant fraction of the Mn and Li ions rapidly occupy tetrahedral sites, forming a metastable intermediate. The second stage involves a more difficult coordinated rearrangement of Mn and Li ions to form spinel. This behavior is contrasted to LixCoO₂. The susceptibility of Mn for migration into the Li layer is found to be controlled by oxidation state which suggests various means of inhibiting the transformation. These strategies could prove useful in the creation of superior Mn based cathode materials. / Singapore-MIT Alliance (SMA)

In this work we have investigated the intercalation of electron-donors between NbS2 slabs in Nb-based layer sulfides. Two series of Sr substituted Nb-based misfit sulfides belonging to the 1.5Q/1H and 1Q/1H series of misfit layer compounds have been synthesised. For large lanthanides (Ln=La, Ce), only the 1Q/1H compounds formed whereas for smaller lanthanides and yttrium, both types of phases can be obtained. The crystal structure of misfit sulfide (Pr0.55Sr0.45S)1.15NbS2 has been refined using the composite approach. In the Q-slab, Pr-atoms are partly replaced by Sr with a random distribution over one cation position. The crystal structure of misfit sulfide [(Sm1/3Sr2/3S)1.5]1.15NbS2 belonging to the 1.5Q/1H series have also been determined. The obtained results suggest a preferred occupancy of the cation positions in the slab where Sr atoms mainly occupy positions on the exterior of the slab while Sm atoms are in the center of the slab. The (La1-xSrxS)1.15NbS2 solid solution (0.1<x<0.9) has also been studied. It was found that the maximum value of Sr substitution is 40-50% and therefore, the minimal value of charge transfer to stabilize this structure type is about 0.6ē per Nb atom. An attempt to synthesize SrxNbS2 (0.1≤x≤0.5) intercalates was made but single phases were not obtained and increasing the temperature from 1000оС to 1100оС leads to the decomposition of these intercalates. Single crystals of Sr0.22Nb1.05S2 and Sr0.23NbS2 were found and their structures were determined. The structures belong to two different types of packings with statistical distribution of Sr between layers. A new superconducting sulfide, "EuNb2S5", was investigated by ED and HREM and its structure model consisting of Nb7S14 and (Eu3S4)2 slabs alternating along the c-axis is suggested. An attempt to suggest a model for the structure of "SrNb2S5" by means of X-ray single crystal diffraction was made. The proposed structure consists of two types of slabs: a Nb7S14 and a [Sr6(NbS4)2S] slab with niobium in tetrahedral coordination. It is shown that "SrNb2S5" and "EuNb2S5" are have similar structures. For the first time, single crystals of the complex sulfide BaNb0.9S3 have also been studied by means of X-ray single crystal diffraction. The single crystal refinement and EDX analysis showed the existence of cation vacancies at the niobium position. BaNb0.9S3 has also been studied by ED and no superstructure was found which implies that and the vacancies are statistically distributed. No improvement of the magnetic properties of the studied compounds was observed in comparison to NbS2.

The goal of this research is to develop a robust, continuum-based approach for a three-dimensional, Finite Element Method (FEM) simulation of layered geomaterials. There are two main approaches to the numerical modeling of layered geomaterials; discrete or discontinuous techniques and an equivalent continuum concept. In the discontinuous methodology, joints are explicitly simulated. Naturally, discrete techniques provide a more accurate description of discontinuous materials. However, they are complex and necessitate care in modeling of the interface. Also, in many applications, the definition of the input model becomes impractical as the number of joints becomes large. In order to overcome the difficulties associated with discrete techniques, a continuum-based approach has become popular in some application areas. When using a continuum model, a discrete material is replaced by a homogenized continuous material, also known as an 'equivalent continuum'. This leads to a discretization that is independent of both the orientation and spacing of layer boundaries. However, if the layer thickness (i.e., internal length scale of the problem) is large, the classical continuum approach which neglects the effect of internal characteristic length can introduce large errors into the solution. In this research, a full 3D FEM formulation for the elasto-plastic modeling of layered geomaterials is proposed within the framework of Cosserat theory. The effect of the bending stiffness of the layers is incorporated in the matrix of elastic properties. Also, a multi-surface plasticity model, which allows for plastic deformation of both the interfaces between the layers and intact material, is introduced. The model is verified against analytical solutions, discrete numerical models, and experimental data. It is shown that the FEM Cosserat formulation can achieve the same level of accuracy as discontinuous models in predicting the displacements of a layered material with a periodic microstructure. Furthermore, the method is capable of reproducing the strength behaviour of materials with one or more sets of joints. Finally, due to the incorporation of layer thickness into the constitutive model, the FEM Cosserat formulation is capable of capturing complicated failure mechanisms such as the buckling of individual layers of material which occur in stratified media.

Cette thèse présente un système de traitement des vidéos multi-vues plus profondeur (MVD), en utilisant le concept d'images multi-couches (LDI). Plusieurs contributions sont proposées, à la fois dans le domaine de la synthèse de vues à partir de cartes de profondeur et dans le domaine de la construction et de la compression des LDI. La méthode de synthèse de vues proposée est basée sur une nouvelle technique de projection (JPF). La projection JPF prend en entrée des données de type image plus profondeur et synthétise un nouveau point de vue quelconque ; elle est conçue pour détecter et remplir les fissures et les petits découvrements pendant la projection. Une mesure de confiance sur les pixels est introduite afin d'éviter les artéfacts fantômes au niveau des contours. Cette projection JPF est utilisée d'une part avec une technique de ré-alignement de textures lors de l'interpolation de vues intermédiaires, et d'autre part avec une nouvelle méthode de synthèse de textures lors de l'extrapolation de vues virtuelles. Afin d'encoder efficacement les séquences de LDI, un schéma de compression multi-vues (MVC/AVC) est adapté pour exploiter à la fois les redondances temporelles et inter-couche. Une construction incrémentale des LDI est proposée, qui réduit le taux de remplissage des couches additionnelles. L'efficacité de la compression est améliorée par une réorganisation des couches par objet, assurant ainsi une meilleure cohérence spatiale. Deux méthodes de rendu sont finalement proposées : la première utilise la projection JPF alors que la seconde utilise des maillages 3D pour un affichage temps réel sur un écran multi-scopiques.

The goal of this research is to develop a robust, continuum-based approach for a three-dimensional, Finite Element Method (FEM) simulation of layered geomaterials. There are two main approaches to the numerical modeling of layered geomaterials; discrete or discontinuous techniques and an equivalent continuum concept. In the discontinuous methodology, joints are explicitly simulated. Naturally, discrete techniques provide a more accurate description of discontinuous materials. However, they are complex and necessitate care in modeling of the interface. Also, in many applications, the definition of the input model becomes impractical as the number of joints becomes large. In order to overcome the difficulties associated with discrete techniques, a continuum-based approach has become popular in some application areas. When using a continuum model, a discrete material is replaced by a homogenized continuous material, also known as an 'equivalent continuum'. This leads to a discretization that is independent of both the orientation and spacing of layer boundaries. However, if the layer thickness (i.e., internal length scale of the problem) is large, the classical continuum approach which neglects the effect of internal characteristic length can introduce large errors into the solution. In this research, a full 3D FEM formulation for the elasto-plastic modeling of layered geomaterials is proposed within the framework of Cosserat theory. The effect of the bending stiffness of the layers is incorporated in the matrix of elastic properties. Also, a multi-surface plasticity model, which allows for plastic deformation of both the interfaces between the layers and intact material, is introduced. The model is verified against analytical solutions, discrete numerical models, and experimental data. It is shown that the FEM Cosserat formulation can achieve the same level of accuracy as discontinuous models in predicting the displacements of a layered material with a periodic microstructure. Furthermore, the method is capable of reproducing the strength behaviour of materials with one or more sets of joints. Finally, due to the incorporation of layer thickness into the constitutive model, the FEM Cosserat formulation is capable of capturing complicated failure mechanisms such as the buckling of individual layers of material which occur in stratified media.

No description available.