Rozprawy doktorskie: „Electromagnetism - electricity”

1

Borges, A. Tarisco. "Mental models of electromagnetism". Thesis, University of Reading, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.342567.

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Nascimento, Fernando José Barros. "Sequência de práticas com recursos multimídia para o ensino de eletromagnetismo no EJA e PROEJA". Volta Redonda, 2017. https://app.uff.br/riuff/handle/1/4700.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Universidade Federal Fluminense. Instituto de Ciências Exatas. Programa de Pós-Graduação em Ensino de Física. Volta Redonda, RJ
O ensino de Física utilizando-se apenas a lousa, algumas exemplificações abstratas e matematizações como fundamento se revela inócuo para a modalidade de Ensino de Jovens e Adultos como o EJA e PROEJA, especialmente a eletricidade básica, o Magnetismo e Eletromagnetismo, em que, muito embora sejam interessantes na produção de conhecimento que vise ao entendimento sobre a geração de energia elétrica, tema importante em nossa sociedade atual, pouca ou nenhuma oportunidade é dada ao professor e ao aluno, no sentido de poderem desenvolver esses três temas de forma interligada, concisa e pragmática. Assim, motivar o aluno com o saber fazer, com resultados mais objetivos, ajudando-os na manipulação das práticas com a orientação do professor e o uso de vídeos livremente disponíveis ou editados dos aplicativos mais usuais para o ensino de Física, contribui para aprendizagem mais significativa, permitindo o pleno desenvolvimento das capacidades do alunado. As matérias aqui propostas de forma geral não são ensinados pelos professores na modalidade de ensino referida, até mesmo para os discentes cuja idade e série seguem os padrões considerados regulares ou próprios, o que torna as atividades aqui elaboradas de extrema importância para o ensino de Física no século XXI.
Teaching Physics using only the board, some abstract exemplification and too much mathematics the foundation reveals innocuous to the form of Youth and Adult Education as the EJA and PROEJA, especially the basic electricity, magnetism and electromagnetism, which even though they are interesting in the production of knowledge aimed at the understanding of the generation of electricity, a major issue in our society today, little or no opportunity is given to the teacher and student, in order to be able to develop these three themes interconnected, concise and pragmatic way. So Motivating students with the know-how, more objective results, helping them handling practices with the teacher's guidance and the use freely available videos or of even edited videos of the most common applications for teaching physics, contributes to more meaningful learning allowing the full development of the student body's capabilities. The materials proposed here generally are not taught by teachers in that type of education, even for students whose age series follow the standards considered regular or own, which makes the activities here developed extremely important for teaching physics at XXI century.

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Lombardi, Giulia. "Unified nonlinear electrical interfaces for hybrid piezoelectric-electromagnetic small-scale harvesting systems". Thesis, Lyon, 2020. http://www.theses.fr/2020LYSEI101.

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Ce travail de recherche présente des interfaces électroniques non-linéaires pour des systèmes hybrides de récupération d'énergie combinant des transducteurs piézoélectriques et électromagnétiques. Ces systèmes ont reçu un grand intérêt en raison de leur capacité à convertir les vibrations mécaniques en énergie électrique suffisante pour alimenter des capteurs à faible puissance. Afin d'alimenter ces appareils microélectroniques, une fois l'énergie convertie, une extraction efficace et intelligente doit être mise en place avec une unité dédiée. Les interfaces hybrides non-linéaires proposées dans ce travail, visant à inclure autant de parties électroactives que possible dans le même circuit, permettent une augmentation de la puissance de sortie finale des microgénérateurs concernés, ainsi qu'une solution pour obtenir une valeur commune de charge optimale, même si chacun des éléments traités présentent des principes de fonctionnement et des valeurs de charge optimale différents. Une première solution est dérivée du SSHI (Synchronized Switch Harvesting on Inductor) et se base sur la technique de commutation synchronisée. Cette méthode vise à remplacer l'inductance passive dans l'interface SSHI par un système électromagnétique actif, conduisant à une interface de microgénérateurs entièrement actifs et augmentant la puissance de sortie finale. Une deuxième solution est issue de la combinaison des techniques SECE (Synchronous Electric Charge Extraction) et SMFE (Synchronous Magnetic Flux Extraction), respectivement développées pour les systèmes piézoélectriques et électromagnétiques. Son principe de base consiste à transférer l'énergie de l’élément piézoélectrique vers le transducteur électromagnétique, et ensuite à extraire l'énergie du système électromagnétique, préalablement amplifiée par le transfert de charges issues du dispositif piézoélectrique. La stratégie consistant à inclure autant de parties électroactives que possible dans la même interface électrique ouvre de nouvelles possibilités de combiner plusieurs systèmes électroactifs, constituant des récupérateurs d'énergie hybride, sans inclure des étages supplémentaires dans les circuits, ce qui permet de maintenir une relative simplicité sans perte de puissance significative
In this research work, electronic nonlinear interfaces for hybrid energy harvesting systems combining piezoelectric and electromagnetic transducers are presented. Such systems have received great attention due to their ability to detect mechanical vibrations and convert them into electrical energy sufficient to power low-power sensors. In order to supply these microelectronic devices the generated sinusoidal signal needs to be rectified into a constant DC voltage. In other words, once the energy is converted, a proper and smart extraction of such energy needs to be implemented with a dedicated unit. The proposed nonlinear hybrid interfaces developed in this work, aimed at incorporating as much as electroactive parts as possible in the circuit, not only increase the final output power of the involved transducers but also provide a solution for obtaining a common optimal load value, despite dealing with elements singularly presenting different working principles and values of optimal load, without the use of additional load adaptation stages. A first solution is derived from the previously developed SSHI (Synchronized Switch Harvesting on Inductor) and based on the Synchronized Switching technique. This method aims at replacing the passive inductor in the SSHI interface with an active electromagnetic system, leading to an all-active microgenerators interface and increasing the final output power. A second solution is derived from a combination of the SECE (Synchronous Electric Charge Extraction) and SMFE (Synchronous Magnetic Flux Extraction) techniques. Its main principle consists of transferring the energy from the piezoelectric to the electromagnetic transducer and then extracting the boosted energy from the electromagnetic system. The strategy of including as much as electroactive parts within the same electrical interface open many different possibilities of interfacing more than one electroactive system, constituting hybrid energy harvesters, without including extra circuit stages, thus maintaining a relative simplicity without high power losses

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Henning, Andrew John. "Electromagnetic wave chaos in photonic crystals". Thesis, University of Nottingham, 2009. http://eprints.nottingham.ac.uk/11155/.

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Similarities in the form of the Schrodinger equation that governs the behaviour of electronic wavefunctions, and Maxwell’s equations which govern the behaviour of electromagnetic waves, allow ideas that originated in solid state physics to be easily applied to electromagnetic waves in photonic structures. While electrons moving through a semiconductor experience a periodic variation in charge, in a photonic crystal electromagnetic waves experience a periodic variation in refractive index. This leads to ideas such as bandstructure being applicable to the one and two dimensional photonic crystals used in this work. The following work will contain theoretical and experimental studies of the transmission through, and electric fields within, one dimensional photonic crystals. A slow variation in the structure of these crystals will lead to the bandstructure shifting, with an photonic analogy of electronic Bloch oscillations and Wannier-Stark ladders being seen in these structures. The two dimensional photonic crystals will be shown, through Hamiltonian ray tracing, to support both stable and chaotic ray paths. Examination of the phase space reveals the existence of ‘Dynamical Barriers’, regions in phase space supporting stable ray trajectories that divide separate regions in which the ray trajectories are chaotic. Various manners in which the bandstructure may be varied will be presented, along with a proposed switch that may be made using these structures. While the ray tracing will be carried out in photonic crystals in the limit of infinitesimally thin dielectric sheets, the model will then be developed to show the bandstructure of a photonic crystal made from finite width dielectric sheets, with examples of dispersion surfaces for these structures being presented.

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5

Young, Jeffrey Lee. "Electromagnetic response of thin wires over an homogeneous earth". Diss., The University of Arizona, 1989. http://hdl.handle.net/10150/184906.

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The electromagnetic response of infinitely long, thin wires over a flat earth is presented for two different applications: the shielding properties of an ensemble of parallel wires excited by a plane wave and the electromagnetic coupling of two perpendicular wires excited by a dipole. The shielding study begins with the formulation of the boundary value problem for N wires over a lossy half space. A suitable axial impedance operator is applied to obtain a system of equations whose unknowns are the currents flowing on each wire. Once the currents are determined, the aggregate field produced by the ensemble can be computed by summing N Fourier type integrals. For the specialized case of the infinite planar grid, Floquet's Theorem and Poisson's Summation Formula are invoked, transforming the linear system of equations into a closed form expression for the current flowing on each wire. We show that the electromagnetic response of the planar grid of finite extent and the grid of infinite extent are similar. For non-planar configurations, such as the semi-circular shell, shielding values of 60 dB are possible when the structure is of non-resonant dimensions; otherwise, the performance can degrade to 20 dB. In the case of the crossed wire configuration, the starting point is the development of the integral equations that govern the coupling between wires and the source; the unknowns are the spectral currents flowing in each wire. The equations are given in terms of generalized impedance functions for the situation where the wires are over a stratified earth. However, for the numerical work, only the case where the wires are in an unbounded, homogeneous medium is considered. Two numerical methods, with overlapping regions of validity, are applied: the method of moments and the method of multiple scatterers. By using the method of moments, we can obtain a matrix equation that will determine the spectral currents for any wire spacing. The multiple scatterer method leads to a more convenient matrix series solution and shows that the coupling strength is proportional to 1/d², where d is the wire separation, plus higher order inverse terms.

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Marcum, Frank. "Analysis of a test technique for hardness surveillance". Ohio : Ohio University, 1988. http://www.ohiolink.edu/etd/view.cgi?ohiou1182866116.

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Simmons, Daniel. "Hybrid methods for modelling advanced electromagnetic systems using unstructured meshes". Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/33230/.

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The aim of this project is the conception, implementation, and application of a simulation tool for the accurate modeling of electromagnetic fields within inhomogeneous materials with complex shapes and the propagation of the resulting fields in the surrounding environment. There are many methods that can be used to model the scattering of an electromagnetic field, however one of the most promising for hybridisation is the Boundary Element Method (BEM), which is a surface technique, and the Unstructured Transmission Line Modeling (UTLM) method, which is a volume technique. The former allows accurate description of the scatterer's boundary and the field's radiation characteristics, but cannot model scattering by materials characterized by a non-uniform refraction index. The latter, on the contrary, can model a very broad range of materials, but is less accurate, since it has to rely on approximate absorbing boundary conditions. A method resulting in the hybridisation of BEM and UTLM can be used to construct a tool that takes into account both the interaction with non-uniform tissue and propagation in its environment. The project aims to describe in detail the implementation of the novel method, and deploy it in a heterogeneous distributed computing environment.

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Cobos, Sanchez Clemente. "Forward and inverse analysis of electromagnetic fields for MRI using computational techniques". Thesis, University of Nottingham, 2008. http://eprints.nottingham.ac.uk/10629/.

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MRI has become an invaluable tool for diagnostic medicine. Its operation is based on the principles of electromagnetism that are dictated by Maxwell's equations. MRI relies on the existence of well defined, spatially and temporally controlled magnetic fields, which are usually generated by coils of wire. Human exposure to these fields has become a safety concern, especially with the increase in the strength of the magnetic fields used. In this thesis, problems in electromagnetism relevant to different areas in MRI and involving the calculation of solutions to both forward and inverse problems are investigated using techniques derived for computational mechanics. The first section of the work focuses on the development of an accurate technique for the solution of magnetostatic inverse problems using boundary element methods (BEM) with the aim of designing optimised gradient coils. This approach was found to be an extremely effective method which can be applied to a wide range of coil geometries and is particularly valuable for designs where the coil surface has low symmetry. BEM-based approaches to designing gradient coils that reduce the likelihood of peripheral nerve stimulation due to rapidly switched magnetic fields are also considered. In the second section of the work, a novel BEM tool to allow the calculation of solutions to quasi-static forward problems has been developed, and used for the evaluation of the electric fields induced in the human body by temporally varying magnetic fields, due to either gradient switching or body movements in strong static magnetic fields. This approach has been tested by comparison with analytic solutions for simply shaped objects, exposed to switched gradients or moving in large static fields, showing good agreement between the results of numerical and analytical approaches. The BEM approach has also been applied to the evaluation of the electric fields induced in human body models. This work involved the development of an appropriate theoretical framework for the study of conducting systems moving in magnetic fields. This involved correcting some misconceptions that had propagated in the literature and allowed the development of an effcient implementation of a BEM suited to this problem.

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Knaak, Allison. "3D synthetic aperture for controlled-source electromagnetics". Thesis, Colorado School of Mines, 2015. http://pqdtopen.proquest.com/#viewpdf?dispub=3682006.

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Locating hydrocarbon reservoirs has become more challenging with smaller, deeper or shallower targets in complicated environments. Controlled-source electromagnetics (CSEM), is a geophysical electromagnetic method used to detect and derisk hydrocarbon reservoirs in marine settings, but it is limited by the size of the target, low-spatial resolution, and depth of the reservoir. To reduce the impact of complicated settings and improve the detecting capabilities of CSEM, I apply synthetic aperture to CSEM responses, which virtually increases the length and width of the CSEM source by combining the responses from multiple individual sources. Applying a weight to each source steers or focuses the synthetic aperture source array in the inline and crossline directions. To evaluate the benefits of a 2D source distribution, I test steered synthetic aperture on 3D diffusive fields and view the changes with a new visualization technique. Then I apply 2D steered synthetic aperture to 3D noisy synthetic CSEM fields, which increases the detectability of the reservoir significantly. With more general weighting, I develop an optimization method to find the optimal weights for synthetic aperture arrays that adapts to the information in the CSEM data. The application of optimally weighted synthetic aperture to noisy, simulated electromagnetic fields reduces the presence of noise, increases detectability, and better defines the lateral extent of the target. I then modify the optimization method to include a term that minimizes the variance of random, independent noise. With the application of the modified optimization method, the weighted synthetic aperture responses amplifies the anomaly from the reservoir, lowers the noise floor, and reduces noise streaks in noisy CSEM responses from sources offset kilometers from the receivers. Even with changes to the location of the reservoir and perturbations to the physical properties, synthetic aperture is still able to highlight targets correctly, which allows use of the method in locations where the subsurface models are built from only estimates. In addition to the technical work in this thesis, I explore the interface between science, government, and society by examining the controversy over hydraulic fracturing and by suggesting a process to aid the debate and possibly other future controversies.

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Barnum, Benjamin Heath. "Electromagnetic and optical characteristics of lightning measured in the earth's ionosphere /". Thesis, Connect to this title online; UW restricted, 1999. http://hdl.handle.net/1773/6843.

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Assi, Fadi Ismail 1963. "Electromagnetic wave scattering by a sphere on a layered substrate". Thesis, The University of Arizona, 1990. http://hdl.handle.net/10150/277882.

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The problem of electromagnetic scattering by a sphere on a layered substrate is treated and numerical results are presented. Initially, the Rayleigh limit, where the sphere radius is small compared to the wavelength, is considered. Closed-form expressions for the far-zone scattered fields are derived from the radiation of the induced dipoles in the sphere in the presence of the substrate, and these incorporate in a rather explicit manner the various parameters of the problem. The general case, where the ratio sphere radius/wavelength is arbitrary, is also considered. A rigorous formulation is used based on the Mie solution for the scattering by a sphere in a homogeneous medium and an extension of Weyl's method for dipole radiation in the presence of a flat surface. Numerical results obtained using the rigorous formulation for electrically small spheres are in very good agreement with those obtained using the Rayleigh approximate method.

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Lee, Robert 1962. "Electromagnetic coupling by a wire penetrating a circular aperture in an infinite planar conducting screen". Thesis, The University of Arizona, 1988. http://hdl.handle.net/10150/276731.

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We consider the problem of electromagnetic propagation along a wire which passes through an aperture. We begin by formulating an integral equation in terms of the electric field at the aperture. The solution of the integral equation allows us to determine parameters of interest such as the equivalent admittance, the current on the wire, and the electric field. We solve the integral equation using both a zeroth order and method of moments approximation for the aperture field. From this we are able to compute the admittance and current in the frequency domain. We next calculate the current response to a transient pulse excitation. The results show that the screen provides little shielding from pulse penetration along the wire. Finally, we calculate solutions for the electric field. The results are given in several contour plots.

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Wright, Diana Beth 1963. "Electromagnetic coupling by a wire through a cavity-backed circular aperture in an infinite screen". Thesis, The University of Arizona, 1988. http://hdl.handle.net/10150/276823.

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The problem of a wire penetrating a circular aperture in an infinite screen and coupling energy into a cavity behind that screen is considered. We formulate an integral equation in terms of the electric field in the aperture. This integral equation is solved using two approximate methods: a zeroth-order approximation valid at low frequencies, and the method of moments. In addition, we introduce an equivalent circuit model to aid in our physical interpretation of the problem. Numerical results for the interior current on the wire and for the equivalent circuit admittance parameters are presented in order to provide a comparison between the two approximations. Inside the cavity, we examine the components of the electric field as a function of position. Finally, the exterior magnetic field far from the aperture is studied as a function of frequency. We examine the relationship between interior resonance features associated with the presence of the cavity and observations of the exterior field.

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Juan, Garcia Pablo. "Evaluación no destructiva de estructuras de obra civil mediante métodos electromagnéticos". Doctoral thesis, Universitat Politècnica de Catalunya, 2011. http://hdl.handle.net/10803/110467.

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Los métodos de evaluación no destructiva en el campo de la ingeniería civil suponen una alternativa a los métodos de control tradicionales. Disponer de técnicas que permitan obtener información relevante acerca de las características físicas de una estructura (o de la evolución de ésta) sin modificar la muestra supone a la vez un ahorro económico y una mayor fiabilidad desde el punto de vista constructivo. La presente tesis desarrolla un método para medir estructuras de hormigón mediante sensores planos, los cuales permiten caracterizar las propiedades electromagnéticas del material poniendo el sensor en contacto con una única cara de la estructura o probeta, lo cual permite medir en múltiples contextos reales como presas o túneles, en los que únicamente se tiene acceso a una cara del material. El principio de funcionamiento de los sensores desarrollados se basa en la transmisión de una onda electromagnética de banda ancha a lo largo de una línea de transmisión plana. La medida del tiempo de propagación y la atenuación que sufre la onda permiten determinar la permitividad, permeabilidad y conductividad del material, que actúa como dieléctrico del sistema. Así, dichas magnitudes actúan como indicadores de las características estructurales del hormigón: en la presente tesis se aplican por un lado a monitorizar el nivel de humedad presente en una muestra de hormigón, y por otro lado, la dosificación de fibras en hormigón reforzado con fibras de acero (HRFA). Desde un punto de vista electromagnético, tanto la presencia de agua como la adición de fibras de acero se modelan de forma análoga, dado que ambos fenómenos implican un incremento de la conductividad resultante, y de la permitividad efectiva, lo cual se justifica mediante los diferentes modelos teóricos de materiales heterogéneos que se han estudiado. De igual forma, los modelos teóricos de las líneas planas permiten extraer las propiedades electromagnéticas a partir de la medida, en el dominio temporal o frecuencial, de la propagación de una onda electromagnética. El desarrollo del método conlleva establecer la geometría adecuada de los sensores empleados en función de cada aplicación, para lo que se ha recurrido al estudio de baluns de banda ancha. Posteriormente, una vez establecida la configuración de medida, la instrumentación empleada, y el procesado de las señales medidas, se ha desarrollado un método de calibración (TYS), adecuado para sensores planos, para los que otros métodos presentan ciertas dificultades. Finalmente, se presenta la aplicación del método propuesto, en primer lugar a la monitorización del fraguado de una losa de mortero durante 28 días, como muestra de la disminución de agua en el hormigón, y en segundo lugar a la medida de la dosificación de fibras de acero en probetas de hormigón, mostrando resultados prometedores de cara a la aplicación de dicha técnica en medidas de campo.
Non-destructive testing and evaluation methods in the field of civil engineering provide an alternative to traditional control methods. The availability of techniques to obtain relevant information about the physical characteristics of a structure (or its evolution) without modifying the sample results in both financial savings and greater reliability from the construction point of view. This thesis develops a method for measuring concrete structures using planar sensors, which can characterise the electromagnetic properties of the material by placing the sensor in contact with only one side of the structure or sample, which allows to measure in many real situations as dams or tunnels, in which only one side of the material can be accessed. The operating principle of the sensors developed is based on the transmission of a broadband electromagnetic wave along a planar transmission line. The measurement of the propagation time as well as the attenuation of the wave, determines the permittivity, permeability and conductivity of the material, which acts as the dielectric of the system. Thus, these variables are indicators of the structural characteristics of concrete: in this thesis they are applied to monitor the humidity level in a sample of concrete, as well as the dosage of fibres in steel fibre reinforced concrete (SFRC). From an electromagnetic point of view, both the presence of water and the addition of steel fibres are modelled similarly, since both phenomena involve an increase in the conductivity and effective permittivity, which is justified by the different theoretical models of composite materials that have been studied. Similarly, the electromagnetic properties of the measurement of the electromagnetic wave propagation can be obtained by means of the theoretical models of planar lines, both in the time or frequency domain. The development of the method involves establishing the appropriate geometry of the sensors used on each application, which entails the study of broadband baluns. Later on, once the measurement configuration and the instrumentation used are established, as well as the processing of the measured signals, a calibration method (TYS) suitable for planar sensors has been developed, for which other methods showed some difficulties. Finally, the application of the proposed method is reported. Firstly, the setting of a mortar slab was monitored for 28 days, showing the decrease of water in the sample, and secondly the dosage of steel fibres in concrete specimens was measured, showing promising results in order to the apply this technique in field measurements.

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Iqbal, Muhammad Zahir. "Structural and electrical characterization of doped graphene and carbon nanotube networks". Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/277570.

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Graphene, a two-dimensional sp2-hybridized network of carbon atoms has received a remarkable cornucopia of new physics and served as a unique model system, due particularly to its electronic properties, which could have interesting applications in electronic, spintronic or quantum devices. The first part of the thesis describes the modulation of graphene¿s structural and electrical properties with various kinds of doping; such as deep ultraviolet irradiation in ambient atmosphere, deep ultraviolet light irradiation in different gaseous environments, and electron beam irradiation. We have fabricated graphene (exfoliated and chemical vapor deposition grown graphene) field effect transistors using photolithography and electron beam lithography and characterized with AFM, Raman spectroscopy and transport measurement using low noise standard lock-in amplifier technique. We have explored how the ultraviolet light exposure tunes the electrical properties of graphene in an ambient atmosphere, confirmed by the shift of Dirac point position towards positive gate voltage, revealing p-type doping for graphene without degradation of mobility. We found that the doping is stable for a time scale of months. This method became more useful when half the graphene device was exposed by ultraviolet light, while the other half part was covered by a mask to make a sharp p-n junction. The doping effect became more prominent and controllable when it was made in an oxygen environment. The most interesting phenomena were observed when doped graphene was restored to a pristine state using ultraviolet light irradiation in a nitrogen environment. Furthermore, we have investigated the doping tunability with ultraviolet light irradiation on mechanically exfoliated single-, bi-, and trilayer graphenes without significantly degrading its charge carrier mobility. In a further study, the structural deformation of graphene was investigated by irradiation of an electron beam. The graphene structure changes its phase in various stages, where graphene transforms gradually from a crystalline to a nanocrystalline form and after a certain irradiation time into an amorphous form. This irradiation effect acts as an n-type dopant for graphene. In this case, mobility decreases with the gradual increase of irradiation dose, which implies the formation of localized states. The second part of the thesis describes carbon nanotube networks as flexible and transparent electrodes for electronic devices, particularly for high frequency applications. The observed results show that at low frequencies, the impedance increases as the density of nanotube networks decreases, as expected. Both the real and imaginary parts of impedance (measured up to 20 GHz) abruptly decrease as the frequency increases over the cut-off frequency. The cut-off frequency not only depends on the carbon nanotube density of the network, but also on the sample geometry. The Nyquist diagram suggests a simple equivalent circuit composed of a parallel combination of a resistor and a capacitor. The experimental results are in line with calculations made by electrochemical spectroscopy simulations. The results show that the electrical behavior is mostly determined by the contact resistance between the nanotubes, which are in a completely disordered distribution in the network. We show that carbon nanotube flexible conducting films, which may be transparent, could be competitive for some applications, such as displays, photovoltaic solar cells or selective sensors.
El grafè, considerat com una xarxa bidimensional d’àtoms de carboni units per enllaços híbrids sp2, és un tema de recerca molt prolífer en els últims anys, com a model de sòlid bidimensional, i molt particularment degut a les seves propietats electròniques, que poden tenir aplicacions interessants en dispositius electrònics, spintrònics o quàntics. La primera part de la Tesi descriu la modificació de les propietats estructurals i elèctriques del grafè utilitzant diferents mètodes per a dopar-lo: radiació ultraviolada d’alta energia (DUV) en atmosfera ambient, DUV en diferents gasos tals com oxigen o nitrogen, o irradiant amb un feix d’electrons (e-beam). Hem fabricat transistors d’efecte de camp (FET) amb grafè (exfoliat a partir del grafit, o bé obtingut per deposició química en fase vapor, CVD) utilitzant fotolitografia i e-beam litografia, i els hem caracteritzat mitjançant AFM, espectroscòpia Raman i mesures de transport elèctric, per a les que hem utilitzat la tècnica d’amplificació de baix soroll, el lock-in. Hem investigat com l’exposició a la llum ultraviolada en atmosfera ambient, modula les propietats elèctriques del grafè, de manera que la posició del punt de Dirac es desplaça cap a tensions de porta positives, cosa que implica dopatge de tipus-p, sense que hi hagi degradació de la mobilitat. El dopatge és estable al menys durant mesos. Amb el mateix mètode, quan només la meitat del dispositiu és exposat a la radiació ultraviolada mentre l’altre meitat és recobert per una màscara metàl·lica, hem obtingut una unió p-n. L’efecte de dopatge és més important i controlable, quan és fet en atmosfera d’oxigen. L’efecte més interessant que hem observat és la reversibilitat, quan el grafè dopat retorna al seu estat primitiu, en ser irradiat amb llum ultraviolada en atmosfera de nitrogen. També hem investigat el dopatge amb llum ultraviolada del grafè exfoliat mecànicament, de una, dues o tres capes, observant que es produeix sense una degradació significativa de la mobilitat dels portadors de càrrega. Posteriorment hem estudiat la deformació estructural del grafè quan és irradiat amb un feix d’electrons. Hem observat canvis estructurals en diferents etapes: el grafè evoluciona gradualment, a partir de la forma cristal·lina, cap a una fase d’estructura nanocristal·lina i finalment, després d’una certa dosi de irradiació, presenta una estructura amorfa. L’efecte d’ irradiar el grafè amb electrons actua com a dopant tipus-n, però en aquest cas la mobilitat decreix en incrementar la dosi, això implica que hi ha formació d’estats localitzats. La segona part de la Tesi tracta de capes primes de nanotubs de carboni, com a elèctrodes flexibles i transparents per a dispositius electrònics, en particular per aplicacions d’alta freqüència. Els resultats obtinguts mostren que, a baixes freqüències, la impedància augmenta en disminuir la densitat de nanotubs, tal com cal esperar. Tan la part real com la part imaginària de la impedància (mesurada fins a 20 GHz) decreixen abruptament en augmentar la freqüència més enllà de la freqüència de tall. La freqüència de tall no depèn únicament de la densitat de nanotubs en la capa, sinó també de la geometria de la mostra. El diagrama de Nyquist es pot interpretar amb un circuit equivalent consistent simplement en una resistència i un condensador en paral·lel. Els resultats experimentals s’ajusten bé a les simulacions fetes per espectroscòpia d’impedàncies (EIS). Els resultats posen en evidència que el comportament elèctric queda majoritàriament determinat per la resistència de contacte entre els nanotubs, que formen la xarxa amb una distribució totalment desordenada. Hem vist que capes primes de nanotubs de carboni conductores i flexibles, que poden ser també transparents, poden ser competitives en diferents aplicacions, com ara pantalles, cel·les solars fotovoltaiques o sensors selectius

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16

Rigla, Pérez Juan Pablo. "Design and characterization of magnetic systems in race-track microtrons". Doctoral thesis, Universitat Politècnica de Catalunya, 2013. http://hdl.handle.net/10803/128671.

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During last four or five decades there has been a growing demand in particle accelerators which can provide electron beams in the energy ranging from 2 MeV to 100 MeV with high energy resolution and good dose control. Other important requirements are that the machines must be compact, of low power consumption, low price and relatively low maintenance cost. There is a variety of sectors interested in such particle accelerators ranging from industry (industrial radiography) to nuclear physics experiments. One type of machines that meet all these requirements are the electron accelerators with beam recirculation. Fair representatives of this class of accelerators are race-track microtrons (RTM). These sources of electron beam are the most efficient equipment for applications with a relatively low beam current and medium energies ranging from 2 MeV to 100 MeV. The aim of the present thesis is to perform studies of some aspects of the RTMs. One part of the thesis is devoted to the design and development of magnetic elements with permanent magnets of two RTMs for different applications. The first one, which is currently under construction at the UPC (Universidad Politécnica de Cataluña), is a novel accelerator with the beam energy variable between 6 MeV and 12 MeV for medical applications (Intraoperative Radiation Therapy treatments). The other machine is a 55 MeV RTM for the detection of explosives by means of photonuclear reactions, which is at the stage of tests at the Skobeltsyn Institute of Nuclear Physics (SINP). The magnetic field in the designed magnets is generated by rare earth permanent magnet (REPM) materials. This allows to get quite compact magnetic systems compatible with high vacuum environment. In the thesis the design and magnetic properties characterization of the magnetic system of these RTMs are carried out. The calculations were performed by means of 2D and 3D simulations using the POISSON, FEMM and ANSYS codes. In the case of the UPC RTM the design of the 180º dipoles, extraction magnets and quadrupole magnet are carried out. For the SINP 55 MeV RTM the optimization of the magnetic field shielding with the aim to reduce the stray magnetic field generated by the extraction magnet is presented. The results of the simulations were confirmed by experimental measurements of the magnetic field of the magnet with the optimized magnetic field shielding. In the other part of the thesis some aspects of the beam dynamics in RTM magnetic systems are studied. A detailed analysis of the fringe - field focusing in RTM dipole magnets is carried out. Equations for calculation of the fringe - field effect on electron beam trajectories are derived and are applied for a study of the end magnets of the UPC 12 MeV RTM. A general formalism for describing the longitudinal beam dynamics in RTMs for electron beams with arbitrary energy and end magnets with arbitrary magnetic field profile is also developed. This formalism is used for the calculation of the phase-slip effect in RTMs with low energy injection.

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17

Taminiau, Tim Hugo. "Optical antennas for single emitters". Doctoral thesis, Universitat Politècnica de Catalunya, 2011. http://hdl.handle.net/10803/132097.

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The interaction of light with matter is a central topic in both fundamental science and applied technology. At the heart of this interaction lies the absorption or emission of a photon by an electronic transition in for example an atom, molecule or semiconductor. Because such quantum emitters are generally much smaller than the wavelength of light, they interact slowly and omnidirectionally with light, limiting their absorption and emission. At radio frequencies similar issues were encountered and addressed long ago. Electrical circuits radiate little because they are much smaller than the corresponding wavelength. To enable wireless communication, they are connected to antennas that have dimensions in the order of the wavelength. These antennas are designed to effectively convert electrical signals into radiation and vice versa. The same concept can be applied in optics. The central idea of this thesis is that the interaction of a quantum emitter with light can be improved by near-field coupling it to the resonant plasmon modes of a metal nano-particle, which then acts as an optical antenna. In this way, excitation and emission rates can be enhanced, and the angular, polarization, and spectral dependence controlled. Chapter 1 of this thesis outlines these concepts and introduces optical antennas for single emitters. The experimental demonstration of optical antennas requires the near-field coupling of a single emitter to a resonant optical antenna. We fabricated optical monopole antennas on scanning probes, so that they can be precisely positioned near single fluorescent molecules. In this way we directly mapped the changes in the excitation and emission of a single quantum emitter as it is scanned near the antenna. Chapter 2 presents the results for the excitation part of the interaction. The enhanced excitation field at the antenna is highly confined (within 25 nm); the emitter only interacts with the antenna mode over this short distance. The antenna resonances were probed directly in the near-field and show that the antenna is indeed an optical analog of a monopole antenna. The experiments in Chapter 3 demonstrate how the antenna controls the emission. If the emitter is placed at the right position and if the antenna is tuned to resonance, the angular emission of the coupled system is determined by the antenna mode, regardless of the orientation of the emitter. In Chapter 4, we exploit that fact. We demonstrate, theoretically and experimentally, that the radiation from a single emitter coupled to a multi-element optical Yagi-Uda antenna is highly directed. We show that by reciprocity such a high directivity both enhances the excitation field and the collection efficiency. An intuitive way to understand optical antennas is as cavities for surface plasmon polaritons. In chapter 5, I present an extended description of the interaction of dipolar emitters with radiation through nano-rod antenna modes, by treating the antenna as a cavity. The results demonstrate how the properties of the antenna modes evolve from macroscopic perfectly conducting antennas to nanoscale plasmonic antennas, and highlight the similarities and differences between optical and conventional antennas. The results presented in this thesis show that optical antennas provide a new way to link single emitters to light. By designing the antenna the absorption and emission properties of the emitter can be tailored. More generally, optical antennas enhance and control light-matter interaction on the nano-scale, making them promising tools for applications in topics as diverse as high resolution near-field scanning optical microscopy, non-linear optics and spectroscopy, and photovoltaic devices.
La interacción entre luz y materia es fundamental tanto en ciencia básica como en tecnología aplicada. En el corazón de esta interacción están la emisión y absorción de fotones en transiciones electrónicas de, por ejemplo, átomos, moléculas o semiconductores. Tales emisores cuánticos son más pequeños que la longitud de onda de la radiación con la que interaccionan. La interacción es entonces lenta y omnidireccional, lo que limita los procesos de absorción y emisión. En radio frecuencias este mismo problema fue resuelto tiempo atrás. Los circuitos eléctricos radián poco por ser más pequeños que las ondas de radio. La comunicación inalámbrica es posible sólo si los circuitos están conectados a antenas con dimensiones del orden de la longitud de onda. Las antenas son diseñadas para convertir efectivamente señales eléctricas en radiación y viceversa. Este principio se extender a la óptica. La idea central de esta tesis es que la interacción entre la luz y un emisor cuántico incrementa cuando éste es acoplando, en el campo cercano, a los modos plasmónicos resonantes de una nano-partícula metálica. La partícula actúa entonces como una antena óptica. Es posible entonces aumentar las tasas de excitación y emisión, y controlar la dependencia angular, espectral y en polarización. El capítulo 1 de ésta tesis explica estos conceptos e introduce las antenas ópticas para emisores individuales. Para implementar experimentalmente una antena óptica es necesario acoplar en campo cercano un emisor individual a una antena resonante. Como las antenas ópticas monopolares fueron fabricadas sobre sondas de barrido, podemos ubicarlas con precisión cerca a una molécula fluorescente. Es así como escaneando un emisor cuántico singular cerca a la antena es posible mapear los cambios en la excitación y la emisión. El capítulo 2 presenta los resultados relativos a la parte de la interacción correspondiente a la excitación. El campo excitado en la antena está altamente confinado (25 nm); el emisor solo interactúa con los modos de la antena dentro de esta pequeña región. Las resonancias, probadas directamente en el campo cercano, muestran que en efecto la antena es el análogo óptico a una antena monopolar. Los experimentos en el capítulo 3 muestran como la antena controla la emisión. Cuando el emisor se ubica en la posición correcta y la antena está en resonancia, la emisión del sistema acoplado es determinada por el modo de la antena, independientemente de la dirección del emisor. El capítulo 4 explora esta característica. Teórica y experimentalmente, hemos demostrado la alta direccionalidad de la radiación de un emisor individual cuando es acoplado a una antena Yagi-Uda de múltiples elementos. Por reciprocidad, esta direccionalidad incrementa tanto el campo de excitación como la eficiencia de acoplamiento. En una forma intuitiva las antenas ópticas se pueden entender como cavidades para los plasmones-polaritones de superficie. Tratando las antenas como cavidades, el capítulo 5 presenta una descripción de la interacción entre los emisores dipolares y la radiación mediada por los modos de las nano-antenas. Los resultados muestran como las propiedades de estos modos evolucionan desde las antenas macroscópicas perfectamente conductoras hasta las nano-antenas plasmónicas. Los resultados también explican las diferencias entre las antenas ópticas y las convencionales. Los resultados presentados en esta tesis prueban que las antenas ópticas son una nueva alternativa para acoplar luz a emisores cuánticos individuales. Las propiedades de absorción y emisión del emisor pueden ser controladas diseñando adecuadamente las antenas. Las antenas ópticas permiten amplificar y controlar la interacción entre radiación y materia en la escala nanométrica, convirtiéndolas en herramientas importantes en campos muy diversos. Por ejemplo, en microscopia óptica de campo cercano, en información cuántica, en óptica no lineal, en espectroscopia y en dispositivos fotovoltaicos

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18

Kleban, Elena. "Investigating electromagnetic properties of brain tissue using 7 T MRI". Thesis, University of Nottingham, 2018. http://eprints.nottingham.ac.uk/51795/.

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Magnetic Resonance Imaging allows electromagnetic properties of the brain to be measured in vivo, providing new markers of structure at a microscopic level. Evaluation of the local complex signal evolution observed using a multi-echo gradient-echo (MEGE) sequence can allow the electromagnetic and relaxivity properties of individual tissue compartments to be accessed. The phase evolution carries valuable information about the different compartments, but is dominated by non-local, large-length-scale field variations which present the main challenge in processing complex MEGE data. In this work 2-compartment and 3-pool models are used to describe signal evolution from a mixed tissue and venous compartment and from white matter, respectively. A new method for removing the effects of non-local field variations without corrupting local non-mono-exponential phase evolution is proposed. The 2-compartment model of venous blood complex signal has been used to access the vascular dependence on orientation and oxygenation levels, and has been validated against existing methods on large distinguishable veins. A phantom consisting of capillary tubes filled with ferritin solution immersed in a water bath has been used to validate the 2-compartment model and the estimated frequency offsets between the ferritin and water compartments at multiple orientations to B⃗0 shown to agree with predictions of theory. White matter in the corpus callosum has been investigated using a saturation-recovery MEGE sequence with variable flip angles with the aim of revealing differences in the T1-relaxation properties of the myelin, the external and the intra-axonal water compartments. The results show that the relative saturation of the myelin water compartment decreases with increasing flip angle and is consistent with there being a short-T1 component associated with myelin water. However, the observed signal behaviour shows less contrast than expected based on the findings from the previous studies. This could be related to differences in exchange rates between compartments. Finally, diffusion-weighted, asymmetric spin-echo imaging was used to simultaneously investigate the diffusivity and electromagnetic properties of the external and intra-axonal compartments of white matter. This approach could provide additional information about white matter microstructure. Asymmetry of the spin echo sequence was achieved by delaying the acquisition by ∆t. The preliminary results show an increase in the scaled magnitude with echo delay at a constant b-value in some regions of the corpus callosum. The preliminary results are promising, but further investigation and method development are required. This thesis has investigated a number of novel methods of studying white matter structure and cerebral microvasculature.

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19

Allgood, Michael David Baginski Michael E. "Finite element analysis of the mesosphere's electromagnetic response to large scale lightning associated with sprites and other transient luminous events". Auburn, Ala, 2008. http://repo.lib.auburn.edu/EtdRoot/2008/SPRING/Electrical_and_Computer_Engineering/Thesis/Allgood_Michael_37.pdf.

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20

Orlando, Andrea. "Numerical modeling and analysis of complex electromagnetic structures, including those containing ferrites". Diss., Rolla, Mo. : Missouri University of Science and Technology, 2009. http://scholarsmine.mst.edu/thesis/pdf/Orlando_09007dcc80705498.pdf.

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Thesis (M.S.)--Missouri University of Science and Technology, 2009.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed November 17, 2009) Includes bibliographical references (p. 83-84).

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21

Turkyilmaz, Serol. "Design And Prototyping Of An Electromagnetic Mems Energy Harvester For Low Frequency Vibrations". Master's thesis, METU, 2011. http://etd.lib.metu.edu.tr/upload/12613637/index.pdf.

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This thesis study presents the design, simulation, and fabrication of a low frequency electromagnetic micro power generator. This power generator can effectively harvest energy from low frequency external vibrations (1-100 Hz). The main objective of the study is to increase the efficiency of the previously proposed structure in METU-MEMS Center, which uses the frequency up-conversion technique to harvest energy from low frequency vibration. The proposed structure has been demonstrated by constructing several macro scale prototypes. In one of the constucted prototypes, the diaphragms are connected to a fixed frame via metal springs. The upper diaphragm having lower resonance frequency carries a magnet, and the lower diaphragm carries a hand wound coil and a magnetic piece for converting 6 Hz external vibrations up to 85 Hz, resulting a maximum voltage and power levels of 11.1 mV and 5.1 µ
W, respectively. In an improved prototype, the metal springs are replaced with rubber ones, providing higher energy conversion efficiency and flexibility to tune the resonance frequency of both diaphragms to desired values. This prototype provides 104 µ
W maximum power and 37.7 mV maximum voltage in response to vibration levels of 30 Hz. The proposed structure is also suitable to be realized by using microfabrication techniques. Hence, the structure to be microfabricated is studied and optimized for this purpose. When scaled to microelectromechanical dimensions, the expected maximum power and voltage from the 10 x 8.5 x 2.5 mm3 generator is 119 nW and 15.2 mV, respectively. A microfabrication process has also been designed for the proposed generator structure. According to this process, the structure consists of a stack of two pieces, each carrying different diaphragms. The diaphragms are made of parylene, and the coil and the magnetic piece are electroplated copper and nickel, respectively. As a result of this study, a new topology is proposed for harvesting energy at low frequency vibrations by the frequency up-conversion technique, and an efficiency improvement is expected with more than three orders of magnitude (119 nanoWatts output for the same size) compared to the study realized in our laboratory in converting low frequency (70-150 Hz) environmental vibrations to electrical energy.

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22

Mallorquí, Jordi J. (Jordi Joan). "Métodos numéricos para aplicaciones biomédicas: problemas directo e inverso electromagnéticos". Doctoral thesis, Universitat Politècnica de Catalunya, 1995. http://hdl.handle.net/10803/6934.

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El objetivo básico de la tesis ha sido el desarrollo de algoritmos de reconstrucción que permiten obtener imágenes de cuerpos biológicos de alto contraste. Los datos de entrada de los distintos algoritmos son los campos dispersados por el cuerpo para distintas ondas incidentes a frecuencias de microondas. Además, se han desarrollado algoritmos para el calculo de los campos dispersados por modelos numéricos, o resolución del problema directo, necesarios para el estudio del problema inverso, o reconstrucción.

Se incluye una extensa recopilación de resultados, tanto simulados como obtenidos a partir de medidas reales, que permiten conocer el comportamiento de los distintos algoritmos en función de los parámetros de diseño de un tomógrafo.

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Tyler, Robert H. "Electromagnetic fields generated by ocean currents and the potential for using geomagnetic data in ocean and climate studies". Thesis, McGill University, 1995. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=39884.

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The ocean currents flowing through the earth's main magnetic field are known to induce secondary magnetic fields. Hence, variations in the ocean circulation induce variations in the net magnetic field. This research is aimed at exploring the potential for using geomagnetic data to study variability in ocean circulation and climate.
First, general relativity theory is used to formally establish the proper set of electromagnetic equations to be used for observers in a rotating (accelerating) frame of reference observing a medium (the ocean, in this case) with relative velocity. Extra terms due to rotation are derived and described and a generalized Schiff's charge density is shown to be potentially significant for the application to ocean circulation.
We extend the theory of electromagnetic fields generated by ocean currents. Many analytical solutions are found for idealized ocean features including sheared flow, jets, and a Stommel gyre. Results indicate that the ocean-induced magnetic fields will typically have magnitudes of 10's-100's of nT within the ocean. Outside of the ocean, the magnitudes are smaller (typically 1-10 nT) but decay on scales set by the horizontal scale of the ocean feature.
We investigate the time-scales associated with the adjustment of electromagnetic fields generated by low-frequency ocean currents. We find that the time scales can be quite long, prohibiting a quasistatic assumption in the treatment of the electromagnetic fields generated by the important tidal, inertial, and diurnal-frequency ocean currents.
Three-dimensional explicit time-dependent and steady-state finite-difference numerical models are constructed to study the electromagnetic fields generated by more realistic ocean current and conductivity features.
The ocean currents generate electromagnetic forces on the fluid at the surface of the earth's core. If these forces lead to significant core motion, the effect of the oceans on the generation and variability of the earth's magnetic field may be nontrivial. We estimate the form and magnitude of these forces and make comparisons with observations. Despite many uncertainties, we find evidence to suggest the ocean forcing mechanism may be significant and conclude that this process should be further investigated in the context of a larger study.
This work indicates that it is likely that the geomagnetic record has captured oceanic signals. From a preliminary data analyses we find that aside from the oceanic tidal signals, the magnetic record shows other signals of possible oceanic origin including an apparent correlation between magnetic records from the equatorial Pacific and the Southern Oscillation Index. We discuss the prerequisites that are necessary to extract information about ocean circulation variability from the geomagnetic record.

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Berg, Matthew James. "A microphysical model of scattering, absorption, and extinction in electromagnetic theory". Diss., Manhattan, Kan. : Kansas State University, 2008. http://hdl.handle.net/2097/1040.

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Ding, Zhiguang. "Electromagnetic Shielding Properties of Iron Oxide Impregnated Kenaf Bast Fiberboard". Thesis, University of North Texas, 2014. https://digital.library.unt.edu/ark:/67531/metadc699998/.

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The electromagnetic shielding effectiveness of kenaf bast fiber based composites with different iron oxide impregnation levels was investigated. The kenaf fibers were retted to remove the lignin and extractives from the pores in fibers, and then magnetized. Using the unsaturated polyester and the magnetized fibers, kenaf fiber based composites were manufactured by compression molding process. The transmission energies of the composites were characterized when the composite samples were exposed under the irradiation of electromagnetic (EM) wave with a changing frequency from 9 GHz to 11 GHz. Using the scanning electron microscope (SEM), the iron oxide nanoparticles were observed on the surfaces and inside the micropore structures of single fibers. The SEM images revealed that the composite’s EM shielding effectiveness was increased due to the adhesion of the iron oxide crystals to the kenaf fiber surfaces. As the Fe content increased from 0% to 6.8%, 15.9% and 18.0%, the total surface free energy of kenaf fibers with magnetizing treat increased from 44.77 mJ/m2 to 46.07 mJ/m2, 48.78 mJ/m2 and 53.02 mJ/m2, respectively, while the modulus of elasticity (MOE) reduced from 2,875 MPa to 2,729 MPa, 2,487 MPa and 2,007 MPa, respectively. Meanwhile, the shielding effectiveness was increased from 30-50% to 60-70%, 65-75% and 70-80%, respectively.

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Dias, Bruno José da Cruz Teixeira. "Application of conducting polymers in nanobiotechnology". Doctoral thesis, Universitat Politècnica de Catalunya, 2011. http://hdl.handle.net/10803/131948.

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Esta tesis describe diferentes aplicaciones en el ámbito de la nanobiotecnología de los polímeros conductores generados electroquímicamente. El objetivo principal de la Tesis, que además es el nexo común de los diferentes estudios que se presentan en los Capítulos 4 al 6, es el análisis de las interacciones entre los polímeros conductores sintéticos y diversas entidades biológicas (ADN, proteínas, polisacáridos, pequeños péptidos, drogas, y células). Uno de los retos más importantes de esta Tesis ha sido la caracterización de las interacciones entre los polímeros conductores y el ADN. Los estudios descritos a lo largo del Capítulo 4 ponen en evidencia la existencia de interacciones específicas por puentes de hidrógeno entre plásmidos de ADN y polímeros conductores con sustituyentes polares. Estas interacciones específicas actúan reforzando las interacciones electrostáticas convencionales intrínsecas a la naturaleza cargada de ambos sistemas. Con la intención de obtener la máxima información acerca de estas interacciones, los estudios se han centrado esencialmente en el poli(3,4-etilendioxitiofeno), un polímero conductor con excelentes propiedades y un amplio número de aplicaciones tecnológicas. Los cambios estructurales que se producen en el ADN como consecuencia de la formación de interacciones específicas, las preferencias del polímero conductor por secuencias nucleotídicas bien definidas, así como por con algunas bases nitrogenadas, han permitido proponer un mecanismo que explica la interacción entre las dos sistemas. Este mecanismo, que también se apoya en cálculos teóricos, es totalmente consistente con todos los datos experimentales descritos en esta Tesis. El Capítulo 5 está dedicado al análisis de la interacción entre las moléculas de morfina y los polímeros conductores. Dentro de este contexto general, este el proposito general de este bloque de resultados ha sido optimizar las condiciones necesarias para la detección de esta droga con poli(N-metilpirrol) y con poli(3,4-etilendioxitiofeno). Los estudios se han realizado teniendo en cuenta el efecto de: la morfología del polímero (es decir, se han examinado las tanto películas planas como películas formadas por microestructuras huecas con morfología tipo rosquilla), el tiempo de incubación con la morfina, y el pH del entono. En todos los casos la detección se ha realizado mediante técnicas electroquímicas, que incluyen tanto la espectroscopia de impedancia electroquímica, como la voltametría cíclica. Los resultados reflejan que, en condiciones controladas, los polímeros estudiados muestran no solo una elevada capacidad para capturar las moléculas de morfina, sino también para retenerlas por un período de tiempo largo. Se ha propuesto la fabricación de un dispositivo portátil para la detección de drogas basado en polímeros conductores. El Capítulo 6 es dedicado a la preparación y caracterización de nuevos biocomposites conductores híbridos para aplicaciones en nanobiotecnología y biomedicina. Concretamente, este bloque de resultados presenta la electropolimerización del poli(3,4-etilendioxitiofeno) con diferentes biomoléculas (es decir, un enzima, poli- y oligosacáridos, y un péptido) y la interacción de estos nuevos materiales híbridos con células. Se ha descubierto que, estos biocomposites conservan las propiedades electroquímicas del polímero conductor y que, además, muestran una prominente actividad celular. Los biocomposites que contienen lisozima, forman películas compactas y estables, muestran una elevada actividad bactericida contra las bacterias Gram (+), la cual es debida a que el enzima se libera de forma controlada. La actividad celular observada en los materiales que contienen dextrinas, sugiere que estos biocomposites son candidatos potenciales para la fabricación de soportes celulares. La incorporación de una baja concentración del CREKA en el polímero, tiene un efecto muy positivo en las propiedades electroquímicas del polímero conductor, mejorandolas.
This Thesis reports different nanobiotechnological applications of electrochemically and electrically conducting polymers. The central, and also common, focus of all the studies and experiments described in Chapters 4 to 6 is the interaction between synthetic conducting polymers and bioentities (DNA, proteins, polysaccharides, small peptides, drugs and cells). One of the major challenges of this Thesis consisted on the characterization of the interactions between conducting polymers and DNA. The studies described in Chapter 4 evidenced that specific hydrogen bonding interactions, in addition to conventional electrostatic interactions, are formed between conducting polymers bearing polar groups and plasmid DNA. In order to get more information about such interactions, studies were essentially focused on poly(3,4-ethylenedioxythiophene), a conducting polymer with excellent technological properties. On the basis of both the structural changes undergone by the DNA upon the formation of specific interactions with the conducting polymer and the preferences of the latter towards well-defined nucleotide sequences and bases, a mechanism has been proposed to explain the interaction between the two macromolecules. This mechanism, which is supported by theoretical calculations, is consistent with all the experimental data reported in this Thesis. Chapter 5 is devoted to examine the interaction between morphine molecules and conducting polymers. Within this general context, this block of results is essentially focused on the optimization of the conditions necessary for the detection of this drug using poly(Nmethylpyrrole) and poly(3,4- ehylenedioxythiophene). Studies have been performed considering the effect of: the polymer morphology (i.e. both flat films and films containing hollow microstructures with doughnut-like morphologies have been examined), the time of incubation with morphine, and the pH of the environment. In all cases detection was carried out using electrochemical techniques, which include electrochemical impedance spectroscopy and cyclic voltammetry. Results reflect that, under controlled conditions, the investigated conducting polymers exhibit a high ability to capture morphine molecules, retaining them for a long period of time. In addition, the fabrication of a portable drug detector device based on conducting polymers has been proposed. Finally, Chapter 6 is devoted to the preparation and characterization of new hybrid conducting biocomposites for nanobiotechnological and biomedical purposes. More specifically, this block of the work presents the electropolymerization of poly(3,4-ethylenedioxythiophene) with different biomolecules (i.e. an enzyme, poly- and oligosaccharides, and a small peptide) and the interaction of these new hybrid materials with cells. It was found that, in general, the prepared biocomposites retain the electrical and electrochemical properties of the individual conducting polymer and, in addition, show a prominent cellular activity. Lysozyme-containing biocomposites, which form compact and stable films, exhibit a high bactericidal activity against Gram (+) bacteria, which is promoted by a controlled release of the enzyme. The remarkable cellular activity of dextrins-containing materials suggests that they are potential candidates for the fabrication of cellular scaffolds. Finally, the incorporation of a low concentration of CREKA peptide into the polymer matrix resulted in a very positive effect on the electrochemical properties of the conducting polymer, which were considerably enhanced.

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Planas, Badenas Ramon. "Stabilized finite element formulations for solving incompressible magnetohydrodynamics". Doctoral thesis, Universitat Politècnica de Catalunya, 2013. http://hdl.handle.net/10803/134740.

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Magnetohydrodynamics (MHD) is the physics branch that studies electrically conducting fluids under external magnetic fields. This thesis deals with the numerical approximation using stabilized finite element methods of two different formulations to model incompressible MHD, namely the resistive and inductionless MHD problems. Further, the linear systems of equations resulting from the application of these discrete formulations to simulate real cases are typically ill-conditioned and can have as many as 10^6-10^9 degrees of freedom. An efficient and scalable solver strategy is mandatory in these cases. On one hand, a new stabilized finite element formulation for the approximation of the resistive magnetohydrodynamics equations has been proposed. The novelty of this formulation with respect to existing ones is that it always converges to the physical solution, even when it is singular, which has been proved through a detailed stability and convergence analysis of the formulation. Moreover, it is inferred from the convergence analysis that a particular type of meshes with a macro-element structure is needed, which can be easily obtained after a straight modification of any original mesh. Finally, different operator splitting schemes have been proposed for solving the transient incompressible resistive MHD system that are unconditionally stable. Two levels of splitting have been considered. On the first level, the segregation of the Lagrange multipliers, the fluid pressure and the magneric pseudo-pressure, from the vectorial fields computation is achieved. On the second level, the fluid velocity and induction fields are also decoupled. This way, the fully coupled indefinite multiphysics system is transformed into smaller uncoupled one-physics problems. On the other hand, a stabilized formulation to solve the inductionless magnetohydrodynamic problem using the finite element method is presented. The inductionless MHD problem models the flow of an electrically charged fluid under the influence of an external magnetic field where the magnetic field induced in the fluid by the currents is negligible with respect to the external one. This system of partial differential equations is strongly coupled and highly nonlinear for real cases of interest. Therefore, solving the multiphysics linear systems of equations resulting from the discretization of these equations with finite element methods is a very challenging task which requires efficient and scalable preconditioners. A new family of recursive block LU preconditioners has been designed to improve the convergence of iterative solvers for this problem. These preconditioners are obtained after splitting the fully coupled matrix into one-physics problems for every variable (velocity, pressure, current density and electric potential) that can be optimally solved, e.g. using preconditioned domain decomposition algorithms. Furthermore, these ideas have been extended for developing recursive block LU preconditioners for the thermally coupled inductionless MHD problem.

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Napolitano, Mario. "Interaction-based nonlinear quantum metrology with a cold atomic ensemble". Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/144558.

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In this manuscript we present an experimental and theoretical investigation of quantum-noise-limited measurement by nonlinear interferometry, or from another perspective, quantum-noise-limited interaction-based measurement. The experimental work is performed using a polarization-based quantum interface between propagating light pulses and cold rubidium-87 atoms trapped in an optical dipole trap. We first review the theory of quantum metrology and estimation theory, and we describe theoretical proposals for nonlinear quantum metrology as developed by the group of Carlton M. Caves in the University of New Mexico. We then describe our proposal, made in 2010, to implement the Caves group's ideas using nonlinear optical interactions in a cold atomic ensemble to implement a nonlinear spin measurement. To evaluate this proposal we develop two theoretical approaches, first an extension of the collective quantum variables approach, often employed to describe quantum interfaces and atomic spin ensembles, to nonlinear optical processes. This results in an effective Hamiltonian containing nonlinear terms of the form described by the Caves group, and demonstrates a qualitative equivalence of the two schemes. The second approach uses the Maxwell-Bloch equations to describe nonlinear propagation of pulses through an atomic spin ensemble, including inhomogeneities and relaxation effects. This latter method makes quantitative predictions about optical rotation signals under realistic experimental conditions. We then describe the implementation of the proposal in a polarization-based light-atom quantum interface. We describe the existing trapping and probing system, focusing on the characteristics that make it suitable for shot-noise-limited and projection-noise-limited atomic spin measurements. We then describe adaptations to use the apparatus with shorter, higher-intensity pulses as required for nonlinear measurement, as well as characterization of the photodetection system under these modified conditions. Calibration of the nonlinear polarization rotation versus probe laser detuning allows us to produce a nearly pure nonlinear rotation signal. Finally, experimental results are presented showing shot-noise-limited nonlinear rotation signals over three orders of magnitude in photon number N. The results are consistent with our theoretical models and confirm a major prediction of the Caves group's work, in that a two-photon interaction gives a scaling for the measurement sensitivity as N^{-3/2}. A brief discussion relates this experimental observation to theoretical discussions of the ¿Heiseinberg limit¿ of quantum metrology, and possible further applications of nonlinear measurement techniques.
En aquest manuscrit presentem una recerca experimental i teòrica sobre mesures limitades pel soroll quàntic fetes mitjançant interferometria no lineal, o des de un altra perspectiva, mitjançant interacció. En el treball experimental es va fer servir una interfície quàntica de polarització entre polsos de llum en propagació i àtoms freds de rubidi-87 atrapats en una trampa òptica de dipol. Primer, farem un repàs de la teoria de la metrologia quàntica i de la teoria de la estimació, descriurem la proposició teòrica sobre metrologia quàntica no lineal tal i com la va desenvolupar el grup de Carlton M. Caves al Universitat de Nou Mèxic. A continuació descriurem la nostra proposta, feta al 2010, de com implantar la idea del grup de Caves fent servir interaccions òptiques no lineals en un conjunt d’àtoms freds amb la finalitat d’efectuar una mesura no lineal de spin. Per avaluar aquesta proposta vam desenvolupar dues aproximacions teòriques fent ús de dos mètodes diferents. En primer lloc vam estendre la tècnica de variables quàntiques col lectives cap als processos òptics no lineals, aquesta tècnica sovint és utilitzada per descriure interfícies quàntiques i conjunts de spin atòmics. Això dóna com a resultat un Hamiltonià efectiu que conté termes no lineals de la forma descrita pel grup de Caves, i demostra una equivalència qualitativa entre el nostre esquema i el seu. El segon mètode fa ús de les equacions de Maxwell-Bloch per descriure la propagació no lineal dels polsos a través del conjunt de spins atòmics, tenint en compte deshomogeneïtats i efectes de relaxació. D’aquesta manera podem fer prediccions quantitatives sobre senyals de rotació de polarització òptica en les condicions d’un experiment real. Seguirem amb la descripció de com vam implementar al laboratori la nostra proposta teòrica mitjançant una interfície quàntica de polarització entre llum i àtoms. Descriurem el ja existent sistema de confinament i sondeig dels àtoms, concentrant-nos en les característiques que permeten fer mesures al limit del soroll quàntic i del soroll de projecció. Aleshores detallarem com vam adaptar el sistema per l’ús amb polsos més curts i intensos, tal i com requereix la mesura no lineal, i al mateix temps com vam calibrar el sistema de detecció de llum en aquestes diferents condicions. El calibratge de la rotació no lineal de polarització en funció de la freqüència del làser de sonda, ens permet obtenir un senyal de rotació casi purament no lineal. Finalment, presentarem els resultats experimentals que mostren senyals de rotació no lineal limitats pel soroll quàntic al llarg de tres ordres de magnitud en el número N de fotons. Tals resultats son consistents amb els nostres models teòrics i confirmen una important predicció del treball del grup de Caves, és a dir que la interacció de dos fotons dóna una llei d’escala de N-3/2 per a la sensibilitat de la mesura. Per concloure, una concisa discussió relaciona aquesta observació experimental amb discussions teòriques sobre el “limit d’Heisenberg” de la metrologia quàntica, i amb d’altres possibles aplicacions de tècniques de mesura no lineal

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Ebertsohn, Nolan Wade. "Cable trays and EMC : modelling and measurement". Thesis, Stellenbosch : Stellenbosch University, 2005. http://hdl.handle.net/10019.1/50293.

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Thesis (MScEng)--University of Stellenbosch, 2005.
ENGLISH ABSTRACT: Common mode currents are a major source of interference in electrical and electronic systems. A universal solution to counteract common mode interference is to introduce an electromagnetic shield with known characteristics. Cable trays are often used to shield cables from unwanted electromagnetic interference (EM!) and its shielding characteristics are defined in terms of its transfer impedance. This thesis pursues the modelling and measurement of the transfer impedance and mutual inductance of non-magnetic cable trays. Theoretical analysis is firstly employed by means of Maxwell's equations after which method of moments (MoM) simulations are performed in order to determine the transfer impedance and mutual inductance within the interior region of a cable tray. The results obtained through simulation are then validated with measurements conducted with an automatic network analyser (ANA). The computational and measured data are in good agreement and the developed model can be used to predict the transfer impedance in the cross-section of non-magnetic cable trays.
AFRIKAANSE OPSOMMING: Gemenernodus strome is 'n bron van interferensie in elektriese en elektroniese stelsels. 'n Universele oplossing om hierdie gemenernodus interferensie teen werk is om 'n elektromagnetiese skerm met bekende eienskappe te implementeer. Geleier leikanale word dikwels gebruik om kabels teen elektromagnetiese interferensie te beskerm en die afskermings eienskappe word in terme van die kanaal se oordragsimpedansie gedefinieer. Hierdie tesis ondersoek die modelering en meting van die wedersydse induktansie en oordragsimpedansie van nie-magnetiese geleier leikanale. 'n Teoretiese analise word eers uitgevoer deur middel van Maxwell se vergelykings waarna metode van momente (MvM) simulasies uitgevoer word om die oordragsimpedansie en wedersydse induktansie in die interne gebied van 'n leikanaal te bepaal. Die resultate verkry deur simulasie word dan bevestig deur meting wat uitgevoer word met behulp van 'n automatiese netwerk analiseerder (ANA). Die numeriese en gemete data stem goed ooreen en die ontwikkelde model kan deurgaans gebruik word om die oordragsimpedansie in die deursnit area van nie-magnetiese geleier leikanale te voorspel.

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Kably, Khalil. "Microdecharges entre hydrometeores : initialisation de l'eclair et rayonnement electromagnetique submicroseconde associe". Toulouse 3, 1988. http://www.theses.fr/1988TOU30142.

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Des hypotheses recentes convergent toutes vers l'idee de l'existence de petites sources localisees a l'interieur du nuage orageux et responsables d'un rayonnement electromagnetique submicroseconde. Des investigations selon deux volets, le premier experimental (en laboratoire) et le second consistant en une modelisation numerique, nous ont permis aujourd'hui d'explorer les diverses possibilites et configurations de microdecharges entre hydrometeores. Ces dernieres sont tres probablement responsables, avec les phenomenes de jonction dans le canal foudre, de la majeure partie du rayonnement rapide lie a la decharge atmospherique. Elles constituent, en outre, la phase d'initialisation de toute decharge prenant naissance a l'interieur du nuage. Les experimentations ont ete menees dans le but de mesurer directement le courant mais egalement le champ electromagnetique rayonne par la microdecharge, permettant en outre, une determination indirecte du meme courant. L'analyse spectrale du champ rayonne a egalement ete effectuee. Ces mesures ont montre que la distance sur laquelle s'effectue la microdecharge joue un role considerable sur l'allure de la signature du courant ainsi que celle du champ rayonne. La modelisation tient compte de la deformation des hydrometeores (due a l'existence d'un champ electrique intense dans le nuage) de leur charge (soit la charge propre, soit celle qui apparait par influence electrostatique) ainsi que de leurs dimensions geometriques (petites ou grosses gouttes d'eau, cristaux de glace,. . . ). La phenomenologie de la microdecharge (corona, arc transitoire) est egalement prise en compte

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31

Bayerl, Albin. "Variability and reliability at the nanoscale of gate dielectrics of MOS devices and graphene based structures". Doctoral thesis, Universitat Autònoma de Barcelona, 2013. http://hdl.handle.net/10803/126515.

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En el primer capítulo de ésta tesis, se les dará un resumen del transistor MOSFET y de las consecuencias del escalado de dispositivos electrónicos. También se explican las alternativas posibles para permitir mantener dicha tendencia, como la introducción de dieléctricos high-k y el potencial del grafeno para aplicaciones en nanoelectrónica. El segundo capítulo se dedica a describir con más detalle el AFM (Atomic Force Microscope), que se ha utilizado para investigar las propiedades eléctricas de los diferentes materiales en la nanoescala. En el tercer capítulo, diferentes condiciones de fabricación de dispositivos basados en capas de HfO2, tales como la temperatura de recocido (y polycrystallization), el espesor y el precursor con el que se crece dichas capas, serán investigadas en la nanoescala. La influencia de una tensión eléctrica a nivel del dispositivo también se estudió. En el capítulo 4, el impacto de las diferentes estreses eléctricos en las propiedades eléctricas de los MOSFETs basados en capas de SiON ultrafinas se investigó a escala manométrica. El uso de un CAFM, el óxido de puerta ha sido analizada después de una NBTI (Bias Temperature Instability) y CHC estrés (Canal portadores calientes). Puesto que con la punta CAFM áreas muy pequeñas pueden ser estudiados, se analizó la degradación inducida a diferentes regiones del óxido de puerta a lo largo del canal. Capítulo 5 describe la invención de un enfoque completamente nuevo para mejorar significativamente las propiedades mecánicas y eléctricas intrínsecas de puntas CAFM disponibles en el mercado por recubrimiento de las mismas con grafeno. Por último, en el capítulo 6, el uso de grafeno para aplicaciones en nanoelectrónica será investigado en la nanoescala. Un aspecto que se analizará es la presencia de ondulaciones, las arrugas y los límites de grano, que se muestran para aumentar la variabilidad de dispositivo a dispositivo. También se estudió el impacto del sustrato sobre el que se transfiere a grafeno. Dado que el grafeno se ha empezado a ser utilizado como electrodo superior en los dispositivos de memoria, en la segunda parte de este capítulo, la variabilidad y la fiabilidad de grafeno-aislante-semiconductor estructuras (GIS) basado en HfO2 serán previamente investigados.
In the first chapter of this thesis, the MOSFET transistor and an overview of the implications of ongoing device shrinking will be given. Possible alternatives to allow the scaling down such as the introduction of high-k dielectrics and the potential of graphene for nanoelectronic applications are also explained. The second chapter will be devoted to describe in more detail the AFM (Atomic Force Microscope), which has been used to investigate the electrical properties of different materials at the nanoscale. In the third chapter, different fabrication conditions of HfO2 layers based devices, such as the annealing temperature (and polycrystallization), the thickness and the precursor with which it was grown will be investigated at the nanoscale. The influence of an electrical stress at the device level is also studied. In chapter 4, the impact of different electrical stresses on the nanoscale electrical properties of ultra-thin SiON based MOSFETs is investigated. Using a CAFM, the gate oxide has been analyzed after a BTI (Bias Temperature Instability) and CHC (Channel Hot Carriers) stress. Since with the CAFM tip very small areas can be studied, the degradation induced at different regions of the gate oxide along the channel was analyzed. Chapter 5 describes the invention of a completely new approach to significantly improve the intrinsic mechanical and electrical properties of commercially available CAFM tips by coating them with graphene. Finally, in chapter 6, the usability of graphene for nanoelectronic applications will be investigated at the nanoscale. One aspect that will be analyzed is the presence of corrugations, wrinkles and grain boundaries, which are shown to increase the device-to-device variability. The impact of the substrate on which graphene is transferred to will be also studied. Since graphene has been recently started to be used as top electrode in memory devices, in the second part of this chapter, the variability and reliability of Graphene-Insulating-Semiconductor (GIS) structures based on HfO2 will be preliminarily investigated.

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Kong, Kang-Chuen. "Real time simulations of EMTP results". Thesis, This resource online, 1993. http://scholar.lib.vt.edu/theses/available/etd-07292009-090324/.

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Zago, Fernando. "O impacto do aumento da urbanização nos niveis das tensões induzidas por descargas atmosfericas nas proximidades de linhas de transmissão". [s.n.], 2009. http://repositorio.unicamp.br/jspui/handle/REPOSIP/260911.

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Orientador: Jose Pissolato Filho
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação
Made available in DSpace on 2018-08-13T22:10:32Z (GMT). No. of bitstreams: 1 Zago_Fernando_D.pdf: 3563533 bytes, checksum: 6e24082f3d66f620688c4145ed029895 (MD5) Previous issue date: 2009
Resumo: Alguns dos principais danos causados por descargas atmosféricas são de origem indireta, ou seja, originam-se dos campos eletromagnéticos provenientes deste tipo de fenômeno natural resultando em problemas de compatibilidade eletromagnética ou em desafios para os projetistas de sistemas de proteção de surtos e de blindagens contra este tipo de interferência. Este trabalho trata de um destes efeitos indiretos, tensões induzidas em linhas aéreas por descargas atmosféricas próximas, utilizando-se das mais recentes pesquisas na área de campos eletromagnéticos provenientes deste tipo de fenômeno natural. Através de uma rigorosa formulação matemática do canal da descarga atmosférica, assim como, dos campos eletromagnéticos provenientes dele, no caso de conexão direta com o solo ou com um objeto elevado (torre ou prédio aterrados), e da consideração de diferentes expressões para o cálculo do raio de atração dos objetos elevados, foi possível realizar um estudo comparativo dos efeitos da urbanização nas proximidades de uma linha aérea com relação ao aumento ou diminuição dos níveis de tensões e correntes induzidas quando da ocorrência de descargas atmosféricas próximas. Estes efeitos foram analisados através de simulações computacionais considerando variações em diversos parâmetros associados à urbanização, tais como: quantidade de objetos elevados, altura média dos objetos elevados, modelo de raio de atração e densidade de ocupação ou de urbanização. A partir da pesquisa realizada durante este trabalho e do programa computacional desenvolvido, são propostas metodologias para se avaliar as modificações nos padrões de ocorrência dos níveis de tensões induzidas de origem atmosférica em linhas aéreas localizadas em diferentes cenários de ocupação urbana ou nas proximidades de objetos elevados como, por exemplo, torres de telecomunicações.
Abstract: Some of the worst damage caused by lightning is indirect, that is, it is caused by electromagnetic fields radiated from this kind of natural phenomenon resulting in electromagnetic compatibility problems or in challenges for designers of surge protection systems and shields against these interferences. This work considers one of these indirect effects, induced voltages on overhead lines by nearby atmospheric lightning discharges, using some of the most recent research in the area of electromagnetic fields generated by these natural phenomena. Starting from a rigorous mathematical formulation for a lightning channel, as well as for the electromagnetic fields generated by them, for the case of lightning striking the soil directly or an elevated grounded object, it was possible to carry out a comparative study of the urbanization effects close to an overhead line with respect to the increase or decrease of the induced voltages and currents when nearby lightning discharges occur. These effects were analyzed using computational simulations considering variations in some parameters associated with urbanization, such as: number of elevated objects, elevated objects average height, different models for the calculation of attractive radius and occupation or urbanization density. Taking into account the research and the computational program developed in this work, methodologies to evaluate the modifications on the occurrence patterns of lightning induced voltages on overhead lines localized at different urban occupation landscapes or at the vicinity of elevated objects like telecommunication towers, were proposed.
Doutorado
Energia Eletrica
Doutor em Engenharia Elétrica

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Llatser, Martí Ignacio. "On the scalability limits of communication networks to the nanoscale". Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/144561.

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Nanosystems, integrated systems with a total size of a few micrometers, are capable of interacting at the nanoscale, but their short operating range limits their usefulness in practical macro-scale scenarios. Nanonetworks, the interconnection of nanosystems, will extend their range of operation by allowing communication among nanosystems, thereby greatly enhancing their potential applications. In order to integrate communication capabilities into nanosystems, their communication subsystem needs to shrink to a size of a few micrometers. There are doubts about the feasibility of scaling down current metallic antennas to such a small size, mainly because their resonant frequency would be extremely high (in the optical domain) leading to a large free-space attenuation of the radiated EM waves. In consequence, as an alternative to implement wireless communications among nanosystems, two novel paradigms have emerged: molecular communication and graphene-enabled wireless communications. On the one hand, molecular communication is based on the exchange of molecules among nanosystems, inspired by communication among living cells. In Diffusion-based Molecular Communication (DMC), the emitted molecules propagate throughout the environment following a diffusion process until they reach the receiver. On the other hand, graphene, a one-atom-thick sheet of carbon atoms, has been proposed to implement graphene plasmonic RF antennas, or graphennas. Graphennas with a size in the order of a few micrometers show plasmonic effects which allow them to radiate EM waves in the terahertz band. Graphennas are the enabling technology of Graphene-enabled Wireless Communications (GWC). In order to answer the question of how communication networks will scale when their size shrinks, this thesis presents a scalability analysis of the performance metrics of communication networks to the nanoscale, following a general model with as few assumptions as possible. In the case of DMC, two detection schemes are proposed: amplitude detection and energy detection. Key performance metrics are identified and their scalability with respect to the transmission distance is found to differ significantly from the case of traditional wireless communications. These unique scaling trends present novel challenges which require the design of novel networking protocols specially adapted to DMC networks. The analysis of the propagation of plasmonic waves in graphennas allows determining their radiation performance. In particular, the resonant frequency of graphennas is not only lower than in metallic antennas, but it also increases more slowly as their length is reduced to the nanoscale. Moreover, the study of parameters such as the graphenna dimensions, the relaxation time of graphene and the applied chemical potential shows the tunability of graphennas in a wide frequency range. Furthermore, an experimental setup to measure graphennas based on feeding them by means of a photoconductive source is described. The effects of molecular absorption in the short-range terahertz channel, which corresponds to the expected operating scenario of graphennas, are analyzed. Molecular absorption is a process in which molecules present in the atmosphere absorb part of the energy of the terahertz EM waves radiated by graphennas, causing impairments in the performance of GWC. The study of molecular absorption allows quantifying this loss by deriving relevant performance metrics in this scenario, which show novel scalability trends as a function of the transmission distance with respect to the case of free-space propagation. Finally, the channel capacity of GWC is found to scale better as the antenna size is reduced than in traditional wireless communications. In consequence, GWC will require lower transmission power to achieve a given performance target. These results establish a general framework which may serve designers as a guide to implement wireless communication networks among nanosystems.

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Koldan, Jelena. "Numerical solution of 3-D electromagnetic problems in exploration geophysics and its implementation on massively parallel computers". Doctoral thesis, Universitat Politècnica de Catalunya, 2013. http://hdl.handle.net/10803/130833.

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The growing significance, technical development and employment of electromagnetic (EM) methods in exploration geophysics have led to the increasing need for reliable and fast techniques of interpretation of 3-D EM data sets acquired in complex geological environments. The first and most important step to creating an inversion method is the development of a solver for the forward problem. In order to create an efficient, reliable and practical 3-D EM inversion, it is necessary to have a 3-D EM modelling code that is highly accurate, robust and very fast. This thesis focuses precisely on this crucial and very demanding step to building a 3-D EM interpretation method. The thesis presents as its main contribution a highly accurate, robust, very fast and extremely scalable numerical method for 3-D EM modelling in geophysics that is based on finite elements (FE) and designed to run on massively parallel computing platforms. Thanks to the fact that the FE approach supports completely unstructured tetrahedral meshes as well as local mesh refinements, the presented solver is able to represent complex geometries of subsurface structures very precisely and thus improve the solution accuracy and avoid misleading artefacts in images. Consequently, it can be successfully used in geological environments of arbitrary geometrical complexities. The parallel implementation of the method, which is based on the domain decomposition and a hybrid MPI-OpenMP scheme, has proved to be highly scalable - the achieved speed-up is close to the linear for more than a thousand processors. Thanks to this, the code is able to deal with extremely large problems, which may have hundreds of millions of degrees of freedom, in a very efficient way. The importance of having this forward-problem solver lies in the fact that it is now possible to create a 3-D EM inversion that can deal with data obtained in extremely complex geological environments in a way that is realistic for practical use in industry. So far, such imaging tool has not been proposed due to a lack of efficient, parallel FE solutions as well as the limitations of efficient solvers based on finite differences. In addition, the thesis discusses physical, mathematical and numerical aspects and challenges of 3-D EM modelling, which have been studied during my research in order to properly design the presented software for EM field simulations on 3-D areas of the Earth. Through this work, a physical problem formulation based on the secondary Coulomb-gauged EM potentials has been validated, proving that it can be successfully used with the standard nodal FE method to give highly accurate numerical solutions. Also, this work has shown that Krylov subspace iterative methods are the best solution for solving linear systems that arise after FE discretisation of the problem under consideration. More precisely, it has been discovered empirically that the best iterative method for this kind of problems is biconjugate gradient stabilised with an elaborate preconditioner. Since most commonly used preconditioners proved to be either unable to improve the convergence of the implemented solvers to the desired extent, or impractical in the parallel context, I have proposed a preconditioning technique for Krylov methods that is based on algebraic multigrid. Tests for various problems with different conductivity structures and characteristics have shown that the new preconditioner greatly improves the convergence of different Krylov subspace methods, which significantly reduces the total execution time of the program and improves the solution quality. Furthermore, the preconditioner is very practical for parallel implementation. Finally, it has been concluded that there are not any restrictions in employing classical parallel programming models, MPI and OpenMP, for parallelisation of the presented FE solver. Moreover, they have proved to be enough to provide an excellent scalability for it.

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Martí, Diaz Zeus. "Disseny i caracterització d'un ondulador helicoïdal per a la producció de llum de sincrotró, i millores en el seu procés de construcció". Doctoral thesis, Universitat Autònoma de Barcelona, 2006. http://hdl.handle.net/10803/3384.

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Aquest estudi es presenta com a resultat de la major part de la recerca que he portat a terme en els quatre anys del meu doctorat (2002-2006), el qual ha estat finançat per l'AGAUR (acrònim de Agència de gestió d'ajuts universitaris i de recerca).
Els estudis que es presenten s'han dut a terme en associació al LLS ("Laboratori de Llum de Sincrotró": consorci format per a la Universitat Autònoma de Barcelona, la Generalitat i l'Estat).
En el capítol 1 es presenten els onduladors dins del marc dels sincrotrons, així com de forma resumida les propietats principals dels primers. Es tracta doncs d'una introducció teòrica.
El contingut dels capítols 2 i 4 va ser recollit i desenvolupat durant la meva estada al sincrotró Elettra de Trieste, Itàlia.
El capítol 2 conté una descripció intensiva i exhaustiva de les tècniques i mètodes de caracterització i implementació dels onduladors. Es tracta d'una descripció del mètode experimental.
El capítol 3 conté una sèrie d'estudis de diferents mètodes per al disseny d'un ondulador. Aquests mètodes han estat desenvolupats per l'autor per tal d'implementar un dels 4 onduladors que s'instal·larà en el futur sincrotró ALBA a Cerdanyola, Espanya.
En el capítol 4 descriu el muntatge d'un ondulador de polarització helicoïdal. En aquest apartat ressaltarem els problemes originats per les inhomogeneïtats dels blocs magnètics.
En el capítol 5 es planteja un nou mètode d'ordenació de blocs per al muntatge d'onduladors basat en el desenvolupament multipolar. Amb aquest mètode es pretén solucionar les mancances d'altres mètodes existents com els utilitzats en el capítol 3, en particular les mancances relacionades amb les inhomogeneïtats dels blocs.
En el capítol 6 es presenten les conclusions resumides de tots aquests estudis.
Este estudio se presenta como resultado de la mayor parte de la búsqueda que he llevado a cabo en los cuatro años de mi doctorado (2002-2006), el cual ha estado financiado por el AGAUR (acrónimo de Agencia de gestión de ayudas universitarias y de búsqueda).
Los estudios que se presentan se han llevado a cabo en asociación al LLS ("Laboratorio de Luz de Sincrotón": consorcio formado por la Universidad Autónoma de Barcelona, la Generalidad y el Estado).
En el capítulo 1 se presentan los onduladores dentro del marco de los sincrotones, así como de forma resumida las propiedades principales de los primeros. Se trata pues de una introducción teórica.
El contenido de los capítulos 2 y 4 fue recogido y desarrollado durante mi estancia al sincrotón Elettra de Trieste, Italia.
El capítulo 2 contiene una descripción intensiva y exhaustiva de las técnicas y métodos de caracterización e implementación de los onduladores. Se trata de una descripción del método experimental.
El capítulo 3 contiene una serie de estudios de diferentes métodos para el diseño de uno ondulador. Estos métodos han estado desarrollados por el autor para implementar uno de los 4 onduladores que se instalará en el futuro sincrotón ALBA a Cerdanyola, España.
En el capítulo 4 describe el montaje de uno ondulador de polarización helicoidal. En este apartado resaltaremos los problemas originados por las inhomogenidades de los bloques magnéticos.
En el capítulo 5 se plantea un nuevo método de ordenación de bloques para el montaje de onduladores basado en el desarrollo multipolar. Con este método se pretende solucionar las deficiencias de otros métodos existentes como los utilizados en el capítulo 3, en particular las deficiencias relacionadas con las inhomogenidades de los bloques.
En el capítulo 6 se presentan las conclusiones resumidas de todos estos estudios.
This study appears as resulting from most of the search which I have carried out in the four years of my doctorate (2002-2006), which has been financed by the AGAUR (acronym of Agència de gestió d'ajuts universitaris i de recerca).
The studies that we present have been carried out in association to the LLS ("Laboratori de Llum de Sincrotró": partnership formed by the Autonomous University of Barcelona, the Catalan local Government and the state Goverment).
In chapter 1 we present the undulators within the frame of synchrotrons, as well as the summarized study of their main properties.
The content of chapters 2 and 4 was gathered and developed during my stay synchrotron Elettra de Trieste, Italy.
Chapter 2 contains an intensive and exhaustive description of the techniques and methods of characterization and implementation of the undulators.
Chapter 3 contains a series of studies of different methods for the design for one undulator. These methods have been developed by the author to implement two of the 4 undulators that will used in the future in synchrotron ALBA in Cerdanyola, Spain.
In chapter 4 it describes the assembly of one undulator of helical polarization. In this section we will emphasize the problems originated by the inhomogeneities of the magnetic blocks.
In chapter 5 a new method of arrangement of blocks for the assembly of undulators considers based on the multipolar development. With this method we try to solve the lacks of other existing methods like the used ones in chapter 3, in particular the lacks related to the inhomogeneities of the blocks. In chapter 6 we summarize the conclusions of all these studies.

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Rouco, Gómez Víctor. "Controlling Vortex Pinning and Dynamics of Nanostructured YBCO Thin Films Grown by Chemical Solution Deposition". Doctoral thesis, Universitat Autònoma de Barcelona, 2014. http://hdl.handle.net/10803/133329.

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Desde el descubrimiento de los Superconductores de Alta Temperatura (SCAT), se ha realizado un gran esfuerzo con tal de optimizar las propiedades eléctricas de estos materiales. A diferencia de los Superconductores convencionales de Baja Temperatura (SCBT), algunos de los SCAT son capaces de alcanzar el estado superconductor a la temperatura de ebullición del nitrógeno (77K) haciéndolos mucho más atractivos en cuanto a aplicaciones tecnológicas dados los reducidos costes de operación. Una de las principales aplicaciones de los SCAT es el transporte eléctrico. La ausencia de resistencia eléctrica de estos materiales cuando están por debajo de su temperatura crítica, Tc, hacen que puedan transportar hasta 10 veces más de potencia que los cables convencionales, o proporcionar una misma potencia con niveles muy inferiores de voltaje. Por otro lado, los SCAT han sido profundamente estudiados en el ámbito de la electrónica. En particular, nanohilos de SCAT pueden ser escalados a tamaños menores debido a sus pequeñas longitudes intrínsecas. Además, sus rápidos coeficientes de relajación permiten altos niveles de adquisición en experimentos de fotodetección cuando éstos son comparados con SCBT. No obstante, una de las características más importantes en el estudio de los SCAT es la presencia de líneas de flujo magnético cuantificadas, denominadas vórtices, en su diagrama de fase. Su mayor diferencia con respecto a los SCBT surge cuando se tienen en cuenta las altas excitaciones térmicas. Como consecuencia, una gran variedad de defectos de anclaje son necesarios con la finalidad de evitar (o controlar) la dinámica de los vórtices y por lo tanto, la disipación eléctrica resultante. El control del anclaje y la dinámica de los vórtices es la finalidad de esta tesis donde los principales resultados están presentados en los capítulos 3, 4 y 5. Previamente, en el capítulo 1 presentamos las principales propiedades superconductoras y en particular, las referidas al comportamiento de los vórtices en muestras de YBa2Cu3O7-x (YBCO -superconductor con el más amplio rango de operación-) crecidas por el método de deposición de soluciones químicas (DSQ). En el capítulo 2 exponemos las técnicas experimentales utilizadas así como los procedimientos de medida. En el capítulo 3, un gran análisis sobre el anclaje de vórtices es presentado. Un fenómeno sin precedentes en el anclaje de vórtices es demostrado el cual permite mejorar la densidad de corriente crítica y su anisotropía en presencia de fases secundarias no-superconductoras dentro de la matriz de YBCO. Además, un análisis de la dinámica de los vórtices con respecto a los defectos naturales de muestras de YBCO-DSQ es expuesto. En los capítulos 4 y 5 nos focalizamos en el control de la dinámica de los vórtices en capas finas de YBCO. En el capítulo 4 demostramos un sentido preferencial para el movimiento de los vórtices cuando nanoestructuras asimétricas son diseñadas en la capa de YBCO. Como consecuencia, una rectificación AC-DC es obtenida. En el capítulo 5, los estudios están centrados en la interacción entre materiales superconductores y ferromagnéticos. Se demuestra como introduciendo nanocilindros ferromagnéticos dentro de la matriz del YBCO, un enorme control sobre la densidad de corriente crítica es posible en función del estado magnético de los nanocilindros. Las conclusiones generales están expuestas y resumidas en el último capítulo. Finalmente, el procedimiento analítico utilizado en una simulación teórica presentada en el capítulo 5 es explicado en el apéndice.
Since the discovery of High-Temperature Superconductors (HTS) a huge effort has been devoted in order to optimize the electric power properties of these materials. As a difference from conventional Low-Temperature Superconductors (LTS), some HTS are able to achieve the superconducting state at the boiling point of nitrogen (77K), making these materials much more attractive for technological applications due to their reduced operating costs. One of the main applications of HTS is the electrical transport. Due to the zero electrical resistance of these materials when they are below the critical temperature, Tc, they can transmit up to 10 times more power than conventional cables or can carry equivalent power at much lower voltages. On the other hand, HTS materials have been deeply studied in the field of electronics. In particular, HTS nanowires can be functionally scaled to smaller sizes due to their extremely short intrinsic lengths. Furthermore, their characteristic fast relaxation rates offer higher counting rates in photodetection experiments when compared to LTS. However, one of the most important issues to take into account in the study of HTS, is the presence of quantified magnetic flux lines, referred as vortex, into their phase diagram. The main difference from LTS concerning the vortex behavior arises from the high thermal excitations. Consequently a great variety of different pinning centers are needed in order to avoid (or control) the vortex dynamics and therefore, the resulting electrical dissipation. Controlling vortex pinning and dynamics is the aim of this thesis where the main results are presented in chapters 3, 4 and 5. Previously, in chapter 1 we present the main superconducting properties, and in particular, the regarding to vortex behavior in YBa2Cu3O7-x (YBCO -HTS with the widest operating range-) samples grown following the Chemical Solution Deposition (CSD) technique. In chapter 2 we show the experimental techniques used in our analysis and the measurement procedure. In chapter 3, a huge analysis of vortex pinning is presented. An unprecedented phenomenon for vortex pinning is demonstrated which allows improving the critical current density and its anisotropy in the presence of non-superconducting secondary phases into the YBCO matrix. Furthermore, an analysis of the vortex dynamics with YBCO-CSD natural defects is performed. In chapters 4 and 5 we have focused in controlling vortex dynamics in YBCO thin films. In chapter 4 a preferential sense for the vortex movement is demonstrated when asymmetric nanostructures are patterned in the YBCO layer. Consequently, a controlled AC-DC rectification is obtained. In chapter 5, the studies are centered in the interaction between superconducting and ferromagnetic materials. It is demonstrated that by inserting ferromagnetic nanorods into the YBCO matrix, a huge control of the critical current density is possible as a function of the magnetic state of the nanorod. General conclusions of the thesis are presented and summarized in the last chapter. Finally, the analytical procedure used in a theoretical simulation presented in chapter 5 is exposed in the appendix.

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Massana, Gracia Valentí. "Optimització del procés de construcció d'un ondulador d'imants permanents per a una font de llum sincrotró". Doctoral thesis, Universitat Autònoma de Barcelona, 2013. http://hdl.handle.net/10803/120236.

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Aquesta tesi és un treball d’investigació sobre les inhomogeneïtats dels imants permanents que es fan servir per construir dispositius d’inserció en fonts de llum de sincrotró, en especial onduladors. El treball té dos objectius. En primer lloc, intentar modelitzar l’efecte d’aquestes inhomogeneïtats en els camps magnètics generats pels imants. Principalment s’ha estudiat les seves característiques magnètiques a partir de mesures directes dels blocs, i hem focalitzat l’estudi en les inhomogeneïtats de les components minoritàries del vector magnetització, ja que aquestes representen una de les més grans limitacions en el rendiment dels dispositius d’inserció. En segon lloc, a partir de la mesura d’aquestes característiques magnètiques, el treball s’ha centrat en la manera en que es poden minimitzar els efectes de les inhomogeneïtats als dispositius d’inserció. Normalment, això es fa a través d’una ordenació determinada dels blocs que compensi els errors generats per les inhomogeneïtats. En aquesta part s’han aplicat els mètodes numèrics d’optimització de funcions multivariants. En una primera part es descriu tot l’equip experimental que s’ha usat per fer les mesures magnètiques i mecàniques dels blocs i on s’explica com s’han caracteritzat tots els bancs del laboratori de dispositius d’inserció del sincrotró Alba, determinant l’error associat a cadascun d’ells. Els blocs magnètics individuals, que són els qui formen l’estructura magnètica global de l’ondulador, s’han mesurat amb unes bobines Helmholtz per a obtenir el vector magnetització. Amb aquests resultats, i aplicant un algorisme que minimitzi la component principal de camp, s’han agrupat els imants en grups (mòduls) d’ 1 (singlets) o 3 blocs (triplets). Aquests mòduls s’han mesurat amb bancs de mesura basats en el principi d’inducció de Faraday per determinar la seva signatura, és a dir, la dependència transversal de la integral de camp. Novament, aplicant un algorisme que minimitzi la primera integral de camp total del dispositiu, s’ha determinat l’ordre dels mòduls dins de l’estructura magnètica global. En una segona part, es descriuen els fonaments teòrics que permeten crear models matemàtics dels imants i que permeten implementar programes per ajustar models morfològics parametritzats d’inhomogeneïtats, com també optimitzar l’ordenació dels blocs i optimització final dels dispositius d’inserció. L’objectiu en aquesta segona part ha estat l’elaboració d’un algorisme que permeti construir segments d’ondulador amb baix error a partir d’imants amb un cert grau d’inhomogeneïtat. Finalment, el treball s’ha centrat en fer un seguiment in-situ del muntatge i verificar que les prediccions fetes en base a aquest model tenen una gran concordança amb les mesures experimentals.
This thesis is a research work on the inhomogeneities of permanent magnets that are used to construct insertion devices in synchrotron light sources, especially undulators. The work has two objectives. First, try to model the effect of these inhomogeneities in the magnetic fields generated by magnets. Mainly we studied their magnetic characteristics from direct measurements of the blocks, and we have focused the study of inhomogeneities in the minority components of the magnetization vector, as these are one of the biggest limitations in the performance of the insertion devices. Secondly, based on the measurements of these magnetic characteristics, work has focused on how you can minimize the effects of inhomogeneities in the insertion devices. This is typically done through a specific arrangement of the blocks to compensate errors caused by the inhomogeneities. In this part, numerical methods have been applied multivariate optimization functions. In the first part of the thesis is described all experimental equipment that has been used to make the measurements magnetic and mechanical of blocks and where explains how to have been characterized all the benches of the insertion devices laboratory at Alba synchrotron, determining the error associated with each of them. The individual magnetic blocks, which are the ones that form the whole structure of the undulator have been measured with Helmholtz coils to get the magnetization vector. With these results, and using an algorithm that minimizes the main component of the field, magnets are grouped into groups (modules) of 1 (singlets) and three blocks (triplets). These modules have been measured with benches based on the principle of Faraday's induction to determine the signature, that is, the transversal dependence of the integral field. Again, using an algorithm that minimizes the total field integral of the device, the order of the modules within the overall magnetic structure has been determined. The second part describes the theoretical foundations that create mathematical models for magnets and allow implementing programs to fit parameterized models of morphological inhomogeneities, and optimize the magnet sorting and the final optimization of the insertion devices. The aim of this second part was the preparation of an algorithm that allows building segments of undulators with low error using magnets with a certain degree of inhomogeneity. Finally, we focused on in-situ monitoring of assembly in order to verify that the predictions made based on this model are very consistent with the experimental measurements.

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Abad, Muñoz Libertad. "Efectos Estructurales y de Interfase en Capas Finas de La(2/3)Ca(1/3)MnO(3)". Doctoral thesis, Universitat Autònoma de Barcelona, 2007. http://hdl.handle.net/10803/3403.

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En el desarrollo de esta tesis hemos analizado distintos aspectos preliminares relacionados con la utilización de óxidos complejos, tales como las perovskitas de manganeso, en la implementación de dispositivos con potenciales aplicaciones en magnetoelectrónica. En concreto, hemos abordado a fondo distintos aspectos relacionados con la fabricación de uniones túnel magnéticas basadas en el sistema La2/3Ca1/3MnO3 (LCMO)/SrTiO3 (STO).
Nuestro trabajo ha puesto de manifiesto que la técnica de pulverización catódica de RF es adecuada para el crecimiento de capas epitaxiales de LCMO de alta calidad sobre sustratos monocristalinos diversos (STO, LaAlO3 (LAO)). Las heteroestructuras LCMO/STO y LCMO/LAO han sido objeto de un detallado análisis poniendo de manifiesto que el desacoplo de parámetros de red juega un papel muy importante tanto sobre el tipo de crecimiento de las heteroestructuras como sobre las propiedades físicas. El sistema LCMO/STO (δ=1.2%) crece tensionado en el plano y permite crecer capas de hasta unos 100 nm sin que haya relajación parcial de la tensión estructural. En cambio, el sistema LCMO/LAO (δ=-1.8%) crece comprimido en el plano y muestra señales de relajación parcial de la tensión estructural incluso para espesores en torno a los 10 nm.
El análisis de las propiedades magnéticas y de transporte muestran que ambos sistemas LCMO/STO y LCMO/LAO adolecen de una notable disminución tanto de su temperatura de transición magnética, TC, como de su magnetización de saturación, MS, al disminuir el espesor de las capas, siendo éste más acusado en el caso del LCMO/STO. Tratamientos térmicos al aire a alta temperatura permiten una mejora sustancial de las propiedades magnéticas y de transporte en ambos sistemas. La mejora es mucho más rápida en el sistema LCMO/LAO. Así mismo, el aumento del espesor de las capas también promueve una mejora de las propiedades, que se aproximan a las del material en forma masiva conforme el grosor aumenta. Esta evolución se ha relacionado con la aparición de una capa no magnética en la superficie de las capas de LCMO que se pone de manifiesto, predominantemente para las capas de LCMO/LAO ultra delgadas. Por otro lado, hemos visto como esta capa inerte puede ser reducida hasta un orden de magnitud mediante tratamientos térmicos de las muestras a alta temperatura.
Nuestro trabajo ha puesto de manifiesto que la tensión estructural juega un papel fundamental y que las mejoras de TC y MS con los tratamientos térmicos están fundamentalmente ligadas a una relajación parcial de la tensión estructural. Otras posibilidades, tales como la incorporación de oxigeno o la variación de la microestructura de las muestras han sido descartadas en base a medidas de efecto Hall y análisis de microscopía electrónica de alta resolución.
Para el estudio de las propiedades de transporte en las interfases hemos desarrollado un procedimiento experimental basado en la utilización de un microscopio de fuerzas atómicas trabajando en modo corriente permitiendo la caracterización de barreras aislantes e interfases entre un metal noble y un óxido complejo. Este método permite la caracterización de barreras y superficies soslayando los problemas derivados del desconocimiento preciso del contacto entre la punta de AFM y la superficie de la capa. Se realizaron medidas mediante la definición de nanocontactos de diferente geometría y tamaño realizados por distintas técnicas de nanofabricación. Mediante el estudio de las curvas I(V) en nanocontactos de LCMO/metal noble (Au, Pt) constatamos la existencia de una barrera superficial aislante (túnel) para capas finas vírgenes de LCMO/LAO. Mediante el ajuste de las curvas I(V) utilizando el modelo de Simmons hemos derivado valores para la capa aislante que concuerdan muy bien con otros datos reportados, además esta capa aislante puede ser reducida drásticamente mediante tratamientos térmicos en atmósfera ambiente.
In this Thesis we have analyzed different preliminary aspects related to the use of complex oxides, such as manganes perovskites, for implementation of functional devices with potential application on magnetoelectronics. To be precise, we have faced in deep the different aspects related with the fabrication of magnetic tunnel junctions based on the La2/3Ca1/3MnO3 (LCMO)/SrTiO3 (STO) system.
Our work makes evident the suitability of RF sputtering technique for growing high quality epitaxial layers of LCMO onto several single-crystalline substrates (STO, LaAlO3 (LAO)). The heterostructures LCMO/STO and LCMO/LAO have been object of a detailed analysis showing up that structural misfit plays a significant role both on the heterostructures growth process and on the physical properties. The system LCMO/STO (δ=1.2%) is under in-plane tensile strain but, thin films up to 100 nm thick can be grown without any partial relieve of the structural strain. In contrast, the system LCMO/LAO (δ=-1.8%) grows under in-plane compressive strain and shows clear traces of partial relaxation of structural strain even for thicknesses around 10nm.
The analysis of the magnetic and transport properties indicates that both systems have an outstanding depression on both the temperature transition, TC, and the saturation magnetization, MS, when decreasing the thin film thickness, being this phenomenon more pronounced in the case of LCMO/STO. High temperature annealing processes in air promote a remarkable improvement of the magnetic and transport properties in both systems. This improvement is faster and more evident in the LCMO/LAO system.
In addition, we have verified that the increment of the film thicknesses also promotes an improvement of the physical properties, approaching that of bulk samples. This evolution has been correlated with the existence of a non magnetic layer at the topmost layers of the LCMO thin films, whose effect becomes more relevant as the film thickness decreases and being dominant for ultra-thin films in the LCMO/LAO system. It has been observed that this dead layer can be reduced up to one order of magnitude by high temperature annealing processes.
Our work has demonstrated that structural strain plays a fundamental role on the magnetic and transport properties of these systems and the improvements of TC and MS with annealing processes is strongly linked to a partial relaxation of the structural strain. Other possibilities, such as oxygen uptake or modifications of the microstructure of the samples have been ruled out in base of Hall Effect measurements and high resolution transmission electron microscopy analysis.
To study transport properties at the interfaces we have developed an experimental procedure based on the use of an atomic forces microscope (AFM) working in current sensing mode, permiting the electrical characterization of insulating barriers and the interfaces between the precious metal and the complex oxide. This method allows characterizing transport properties of barriers and interfaces avoiding the problems related to the uncertainty of the exact nature of the contact between the AFM tip an the analyzed surface. Measurements were realized by defining nanocontacts, with different geometries and sizes, using different nanofabrication techinques.

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Fondevilla, Sala Núria. "Desenvolupament de microgeneradors inercials electromagnètics en tecnologia de microsistemes per a la recuperació d’energia mecànica residual de l’ambient". Doctoral thesis, Universitat de Barcelona, 2012. http://hdl.handle.net/10803/124219.

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Aquesta tesi descriu el desenvolupament d’un microgenerador ressonant compatible amb la tecnologia de Si, especialment adequat per extreure energia mecànica residual de l’ambient. Aquesta mena de font “perpètua” d’energia pot ser molt adient per alimentar sensors abandonats. El disseny d’aquest tipus de microgeneradors electromagnètics consta d’un sensor de moviment i d’un transductor electromagnètic amb un moviment relatiu entre l’imant i la bobina. El principal avantatge d’aquest dispositiu és la simplicitat del disseny per aprofitar les vibracions provinents del motor d’una màquina caracteritzades per presentar una freqüència ben definida però de baixa amplitud. Per poder amplificar-ne l’amplitud, es treballa en condicions de ressonància i s’ajusta la freqüència de ressonància de l’estructura a la de les vibracions. El dispositiu està format per una bobina micromecanitzada i un imant mòbil que actua com a massa inercial en una estructura ressonant. El disseny del microgenerador electromagnètic ressonant es realitza a partir de l’estudi del principi d’operació, que permet modelitzar tant el moviment de la massa inercial com la distribució de les línies del camp magnètic. Mitjançant simulacions per elements finits (ANSYS), s’analitza per una banda la influència de la geometria i materials del ressonador i per altra banda, els paràmetres geomètrics i del material conductor de la microbobina en la tensió i la potència generades. A partir dels resultats obtinguts amb una sèrie de mesures experimentals amb ressonadors amb membranes de Kapton de diferents mides amb la microbobina s’ha realitzat una sèrie de mesures experimentals, per a caracteritzar-los tant a nivell elèctric com a nivell mecànic, i mitjançant diferents simulacions addicionals del dispositiu es realitza un “roadmap” per a l’optimització. Basant-se aquestes primeres conclusions es fabrica un altre joc de prototips amb unes bobines micromecanitzades de Cu en les que s’han inclòs algunes de les optimitzacions. D’una banda, s’ha disminuït l’esmorteïment paràsit, per millorar-ne la ressonància i d’altra banda s’ha incrementat la densitat d’espires i s’ha disminuït la resistència en sèrie de la microbobina, per augmentar-ne la variació de flux. Aquesta reducció de la resistència s’ha aconseguit amb la implementació de processos electroquímics per a fabricar bobines amb pistes més gruixudes. Els resultats obtinguts amb la seva caracterització, se’ls ha comparat amb els d’uns altres prototips amb els mateixos ressonadors però emprant una bobina de fil de Cu convencional enlloc de la microbobina per tal d’estudiar-ne la influència tant a nivell de tensió com de voltatge. Les dades de tensió i potència generades experimentalment amb el microgenerador ja es troben en el rang de les esperades segons l’estat de l’art. De totes maneres, s’analitza la possibilitat de millorar-ho en futures optimitzacions. Les mesures experimentals, a més a més de corroborar el potencial d’aquests prototips, evidencien l’existència d’efectes no lineals. Malgrat que els fenòmens de no linealitat, característics dels polímers, permeten obtenir un increment en l’energia generada, presenten la dificultat de necessitar treballar en condicions de freqüència d’excitació creixent degut al comportament metaestable del ressonador. Donada la importància d’aquests fenòmens sobre el comportament i fins i tot sobre l’aplicabilitat dels dispositius s’estudien els mecanismes que intervenen en la elasticitat dels materials per tal de realitzar una primera anàlisi i modelització dels efectes no lineals i histèresi de les membranes polimèriques. La resolució de les equacions del moviment reescrites per a incloure un terme no lineal en la força recuperadora, ha permès reproduir el comportament observat experimentalment i donar una estimació dels diversos paràmetres que el caracteritzen.
This thesis describes the design and optimization of a ressonant electromagnetic inertial microgenerator for energy scavenging applications, compatible with Si technology and a relatively high electromechanical coupling with simple designs. It consists of a fixed micromachined coil and a movable magnet (inertial mass) mounted on a resonant structure. This device is well suited for harvesting of mechanical energy from vibrations induced by operating machines which are characterized by a well defined frequency and low displacement amplitudes. Adjusting the resonant frequency of the system to that of the targeted vibrations allows amplification of these low amplitude displacements. ANSYS simulations are carried out to investigate the influence of the coil parameters (density of tracks and tracks cross section) and resonator geometry (resonant frequency and parasitic damping) on the device performance. The experimental mechanical and electrical characterization of a first prototype fabricated with a modular manufacturing process (not optimized) has allowed the validation of the model developed for the inertial electromagnetic microgenerators, which is then used as a roadmap for a number of optimizations for the final device design. Generator prototypes have been fabricated with resonant membranes with different dimensions, using in all cases the same design of electromagnetic transducer. This has allowed investigating the influence of the membrane geometry on the parasitic damping which is a key parameter for the generator performance. In relation to the first prototypes a significant increase in the generated power has been achieved by the implementation of electrochemical processes for the fabrication of coils with thicker metal tracks. The results obtained have shown the ability of these devices to generate power and output voltages in the range of state of the art. The characterization of the fabricated prototypes has allowed to observe the presence of non-linear effects that lead to the appearance of hysteretic vibrational phenomenon and strongly affect the output of the microgenerator. These effects are likely related to the mechanical characteristics of the polymeric membrane, and determine an additional dependence of vibration frequency on the excitation amplitude. Rewriting and solving the equations of movement including a nonlinear term in restoring force leads to a simple model that successfully reproduces the hysteresis of the resonator.

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Gertrudes, João Bosco. "Influência da condutividade e permissividade do solo em função da frequência no cálculo da impedância longitudinal e admitância transversal de linhas aéreas de transmissão". [s.n.], 2010. http://repositorio.unicamp.br/jspui/handle/REPOSIP/260993.

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Orientador: Maria Cristina Dias Tavares
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de Computação
Made available in DSpace on 2018-08-16T03:57:35Z (GMT). No. of bitstreams: 1 Gertrudes_JoaoBosco_D.pdf: 5500576 bytes, checksum: ef85206048f845d1bff89f7c8948d288 (MD5) Previous issue date: 2010
Resumo: Esta tese tem como objetivo o desenvolvimento de modelos mais precisos para o cálculo de parâmetros elétricos de linhas aéreas de transmissão para estudos de transitórios na faixa de frequência 0 a 2 MHz. Esta faixa de frequência cobre a maioria das perturbações eletromagnéticas nos sistemas elétricos: energização, faltas simétricas e assimétricas, rejeições de carga, descargas atmosféricas, perturbações harmônicas, etc. Na faixa de frequência citada, em consequência da dependência dos parâmetros do solo com a frequência, a condutividade elétrica do solo (sg) tem a mesma ordem de grandeza que o produto entre a frequência angular do sinal incidente no solo (w) e a permissividade elétrica do solo (eg). Portanto, as suposições de baixa frequência tradicionalmente adotadas - sg constante e weg desprezível (sg >> weg) - podem levar a modelos que não representam de forma adequada a resposta da linha no caso de transitórios rápidos (com espectro de frequência acima de 1 kHz). O objetivo central desta tese é verificar a influência da dependência dos parâmetros do solo no cálculo da impedância longitudinal e admitância transversal por unidade de comprimento em comparação com os modelos tradicionais de cálculo. As análises são feitas inicialmente para o caso de um único condutor acima do solo e posteriormente através de um estudo para uma linha de transmissão trifásica, 440 kV, em operação no sistema elétrico brasileiro. As impedâncias longitudinais e as admitâncias transversais de retorno pelo solo são avaliadas através de métodos tradicionais de cálculo e através de integração numérica das formulações de Carson modificadas para inclusão do modelo de solo que considera a depêndencia de sg e weg em função da frequência
Abstract: This thesis aims to propose more accurate models for calculating the electrical parameters of overhead power transmission lines. The influence of earth's conductivity and permittivity frequency dependence is evaluated when calculating transversal and longitudinal transmission lines' parameters in the frequency range 0 to 2 MHz. This frequency range covers the majority of the electromagnetic transients in electrical systems (switching transients, faults' transients, load rejections, harmonic disturbances, etc). Between 1 kHz to 2 MHz, the product of the signal angular frequency (w) by the dielectric constant (eg) of the soil may have the same order of magnitude as the conductivity (sg), due to the variation of these parameters with frequency. Therefore, the assumptions of low frequency traditionally used - the soil conductivity (sg) considered as constant and weg that can be negligible (sg >> weg) - can lead to incorrect models that do not adequately represent the transmission line's response, in cases of fast transients phenomena (with frequency spectrum above 1 kHz). The importance of properly considering the frequency dependent soil model is presented for the one-conductor case and for a single three-phase transmission line (440 kV, considered ideally transposed). The aim is to compare the transmission line parameters calculated considering the conductivity and weg frequency dependence ground model in relation to the common ground representation, with constant conductivity and weg that may be neglected. It is also compared the results obtained from the complex plane method with those obtained from the Carson's modified expressions for longitudinal parameters and between Carson's modified potential-coefficient correction factors and from the most common approximated methods for transversal admittance parameters
Doutorado
Energia Eletrica
Doutor em Engenharia Elétrica

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Gàzquez, Alabart Jaume. "TEM investigation of growth mechanisms and microstructure of model YBCO coated conductor architectures deposited by metalorganic decomposition". Doctoral thesis, Universitat Autònoma de Barcelona, 2007. http://hdl.handle.net/10803/3401.

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This thesis is divided in six chapters. In the first one, we expose an amenable brief introduction to the YBCO compound, as well as to the CSD method. Then, in the second chapter we describe briefly the experimental techniques used for the physical
characterization of the CSD thin films presented in this work. The results, chapters 3, 4 , 5 and 6, are presented in two parts: Part I reports many aspects governing the growth mechanisms of CSD films, in particular the identification of those factors controlling the evolution of microstructures, which remain poorly established in comparison with vacuum deposited films. We give first an account of the evolution from a partially oriented granular microstructure to a dense epitaxial one in CeO2 films deposited from chemical solutions (chapter 3), and second the microstructural evolution of YBCO from trifluoroacetate precursors, which follow a complex compositional trajectory (chapter 4). In part II the microstructure of TFA-YBCO thin films with high critical current densities is analysed. We depict intrinsic structural defects occurring within YBCO films, focusing in extended defects which can lead to strong flux-pinning and high critical current density (chapter 5). The microstructure of TFA-YBCO films with artificial defects has been also studied (chapter 6). The inclusion of BaZrO3 results in strong increase of the critical current density, demonstrating that chemical solution growth is a very flexible methodology to nanostructure YBCO films and coated conductors.
Finally, we present the general conclusions of this study.

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Saureu, Artesona Sergi. "From mononuclear to dinuclear: magnetic properties of transition metal complexes". Doctoral thesis, Universitat Rovira i Virgili, 2016. http://hdl.handle.net/10803/386451.

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En les darreres dècades, el món de la tecnologia i el desenvolupament de nous aparells electrònics s'han convertit en vitals per la nostra societat. Considerant la creixent demanda per la interpretació de resultats experimentals, la millora dels mètodes teòrics i el creixement dels recursos computacionals ens han permés un millor enteniment del comportament magnètic en sistemes amb metalls de transició. L'objectiu d'aquesta tesi és contribuir en aquest camp d'investigació amb l'estudi de materials magnètics utilitzant eines computacionals (DFT, TD-DFT, CASSCF, CASPT2, DDCI, etc.), i en alguns casos, combinant-ho amb resultats experimentals. La primera part (Capítols 3 i 4) inclou l'estudi dels estats electronics de complexes de spin-crossover de Fe(II) i Fe(III) combinant la teoria funcional de la densitat (DFT i TD-DFT) amb mètodes multiconfiguracionals (CASSCF, CASPT2). A més a més, utilitzant la mateixa combinació, hem descrit el fenomen LIESST en complexes de Fe(III). La segona part (Cap. 5 i 6) exposa l'estudi de les propietats magnètiques associades a l'acoblament magnètic utilitzant mètodes variacionals (DDCI, DDCI-2), en un complex de Fe(IV) i un complex bimetàl·lic [MnCr]-oxalat, y com els canvis estructurals afecten a aquest acoblament. Altrament, hem fet un rigurós anàlisi de l'estructura electrònica del complex de Fe(IV) per proporcionar més informació en la descripció més adequada del sistema.
En las últimas décadas, el mundo de la tecnologia y el desarrollo de nuevos aparatos electrónicos se han convertido en vitales para nuestra sociedad. Considerando la creciente demanda para la interpretación de resultados experimentales, la mejora de los métodos teóricos y el crecimiento de los recursos computacionales nos han permitido un mejor entendimiento de los comportamientos magnéticos en los sistemas con metales de transición. El objetivo de esta tesis es contribuir a este campo de investigación con el estudio de materiales magnéticos usando herramientas computacionales (DFT, TD-DFT, CASSCF, CASPT2, DDCI, etc.), y en algunos casos, combinando con resultados experimentales. La primera parte (Capítulo 3 y 4) incluye el estudio de los estados electrónicos de los complejos de spin-crossover de Fe(II) y Fe(III) combinando la teoria funcional de la densidad (DFT y TD-DFT) con métodos multiconfiguracionales (CASSCF, CASPT2). Además, usando la misma combinación, hemos descrito el fenómeno LIESST en complejos de Fe(III). La segunda parte (Cap. 5 y 6) expone el estudio de las propiedades magnéticas asociadas al acoplamiento magnético utilizando metodos variacionales (DDCI, DDCI-2), en un complejo de Fe(IV) y un complejo bimetálico [MnCr]-oxalato, y como los cambios estructurales afectan a ese acoplamiento. Por otra parte, hemos hecho un riguroso analisis de la estructura electrónica del complejo de Fe(IV) para aportar la información para la descripción mas adecuada del sistema.
Over the last decades the world of technology and the development of new devices have become vital for our society. Considering the growing demand for interpretation of experimental observations, the improvement of theoretical methods and the increasing of the computational resources has allowed us to deepen the understanding of magnetic beahvior of metal transitions architectures. The aim of this thesis is to contribute to this research field with the study of magnetic materials by using computational tools (DFT, TD-DFT, CASSCF, CASPT2, DDCI, etc.), and in some cases combining it with experimental results. The first part (Chapters 3 and 4) includes the study of the electronic states of Fe(II) and Fe(III) spin-crossover complexes combining the density functional theory (DFT and TD-DFT) with multiconfigurational methodologies (CASSCF, CASPT2). In addition, we have described the LIESST phenomenon in Fe(III) using the same combination. The second part (Chapters 5 and 6) exposes the study of the magnetic properties related to the magnetic coupling using variational methods (DDCI, DDCI-2) of a Fe(IV) complex and bimetallic [MnCr] oxalate-based complexes and how changes can influence to the coupling. Moreover, a rigorous analysis of the electronic structure of the Fe(IV) system has been performed to provide more information about the most adequate description of the system in terms of intuitive chemical concepts.

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Heideier, Raphael Bertrand. "Metodologia para avaliar o emprego de técnicas de seleção e conversão da radiação solar sobre a produção de energia elétrica de células fotovoltaicas". Universidade de São Paulo, 2017. http://www.teses.usp.br/teses/disponiveis/3/3143/tde-05122017-151838/.

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Esta tese propõe uma metodologia para avaliar o emprego de técnicas de seleção e conversão da radiação solar para aumentar a produção de energia elétrica de células fotovoltaicas. Foi feita uma revisão dos mecanismos fotovoltaicos para produção de energia elétrica e os entraves para aumentar sua eficiência, em especial os impactos da temperatura e da absorção do espectro solar. Uma metodologia foi criada para definir um filtro óptico ótimo a ser utilizado em uma região na presença ou não de conversão do espectro, através de simulações computacionais, que define a faixa do espectro solar que deve ser absorvida pela célula para se ter o melhor equilíbrio entre absorção e aquecimento. Um modelo matemático foi criado e implementado computacionalmente, para calcular a produção de energia de um módulo fotovoltaico, em uma dada região com ou sem a aplicação de técnicas de conversão e seleção da radiação solar. Através de um estudo de caso com dados da cidade de Petrolina, PE, Brasil, concluiu-se que o ganho esperado para utilização de um filtro óptico que reflete ondas de comprimento superiores a 1000 nm é cerca de 1% da energia gerada. Ensaios empíricos permitiram validar a metodologia proposta, comparando os dados reais com os dados teóricos obtidos pelas simulações do modelo matemático. Após a calibração do modelo matemático, verificou-se que os resultados de potência instantânea calculados com o modelo matemático variavam cerca de 5% tanto para o módulo com filtro quanto para o módulo sem filtro. Para contornar a restrição construtiva do filtro óptico e verificar outros ganhos possíveis são propostos experimentos com compostos de conversão de radiação solar associados com filtros ópticos para seleção da radiação solar. Com isto, espera-se aumentar significativamente a produção de energia elétrica de células fotovoltaicas de Silício cristalino.
This thesis proposes a methodology to evaluate the use of solar radiation selection and conversion techniques to increase the production of electric energy of photovoltaic solar cells. A review of the photovoltaic mechanisms for the production of electrical energy and the obstacles to increase its efficiency, especially the impacts of temperature and the absorption of the solar spectrum was done. A methodology was developed to define an optimal optical filter to be used in a region in the presence or not of solar spectrum conversion, through computational simulations, which defines the range of the solar radiation that must be absorbed by the cell in order to have the best balance between absorption and heating. A mathematical model was created and implemented computationally to calculate the energy production of a photovoltaic module in a given region with or without the application of solar radiation conversion and selection techniques. A case study with data from the city of Petrolina, PE, Brazil, concluded that the expected gain for the use of an optical filter that reflects wavelengths above 1000 nm is about 1% of the power generated. Empirical tests allowed validating the proposed methodology comparing the actual data with the theoretical data obtained by the simulations of the mathematical model. After calibration of the mathematical model, it was verified that the instantaneous power results calculated with the mathematical model vary about 5% for both the filter module and the unfiltered module. To avoid the constructive constraint of the optical filter and to verify other possible gains are proposed experiments with solar radiation conversion compounds associated with optical filters to select the solar radiation. With this, it is expected to significantly increase the electricity production of crystalline silicon photovoltaic cells.

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Sort, Viñas Jordi. "Magnetic hardening induced by ferromagnetic-antiferromagnetic coupling". Doctoral thesis, Universitat Autònoma de Barcelona, 2002. http://hdl.handle.net/10803/3340.

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Es bien sabido que las interacciones de canje inducidas entre materiales ferromagnéticos (FM) y antiferromagnéticos (AFM) cuando son enfriados desde una temperatura superior a la temperatura de Néel, TN, del AFM, hacen aumentar la coercividad, HC, del componente FM. Nosotros hemos analizado los efectos de hacer una molienda mecánica de polvos FM (Co o SmCo5) con AFM (con TN, ya sea por encima de temperatura ambiente, por ejemplo el NiO, o por debajo, por ejemplo el CoO). Se han estudiado varias combinaciones (Co + NiO, SmCo5 + NiO y SmCo5 + CoO), dando especial énfasis a los sistemas FM + NiO. Los distintos comportamientos estructurales y magnéticos del Co y del SmCo5 durante la molienda y los tratamientos térmicos hacen necesario adaptar la ruta de procesado adecuadamente para cada caso. Así, en los sistemas Co + AFM los efectos de las interacciones FM-AFM se inducen mayoritariamente después de un enfriado en campo de los polvos molidos desde T > TN. Contrariamente, en polvos SmCo5 + AFM, los tratamientos térmicos deterioran enormemente las propiedades magnéticas duras del SmCo5. De todas formas, un análisis comparativo de las propiedades del SmCo5 molido con NiO o con CoO parece indicar que, en cierto grado, las interacciones de canje pueden, de hecho, inducirse en el SmCo5 + NiO durante la molienda. Las interacciones de canje FM-AFM se manifiestan en un desplazamiento del ciclo de histéresis y unos aumentos de HC y de la razón de cuadratura, MR/MS. Estos efectos se observan a temperatura ambiente siempre que TN sea mayor que la temperatura ambiente (por ejemplo, con NiO). Además, si se optimizan las proporciones de FM a AFM, así como las condiciones de molienda, también es posible aumentar, hasta cierto punto, el producto de energía, (BH)Max. Por un lado, el aumento del contenido de AFM hace aumentar las interacciones FM-AFM (induciendo un aumento de HC y MR/MS). Sin embargo, como la imanación de un AFM es prácticamente nula, la imanación de saturación del material compuesto FM-AFM se ve también reducida proporcionalmente al porcentaje de AFM. Como consecuencia de estos dos efectos competitivos, se obtiene que las composiciones que maximizan HC no son las óptimas para aumentar (BH)Max. Por tanto, tanto el tiempo de molienda como las proporciones FM:AFM deben optimizarse adecuadamente dependiendo de las propiedades magnéticas deseadas para los productos finales.
Ferromagnetic (FM) - antiferromagnetic (AFM) exchange interactions induced after field cooling FM-AFM composites through the Néel temperature of the AFM are known to increase the coercivity, HC, of the FM component. We have studied the effect of milling different FM (Co or SmCo5) with AFM powders (with TN, either above room temperature, e.g. NiO, or below room temperature, e.g. CoO). Several combinations (i.e. Co + NiO, SmCo5 + NiO and SmCo5 + CoO) have been studied, with special focus on FM + NiO composites. The different structural and magnetic behaviors of Co and SmCo5 during ball milling or heat treatments make it necessary to properly adapt the processing route for each case. Thus, in Co + AFM composites the effects of FM-AFM exchange interactions are mainly induced after field cooling the as-milled powders from above TN. Contrarily, in SmCo5 + AFM powders, heating results in deterioration of the hard magnetic properties. However, comparison of the magnetic properties of SmCo5 milled with NiO or with CoO indicates that for SmCo5 + NiO some exchange interactions can be actually induced during the milling. FM-AFM exchange interactions result in a shift of the hysteresis loop and in enhancements of HC and the squareness ratio, MR/MS. These effects are observed at room temperature, provided that TN is above room temperature (i.e. for NiO). In addition, an enhancement of the energy product, (BH)Max, can also be accomplished, to a certain extent, after optimization of the FM:AFM ratio and milling conditions. On the one hand, an increase of the AFM content brings about an increase of FM-AFM exchange interactions (i.e. an increase of HC and MR/MS). Nevertheless, on the other hand, since the AFM has a zero net magnetization, the overall saturation magnetization, MS, of the FM-AFM composites decreases proportionally to the amount of AFM. As a consequence of these competing effects, it has been found that the compositions resulting in maximum coercivities are not the optimum ones to enhance (BH)Max. Therefore, both the milling time and the FM-AFM ratio need to be optimized depending on the desired properties for the final materials.

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Sancristóbal, Alonso Belén de. "Neuronal oscillations: from single-unit activity to emergent dynamics and back". Doctoral thesis, Universitat Politècnica de Catalunya, 2013. http://hdl.handle.net/10803/129072.

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L’objectiu principal d’aquesta tesi és avançar en la comprensió del processament d’informació en xarxes neuronals en presència d’oscil lacions subumbrals. La majoria de neurones propaguen la seva activitat elèctrica a través de sinapsis químiques que són activades, exclusivament, quan el corrent elèctric que les travessa supera un cert llindar. És per aquest motiu que les descàrregues ràpides i intenses produïdes al soma neuronal, els anomenats potencials d’acció, són considerades la unitat bàsica d’informació neuronal, és a dir, el senyal mínim i necessari per a iniciar la comunicació entre dues neurones. El codi neuronal és entès, doncs, com un llenguatge binari que expressa qualsevol missatge (estímul sensorial, memòries, etc.) en un tren de potencials d’acció. Tanmateix, cap funció cognitiva rau en la dinàmica d’una única neurona. Circuits de milers de neurones connectades entre sí donen lloc a determinats ritmes, palesos en registres d’activitat colectiva com els electroencefalogrames (EEG) o els potencials de camp local (LFP). Si els potencials d’acció de cada cèl lula, desencadenats per fluctuacions estocàstiques de les corrents sinàptiques, no assolissin un cert grau de sincronia, no apareixeria aquesta periodicitat a nivell de xarxa. Per tal de poder entendre si aquests ritmes intervenen en el codi neuronal hem estudiat tres situacions. Primer, en el Capítol 2, hem mostrat com una cadena oberta de neurones amb un potencial de membrana intrínsecament oscil latori filtra un senyal periòdic arribant per un dels extrems. La resposta de cada neurona (pulsar o no pulsar) depèn de la seva fase, de forma que cada una d’elles rep un missatge filtrat per la precedent. A més, cada potencial d’acció presinàptic provoca un canvi de fase en la neurona postsinàptica que depèn de la seva posició en l’espai de fases. Els períodes d’entrada capaços de sincronitzar les oscil lacions subumbrals són aquells que mantenen la fase d’arribada dels potencials d’acció fixa al llarg de la cadena. Per tal de què el missatge arribi intacte a la darrera neurona cal, a més a més, que aquesta fase permeti la descàrrega del voltatge transmembrana. En segon cas, hem estudiat una xarxa neuronal amb connexions tant a veïns propers com de llarg abast, on les oscil lacions subumbrals emergeixen de l’activitat col lectiva reflectida en els corrents sinàptics (o equivalentment en el LFP). Les neurones inhibidores aporten un ritme a l’excitabilitat de la xarxa, és a dir, que els episodis en què la inhibició és baixa, la probabilitat d’una descàrrega global de la població neuronal és alta. En el Capítol 3 mostrem com aquest ritme implica l’aparició d’una bretxa en la freqüència de descàrrega de les neurones: o bé polsen espaiadament en el temps o bé en ràfegues d’elevada intensitat. La fase del LFP determina l’estat de la xarxa neuronal codificant l’activitat de la població: els mínims indiquen la descàrrega simultània de moltes neurones que, ocasionalment, han superat el llindar d’excitabilitat degut a un decreixement global de la inhibició, mentre que els màxims indiquen la coexistència de ràfegues en diferents punts de la xarxa degut a decreixements locals de la inhibició en estats globals d’excitació. Aquesta dinàmica és possible gràcies al domini de la inhibició sobre l’excitació. En el Capítol 4 considerem acoblament entre dues xarxes neuronals per tal d’estudiar la interacció entre ritmes diferents. Les oscil lacions indiquen recurrència en la sincronització de l’activitat col lectiva, de manera que durant aquestes finestres temporals una població optimitza el seu impacte en una xarxa diana. Quan el ritme de la població receptora i el de l’emissora difereixen significativament, l’eficiència en la comunicació decreix, ja que les fases de màxima resposta de cada senyal LFP no mantenen una diferència constant entre elles. Finalment, en el Capítol 5 hem estudiat com les oscil lacions col lectives pròpies de l’estat de son donen lloc al fenomen de coherència estocàstica. Per a una intensitat òptima del soroll, modulat per l’excitabilitat de la xarxa, el LFP assoleix una regularitat màxima donant lloc a un període refractari de la població neuronal. En resum, aquesta Tesi mostra escenaris d’interacció entre els potencials d’acció, característics de la dinàmica de neurones individuals, i les oscil lacions subumbrals, fruit de l’acoblament entre les cèl lules i ubiqües en la dinàmica de poblacions neuronals. Els resultats obtinguts aporten funcionalitat a aquests ritmes emergents, agents sincronitzadors i moduladors de les descàrregues neuronals i reguladors de la comunicació entre xarxes neuronals.
The main objective of this thesis is to better understand information processing in neuronal networks in the presence of subthreshold oscillations. Most neurons propagate their electrical activity via chemical synapses, which are only activated when the electric current that passes through them surpasses a certain threshold. Therefore, fast and intense discharges produced at the neuronal soma (the action potentials or spikes) are considered the basic unit of neuronal information. The neuronal code is understood, then, as a binary language that expresses any message (sensory stimulus, memories, etc.) in a train of action potentials. Circuits of thousands of interconnected neurons give rise to certain rhythms, revealed in collective activity measures such as electroencephalograms (EEG) and local field potentials (LFP). Synchronization of action potentials of each cell, triggered by stochastic fluctuations of the synaptic currents, cause this periodicity at the network level.To understand whether these rhythms are involved in the neuronal code we studied three situations. First, in Chapter 2, we showed how an open chain of neurons with an intrinsically oscillatory membrane potential filters a periodic signal coming from one of its ends. The response of each neuron (to spike or not) depends on its phase, so that each cell receives a message filtered by the preceding one. Each presynaptic action potential causes a phase change in the postsynaptic neuron, which depends on its position in the phase space. Those incoming periods that are able to synchronize the subthreshold oscillations, keep the phase of arrival of action potentials fixed along the chain. The original message reaches intact the last neuron provided that this phase allows the discharge of the transmembrane voltage.I the second case, we studied a neuronal network with connections to both long range and close neighbors, in which the subthreshold oscillations emerge from the collective activity apparent in the synaptic currents. The inhibitory neurons provide a rhythm to the excitability of the network. When inhibition is low, the likelihood of a global discharge of the neuronal population is high. In Chapter 3 we show how this rhythm causes a gap in the discharge frequency of neurons: either they pulse single spikes or they fire bursts of high intensity. The LFP phase determines the state of the neuronal network, coding the activity of the population: its minima indicate the simultaneous discharge of many neurons, while its maxima indicate the coexistence of bursts due to local decreases of inhibition at global states of excitation. In Chapter 4 we consider coupling between two neural networks in order to study the interaction between different rhythms. The oscillations indicate recurrence in the synchronization of collective activity, so that during these time windows a population optimizes its impact on a target network. When the rhythm of the emitter and receiver population differ significantly, the communication efficiency decreases as the phases of maximum response of each LFP signal do not maintain a constant difference between them.Finally, in Chapter 5 we studied how oscillations typical of the collective sleep state give rise to stochastic coherence. For an optimal noise intensity, modulated by the excitability of the network, the LFP reaches a maximal regularity leading to a refractory period of the neuronal population.In summary, this Thesis shows scenarios of interaction between action potentials, characteristics of the dynamics of individual neurons, and the subthreshold oscillations, outcome of the coupling between the cells and ubiquitous in the dynamics of neuronal populations . The results obtained provide functionality to these emerging rhythms, triggers of synchronization and modulator agents of the neuronal discharges and regulators of the communication between neuronal networks.

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Llobet, Megías Anna. "Contribution à l'étude du magnéto-transport et aux phénomènes de ségrégation de phase dans les manganites". Doctoral thesis, Universitat Autònoma de Barcelona, 2000. http://hdl.handle.net/10803/3332.

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Svozilík, Jiŕí. "Photonic entanglement : new sources and new applications". Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/284225.

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Non-classical correlations, usually referred as entanglement, are ones of the most studied and discussed features of Quantum Mechanics, since the initial introduction of the concept in the decade of 1930s. Even nowadays, a lot of efforts, both theoretical and experimental, are devoted in this topic, that covers many distinct areas of physics, such as a quantum computing, quantum measurement, quantum communications, solid state physics, chemistry and even biology. The fundamental tasks that one should consider related to the entanglement are: -How to create quantum entangled states. -How to maintain entanglement during propagation against sources of decoherence. -How to effectively detect it. -How to employ the benefits that entanglement offers. This thesis, divided into four chapters, concentrates on the first and last tasks considered above. In Chapter 1, a brief introduction and overview of what it is entanglement is given, starting with the famous paper of Einstein, Podolsky and Rosen, and continuing with John Bell's formulation of the so-called Bell's inequalities. We define here general concepts about entangled quantum states and introduce important entanglement measures, that are later used all over the thesis. In this chapter, sources of entangled particles (namely photons) are also mentioned. The importance is put on sources based on the nonlinear process of spontaneous parametric down-conversion. The last part of this chapter is then dedicated to a list of applications that benefit from the use of engangled states. Chapter 2 is devoted to the systematic study of the generation of entangled and non-entangled photon pairs in semiconductor Bragg reflection waveguides. Firstly, we present a source of photon pairs with a spectrally uncorrelated two-photon amplitude, achieved by a proper tailoring of the geometrical and material dispersions via structural design of waveguides. Secondly, Bragg reflection waveguides are designed in a scuh way, that results in the generation of spectrally broadband paired photons entangled in the polarization degree of freedom. Finally, we present experimental results of entangled photon pairs generation in this type of structures. In Chapter 3, we explore the feasibility of the generation of photon pairs entangled in the spatial degree of freedom, i.e. in the orbital angular momentum (OAM). Firstly, we examine how to create a highly multidimensional Hilbert space using OAM modes obtained in a chipred-poled nonlinear bulk crystals. Here, we show, how an increase of the chirp of the poling can effectively increase the Schmidt number by several orders of magnitude. Secondly, we investigate periodically poled silica glass fibres with a ring-shpaed core, that are capable to support the generation of simple OAM modes. The final Chapter 4 is dedicated to the Anderson localization and quantum random walks. At the beginning of this chapter, we present an experimental proposal for the realization of a discrete quantum random walks using the multi-path Mach-Zehnder interferometer with a spatial light modulator, that allows us to introduce different types of statistical or dynamical disorders. And secondly, we show how the transverse Anderson localization of partially coherent light, with a variable first-order degree of coherence, can be studied making use of entangled photon pairs.
Las correlaciones cuánticas, normalmente conocidas como entanglement, son uno de los temas más estudiados y discutidos de la Mecánica Cuántica, desde la introducción del concepto en la década de 1930. Incluso hoy en día, una gran cantidad de esfuerzos, tanto teóricos como experimentales, se dedican en este tema, que cubre muchas áreas distintas de la física, tales como medición cuántica (quantum metrology), computación cuántica (quantum computing), comunicaciones cuánticas (quantum communications), física de estado sólido, química e incluso biología. Las tareas fundamentales de investigación que uno debe considerar en relación con entrelazamiento son: -Cómo crear estados cuánticos entangled. -Cómo mantener el entanglement durante la propagación, en contra de las fuentes que pueden crear de-coherencia. -Cómo emplear los beneficios que el entanglement ofrece. Esta tesis, dividida en cuatro capítulos, se centra en la primera y últimas tareas consideradas. En el capítulo 1, se da una breve introducción y una visión general de lo que es el entrelazamiento (entanglement), empezando por el famoso artículo de Einstein, Podolosky y Rosen, y continuando con la formulación de John Bell de las llamados desigualdades de Bell. Definimos aquí conceptos generales acerca de los estados cuánticos enrelazados e introducimos algunas medidas de entrelazamiento importantes, que se utilizan posteriormente a lo largo de toda la tesis. En este capítulo, algunas fuentes de partículas entrelazadas (fotones) se mencionan brevemente. La importancia se pone en fuentes basadas en el proceso no lineal de generación paramétrica espontánea (SPDC, Spontaneous Parametric Down Conversion). La última parte de este capítulo está dedicado a mencionar algunas aplicaciones que se benefician de la utilización de estados entrelazados. El capítulo 2 se dedica al estudio sistemático de la generación de pares de fotones entrelazados, o no, en guías semiconductores de tipo Bragg. En primer lugar, se presenta una fuente de pares de fotones espectralmente no correlacionados, lo que se puede conseguir utilizando la geometría adecuada y la adecuada dispersión del material, a través del diseño estructural de las guías de onda. En segundo lugar, las guías de onda de Bragg se diseñan de manera que dan como resultado la generación de pares de fotones entrelazados en el grado de libertad de polarización con un ancho de banda grande. Finalmente, se presentan resultados experimentales de pares de fotones generados en este tipo de estructuras. En el capítulo 3, se explora la viabilidad de la generación de pares de fotones entrelazados en el grado espacial de libertad, es decir, en el momento angular orbital (OAM). En primer lugar, se investiga cómo crear un espacio de Hilbert altamente multidimensional utilizando modos OAM Para ello se ahce uso de materiales no lineales con chirped-qausi-phase-matching. Aquí mostramos cómo un aumento del chirp puede aumentar efectivamente el número de Schmidt en varios órdenes de magnitud. En segundo lugar, se investiga como fibras de vidrio con un núcleo en forma de anillo son capaces de generar y propagar los modos con OAM más simples. El Capítulo 4 se dedica a la localización Anderson y a los llamados paseos aleatorios cuánticos (Quantum random walks). En primer lugar se presenta una propuesta experimetnal para la realización de un paseo aleatorio discreto cuántico utilizando un interferómetro MAch-Zehnder con un modulador espacial de luz, que nos permite introducir diferentes tipos de ruido con diferentes tipos de estadística. En segundo lugar, se muestra cómo la localización transveral Anderson de luz parcialmente coherente se puede estudiar haciendo uso de pares de fotones entrelazado

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Madinehei, Milad. "Improvement of corrosion and mechanical properties of amorphous steels by microalloying and nanocrystallization". Doctoral thesis, Universitat Politècnica de Catalunya, 2015. http://hdl.handle.net/10803/319456.

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Los aceros amorfos que contienen cantidades óptimas de cromo son muy atractivos para aplicaciones prácticas debido a su comportamiento frente a la corrosion. Estas aleaciones tienen extraordinarias propiedades de resistencia a la corrosión incluso en ambientes hostiles. Todas las composiciones de aceros amorfos estudiados hasta ahora por su resistencia a la corrosión tienen una cantidad relativamente grande de Cr y una consecuencia es la reducción de la capacidad de fromación del vidrio (GFA). También las propiedades mecánicas de los vidrios metálicos son altamente sensibles a los cambios de composición. Se ha hecho un estudio detallado sobre el efecto de la baja cantidad de Cr en la GFA, la resistencia a la corrosión y el comportamiento mecánico con el fin de determinar la cantidad óptima de Cr necesario para obtener una buena resistencia a la corrosión y buenas propiedades mecánicas, manteniendo la GFA. La composición mostró drásticamente diferentes efectos en las diferentes propiedades de la aleación en función de la cantidad de Cr. Los resultados muestran que las aleaciones de Fe-Cr-Mo-C-B con notable GFA, alta estabilidad térmica, alta dureza, así como superior resistencia a la corrosión y resistencia al desgaste se pueden conseguir con adiciones de Cr con cerca de 5 at.%. Aleaciones con contenido superior de Cr (Cr6% at.) son mejores candidatos sólo para aplicaciones donde la plasticidad no es un problema y el material se expone a ambientes agresivos. Aunque las aleaciones con contenidos relativamente bajos de Cr exhibieron la mejor plasticidad entre todas las aleaciones estudiadas, su alta tendencia a corroer limita sus aplicaciones a entornos inertes. Las propiedades químicas de los vidrios metálicos están estrechamente relacionadas con su estructura única amorfa, por eso fueron investigadas las transformaciones estructurales térmicamente inducidas (relajación y cristalización) y su efecto sobre la resistencia a la corrosión. Mediciones electroquímicas de las muestras tratadas térmicamente revelaron una fuerte dependencia de la resistencia a la corrosión en la transformación structural. Aunque la relajación estructural mejoró la resistencia a la corrosión mediante la formación de un vidrio más ideal, la mejor resistencia a la corrosión se obtuvo para la muestra con nanocristales ricos en Cr incrustados en una matriz amorfa. La precipitación de una segunda fase cristalina (rica en Mo) tuvo un efecto perjudicial sobre la resistencia a la corrosión pero las aleaciones exhibieron buenas propiedades anticorrosivas hasta que los granos ricos en Mo excedieron el límite crítico de 50 nm y el capa pasiva rica en Cr no era capaz de cubrirlos completamente.
Amorphous steels containing optimal quantities of chromium are very attractive for practical applications due to their corrosion behavior. These alloys have extraordinary corrosion resistant properties even in hostile environments. All the amorphous steels compositions studied so far for their corrosion resistance have a relatively large amount of Cr and one consequence is the reduction of the glass forming ability (GFA). Also mechanical properties of metallic glasses are highly sensitive to composition change. A detailed study on the effect of low amount of Cr on the glass forming ability, corrosion resistance and mechanical behavior has been done in order to determine the optimal amount of Cr needed to obtain good corrosion and mechanical properties while maintaining a high GFA. The composition showed drastically different effects on the different properties of the alloy depending on the amount of Cr. The results show that Fe–Cr–Mo–C–B alloys with remarkable GFA, high thermal stability, high hardness, as well as superior corrosion and wear resistance can be obtained with Cr additions near 5 at.%. Higher Cr-content (Cr˃6 at.%) alloys are better candidates only for applications where plasticity is not an issue and the material is exposed to aggressive environments. Although alloys with relatively low Cr contents exhibited the best plasticity among all the studied alloys, their high tendency to corrode limits their applications to inert environments. The chemical properties of metallic glasses are closely related to their unique amorphous structure so thermally induced structural transformations (structure relaxation and crystallization) and their effect on the corrosion resistance were investigated. Electrochemical measurements of the heat treated samples unveiled a strong dependence of corrosion resistance on structural transformation. Although structural relaxation improved the corrosion resistance by forming a more ideal glass, the best corrosion resistance was obtained for the sample showing a structure consisting of nanocrystalline Cr-rich phase embedded in an amorphous matrix. Precipitation of a second crystalline phase (Mo-rich) had a detrimental effect on corrosion resistance but the alloys exhibited good anticorrosion properties until the Mo-rich grains exceed the critical limit of 50 nm, where the Cr-rich passive film was not able to cover them completely

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Pérez, Tomàs Amador. "Novel materials and processes for gate dielectrics on Silicon carbide". Doctoral thesis, Universitat Autònoma de Barcelona, 2005. http://hdl.handle.net/10803/3381.

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There is considerable evidence of the need for a semiconductor technology which exceeds the limitations imposed by silicon across a wide spectrum of industrial applications. Wide bandgap semiconductor, such as silicon carbide (SiC), gallium nitride (GaN) and diamond, offer the potential to overcome both the temperature and voltage blocking limitations of Si. SiC is nowadays the most attractive candidate, offering significant potential advantages at both high temperature and high voltage levels whilst benefiting from tractable materials technology. Moreover, SiC is the only that can be thermally oxidized to form a high quality native oxide (SiO2), which enables the fabrication of MOS based devices.
However, very near the definitive emergence, the SiC technology needs to address two fundamental limitations: The price of the wafers and the poor SiC/SiO2 interface. The high density of imperfections encountered at the SiC/oxide interface represents a major obstacle in the development of functional SiC devices. The main efforts of this thesis have been directed to the detection and reduction of interface traps in the oxide/SiC interface. To achieve this demanding objective, two different ways have been contemplated: (1) Investigations have been carried out to improve the thermal oxidation or even to improve the formation of the interface with alternative techniques as nitridation or deposited oxides. (2) The classical insulator made up with SiO2 has been replaced by other innovative dielectrics.
Innovative gate fabrication processes have been proposed in this thesis using deposited SiO2 gate oxides from PECVD with silane and TEOS as precursors. SiO2-TEOS deposited oxides are an alternative to thermal oxidation. 4H-SiC MOSFET with mobilities up to 38-45 cm2/Vs [(0001) face] and 216 cm2/Vs [(11-20) face] have been fabricated.
We have demonstrated that the thermal oxidation of Ta2Si is a simple way to achieve a high-k dielectric on SiC (and on Si). We have fabricated one of the first well behaved high-k MOSFET on SiC with a mobility peak up to 45 cm2/Vs
In the last section, a field-effect mobility model including Coulomb scattering at interface traps has been proposed fitting the experimental channel mobility of SiC MOSFETs and the device behavior depending on the density of interface traps, the substrate doping level and the temperature.

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