La production des dipôles supraconducteurs de la machine LHC du CERN s'est terminée en automne 2006. Les aimants fonctionnent à la température cryogénique de 1.9 K et doivent produire un champ magnétique très uniforme permettant de conduire les protons dans la machine. Le champ magnétique a été contrôlé avec beaucoup de rigueur et différentes mesures ont été réalisées pendant les différentes phases de l'assemblage des dipôles. Le but a été la découverte de défauts de production et d'assemblage qui prouvent limiter les performances des aimants. Dans le travail de thèse les effets de la variation de la géométrie des composantes mécaniques comme les câbles supraconducteur, les cales et les colliers de soutien des bobines sur l'uniformité du champ magnétique ont été étudiés. Une méthode pratique pour identifier et corriger les problèmes d'usinage a été développée et utilisée dans la phase de production. La thèse consiste en trois parties principales :<br />-Analyse de la production des principales composantes mécaniques des dipôles et étude de l'influence de la géométrie et des procédures d'assemblage sur la qualité du champ magnétique.<br />-Utilisation pratique des mesures effectuées sur les dipôles dans l'atelier d'assemblage pour résoudre les problèmes de production et comprendre le comportement de la géométrie des bobines pendant les phases d'assemblage.<br />-Etude théorique des composantes aléatoires des harmoniques du champ magnétique pour qualifier la production des dipôles.

The Earth’s magnetic field is generated by fluid motion of liquid iron in the outer core. Flows at the top of the outer core are believed to be responsible for the secular variation (SV) observed at the surface of the Earth. Modelling of this flow is open to considerable ambiguity, though methods adopting different physical assumptions do lead to similar flow velocity regimes. Some aspects of the ambiguities are investigated in this thesis. The last decade has seen a significant improvement in the capability to observe the global field at high spatial resolution. Several satellite missions have been launched, providing a rich new set of scalar and vector magnetic measurements from which to model the global field in detail. These data complement the existing record of groundbased observatories, which have continuous temporal coverage at a single point. I exploit these new data to model the secular variation (SV) globally and attempt to improve the core flow models that have been constructed to date. Using the approach developed by Mandea and Olsen (2006) I create a set of evenly distributed ‘Virtual Observatories’ (VO), at 400km above the Earth’s surface, encompassing satellite measurements from the CHAMP satellite over seven years (2001-2007), inverting the SV calculated at each VO to infer flow along the core-mantle boundary. Direct comparison of the SV generated by the flow model to the SV at individual VO can be made. Thus, the residual differences can be investigated in detail. Comparisons of residuals from flow models generated from a number of VO datasets provide evidence that they are consistent with internal and external field effects in the satellite data. I also show that the binning and processing of the VO data can induce artefacts, including sectorial banding, into the residuals. By employing the core flows from the inversion of SV data it may be possible to forecast the change of the present magnetic field (as measured) forwards in time for a short time period (e.g. less than five years) within an acceptable error budget. Using simple advection of steady or non-steady flows to forecast magnetic field change gives reasonably good fit to field models such as GRIMM, POMME or xCHAOS (< 50nT root mean square difference after five years). The forecast of the magnetic field change can be improved by optimally assimilating measurements of the field into the forecast from flow models at discrete points in time (e.g. annually). To achieve this, an Ensemble Kalman Filter (EnKF) can be used to the capture non-linearity of the model and delineate the error bounds by means of a Monte Carlo representation of the field evolution over time. In the EnKF model, an ensemble of probable state vectors (Gauss coefficients) evolve over time, driven by SV derived from core flows. The SV is randomly perturbed at each step before addition to the state vectors. The mean of the ensemble is chosen as the most likely state (i.e. field model) and the error associated with the estimate can be gauged from the standard deviation from the mean. I show an implementation of the EnKF for steady and non-steady flows generated from ‘Virtual Observatory’ field models, compared to the field models GRIMM and xCHAOS over the period 2002–2008. Using the EnKF, the maximum difference never exceeds 25nT over the period. This promising approach allows measurements to be included into model predictions to improve the forecast.

The influence of the horizontal magnetic field has not been sufficiently studied in contrast to study activity on the influence of the vertical magnetic field by researchers. The reason was that the influence of horizontal magnetic field to change the wear mass loss of ferromagnetic materials is smaller compared to the vertical magnetic field. However, the influence of horizontal magnetic field on rolling contact changes the subsurface crack initiation point toward surface is postulated by a researcher. Therefore, it is significance finding out how the horizontal magnetic field influences the tribological characteristics. This thesis presents a study on the influence of the horizontal magnetic field on wear in sliding contacts contributes for ascertainment the effect and mechanism of horizontal magnetic field on tribological characteristics of sliding contacts, through the experimental approach.The static magnetic field with densities of 0 and 1.1 Tesla and different orientations was applied to different contact conditions, different surface modifications and two sliding frequencies, using a ball-on-plate contact configuration. In conclusion, the presence of magnetic field enhances the chemical adsorption between iron or oxide iron and oxygen, and causes the transition of adhesive wear to oxidative wear. Besides, the presence of magnetic field combined with low sliding frequency forms the bulging on the wear surface and weakens the prevailing wear mechanism due to the low frictional temperature. On the other hand, the presence of magnetic field combined with high sliding frequency induces the transition to the oxidative wear mechanism and reduces the wear. Also, distinctly different appearances of wear surface are created by different magnetic field orientations. In the lubricated sliding contact, the magnetic field causes the reduction of wear and induction of oxide. It is postulated that the presence of magnetic field enhances the oxygen adsorption on the wear track by iron wear particles and hence varies the tribological behaviour. The influence of magnetic field on carbon steel coating consists in changes of oxide iron layer and steel layer, alterations of mechanical properties of the coating, and decrease in the mass loss and the surface roughness on the dry sliding contact. All these could be suggested the influence of adhesive strength of the interface between the base material and coating.

Student Number : 9807515F - MSc Dissertation - School of Computational and Applied Mathematics - Faculty of Science / Statistical and algebraic techniques of subspace tracking were tested for filtering the earth’s response from airborne magnetic field gradients in order to discriminate the relatively small response (dipole) of objects on the earth’s surface, such as UXO. Filtering the data was not very effective with these methods but a subspace was found in the data for the magnitude of the magnetic moment of the dipole. This subspace is easily obtained using the singular value decomposition and can be used for an approximate location, without depth estimation, as well as the relative size of the dipole.

Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of great interest to contemporary scientists because of their ability to directly convert temperature differences into electricity, and are regarded as a promising mode of alternative energy. The TE conversion efficiency is determined by the Carnot efficiency, η_C and is relevant to a commonly used figure of merit ZT of a material. Improving the value of ZT is presently a core mission within the TE field. In order to advance our understanding of thermoelectric materials and improve their efficiency, this dissertation investigates the low-temperature behavior of the p-type thermoelectric Cu2Se through chemical doping and nanostructuring. It demonstrates a method to separate the electronic and lattice thermal conductivities in single crystal Bi2Te3, Cu, Al, Zn, and probes the electrical transport of quasi 2D bismuth textured thin films. Cu2Se is a good high temperature TE material due to its phonon-liquid electron-crystal (PLEC) properties. It shows a discontinuity in transport coefficients and ZT around a structural transition. The present work on Cu2Se at low temperatures shows that it is a promising p-type TE material in the low temperature regime and investigates the Peierls transition and charge-density wave (CDW) response to doping [1]. After entering the CDW ground state, an oscillation (wave-like fluctuation) was observed in the dc I-V curve near 50 K; this exhibits a periodic negative differential resistivity in an applied electric field due to the current. An investigation into the doping effect of Zn, Ni, and Te on the CDW ground state shows that Zn and Ni-doped Cu2Se produces an increased semiconducting energy gap and electron-phonon coupling constant, while the Te doping suppresses the Peierls transition. A similar fluctuating wave-like dc I-V curve was observed in Cu1.98Zn0.02Se near 40 K. This oscillatory behavior in the dc I-V curve was found to be insensitive to magnetic field but temperature dependent [2]. Understanding reducing thermal conductivity in TE materials is an important facet of increasing TE efficiency and potential applications. In this dissertation, a magnetothermal (MTR) resistance method is used to measure the lattice thermal conductivity, κ_ph of single crystal Bi2Te3 from 5 to 60 K. A large transverse magnetic field is applied to suppress the electronic thermal conduction while measuring thermal conductivity and electrical resistivity. The lattice thermal conductivity is then calculated by extrapolating the thermal conductivity versus electrical conductivity curve to a zero electrical conductivity value. The results show that the measured phonon thermal conductivity follows the e^(Δ_min⁄T) temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. These low-temperature experimental data and analysis on Bi2Te3 are important compliments to previous measurements and theoretical calculations at higher temperatures, 100 – 300 K. The MTR method on Bi2Te3 provides data necessary for first-principles calculations [4]. A parallel study on single crystal Cu, Al and Zn shows the applicability of the MTR method for separating κ_e and κ_ph in metals and indicates a significant deviation of the Lorenz ratio between 5 K and 60 K [3]. Elemental bismuth is a component of many TE compounds and in this dissertation magnetoresistance measurements are used investigate the effect of texturing in polycrystalline bismuth thin films. Electrical current in bismuth films with texturing such that all grains are oriented with the trigonal axis normal to the film plane is found to flow in an isotropic manner. By contrast, bismuth films with no texture such that not all grains have the same crystallographic orientation exhibit anisotropic current flow, giving rise to preferential current flow pathways in each grain depending on its orientation. Textured and non-textured bismuth thin films are examined by measuring their angle-dependent magnetoresistance at different temperatures (3 – 300 K) and applied magnetic fields (0 – 90 kOe). Experimental evidence shows that the anisotropic conduction is due to the large mass anisotropy of bismuth and is confirmed by a parallel study on an antimony thin film [5]. [1] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril Opeil, J. Alloys Compd. 699, 718 (2017). [2] Mengliang Yao, Weishu Liu, Xiang Chen, Zhensong Ren, Stephen Wilson, Zhifeng Ren, and Cyril P. Opeil, J. Materiomics 3, 150 (2017). [3] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal metals: Al, Cu and Zn, Mengliang Yao, Mona Zebarjadi, and Cyril P. Opeil, under review. [4] Experimental determination of phonon thermal conductivity and Lorenz ratio of single crystal bismuth telluride, Mengliang Yao, Stephen Wilson, Mona Zebarjadi, and Cyril Opeil, under review. [5] Albert D. Liao, Mengliang Yao, Ferhat Katmis, Mingda Li, Shuang Tang, Jagadeesh S. Moodera, Cyril Opeil, Mildred S. Dresselhaus, Appl. Phys. Lett. 105, 063114 (2014). / Thesis (PhD) — Boston College, 2017. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Este trabalho consistiu na implementação de um método de homogeneização de campo, denominado shimming passivo. Esse método é caracterizado pela inserção de pequenas peças ferromagnéticas no interior do magneto; a interação destas peças com o campo magnético principal produz campos locais capazes de corrigir inomogeneidades indesejadas, quando várias peças são estrategicamente combinadas. Embora esse método já tenha sido bem discutido, implementado e publicado por D.I.Hoult na década de 80, ele ainda não havia sido desenvolvido para um magneto resistivo com núcleo de ferro e peças polares, como é o caso do Artro-ToRM. Nosso objetivo era, através do Artro-ToRM, encontrar uma metodologia de modelagem e correção de campo que fosse útil para magnetos com a mesma geometria. Foram utilizados métodos computacionais de ajuste numérico que, a partir de mapas originais de campo, foram capazes de encontrar configurações de peças magnéticas que pudessem reduzir as inomogeneidades presentes. Um dos maiores desafios do trabalho, foi encontrarmos elementos passivos com comportamento previsível quando submetidos ao campo magnético principal, já que os programas de otimização presupunham que trabalhávamos com dipolos magnéticos, para efeitos de simplificação. Finalmente, considerando uma região cilíndrica com raio de 5 cm e comprimento de 10 cm, mostramos uma melhora de 390 ppm para 250 ppm na homogeneidade, após a correção passiva. / This work presents the implementation of a method for field correction, called passive shimming. The method is characterized by the insertion of small iron pieces in magnet bore; the interaction of these pieces with the main magnetic field produces local fields capable of correcting undesired inhomogenity, when some parts are strategically combined. Although this method has been already proposed by D.I.Hoult in the eighties, it still has not been developed for resistive magnets with polar pieces, such as the Artro-ToRM. Our objective were to find a method for field modeling and correction that could be useful for magnets with similar geometry. Computational methods of numerical adjustment were used from the original field maps, it was possible to find the optimal the configurations of magnetic parts for reducing the field inhomogenity. One of the biggest challenges of the work, was to find passive elements with previsible behavior when submitted to magnetic field main, since the optimization programs preassumed that we worked with magnetic dipoles, for the purpose of simplification. Finally, considering a cilindrical region, we show an improvement of 390 ppm to 250 ppm in the homogenity, after the passive correction.

Nesta disserta&ccedil;&atilde;o de mestrado se monitorou in situ e ex situ a corros&atilde;o de corpos de prova de cobre met&aacute;lico por resson&acirc;ncia magn&eacute;tica nuclear no dom&iacute;nio do tempo (RMN-DT). A rea&ccedil;&atilde;o ocorreu em solu&ccedil;&atilde;o aquosa de HCl 1 mol L-1 fornecendo &iacute;ons de cobre Cu2+ como produtos da corros&atilde;o. A corros&atilde;o foi monitorada com um espectr&ocirc;metro RMN de bancada, atrav&eacute;s da correla&ccedil;&atilde;o entre os tempos de relaxa&ccedil;&atilde;o transversal (T2) adquiridos por meio da sequ&ecirc;ncia de pulso CPMG e a concentra&ccedil;&atilde;o de Cu2+ na solu&ccedil;&atilde;o. As rea&ccedil;&otilde;es foram estudadas usando como corpos de prova placas e cilindros de cobre, na presen&ccedil;a e aus&ecirc;ncia de campo magn&eacute;tico e na presen&ccedil;a e aus&ecirc;ncia de potencial el&eacute;trico aplicado. Esses experimentos foram realizados para estudar o efeito das for&ccedil;as magn&eacute;ticas que podem afetar as rea&ccedil;&otilde;es. Os experimentos de RMN-DT-eletroqu&iacute;mica (RMN-DT-EQ) foram executados usando o corpo de prova em formato de placa de cobre como eletrodo de trabalho, fio espiral de platina como contra eletrodo e um eletrodo de refer&ecirc;ncia de Ag/AgCl (KCl 3 mol L-1). Os experimentos sem potencial aplicado foram realizados usando-se somente os corpos de prova de cobre na solu&ccedil;&atilde;o de HCl. Os resultados mostraram que o campo magn&eacute;tico n&atilde;o alterou o efeito da corros&atilde;o quando se aplicou um potencial de 1V. No entanto a rea&ccedil;&atilde;o foi inibida na corros&atilde;o sem potencial el&eacute;trico aplicado. Esses resultados levaram a hip&oacute;tese de que, na rea&ccedil;&atilde;o com potencial aplicado, a for&ccedil;a de Lorentz foi minimizada pelas for&ccedil;as de gradiente de campo magn&eacute;tico e for&ccedil;a de gradiente da concentra&ccedil;&atilde;o de esp&eacute;cies paramagn&eacute;tica. No caso da corros&atilde;o sem potencial el&eacute;trico aplicado, a hip&oacute;tese para a inibi&ccedil;&atilde;o da corros&atilde;o foi que as for&ccedil;as de gradiente mantiveram os &iacute;ons de cobre na interface corpo de prova/solu&ccedil;&atilde;o, o que dificultou a rea&ccedil;&atilde;o. O efeito do campo magn&eacute;tico sobre a superf&iacute;cie do cilindro de cobre ao final do processo de corros&atilde;o tamb&eacute;m foram analisados pelas t&eacute;cnicas de microscopia eletr&ocirc;nica de varredura e microscopia de for&ccedil;a at&ocirc;mica enquanto que a solu&ccedil;&atilde;o resultante do processo de corros&atilde;o teve os valores da concentra&ccedil;&atilde;o de &iacute;ons Cu2+ quantificados pelas espectroscopias de absor&ccedil;&atilde;o no ultravioleta e vis&iacute;vel e de absor&ccedil;&atilde;o at&ocirc;mica com chama. / In this Masters dissertation the corrosion of metallic copper samples was monitored in situ and ex situ by time domain nuclear magnetic resonance (TD-NMR). The reaction was performed in an aqueous solution containing HCl (1 mol L-1), where Cu2+ ions were the corrosion products. A benchtop NMR spectrometer was used to monitor the reaction through the correlation between the transverse relaxation times (T2), acquired with the CPMG pulse sequence, and the concentration of Cu2+ in the solution. The reactions were studied using copper plaques and cylinders in the presence and absence of a magnetic field and in the presence and absence of an applied potential. These experiments were performed to study the effect of the magnetic forces which affect reactions with and without an applied potential. The coupling experiments between TD-NMR and electrochemistry (EC-NMR) were performed using a copper plaque as a working electrode, a platinum wire in a spiral shape as a counter electrode and a Ag/AgCl KCl 3 mol L-1 reference electrode. Experiments in which no potential was applied were performed by inserting the copper sample in an HCl aqueous solution. Results showed that the magnetic field didn&rsquo;t alter the corrosion process when a 1V potential was applied but it did inhibit the corrosion of copper when no potential was applied. These results lead to the hypothesis that, in the reaction with an applied potential, the Lorentz force was minimized by the forces created by the magnetic field and the concentration gradient of the paramagnetic species (Cu2+). In the case of corrosion experiments without an applied potential a possibility is that the forces created by the concentration gradient force copper ions to stay on the interface copper/solution, which hinders the reaction. The effect of the magnetic field on the surface of the copper cylinder at the end of the corrosion process was also analysed by scanning electron microscopy and atomic force microscopy while the concentration of Cu2+ in the solution was measured by UV-vis spectroscopy and flame atomic absorption spectrometry.

Des études expérimentales et théoriques du mécanisme d'électrodéposition de différents revêtements à base de cobalt : le cobalt métallique, les alliages binaires Co-Ni, Co-Cu et l'alliage ternaire Co-Ni-Cu sont menés.Les propriétés de ces films dépendent fortement des paramètres expérimentaux tels que le pH de l'électrolyte, la nature du substrat, etc.. Un champ magnétique jusqu'à 12T est superposé parallèlement à la surface de l'électrode de travail. La convection forcée induite dans l'électrolyte modifie les conditions hydrodynamiques et par conséquent, influe sur la structure et la morphologie des films obtenus. Par ailleurs, la cinétique des processus et la croissance des cristaux sont améliorées dans des conditions magnéto électrochimique. Les microstructures obtenues suite au procédé électrochimique en présence d'un champ magnétique sont caractérisées par différentes propriétés magnétiques trouvant un intérêt potentiel en tant que matériaux magnétiques doux et / ou dur. Dans le cadre du projet ANR COMAGNET, les matériaux subissent ensuite un traitement thermique en présence d'un champ magnétique, des phénomènes de recristallisation et interdiffusion sont mis en évidence induisant des modifications des propriétés magnétiques.Enfin, des calculs de l'énergie d'adsorption de l'atome d'hydrogène et de l'ion H+ sur différentes faces cristallographiques du cobalt sont menés par modélisation DFT (Théorie de la Fonctionnelle de la Densité). Ces résultats, mis en relation avec l'étude expérimentale, montrent le lien entre la formation de certaines structures cristallographiques et l'évolution simultanée de l'hydrogène à la surface de l'électrode pendant l'électrolyse. / The present work is focused on the experimental and theoretical studies of the electrodeposition mechanism of Co-based films, including single Co, binary Co-Ni, Co-Cu and ternary Co-Ni-Cu alloy films. The modeling and structure-properties relationship by a multi-scale approach is investigated.The preliminary study involves determination and optimization of the electrodeposition parameters in order to obtain nanocrystalline films with satisfied surface quality and promising magnetic properties. It is shown that the electrolytic pH, to-be-deposited type of substrate and deposition time are of high importance in the fabrication of nanoscale materials. Among them, the electrolytic pH is the one, which has the greatest effect on the structure formation. The film growth as well as its quality can be strongly affected by a superimposition of an external magnetic field. The electrodeposition process was carried out under parallel to the electrode surface magnetic fields with magnets strength up to 12T. The results reveal that the induced forced convection in the electrolyte changes the hydrodynamic conditions and thus, affects the structure and morphology of the obtained films. Furthermore, the process kinetics and crystal growth are enhanced under magnetic electrodeposition conditions.Microstructure formed by the electrochemical processing is characterized by some significant magnetic properties, which may result in soft and/or hard magnetic materials, depending on their application approach. Additionally, the microstructure of films has been improved by the magnetic annealing treatment. Thus, the recrystallization and interdiffusion phenomena are observed, and modification of the magnetic properties is induced.Considering the electrodeposition process carried out in aqueous solutions the secondary process, which is the hydrogen evolution reaction (HER), needs to be taken into account. The theoretical study based on the tools of quantum mechanics (QM) and density functional theory (DFT) is used to determine the adsorption energies of hydrogen. In this work, the calculation results are related with the experimental study and may explain the structure formation assisted by the simultaneous evolution of hydrogen at the electrode surface.

Nesta disserta&ccedil;&atilde;o de mestrado se monitorou in situ e ex situ a corros&atilde;o de corpos de prova de cobre met&aacute;lico por resson&acirc;ncia magn&eacute;tica nuclear no dom&iacute;nio do tempo (RMN-DT). A rea&ccedil;&atilde;o ocorreu em solu&ccedil;&atilde;o aquosa de HCl 1 mol L-1 fornecendo &iacute;ons de cobre Cu2+ como produtos da corros&atilde;o. A corros&atilde;o foi monitorada com um espectr&ocirc;metro RMN de bancada, atrav&eacute;s da correla&ccedil;&atilde;o entre os tempos de relaxa&ccedil;&atilde;o transversal (T2) adquiridos por meio da sequ&ecirc;ncia de pulso CPMG e a concentra&ccedil;&atilde;o de Cu2+ na solu&ccedil;&atilde;o. As rea&ccedil;&otilde;es foram estudadas usando como corpos de prova placas e cilindros de cobre, na presen&ccedil;a e aus&ecirc;ncia de campo magn&eacute;tico e na presen&ccedil;a e aus&ecirc;ncia de potencial el&eacute;trico aplicado. Esses experimentos foram realizados para estudar o efeito das for&ccedil;as magn&eacute;ticas que podem afetar as rea&ccedil;&otilde;es. Os experimentos de RMN-DT-eletroqu&iacute;mica (RMN-DT-EQ) foram executados usando o corpo de prova em formato de placa de cobre como eletrodo de trabalho, fio espiral de platina como contra eletrodo e um eletrodo de refer&ecirc;ncia de Ag/AgCl (KCl 3 mol L-1). Os experimentos sem potencial aplicado foram realizados usando-se somente os corpos de prova de cobre na solu&ccedil;&atilde;o de HCl. Os resultados mostraram que o campo magn&eacute;tico n&atilde;o alterou o efeito da corros&atilde;o quando se aplicou um potencial de 1V. No entanto a rea&ccedil;&atilde;o foi inibida na corros&atilde;o sem potencial el&eacute;trico aplicado. Esses resultados levaram a hip&oacute;tese de que, na rea&ccedil;&atilde;o com potencial aplicado, a for&ccedil;a de Lorentz foi minimizada pelas for&ccedil;as de gradiente de campo magn&eacute;tico e for&ccedil;a de gradiente da concentra&ccedil;&atilde;o de esp&eacute;cies paramagn&eacute;tica. No caso da corros&atilde;o sem potencial el&eacute;trico aplicado, a hip&oacute;tese para a inibi&ccedil;&atilde;o da corros&atilde;o foi que as for&ccedil;as de gradiente mantiveram os &iacute;ons de cobre na interface corpo de prova/solu&ccedil;&atilde;o, o que dificultou a rea&ccedil;&atilde;o. O efeito do campo magn&eacute;tico sobre a superf&iacute;cie do cilindro de cobre ao final do processo de corros&atilde;o tamb&eacute;m foram analisados pelas t&eacute;cnicas de microscopia eletr&ocirc;nica de varredura e microscopia de for&ccedil;a at&ocirc;mica enquanto que a solu&ccedil;&atilde;o resultante do processo de corros&atilde;o teve os valores da concentra&ccedil;&atilde;o de &iacute;ons Cu2+ quantificados pelas espectroscopias de absor&ccedil;&atilde;o no ultravioleta e vis&iacute;vel e de absor&ccedil;&atilde;o at&ocirc;mica com chama. / In this Masters dissertation the corrosion of metallic copper samples was monitored in situ and ex situ by time domain nuclear magnetic resonance (TD-NMR). The reaction was performed in an aqueous solution containing HCl (1 mol L-1), where Cu2+ ions were the corrosion products. A benchtop NMR spectrometer was used to monitor the reaction through the correlation between the transverse relaxation times (T2), acquired with the CPMG pulse sequence, and the concentration of Cu2+ in the solution. The reactions were studied using copper plaques and cylinders in the presence and absence of a magnetic field and in the presence and absence of an applied potential. These experiments were performed to study the effect of the magnetic forces which affect reactions with and without an applied potential. The coupling experiments between TD-NMR and electrochemistry (EC-NMR) were performed using a copper plaque as a working electrode, a platinum wire in a spiral shape as a counter electrode and a Ag/AgCl KCl 3 mol L-1 reference electrode. Experiments in which no potential was applied were performed by inserting the copper sample in an HCl aqueous solution. Results showed that the magnetic field didn&rsquo;t alter the corrosion process when a 1V potential was applied but it did inhibit the corrosion of copper when no potential was applied. These results lead to the hypothesis that, in the reaction with an applied potential, the Lorentz force was minimized by the forces created by the magnetic field and the concentration gradient of the paramagnetic species (Cu2+). In the case of corrosion experiments without an applied potential a possibility is that the forces created by the concentration gradient force copper ions to stay on the interface copper/solution, which hinders the reaction. The effect of the magnetic field on the surface of the copper cylinder at the end of the corrosion process was also analysed by scanning electron microscopy and atomic force microscopy while the concentration of Cu2+ in the solution was measured by UV-vis spectroscopy and flame atomic absorption spectrometry.

Spin interactions play a key role in the function of molecular materials from naturally occurring biological complexes to synthetic materials for light-harvesting and light-emission. This thesis investigates the spin interactions of spin-1 triplet excitons formed by singlet fission. Singlet fission produces two triplet excitons from one light-induced singlet state and holds promise to enable solar energy generation beyond traditional efficiency limits. As the lifetime of triplet pairs depends sensitively on their spin degree of freedom, in this thesis we deploy spin-sensitive techniques to understand the interactions and evolution of triplet pairs. After introducing the relevant theoretical and experimental background underlying singlet fission and the role of spin, we describe the first observation of strongly exchange coupled, high-spin triplet-pair states ($S=2$) in a solid-state organic semiconductor and show that the singlet fission process allows for the formation of long-lived, strongly coupled spin-two states. We then describe the development and use of photoluminescence-detected avoided level-crossings in applied magnetic fields to quantify the strength of exchange coupling and identify specific optical signatures of exchange-coupled triplet pairs. Using high magnetic fields ($\leq\mbox{60 T}$) we isolate and measure the exchange coupling and optical signatures of multiple distinct triplet pairs in the same material. Finally, we probe the mechanisms of formation and decay of spin polarization from triplet pair states using pulsed spin resonance. The measured dynamics are consistent with polarization driven by fluctuations in exchange coupling between pairs and spin-orbit mediated decay of triplet excitons to the ground state. The combined measurements of the spin parameters and polarization dynamics of triplet pairs from ns to ms timescales provides a quantitative picture of the spin states generated by singlet fission.