Dissertations / Theses on the topic 'Magnetism. Electromagnetism'

This thesis presents a study on core loss calculations in rotating electrical machines. The basic concepts concerning magnetic moments, ferromagnetism, magnetic domains and magnetic hysteresis are introduced. The three-term models for alternating and rotational core losses in electrical steel sheets are presented. Several core loss measurement techniques are reviewed and an experiment is carried out to measure the total core losses in an electrical sheet steel sample under alternating and rotational magnetic fields of various frequencies and amplitudes. The coefficients in the loss models for alternating and rotational core losses are obtained through curve fitting process. The theory of electromagnetic fields is presented through the Maxwell equations and field scalar equations. A detailed review on core loss models for rotating electrical machines is presented. A rotational core loss model is adopted to calculate the core losses in a PM motor. The total core loss in the PM motor is obtained by summing the element losses using a MATLAB program. An experiment is conducted to measure the total core loss in the PM motor. The calculated total core loss in the PM motor is compared with the experimental results. The calculated total core losses are about 19% lower than the tested results. Various possible causes for this discrepancy are discussed
Master of Engineering (Hons)

The thesis deals with the design of a hybrid magnetic bearing. This is an extension of the issue of common bearings in high-speed motors. The work is divided into three parts. A general theory of magnetic bearings is described in the first part. The second part deals with the mathematical description of the bearing. A proposal of specific hybrid magnetic bearing is described in the third part. The bearing for the motor was already designed. It is a 45000rpm motor with a power output of 12 kW. This thesis aims to create a design of hybrid magnetic bearing with magnets to create a permanent magnetic field and coils to regulate forces to stabilize the rotor and limit vibrations. The practical design includes mathematical calculation in Matlab and computer simulation based on the finite element method in ANSYS Maxwell.

This work proposes the development of a technique to growth of single crystals through metallic flux technique at the Sergipe State. Besides, the system YxGd1-xIn3 (0 x 0.5 to pass of 0.10 % ) has been obtained via this technique and its structural and magnetic properties are studied. X-ray diffraction results have been performed in order to verify if the desired crystalline phase was obtained. The experimental results show that it has been grown with success. The magnetic characterization is carried out through magnetic susceptibility data as function of temperature (2 x 300 K) and magnetic field (??7 H 7T). The T-dependence of the magnetization shows a suppression of the magnetic ordering temperature with increasing of the yttrium concentration. The analyses of these experimental data show that the system presents magnetic frustration which is linked to the dilution effect introduced by the yttrium ion in the gadolinium site. The magnetization data as function of magnetic field carried out at 2 K show a usual behavior of a material with antiferromagnetic ordering.
Este trabalho propôs a implementação da técnica de crescimento de monocristais via fluxo metálico, na Universidade Federal de Sergipe (UFS) campus de itabaiana. Além disso, o sistema YxGd1-xIn3 (0 < x < 0.5 a passo de 0.10 % ) foi obtido através dessa técnica e as propriedades estruturais e magnéticas foram estudadas. Medidas de difração de raios X foram realizadas com o intuito de verificar o crescimento da fase cristalina desejada. Os dados mostraram que ela foi obtida com sucesso. A caracterização magnética foi feita através de medidas de susceptibilidades magnética em função da temperatura (2 < x < 300 K) e do campo magnético (-7 < H < 7T). As medidas de magnetização em função da temperatura mostraram uma supressão da temperatura de ordenamento magnético com o aumento da concentração de ítrio. As análises desses dados mostraram que o sistema apresenta frustração magnética que está ligado ao efeito de diluição introduzido pelo íon de ítrio no sitio do gadolínio. As medidas de magnetização em função do campo magnético realizadas a 2 K mostraram um comportamento típico de um material com ordenamento antiferromagnético.

Thesis (M.Sc (Hons)) -- University of Western Sydney, Nepean, 2000.
"Submitted for the degree of Master of Engineering (Hons), School of Mechatronic, Computer & Electrical Engineering, University of Western Sydney, Nepean" Includes bibliographical references (leaves 107-114).

Acompanha: Produto educacional: uma proposta de sequência didática para tópicos de magnetismo e eletromagnetismo
CAPES
O objetivo deste trabalho foi estudar a aplicação de uma sequência didática pautada na atividade experimental por meio de uma abordagem investigativa com uma turma da 4ª Série do Ensino Técnico Integrado em informática de nível Médio, com uma faixa etária entre 15 e 17 anos de uma instituição de ensino da rede pública federal da cidade de Campo Mourão/PR. A implementação da proposta e posteriormente análise dos dados atendeu os pressupostos da pesquisa qualitativa. Com esse trabalho, constatou-se que os alunos traziam suas explicações para o fenômeno físico estudado as quais careciam de respaldo técnico e científico e, a partir do trabalho em grupo durante a implementação da proposta, possibilita a estes uma visão mais científica e menos simplista do conceito estudado.
The objective of this work was to study the application of a didactic sequence based on the experimental activity by an investigative approach with a group of the 4th grade students of an Integrated Technical Course in Computer Science, at age between 15 and 17 years old of a federal institution in Campo Mourão city / PR. The implementation of the proposal and later analysis of the data met the assumptions of the qualitative research. With this work, it was verified that the students brought their explanations for the physical phenomenon studied, which needed technical and scientific support and, the group work, during the implementation of the proposal, allows them a more scientific and less simplistic view of the concept studied.

La resposta dels materials i/o sistemes físics quan interactuen amb la radiació electromagnètica ens dóna informació sobre les propietats físiques d’aquesta classe de materials i/o sistemes, com per exemple la conductivitat elèctrica, la permeabilitat magnètica, l’isospín d’un nucli atòmic, etc. Dos exemples són les ressonàncies magnètiques nuclear i electrònica, que consisteixen en sotmetre un material a radiació electromagnètica en el rang de radiofreqüència i microones, freqüències entre el 10 MHz i 300 GHz, i a un camp magnètic estacionari. En aquesta tesi es presenten els comportaments i/o la resposta de dos materials magnètics en front a la radiació de microones, el qual pertany al rang de freqüències compres entre els 300 MHz i els 300 GHz. Per aquest propòsit, el Grup de Magnetisme, Superconductivitat, Microones i Baixes temperatures de la Universitat de Barcelona, disposa d’un analitzador de xarxes vectorial model E8361A de la companyia Agilent, que permet analitzar senyals electromagnètiques, transmeses i/o reflectides, entre 10 MHz i 110 GHz. Les dues classes de materials magnètics que s’han estudiat son: el granat d’itri-ferro, amb composició química corresponent Y3Fe5O12 (YIG) i la manganita de lantà i calci, amb composició química corresponent La0.25Ca0.75MnO3. En el cas del YIG, encara que és un material magnètic força estudiat, ens vàrem plantejar el seu estudi emprant nous dissenys de ressonadors de microones. Aquest fet, junt amb l’aparició de nous resultats experimentals inesperats, i mancats d’una interpretació, justifiquen el seu estudi en aquesta memòria de tesis. En el cas de la manganita, ens trobàrem en la situació oposada, no hi ha gaires publicacions sobre mesures experimentals de ressonància electrònica de espín en aquest tipus de materials. Juntament amb l’aparició de nous fenòmens magnètics no publicats, quan es va procedir a la seva sistemàtica i completa caracterització magnètica. Fets que justifiquen el seu estudi en aquesta memòria de tesis. Aquesta memòria de tesis està organitzada en dos capítols: El primer capítol està dedicat al granat de d’itri-ferro, material ferrimagnètic a temperatura ambient (la seva temperatura de Curie és de 550 K), i que té les pèrdues d’energia més petites de tots els materials coneguts, quan aquest està sotmès a una senyal electromagnètica de microones. Llavors, aquest material és el més indicat per a l’estudi del fenòmens associats a la ressonància ferromagnètica. Per tant, en aquest capítol es presenta una introducció a la tècnica i a la fenomenologia física associada a la ressonància magnètica; una descripció de les propietats físiques del granat d’itri-ferro i una introducció a l’electrònica de microones. Aquesta última servirà com fonament per comprendre els experiments de ressonància ferromagnètica a temperatura ambient; ja que en ells s’empren tres tipus de ressonadors de microones: dos ressonadors de línia de microtira lambda/2 i un tercer consistent en dos ressonadors lambda/2 de microtira acoblats electrònicament entre ells en els seus centres (en forma de X). Juntament amb l’analizador de xarxes vectorial i un electromagnet, que pot aplicar camps magnètics fins a 0.5 T. La mostra de YIG ha estat crescuda epitaxialment en les dues cares d’un disc de granat de gadolini-gal.li (Gd3Ga5O12). El conjunt té un gruix de 0.5 mm i un diàmetre de 4 mm; les dues capes de YIG tenen un gruix de 49.6 milimicres cadascuna. Els resultats experimentals ens mostren tres espectres de absorció associats a tres classes de modes magnetoestàtics, com conseqüència de tres tipus de distribucions espacials de camp magnètic de microones, que sent el disc de YIG. Observats a quatre freqüències de ressonància diferents. I no únicament a una freqüència fixada, sinó en el rang de freqüències que defineixen cada pic de ressonància de cada ressonador. Els resultats d’aquesta última classe d’experiments mostren una correspondència lineal entre la freqüència de microones i el camp magnètic aplicat, però no correspon amb la relació obtinguda a través de la teoria dels modes magnetoestàtics. Es van realitzar estimacions per descartar qualsevol efecte de potència o acoblament entre les dues capes de YIG, portant a l’única explicació possible que aquests modes magnetoestàtics estan interaccionant amb la radiació electromagnètica que ells mateixos generen en la mostra de YIG. Arrel d’això, s’ha ideat un model basat en aquesta conclusió, per explicar com canvien les relacions lineals entre la freqüència de microones i el camp magnètic aplicat, en presència de radiació electromagnètica. Model que explica satisfactòriament els resultats obtinguts en les mesures experimentals de ressonància ferromagnètica en YIG. El segon capítol està dedicat a l’estudi de la manganita La0.25Ca0.75MnO3 des del punt de vista magnètic i de la ressonància electrònica d’espín. Per això, en aquest capítol se realitza una introducció a la família de manganites La(1-x)CaxMnO3 , atenent a la seva fenomenologia magnètica. Degut a això, també es descriu el fenomen del esbiaix d’intercanvi i la dinàmica de sistemes superparamagnètics i de vidres de espín. Aquests seran els fonaments per entendre els resultats de la caracterització magnètica de dues manganites nanogranulars, que anomenarem A i B; les quals fóren fabricades pel mètode sol-gel y calcinades a 600ºC i a 1000ºC respectivament. Per poder realitzar aquestes mesures experimentals, es disposa d’un magnetòmetre MPMS que permet realitzar mesures de magnetització entre 2 K y 350 K, aplicant camps magnètics entre -5 T i 5 T, a més de realitzar mesures de susceptibilitat magnètica a diferents freqüències de camp magnètic altern entre 0.1 Hz i 1500 Hz. Els resultats obtinguts mostren que els nanogrànuls contenen, des del punt de vista magnètic, un nucli antiferromagnètic i una escorça formada per clústers ferromagnètics, que interactuen a través de l’interacció de intercanvi. A més, en la mostra B s’observa el fenomen de l’ordenament de càrrega a altes temperatures i la manifestació d’un vidre d’espín interficial a baixes temperatures, entre els clústers ferromagnètics i el nucli antiferromagnètic. Fets molt poc habituals en aquest tipus de mostres i que no es donen en la mostra A. Finalment, en les mesures de ressonància electrònica d’espín, es van emprar dues classes de ressonadors de microones: un ressonador coaxial i un ressonador de microtira curtcircuitat i alhora connectat en sèrie a una resistència de 50 Ohms, específica per senyals de radiofreqüència i microones. Amb ells, els resultats experimentals obtinguts per a la mostra A concorden amb el model hidrodinàmic per als vidres d’espín. Fet que permet estimar la dependència de la constant giromagnètica i la constant d’anisotropia uniaxial efectiva de la mostra A, amb la temperatura. La correlació entre l’anisotropia magnètica uniaxial efectiva i la temperatura concorda amb la corresponent als vidres d’espín i/o les nanopartícules magnètiques i a més, permet l’estimació de la seva temperatura d’ ordenament de càrrega, essent aproximadament de 225 K . El factor giromagnètic de la mostra A reflecteix un canvi de la seva estructura cristal.lina, associada als clústers ferromagnètics, mentre va variant la temperatura. Fet que es presenta com una alternativa a l’hora d’estudiar canvis estructurals en els cristalls.
In this doctoral thesis, the response against the microwave electromagnetic radiation of two well differentiated magnetic materials is studied: the yttrium-iron garnet and the nanogranular manganite of chemical composition La0.25Ca0.75MnO3. In the case of the yttrium-iron garnet (YIG), even though it is a well-known magnetic material, we considered its study using a new sort of microwave resonators. This fact, together with the obtaining of unexpected experimental results without physical interpretation, inspires its study in this thesis. In the case of the manganite, we had found the opposite situation, in the literature experimental studies in electron spin resonance (ESR) have hardly ever been reported in this kind of materials. Therefore, we decided to perform its study using different microwave resonators. It was necessary, as well, to characterize magnetically the samples to understand their behavior. Precisely in these last experimental measurements, performed following a complete and systematic procedure, we were able to explain new physical phenomenology never reported previously, until now, in this material. This doctoral thesis, entitled “Study of Magnetic Materials with Microwaves” is arranged in two chapters. The first one is devoted to the study of the ferromagnetic resonance in a disk-shaped YIG sample. The results show the appearance of three types of Magnetostatic Spin Forward Volume Waves (MSFVW), for three kinds of microstrip resonators (two linear ones and one X-shaped), related with three kinds of magnetic spatial distributions. The relationship between the microwave frequency and the applied magnetic field does not follow the theoretical model of MSFVW and, after some estimations, we concluded that this is caused by the interaction between the MSFVW and the electromagnetic radiation generated by them. This new fact is explained theoretically by the development of a new model, considering this last interaction. The second chapter is devoted to the study of nanogranular manganites of chemical composition La0.25Ca0.75MnO3, from the point of view of their magnetism and their ESR response. A complete and systematic magnetic characterization is performed firstly in two samples, named A and B. Both samples manifest exchange bias interaction between ferromagnetic shell clusters and antiferromagnetic core, in each nanograin. Moreover, in sample B, interficial spin-glass is observed at low temperatures and charge ordering transition at high temperatures. These facts are rarely reported in similar magnetic systems. And they are not observed in sample A. ESR experiments were performed in sample A with a coaxial resonator and a short-circuited microstrip resonator connected to a 50 Ohms series resistor. The results follow the hydrodynamic model, allowing the estimation of the effective magnetic anisotropy constant and the gyromagnetic factor at different temperatures. The first one shows the same thermal behavior expected for nanoparticles and/or spin-glasses, and it also allows the estimation of the charge ordering temperature of sample A around 225 K. The thermal dependence of the gyromagnetic factor shows the expected behaviour for the crystalline structure of a La0.25Ca0.75MnO3 nanoparticle system. This means that the calculation of the thermal behavior of gyromagnetic factor is an alternative to X-ray thermal studies.

This master thesis focuses on literature research of problematics linked to power actuators working on electromagnetic principle to accelerate metal projectiles. It’s goal is mathematical analysis and constuction of selected type of electromagntic power actuator – induction coilgun

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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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Thesis (MTech (Electrical Engineering))--Cape Peninsula University of Technology, 2017.
This research project focuses on the implementation of a Reverberation Chamber (RC) by the transformation of an existing electromagnetically shielded room. The reverberation chamber is a kind of shielded room designed to create a statistically random internal electromagnetic environment. The reverberating environment makes it possible to obtain high field strengths from a relatively low input power. The electric fields in the chamber have to be stirred to achieve a statistically uniform field. The first part of this thesis presents an overview of reverberation chamber principles and preliminary calculations are done: the lowest usable frequency is estimated to be close to 300 MHz from empirical criteria. Modelling of the statistical environment is then presented, where electromagnetic quantities are characterised by probability density functions (Gaussian, Rayleigh and exponential); correlation issues are also presented. Measurements are performed in the frequency range of 800 MHz – 4 GHz, dictated by the antennas available for this research study. An investigation of cable losses is conducted, followed by a discussion on measurement accuracy. Mechanical stirrers are designed and manufactured. Electromechanical components are selected based on the literature study. Measurements are obtained through an automated setup using MATLAB®. To verify that the RC, with its in-house designed mechanical stirrers, is well-operated, the stirring ratio is experimentally determined. After this first test, an exhaustive investigation of probability density functions is conducted, taking into account correlation issues. Measurements show that the quality factor of the chamber is close to 2000 at 3 GHz, and that 60 independent stirrer positions at 4 GHz can be used for statistical analyses. Finally, the uniformity test is performed with an improved accuracy using frequency stirring. In conclusion, the CPUT RC passes the validation procedure according to the IEC 61000-4-21 standard by generating the required field uniformity within the accepted uncertainty level.

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.

This thesis is concerned with the synthesis of linear-optimal heave control of electromagnetic suspension systems. The methods of computer-aided analysis and simulation were employed in this research. The intrinsic properties of electromagnetic suspension system were investigated to facilitate the synthesis and to provide guidelines for the design of electromagnets or linear motors suitable for use in suspension systems. The technique of complementary filtering, which resolves the conflicting requirements of high stiffness to load and high compliance for ride-comfort, was further developed. This led to the ability of directly comparing the merits of systems with different configurations and determining the optimal natural frequency. Together with a novel way of removing steady state gap error during the traversing of transition curves, the filtering technique was applied to the system using fixed-gain and sliding-mode variable-structure control methods. The latter method is well known for its abi1ity in maintaining closed-loop characteristics in the presence of disturbance. However, further development was required before it was applied. The resultant suspension system gave a 20% improvement in ride-quality over the best published result which was carried out under similar test conditions. Even so, theoretical analysis showed that a four-fold improvement may be possible.

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.

Thesis (M.S. in Electrical Engineering) Naval Postgraduate School, June 1997.
Thesis advisors, Richard W. Adler, Jovan E. Lebaric. Includes bibliographical references (p. 45). Also available online.

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

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.

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.

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

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.

Thesis (Ph.D)--Mechanical Engineering, Georgia Institute of Technology, 2008.
Committee Chair: Lee, Kok-Meng; Committee Member: Bill Singhose; Committee Member: David G. Taylor; Committee Member: Eric Johnson; Committee Member: Nader Sadegh. Part of the SMARTech Electronic Thesis and Dissertation Collection.

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.

Multifunctional polymer nanocomposites (PNCs) are attractive for the design of tunable RF and microwave components such as flexible electronics, attenuators, and antennas due to cost-effectiveness and durability of polymeric matrices. In this work, three separate PNCs were synthesized. Magnetite (Fe3O4) and cobalt ferrite (CFO) nanoparticles, synthesized by thermal decomposition, were used as PNC fillers. Polymers used in this work were a commercial polymer provided by the Rogers Corporation (RP) and polyvinylidene fluoride (PVDF). PNCs in this thesis consist of Fe3O4 in RP, CFO in RP, and Fe3O4 in PVDF. Characterization techniques for determining morphology of the nanoparticles, and their resulting PNCs, include x-ray diffraction, transmission electron microscopy and magnetometry. All magnetometry measurements were taken using a Quantum Design Physical Property Measurement System with a superconducting magnet. Temperature and external magnetic field magnetization measurements revealed that all samples exhibit superparamagnetic behavior at room temperature. Blocking temperature, coercivity and reduced remnant magnetization do not vary with concentration. Tunable saturation magnetization, based on nanoparticle loading, was observed across all PNCs, regardless of polymer or nanoparticle choice, indicating that this is an inherent property in all similar PNC materials. Tunability studies of the magneto-dielectric PNCs were carried out by adding the PNC to cavity and microstrip linear resonator devices, and passing frequencies of 1-6 GHz through them in the presence of transverse external magnetic fields of up to 4.5 kOe, provided by an electromagnet. Microwave characteristics were extracted from scattering parameters of the PNCs. In all cases, losses were reduced, quality factor was increased, and tunability of the resonance frequency was demonstrated. Strong magnetic field dependence was observed across all samples measured in this study.

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.

Naval submarines and surface ships are regularly subjected to a treatment called "deperming" that seeks to design the vessel???s permanent magnetisation for optimal magnetic camouflage. A scaled model of a magnetic treatment facility (MTF) has been established as a valid system to simulate deperming and used to investigate various aspects of the deperm process including: magnetic anisotropy and demagnetising fields as factors in the physical modelling of magnetism in whole vessels; a comparison of current and alternative deperm procedures; the application of theoretical models of bulk magnetisation to calculate deperm outcomes in the physical model and in actual vessels. A "laboratory MTF" was constructed to imitate the applied field geometry at a naval MTF. The system was calibrated and it was determined that the laboratory MTF could make magnetic measurements on a CU200T-G steel bar sample with an equivalent accuracy (error = ??5%) to that of standard magnetometric equipment. Experiments were conducted with emphasis on a holistic approach to modelling the deperm process and describing magnetisation changes in whole objects. The importance of the magnetic anisotropic changes to steel with cold rolling was confirmed. In CU200T-G steel sheet the initial susceptibility (ci) was found to increase by a factor of 3 ??0.1 in the rolling direction, from a value of ~ 110 in the un-rolled steel sheet (thickness dependent). ci in the rolled sheet transverse to the rolling direction was decreased by a factor of 0.94 ??0.09 to ci in the un-rolled sheet steel. Previous studies on hull steel have neglected to account for this transformation through cold work. The demonstration on mild steel here is expected to have an analogy in the final state of the hull sheet steel as it resides in a submarine pressure hull. Future studies either on hull material or on modelling whole vessels should include the same or similar magnetic anisotropic properties in the steel(s) under investigation. Hollow circular tubes made from CA2S-E and CU200T-G steel sheet were selected as models for vessels. It was shown that these steel tubes were a good choice in this regard: minimising the complexity of the experiment whilst maintaining the validity of a deperm simulation. During a deperm there was an excellent qualitative likeness in the permanent longitudinal magnetisation (PLM) for the steel tubes to PLM in both a submarine and a surface vessel. Permanent vertical magnetisation (PVM) deperm results from the tubes displayed a close qualitative match with PVM in a submarine but not in a surface vessel. A theoretical treatment for demagnetisation factors (Nd) in hollow ellipsoids was used in conjunction with a geometrical approximation to calculate Nd for finite hollow objects of revolution. Subsequent theoretical calculations correlated well with experimental results for measured effective ci (ceff) in hollow circular CU200T-G steel tubes of various lengths and aspect ratios. Using an estimate of 100 as ci for submarine hull steel, the same analysis produces Nd for the axial and transaxial directions in a submarine equal to 5.97??10-3 and 0.0142 respectively. Three items for potential improvement were identified in the current deperm protocol used on naval vessels (Flash-D): redundancy in the protocol; the duration of the deperm and a theoretical basis for predicting the final magnetisation or changes in magnetisation during a deperm. Simulations of a novel "anhysteretic deperm" method, designed to combat these issues, compared favourably to the Flash-D protocol. The standard deviation (s) of the final PVM from 30 Flash-D deperms on steel tubes was 206 A/m; for the final PVM from 30 anhysteretic deperms of the same duration, this was 60 A/m. The s for the final PLM for Flash-D and anhysteretic deperms of the same duration were 416 A/m and 670 A/m respectively. The conclusion is that adopting the anhysteretic deperm on actual vessels would improve the reliability of the PVM outcome. Though the procedure would demand the same duration as Flash-D, there is the advantage of saving time by not having to repeat deperms to obtain the desired result. Additionally the anhysteretic deperm is considerably more amenable to theoretical analysis. A modified version of Langevin???s equation was used to predict the final PLM and PVM results for anhysteretic deperms and to provide a useful analysis of the anhysteretic processes in the Flash-D procedure. Using a Preisach analysis of hysteresis, a mathematical description of bulk magnetic changes that occur to a specific object, within a deperm, has been developed. Theoretical calculations of PLM in a steel tube during and after both types of deperm are in excellent agreement with experimental data. The same theoretical approach was also used to retrospectively model PLM results from previous Flash-D deperms on a submarine with equal success. With this analysis it is proposed that anhysteretic deperm outcomes could be predicted a priori. The influence of magnetic cargo on hull magnetisation was demonstrated to be of significance during and after deperming. "Sympathetic deperming" occurs where a magnetic source is located close to the hull during a deperm. It was found that a vessel or model vessel hull could still be demagnetised even when they contain magnetic cargo that would normally resist the direct application of the same magnetic fields. This was explained using the principles of demagnetising fields and anhysteretic magnetisation. A possible explanation was provided for a PVM measurement anomaly common to the model and vessel deperm results. From measurement, alternating longitudinal applied fields apparently induce corresponding changes in the PVM. This effect could be explained by the depermed object being offset longitudinally from the position expected by the measurement system. This offset could be estimated using an analysis of the changes to PLM and PVM after a longitudinal applied field. The offset displacements calculated for the vessels were too small to be verified experimentally (> 0.1m), but the predicted offset for the steel tubes coincided with the limit of precision for their placement in the laboratory MTF = 0.5mm The aim of this work was to look at the deperm process with reference to a system that demonstrated qualitative similarities to deperms on actual vessels. The laboratory MTF is a unique facility, permitting a useful practical analysis of deperming based on sound magnetostatic measurements The experimental and theoretical results gained here have direct application to future deperms on naval vessels with particular reference to submarines.

Thesis (M.S.)--Missouri University of Science and Technology, 2010.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed Dec. 23, 2009). Includes bibliographical references (p. 60).

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.

This thesis describes work related to the theme of sculpted electromagnetic fields - engineered fields with particular spatial patterns - and their interactions with molecules. We are motivated by the following questions: what are ways of detecting spatial patterns in electromagnetic fields? What are possible applications of spatially engineered fields? Are there molecular transitions that are dark to plane waves but that can be probed by sculpted fields? The first part of this thesis is in the area of magnetic field effects in chemistry. We focus on magnetic field modulated fluorescence, which provides a convenient method for imaging magnetic field strength. We proposed and demonstrated a fluorescence technique that allows imaging through strongly scattering media. We achieve this by exploiting the fact that most materials do not scatter magnetic field. This allows us to project a magnetic field pattern beyond the scattering surface. The magnetic field dependent fluorescence then allows us to map out the object of interest. We constructed a setup that demonstrates 2D imaging using this technique. We synthesized new molecular systems to enhance the sensitivity to magnetic field. We characterized and compared these molecules with steady state fluorescence spectroscopy, transient fluorescence and transient absorption measurements. The results reveal patterns that point to directions for engineering chemical systems to further enhance their magnetic field sensing properties. The second portion of this thesis is a theoretical study of the molecular multipole transitions and their couplings to local electromagnetic quantities. Using a semiclassical approach, we performed a multipole expansion of molecular transitions driven by monochromatic radiation. We derived the local electromagnetic quantities that couple to different multipole transitions and observables such as circular dichroism and magnetic circular dichroism. It was observed that certain transitions are dark to plane waves, but could be probed by simple spatial arrangements such as superpositions of plane waves. Experiments for their detection are also proposed.
Engineering and Applied Sciences

This thesis describes the development of a new type of Magnetic Force Microscope (MFM) probe based on a unique electromagnetic design. In addition the design, construction and testing of a new MFM system, complete in both hardware and software, is also described. The MFM allowed initial tests on prototypes of the new probe, and is to provide a base for future new probe integration. The microscope uses standard MFM micro-cantilever probes in static modes of imaging. A new computer hosted DSP control system, software, and its various interfaces with the MFM have been integrated into the system. The system has been tested using standard probes with various specimens and satisfactory results have been produced. A novel probe has been designed to replace the standard MFM magnetic coated tip with a field generated about a sub-micron aperture in a conducting film. The field from the new probe is modelled and its imaging capability investigated, with iterative designs analysed in this way. The practical construction and potential problems therein, of the probe are also considered. Test apertures have been manufactured, and an image of the field produced when operating is provided as support to the theoretical designs. Future methods of using the new probe are also discussed, including the examination of the probe as a magnetic write mechanism. This probe, integrated into the MFM, can provide a new method of microscopic magnetic imaging, and in addition opens a new potential method of magnetic storage that will require further research.

The Lorentz force law of classical electrodynamics states that the force F exerted by the magnetic induction B on a particle of charge q moving with velocity V is given by F = qV x B Since this force is orthogonal to the direction of motion, the magnetic field is said to be incapable of performing mechanical work. Yet there is no denying that a permanent magnet can readily perform mechanical work by pushing/pulling on another permanent magnet - or by attracting pieces of magnetizable material such as scrap iron or iron filings. We explain this apparent contradiction by examining the magnetic Lorentz force acting on an Amperian current loop, which is the model for a magnetic dipole. We then extend the discussion by analyzing the Einstein-Laub model of magnetic dipoles in the presence of external magnetic fields.

Ferrites are a class of magnetic oxides with superior electromagnetic (EM) properties at microwave frequencies when compared to conventional metallic magnetic materials for use in antenna miniturization and radar absorbers. Metamaterials are also a special group of materials, which are known to provide EM responses not found in nature due to subwavelength structuring. In this thesis, a range of ferrite composite materials and metamaterial structures are exploited to develop new methods for controlling permittivity and permeability up to 4 GHz with a view to producing high refractive index materials and to demonstrate broadband impedance matching to free space. The rst section of the thesis uses composites of powdered MnZn ferrite (as the ller) and PTFE (as the matrix), fabricated by a novel cold pressing technique, to produce composites for a range of volume fractions of MnZn ferrite (between 0-80% vol.). The EM properties for all composites were determined as a function of % vol. and the results were found to be in agreement with the Lichtenecker mixing formula. This study is the rst convincing con rmation of the Lichtenecker mixing formula over a broad range of volume fractions (0-80% vol.). The cold pressing method was found to produce composites with reproducible EM properties, and was extended to use aluminium, barium titanate (as llers) and also cellulose as an alternative matrix. Importantly, with regard to the study of cellulose composites, our work is the rst to explore volume fractions of up to 85% and, the rst to con rm the Lichtenecker mixing formula with these materials. The ferrite particle size, as well as the volume fraction of ferrite, impacts the EM response of the composites. Both the permittivity and permeability increase as a function of ferrite particle size; however, the permeability increases at a much faster rate than the permittivity with particle size. It is shown that by controlling the ferrite particle size in conjunction with the volume fraction of ferrite, broadband impedance matching to free space can be realised for tailored values of refractive index. This is the rst study that demonstrates independent control of the permittivity and permeability of ferrite composites by controlling the ferrite particle size. Alternatively, by adding a third component to the two part composite it is demonstrated that broadband impedance matching to free space can also be realised with a refractive index of 16.1 (between 10-50 MHz). This is the rst time, to the authors knowledge, that three part composites have been used to achieve high refractive index materials that are impedance matched to free space. The second section of this thesis takes the concept of metamaterials to structure ferrite composite material with a further view to gain independent control over the permittivity and permeability. By tailoring the EM response of this metamaterial, which is comprised of anisotropic arrays of ferrite cubes, broadband impedance matching to free space is demonstrated. The refractive index over the impedance-matched frequency range is also very high (9.5). The metamaterial also acts as an excellent non-re ecting subwavelength thickness absorber up to 200 MHz. An analytical description of the permittivity and permeability dependence on the metamaterial parameters is developed to predict the EM response of this metamaterial, and of similar systems. In the last part of this thesis, the concept of cubic metamaterials is extended to more complex metallic meta-atoms, where the permittivity and diamagnetic response of the metamaterial are independently tailored to demonstrate how the refractive index can be tuned over a broad frequency range. By understanding the role of individual cube parameters, the diamagnetic response can be controlled between near zero and unity, which greatly alters the refractive index. The results are the rst experimental validation for showing `design' control of the permittivity and permeability of these metamaterials via geometry tuning of the meta-atom design.

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.

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.