This thesis presents a comprehensive study of the compressive response of hexagonal honeycomb panels from the initial elastic regime to a fully crushed state. Expanded aluminum alloy honeycomb panels with a cell size of 0.375 in (9.53 mm), a relative density of 0.026, and a height of 0.625 in (15.9 mm) are laterally compressed quasi statically between rigid platens under displacement control. The cells buckle elastically and collapse at a higher stress due to inelastic action. Deformation then first localizes at mid-height and the cells crush by progressive formation of folds; associated with each fold family is a stress undulation. The response densifies when the whole panel height is consumed by folds. The buckling, collapse, and crushing events are simulated numerically using finite element models involving periodic domains of a single or several characteristic cells. The models idealize the microstructure as hexagonal, with double walls in one direction. The nonlinear behavior is initiated by elastic buckling while inelastic collapse that leads to the localization observed in the experiments occurs at a significantly higher load. The collapse stress is found to be mildly sensitive to various problem imperfections. For the particular honeycomb studied, the collapse stress is 67% higher than the buckling stress. It was also shown that all aspects of the compressive behavior can be reproduced numerically using periodic domains with a fine mesh capable of capturing the complexity of the folds. The calculated buckling stress is reduced when considering periodic square domains as the compatibility of the buckles between neighboring cells tends to make the structure more compliant. The mode consisting of three half waves is observed in every simulation but its amplitude is seen to be accented at the center of the domains. The calculated crushing response is shown to better resemble measured ones when a 4x4 cell domain is used, which is smoother and reproduces decays in the amplitude of load peaks. However, the average crushing stress can be captured with engineering accuracy even from a single cell domain. / text

Le projet de thèse, mené en collaboration avec le laboratoire Medesis Pharma, porte sur la mise au point de deux formulations pharmaceutiques, une microémulsion inverse et un gel colloïdal, à base de Pécéol®, lécithine, éthanol et eau. Le lithium indiqué dans le traitement de maladies neurodégénératives a été utilisé comme un modèle de principe actif hydrophile dans le cadre de ce projet. Ce travail a été initié par l’étude du comportement de phases du mélange quaternaire Pécéol®/lécithine/éthanol/eau, mettant essentiellement l’accent sur les différents paramètres impliqués dans les transitions de phases, dans le but de déterminer le rôle du Pécéol® et de la lécithine dans la structuration du système.- L’étude des propriétés structurales de la microémulsion, dans le domaine riche en Pécéol®, a montré que la structure du système n’est pas cohérente avec un modèle classique de micelles sphériques, l’hypothèse d’une structure localement lamellaire a été proposée. Les propriétés de solubilisation de la microémulsion ont été évaluées par l’élaboration d’une série de diagrammes de phases pseudo-ternaires, en faisant varier le rapport éthanol/lécithine. Une efficacité optimale a été obtenue pour un rapport éthanol/lécithine égal à 40/60. La formulation optimisée est capable d’incorporer efficacement le lithium (Li), à des concentrations acceptables sans altérer la stabilité du système. L’étude des propriétés d’incorporation du Li dans la microémulsion a montré que le mécanisme d’encapsulation est compatible avec un modèle d’interactions de type drug-surfactant. Les résultats des études de stabilité de la microémulsion vide et chargée en Li menées selon les conditions ICH sont satisfaisants.- La structuration du gel clair, dans le domaine riche en lécithine, a été suivie par des analyses de rhéologie et de microscopie en lumière polarisée en fonction du rapport lécithine/Pécéol®. Les résultats ont montré que les propriétés rhéologiques du gel peuvent être modifiées en ajustant le rapport lécithine/Pécéol®. La caractérisation de la structure du gel clair par diffraction de rayons X aux petits angles (SAXS) et cryofracture conjuguée à la microscopie électronique en transmission (FF-TEM) a mis en évidence une structure anisotrope hexagonale H2. Alors que dans le domaine riche en eau, une structure anisotrope hexagonale inverse H2 et lamellaire Lα a été mise en évidence. Les propriétés rhéologiques modulables du gel dans le domaine riche en lécithine ainsi que sa grande capacité de rétention d’eau, sont particulièrement intéressantes pour une application du gel clair dans le domaine de pharmaceutique et/ou cosmétique. / This thesis project, carried out in collaboration with Medesis Pharma laboratory, is about the development of two pharmaceutical formulations, a reverse microemulsion and a colloidal gel, based on Pécéol®, lécithine, ethanol and water. The indicated lithium in the treatment of neurodegenerative diseases has been used as a hydrophilic principal active model for this project. This work started with the study of the phase behavior of the four component system Pécéol®/lecithin/éthanol/water, focusing primarily on the various parameters involved in phase transitions in order to determine the role of Pécéol® and lecithin in the structuring of the system.- The study of the structural properties of the reverse microemulsion, in the Pécéol® rich domain, showed that the system structure is not coherent with a classical model of spherical micelles, a hypothesis of a local lamellar structure was proposed. The solubilization properties of the microemulsion were evaluated following the development of a series of pseudo-ternary phase diagrams, by varying the ethanol/lecithin ratio. Optimal efficiency was obtained for an ethanol/lecithin ratio of 40/60. The optimized formulation is capable of effectively incorporating lithium (Li) to acceptable concentrations without altering the stability of the system. The study of the incorporation properties of Li in the microemulsion has shown that the encapsulation mechanism is compatible with drug-surfactant binding model. The results of the stability studies of the free and Li loaded microemulsion, conducted according to ICH conditions, are satisfactory.- The gel structuring was followed, in the lecithin rich domain, through rheological and optical microscopy analyses, according to lecithin/Pécéol® ratio. Results showed that the rheological properties of the gel can be modified by adjusting the lecithin/Pécéol® ratio. Moreover, the characterization of the gel structure through Small Angle X-Ray Scattering (SAXS) and Freeze Fracture-Transmission Electron Microscopy (FF-TEM) showed an inverse hexagonal phase H2. While, in the water rich region, an inverse hexagonal H2 and lamellar Lα phases were identified. The adjustable rheological properties of the clear gel, and its great water solubilisation capacity, are particularly interesting for the application of the gel in pharmaceutical and/or cosmetic fields.

Made available in DSpace on 2016-08-18T18:19:28Z (GMT). No. of bitstreams: 1 Katiane Pereira da Silva.pdf: 8268889 bytes, checksum: fe86169cbfe78ea0598321da785e9b93 (MD5) Previous issue date: 2010-07-23 / FUNDAÇÃO DE AMPARO À PESQUISA E AO DESENVOLVIMENTO CIENTIFICO E TECNOLÓGICO DO MARANHÃO / The compounds Cs4W11O35 (CW) and Rb4W11O35 (RW) belong to the class of hexagonal bronzes whose structure originates from KxWO3 superconductor hexatungstate. Charge imbalanced tungsten bronzes are dielectric materials with rich polymorphism, ferroelectric properties and second harmonic generation. In this work we report polarized Raman spectra results for both CW and RW as well as results of high pressure Raman scattering experiments (0.0 11.0 GPa) for Cs4W11O35 system, for which we have observed two structural phase transitions at about 4 GPa and 7.5 GPa. We discuss these transformations and polarized Raman spectra, on basis on lattice dynamics calculation in the related system KNbW2O9. Polarized Raman spectra provide strong indicative that the highest wavenumber modes observed in these systems originates from tungsten or oxygen vacancies. The observation of a soft-like mode indicates that the observed phase transitions exhibit a displacive type behavior, thus further indicating that these transformations are likely related to reorientations of octahedral units. The soft mode nature is discussed as well. Additional, measurement of temperature dependent Raman spectra shows that the structure of CW is stable op to 800 K. Low temperature measurements reveal the existence of a new phase below 190 K. Temperature transformation lead the structure to a higher symmetric phase below 190 K. / Os compostos Cs4W11O35 e Rb4W11O35 pertencem à classe dos bronzes hexagonais cuja estrutura deriva do hexatungstato supercondutor KxWO3. Os bronzes hexagonais são materiais dielétricos com rico polimorfismo, apresentam propriedades ferroelétricas e Geração de Segundo Harmônico. Neste trabalho relatamos resultados de espectros Raman polarizado para ambos os sistemas e resultados de experimentos de espalhamento Raman em altas pressões (com faixa de pressão de 0,0 11 GPa) no sistema Cs4W11O35, para o qual observamos no mínimo duas transições de fase estrutural em aproximadamente, 4 GPa e 7,5 GPa. Discutimos estas transformações, assim como os espectros Raman polarizados destes materiais, com base em cálculo clássico de dinâmica de rede no sistema KNbW2O9, que possui estrutura similar à dos materiais em estudo. Os espectros Raman polarizado fornecem forte indicativos de que os modos de vibração de número de onda mais alto observados nestes sistemas são originados de vacâncias de tungstênio (W) ou de oxigênio (O). Nos resultados de espectroscopia Raman, observamos o aparecimento de modos do tipo soft, indicando que pelo menos umas das transições de fases estruturais observadas apresentam comportamento do tipo deslocamento, o que, além disso, indica que estas transformações são relacionadas provavelmente a reorientações dos octaedros. As comparações da dependência dos modos do tipo soft com a pressão nos materiais contendo Cs e Rb revelam que os raios iônicos influenciam no comportamento soft destes modos. Os estudos do tungstato de césio pra altas temperaturas mostraram que a estrutura deste material é extremamente estável para altas temperaturas. Já para baixas temperaturas foi observada em torno de 190 K uma transição de fase relacionada a fortes modificações na estrutura.

Goal of the thesis is to show the possibilities of using ordinary hobbyist RC servos and means of their control by a computer. For demonstration of the capabilities of servos, we have created Stewart platform which is using hobbyist servos to position the platform. In the thesis there is explained principle of the inner working of a servo, their properties, abilities and also means of their control by a computer. In practical chapters of the thesis there is documented process of designing and implementing of functional Stewart platform along with specific properties of this platform which influence its design. This platform is also reviewed and based on the results we will show technical properties of the platform, advantages and disadvantages of using ordinary hobbyist servos to control the platform. Design of the platform and its documentation is created with respect to intent to make the ddesign, source codes, documentation available to public on wepages focused on aggregating of instructables and tutorials to make usage of outputs possible in practice.

During neural activity, action potentials travel down axons, generating effective charge current pulses, which are central in neuron-to-neuron communication. Consequently, said current pulses generate associated magnetic fields with amplitudes on the order of picotesla (pT) and femtotesla (fT) and durations of 10’s of ms. Magnetoencephalography (MEG) is a technique used to measure the cortical magnetic fields associated with neural activity. MEG limitations include the inability to detect signals from deeper regions of the brain, the need to house the equipment in special magnetically shielded rooms to cancel out environmental noise, and the use of superconducting magnets, requiring cryogenic temperatures, bringing opportunities for new magnetic sensors to overcome these limitations and to further advance neuroscience. An extraordinary magnetoresistance (EMR) tunable graphene magnetometer could potentially achieve this goal. Its advantages are linear response at room temperature (RT), sensitivity enhancement owing to combination of geometric and Hall effects, microscale size to place the sensor closer to the source or macroscale size for large source area, and noise and sensitivity tailoring. The magnetic sensitivity of EMR sensors is, among others, strongly dependent on the charge mobility of the sensing graphene layer. Mechanisms affecting the carrier mobility in graphene monolayers include interactions between the substrate and graphene, such as electron-phonon scattering, charge impurities, and surface roughness. The present work reviews and proposes a material set for increasing graphene mobility, thus providing a pathway towards pT and fT detection. The successful fabrication of large-size magnetic sensors employing CVD graphene is described, as well as the fabrication of trilayer magnetic sensors employing mechanical exfoliation of h-BN and graphene. The magneto-transport response of CVD graphene Hall bar and EMR magnetic sensors is compared to that obtained in equivalent trilayer devices. The sensor response characteristics are reported, and a determination is provided for key performance parameters such as current and voltage sensitivity and magnetic resolution. These parameters crucially depend on the material's intrinsic properties. The Hall cross magnetic sensor here reported has a magnetic sensitivity of ~ 600 nanotesla (nT). We find that the attained sensitivity of the devices here reported is limited by contaminants on the graphene surface, which negatively impact carrier mobility and carrier density, and by high contact resistance of ~2.7 kΩ µm at the metallic contacts. Reducing the contact resistance to < 150 Ω µm and eliminating surface contamination, as discussed in this work, paves the way towards pT and ultimately fT sensitivity using these novel magnetic sensors. Finite element modeling (FEM) is used to simulate the sensor response, which agrees with experimental data with an error of less than 3%. This enables the prediction and optimization of the magnetic sensor performance as a function of material parameters and fabrication changes. Predictive studies indicate that an EMR magnetic sensor could attain a sensitivity of 1.9 nT/√Hz employing graphene with carrier mobilities of 180,000 cm²/Vs, carrier densities of 1.3×10¹¹ cm^-2 and a device contact resistance of 150 Ω µm. This sensitivity increments to 443 pT/√Hz if the mobility is 245,000 cm²/Vs, carrier density is 1.6×10¹⁰ cm^-2, and a lower contact resistance of 30 Ω µm. Such devices could readily be deployed in wearable devices to detect biomagnetic signals originating from the human heart and skeletal muscles and for developing advanced human-machine interfaces.

In the present work the deformation behaviour of magnesium-based alloy AZ31 which was produced by horizontal continual casting is investigated. Samples with two different orientations are deformed in tension and compression at strain rate of 10-3 s-1 in temperature range of 20 řC to 300 řC. Simultaneously, the acoustic emission is recorded and studied with focused on mechanical twinning. The mechanisms of plastic deformation of material and anisotropy between tension and compression are discussed. Dependence of deformation behaviour on temperature is analyzed. The microstructure of original and deformed material is also studied by means of optical microscopy.

Nuclear magnetic resonance is a method, which provides information about the magnitude of hyperfine field present on izotope's nuclei with non-zero spin. The hyperfine field is sensitive to the local ordering of atoms surrounding the nuclei. The hyperfine field changes can be observed in response to changes in surroundings caused by substitution. The NMR spektra of 57Fe nuclei in the hexagonal ferrites with magnetoplumbit structure which contain cation substitution of trivalent cations Nd, Pr in crystallographic position of divalent cations Sr are measured and interpreted in this work. All experiment are measured at 4.2 K and we investigate the influence of the cation substituion on hyperfine field in thr measured oxides.

Title: Preparation and characterization of substituted Y ferrites in the form of ceramics and thin films Author: Dorota Pulmannová Department: Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague Supervisor: RNDr. Daniel Nižňanský, Ph.D. Consultant: Ing. Josef Buršík, CSc. Abstract: In this work we describe a preparation and characterization of a hexagonal ferrite series with composition BaSrCoZnXFe11O22 where X=Fe, Al, Ga, In and Sc. We have prepared these ferrites in the powder and ceramic form using the citrate synthesis and in the thin film form using the chemical solution deposition method. Using the powder neutron diffraction we have found that the sample containing only Fe has collinear magnetic structure that belongs to the C2/m or C2'/m' group. Magnetic structure of the samples substituted with In and Sc is similar, but the magnetic moments of the 18hVI site atoms are not aligned parallely with the other moments. Magnetic structure of Ga-substituted sample is different, it is modulated with a propagation vector k ≈ (0, 0, 3/4). Propagation vector of the Al-substituted ferrite is k ≈ (0, 0, 3/2). Substituting elements show strong preferences for the cation sites. Al and Ga prefer the 3bVI site, Zn prefers the tetrahedral 6cIV and In and Sc prefer the 6cVI site. Room...

Systematic measurements of the magnetic moment in dependence on temperature and magnetic field of hexagonal 6H-BaTiO3 + 0.04 BaO + x/2 Fe 2 O 3 (0.005 x 0.05) ceramics were performed to study the influence of Fe ions on the magnetic properties. While the samples show Curie–Weiss paramagnetism for Fe concentrations 1.0 mol%, antiferromagnetic interactions become manifest for 2.0 and 5.0 mol% iron. With increasing Fe content the antiferromagnetic interaction, which is assumed to be caused by a superexchange mechanism Fe 3+ Ti(1) − O 2− O(2) − Fe3+ Ti(2) , becomes stronger. At external magnetic fields smaller than 1 T a further, ferromagnetic interaction between Fe 3+ ions is detected below 200 K. The interactions between Fe 3+ ions in the samples with 2.0 and 5.0 mol% iron are also manifest in the EPR spectra by numerous lines with low intensity. Q-band EPR investigations of 5.0 mol% Fe doped single crystals confirm the existence of only one type of Fe 3+ –V O associates in the samples.

The development of metals that can overcome the strength-ductility-weight trade-off has been an ongoing challenge in engineering for many decades. A promising option for making such materials are Metal matrix composites (MMCs). MMCs contain dispersions of reinforcement in the form of fibers, particles, or platelets that significantly improve their thermal, electrical, or mechanical performance. This dissertation focuses on reinforcement with two-dimensional (2D) materials due to their unprecedented mechanical properties. For instance, compared to steel, the most well-studied 2D material, graphene, is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa). Examples of reinforcement by graphene have achieved increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. However, the superior mechanical properties of graphene are not fully transferred to the matrix in conventional MMCs, a phenomenon known as the "valley of death." In an effort to develop key insight into how the relationships between composite design, processing, structure, properties, and mechanics can be used to more effectively transfer the intrinsic mechanical properties of reinforcements to bulk composite materials, nanolayered composite systems made of Ni, Cu, and NiTi reinforced with graphene or 2D hexagonal boron nitride h-BN is studied using experimental techniques and molecular dynamics (MD) simulations. / Doctor of Philosophy / The design of new metals with concurrently improved strength and ductility has been an enduring goal in engineering for many decades. The utilization of components made with these new materials would reduce the weight of structures without sacrificing their performance. Such materials have the potential to revolutionize many industries, from electronics to aerospace. Traditional methods of improving the properties of metals by thermomechanical processing have approached a point where only minor performance improvements can be achieved. The development of Metal matrix composites (MMCs) is among the best approaches to achieving the strength-ductility goal. Metal matrix composites are a class of materials containing reinforcements of dissimilar materials that significantly improve their thermal conductivity, electrical conductivity, or mechanical performance. Reinforcements are typically in the form of dispersed fibers, particles, or platelets. The ideal reinforcement materials have superior mechanical properties compared to the metal matrix, a high surface area, and a strong interfacial bond with the matrix. Two-dimensional (2D) materials (materials made up of a single to a few layers of ordered atoms) are attractive for reinforcement in composite materials because they possess unprecedented intrinsic properties. The most well-studied 2D material, graphene, is made of a single layer of carbon atoms arranged in a hexagonal honeycomb pattern. It is nearly forty times stronger (130 GPa) and five times stiffer (1 TPa) than steel. Examples of graphene reinforcing have shown increases in strength of 60% due to load transfer at the metal/graphene interface and dislocation blocking by the graphene. Despite their exceptional mechanical properties, the superior mechanical properties of graphene are not fully transferred to the matrix when incorporated into conventional metal matrix composites. This phenomenon, known as the "valley of death," refers to the loss of mechanical performance at different length scales. One cause of this phenomenon is the difficulty of evenly dispersing the reinforcements in the matrix using traditional fabrication techniques. Another is the presence of dislocations in the metal matrix, which cause very large local lattice strains in the graphene. This atomistic-scale deformation at the interface between the metal and the graphene can significantly weaken it, leading to failure at low strains before reaching its intrinsic failure stress and strain. This dissertation aims to provide insight into how the relationships between composites' design, processing, structure, properties, and mechanics can be used to transfer intrinsic mechanical properties of reinforcements to bulk composite materials more effectively. For this, nanolayered composite systems of Ni and Cu reinforced with graphene or 2D h-BN were studied using experimental techniques and molecular dynamics (MD) simulations to elucidate the underlying mechanisms behind the composites' material structure and mechanical behavior. Additionally, we explore the incorporation of graphene in a metallic matrix that does not deform through dislocations (or shear bands), such as the shape memory alloy nickel-titanium ( Nitinol or NiTi), to avoid low strain failure of the metal/graphene interface. This theoretical strengthening mechanism is investigated by designing and fabricating NiTi/graphene composites.