The purpose of this investigation is to develop techniques of experimentation in the field of electron transport phenomena.

In this work we studied the convective heat transfer in a magnetic fluid in both zero and applied magnetic fields. The natural convection is observed in a quasi-one dimensional magnetic fluid in a horizontal temperature gradient. The horizontal non-homogeneous magnetic fields were applied across the sample cell either parallel or anti-parallel to the temperature gradient. The temperature profile was measured by eight thermocouples and temperature sensitive paint. The flow velocity field and streamlines were obtained by optical flow method. Calculated Nusselt numbers, Rayleigh number, and Grashof number show that the convective flow is the main heat transfer mechanism in applied fields in our geometry. It was found that when the field gradient is parallel with temperature gradient, the fields enhance the convective heat transfer while the fields inhibit it in anti-parallel configuration by analyzing the temperature difference across the sample, flow patterns, and perturbation Q field in applied fields. Magnetic Rayleigh number and magnetic Grashof number show that the thermomagnetic convections dominate in high magnetic fields. It is shown that the physical nature of the field effect is corresponding to the magnetic body force which is perpendicular to the gravity in our experiments. When the direction of the magnetic body force is same with temperature gradient in parallel configuration, the body force increases the convective heat transfer; while it has opposite effect in anti-parallel configuration. Our study will not only shed light on the fundamental mechanisms for thermomagnetic convection but also help to develop the potential field-controlled heat transfer devices.

The purpose of this investigation was to study head-transport phenomena in a single crystal of antimony at liquid helium temperatures. In particular, the longitudinal and transverse components of the thermal resistivity tensor were measured as a function of magnetic field up to eighteen kilogauss.

Motores termomagnéticos são dispositivos capazes de converter calor em energia mecânica através do efeito termomagnético, e são uma alternativa para a conversão de energia de rejeitos térmicos de baixa e baixíssima qualidade. Neste trabalho é proposta uma classificação dos motores termomagnéticos como sendo de dois tipos, os motores tipo Edison e os motores tipo Tesla. Feita a classificação, diferenciou-se o comportamento de operação e os ciclos termodinâmicos desenvolvidos pelos dois tipos de motores, mostrando que motores do tipo Tesla desenvolvem um ciclo termodinâmico que pode ser aproximado por um ciclo Brayton magnético, já motores do tipo Edison descrevem um ciclo mais complexo, não podendo ser aproximado por um ciclo Brayton. Compararam-se os parâmetros de interesse para ambos os motores através de análises termodinâmicas, onde se concluiu que motores do tipo Tesla apresentam melhores respostas de trabalho e eficiência que motores do tipo Edison, quando são consideradas as mesmas condições de operação. Além disso, identificou-se que a equação de força de Kelvin é a equação que corretamente descreve o comportamento da força magnética em um motor termomagnético, essa contribuição é importante, pois vários trabalhos publicados na literatura utilizam equações que não descrevem corretamente o comportamento da força magnética. Mostrou-se que o trabalho produzido em um motor termomagnético é igual ao trabalho produzido pela força magnética resultante no dispositivo. Foi desenvolvida e validada uma metodologia para o cálculo do trabalho específico produzido em um motor do tipo Tesla. Utilizando as metodologias validadas, verificou-se como a temperatura, o campo magnético aplicado, o fator de desmagnetização e o tipo de transição influenciam o comportamento dos motores termomagnéticos tipo Tesla, o que abre caminho para o desenvolvimento de dispositivos mais interessantes do ponto de vista termodinâmico. / Thermomagnetic motors are devices capable of converting heat into mechanical energy through the thermomagnetic effect. These devices are able to operate using low or very low quality thermal waste, being an alternative to avail that range of thermal energy. This work classifies the thermomagnetic motors in two types: The Tesla type and the Edison type thermomagnetic motors, differentiating the operational behavior and the thermodynamic cycles developed in each type. By using thermodynamic approaches, it is shown that the Tesla type thermomagnetic motors have best response in terms of work and efficiency than the Edison type thermomagnetic motors, when the same operating conditions are considered. In addition, an experimental approach is presented, proving that the Kelvin force equation describes the behavior of the force in thermomagnetic motors, and the work produced in a motor is the same that the work produced by the resultant magnetic force in the system. It was developed and validated a method to estimate the work produced by cycle in a Tesla type thermomagnetic motor, and using thermodynamic approaches, the relevance of the temperature, applied magnetic field, demagnetizing factor and transition type in the Tesla type thermomagnetic motor were verified.

Motores termomagnéticos são dispositivos capazes de converter calor em energia mecânica através do efeito termomagnético, e são uma alternativa para a conversão de energia de rejeitos térmicos de baixa e baixíssima qualidade. Neste trabalho é proposta uma classificação dos motores termomagnéticos como sendo de dois tipos, os motores tipo Edison e os motores tipo Tesla. Feita a classificação, diferenciou-se o comportamento de operação e os ciclos termodinâmicos desenvolvidos pelos dois tipos de motores, mostrando que motores do tipo Tesla desenvolvem um ciclo termodinâmico que pode ser aproximado por um ciclo Brayton magnético, já motores do tipo Edison descrevem um ciclo mais complexo, não podendo ser aproximado por um ciclo Brayton. Compararam-se os parâmetros de interesse para ambos os motores através de análises termodinâmicas, onde se concluiu que motores do tipo Tesla apresentam melhores respostas de trabalho e eficiência que motores do tipo Edison, quando são consideradas as mesmas condições de operação. Além disso, identificou-se que a equação de força de Kelvin é a equação que corretamente descreve o comportamento da força magnética em um motor termomagnético, essa contribuição é importante, pois vários trabalhos publicados na literatura utilizam equações que não descrevem corretamente o comportamento da força magnética. Mostrou-se que o trabalho produzido em um motor termomagnético é igual ao trabalho produzido pela força magnética resultante no dispositivo. Foi desenvolvida e validada uma metodologia para o cálculo do trabalho específico produzido em um motor do tipo Tesla. Utilizando as metodologias validadas, verificou-se como a temperatura, o campo magnético aplicado, o fator de desmagnetização e o tipo de transição influenciam o comportamento dos motores termomagnéticos tipo Tesla, o que abre caminho para o desenvolvimento de dispositivos mais interessantes do ponto de vista termodinâmico. / Thermomagnetic motors are devices capable of converting heat into mechanical energy through the thermomagnetic effect. These devices are able to operate using low or very low quality thermal waste, being an alternative to avail that range of thermal energy. This work classifies the thermomagnetic motors in two types: The Tesla type and the Edison type thermomagnetic motors, differentiating the operational behavior and the thermodynamic cycles developed in each type. By using thermodynamic approaches, it is shown that the Tesla type thermomagnetic motors have best response in terms of work and efficiency than the Edison type thermomagnetic motors, when the same operating conditions are considered. In addition, an experimental approach is presented, proving that the Kelvin force equation describes the behavior of the force in thermomagnetic motors, and the work produced in a motor is the same that the work produced by the resultant magnetic force in the system. It was developed and validated a method to estimate the work produced by cycle in a Tesla type thermomagnetic motor, and using thermodynamic approaches, the relevance of the temperature, applied magnetic field, demagnetizing factor and transition type in the Tesla type thermomagnetic motor were verified.

Magnetic fluid flow by thermomagnetic convection with and without buoyancy was studied in experiments and computational simulations. A mineral oil based ferro magnetic fluid was subjected to varying magnetic fields to induce thermomagnetic convection. As such fluids are mainly developed to increase heat transfer for cooling the fundamental effects on magnetic fluid flow was investigated using various magnetic field distributions. Computational simulations of natural and thermomagnetic convection are based on a Finite-Element technique and considered a constant magnetic field gradient, a realistic magnetic field generated by a permanent magnet and alternating magnetic fields. The magnetic field within the fluid domain was calculated by the magneto-static Maxwell equations and considered in an additional magnetic body force known as the Kelvin body force by numerical simulations. The computational model coupled the solutions of the magnetic field equations with the heat and fluid flow equations. Experiments to investigate thermomagnetic convection in the presence of terrestrial gravity used infrared thermography to record temperature fields that are validated by a corresponding numerical analysis. All configurations were chosen to investigate the response of the magnetic fluid to the applied body forces and their competition by varying the magnetic field intensity and its spatial distribution. As both body forces are temperature dependent, situations were analysed numerically and experimentally to give an indication of the degree by which heat transfer may be enhanced or reduced. Results demonstrate that the Kelvin body force can be much stronger than buoyancy and can induce convection where buoyancy is not able to. This was evident in a transition area if parts of a fluid domain are not fully magnetically saturated. Results for the transition from natural convection to thermomagnetic convection suggest that the domain of influence of the Kelvin body force is aligned with the dominance of the respective body force. To characterise the transition a body force ratio of the Kelvin body force to buoyancy was developed that identified the respective driving forces of the convection cells. The effects on heat transfer was quantified by the Nusselt number and a suitable Rayleigh number. A modified Rayleigh number was used when both body forces were active to define an effective body force by taking the relative orientation of both forces into account. Results for the alternating magnetic field presented flow fields that altered with the frequency of the applied magnetic field but with varying amplitude. This affected the heat transfer that alternated with the frequency but failed to respond instantaneously and a phase lag was observed which was characterised by three different time scales.

This research focuses on developing new techniques and designs for highly con- trollable microactuating systems with large force-stroke outputs. A fixed-fixed mi- crobeam is the actuating element in the introduced techniques. Either buckling of a microbridge by thermal stress, lateral deflection of a microbridge by electro- magnetic force, or combined effects of both can be employed for microactuation. The proposed method here is MicroElectroThermoMagnetic Actuation (METMA), which uses the combined techniques of electrical or electro-thermal driving of a mi- crobridge in the presence of a magnetic field. The electrically controllable magnetic field actuates and controls the electrically or electrothermally driven microstruc- tures. METMA provides control with two electrical inputs, the currents driving the microbridge and the current driving the external magnetic field. This method enables a more controllable actuating system. Different designs of microactuators have been implemented by using MEMS Pro as the design software and MUMPs as the standard MEMS fabrication technology. In these designs, a variety of out-of- plane buckling or displacement of fixed-fixed microbeams have been developed and employed as the actuating elements. This paper also introduces a novel actuating technique for larger displacements that uses a two-layer buckling microbridge actu- ated by METMA. Heat transfer principles are applied to investigate temperature distribution in a microbeam, electrothermal heating, and the resulting thermoelas- tic effects. Furthermore, a method for driving microactuators by applying powerful electrical pulses is proposed. The integrated electromagnetic and electrothermal microactuation technique is also studied. A clamped-clamped microbeam carry- ing electrical current has been modeled and simulated in ANSYS. The simulations include electrothermal, thermoelastic, electromagnetic, and electrothermomagnetic effects. The contributions are highlighted, the results are discussed, the research and design limitations are reported, and future works are proposed.

This research focuses on developing new techniques and designs for highly con- trollable microactuating systems with large force-stroke outputs. A fixed-fixed mi- crobeam is the actuating element in the introduced techniques. Either buckling of a microbridge by thermal stress, lateral deflection of a microbridge by electro- magnetic force, or combined effects of both can be employed for microactuation. The proposed method here is MicroElectroThermoMagnetic Actuation (METMA), which uses the combined techniques of electrical or electro-thermal driving of a mi- crobridge in the presence of a magnetic field. The electrically controllable magnetic field actuates and controls the electrically or electrothermally driven microstruc- tures. METMA provides control with two electrical inputs, the currents driving the microbridge and the current driving the external magnetic field. This method enables a more controllable actuating system. Different designs of microactuators have been implemented by using MEMS Pro as the design software and MUMPs as the standard MEMS fabrication technology. In these designs, a variety of out-of- plane buckling or displacement of fixed-fixed microbeams have been developed and employed as the actuating elements. This paper also introduces a novel actuating technique for larger displacements that uses a two-layer buckling microbridge actu- ated by METMA. Heat transfer principles are applied to investigate temperature distribution in a microbeam, electrothermal heating, and the resulting thermoelas- tic effects. Furthermore, a method for driving microactuators by applying powerful electrical pulses is proposed. The integrated electromagnetic and electrothermal microactuation technique is also studied. A clamped-clamped microbeam carry- ing electrical current has been modeled and simulated in ANSYS. The simulations include electrothermal, thermoelastic, electromagnetic, and electrothermomagnetic effects. The contributions are highlighted, the results are discussed, the research and design limitations are reported, and future works are proposed.

Le développement des nanotechnologies a permis à la médecine de progresser là où lesméthodes traditionnelles de diagnostic et de thérapie connaissaient certaines limites. La manipulationet le contrôle de l’infiniment petit permet aujourd’hui de créer des systèmes adaptés à l’environnementcellulaire.Dans ce travail, nous nous sommes intéressés au potentiel des nanoparticules magnétiques d’oxydede fer en nanomédecine et notamment à l’utilisation de leurs propriétés magnétiques particulièrespour la mise au point de nouveaux matériaux pour la délivrance de principe actif par activationthermomagnétique. Notre système est constitué d’un hydrogel physique biocompatible, denanoparticules magnétiques et d’émulsions de mésophases lipidiques (Isasomes). Les Isasomes sontdes dispersions de systèmes auto assemblés qui selon la température peuvent changer de structure(phases hexagonales, cubiques,…). L’ajout d’un principe actif aux Isasomes peut aussi modifier leurstructure interne ; des mesures de SAXS ont permis de confirmer cet effet. Ces émulsionsnanostructurées ont servi de réservoir aux molécules modèles de principe actif (le radical TEMPO).Après activation magnétique, la diffusion contrôlée du principe actif hors de l’hydrogel a été suivie parRPE. Enfin, les nanoparticules ont été fonctionnalisées de façon à concevoir un hydrogel réticulé parles nanoparticules magnétiques. Les diverses étapes de la fonctionnalisation ont été validées pardifférentes techniques expérimentales (Diffraction de rayons X, MET, Raman, IRTF, Zétamétrie, ATG,XPS). / The development of nanotechnology led to significant progress in medicine especially wheretraditional methods of diagnosis and therapy showed limits. The manipulation and control of thephysics at the nanoscale offered new opportunities for creating systems tailored to the cellularenvironment. In this work, we were interested in the high potential of magnetic nanoparticles of ironoxide in medicine. In particular, we would like to use their peculiar magnetic properties for developingnew materials for the delivery of active compounds through thermomagnetic activation. Our systemconsists of a biocompatible hydrogel with confined magnetic nanoparticles and lipid-based emulsions,called Isasomes. Those are dispersions of lipid mesophases (hexagonal, cubic,…) that can be tunedby temperature or composition. The incorporation of an active compound into the Isasomes canequally modify their internal structure as confirmed by SAXS measurements. These nanostructuredemulsions are used here as reservoirs for model molecules (radical TEMPO), which are trapped intothe hydrogel. After magnetic activation, the controlled release of TEMPO outside the hydrogel hasbeen followed by Electron Paramagnetic Resonance (EPR). Finally, magnetic nanoparticles havebeen functionalized and connected to hyaluronic acid in order to design a crosslinked hydrogel. Thevarious steps of functionalization have been checked by various experimental techniques (Xrays,Raman spectroscopy, TEM, FTIR, zetametry, TGA, XPS).

Weyl semimetals feature linear band crossings with non-trivial topological properties. These (Weyl) points can have a strong impact on the transport response, when they are located close to the Fermi level. The Weyl points can lead to, among other things, a large anomalous Hall effect. Co3Sn2S2 is a prototypical candidate of this material class and displays intriguing physics, originating from its topological as well as magnetic properties. Many interesting transport responses, including the anomalous Hall effect and the anomalous Nernst effect have been found to be realised in this compound. Previous experiments have been carried out exclusively in (bulk) single crystals of Co3Sn2S2. It is demonstrated in this thesis, that microsturctures with well defined contacts can fabricated from bulk single crystals using focused ion beam cutting. Laser lithography is employed to define the electrical contacts to the micro ribbons. The microstructured devices are used to study the evolution of the magnetoresistance and magnetothermopower with magnetic field and temperature. The magnetoelectric transport response in Co3Sn2S2 is addressed in the first part of this thesis. Co3Sn2S2 exhibits a particularly large anomalous Hall effect in the transverse magnetoresistance. The anomalous Hall conductivity is found to be independent of the longitudinal conductivity, suggesting an intrinsic origin of the anomalous Hall effect. This notion is further corroborated by comparing the experimental results with band structure calculations, using the Berry curvature. Furthermore, the agreement between measurements on the microstructures with measurements on single crystals demonstrates the high quality of the microstructured devices. Additionally to the transverse magnetoresistance also the longitudinal magnetoresistance of Co3Sn2S2 is studied. The magnetoresistance decreases with increasing magnetic field for temperatures above 100 K. Futhermore, the magnetoresistance is found to be highly anisotropic with respect to the direction of the magnetic field. The negative magnetoresistance can be consistently explained invoking the magnon magnetoresistance. This effect is present in the ferromagnetic as well as the paramagnetic phase and is the dominant magnetoresistance effect above 100 K. The remarkably strong magneto-crystalline anisotropy of Co3Sn2S2 along its crystalline c-axis leads to the interesting observation, that the magnon magnetoresistance appears to depend on the projection of the external magnetic field on the magnetization. At low temperatures, magnon modes are frozen out and the magnon magnetoresistance vanishes. However, the anisotropy of the carrier mobilities gives rise to an anisotropic orbital magnetoresistance. Interestingly, no clear signs of crystalline/non-crystalline anisotropic magnetoresistance, depending on the magnetization direction, can be observed. The salient features in the magnetoelectric response can be understood through the comprehensive investigations reported in this thesis. In the second part, the thermal counterparts of the magnetoelectric transport responses are investigated. Again, a surprisingly large transverse (Nernst) effect is found. The Nernst conductivity reaches up to 4 A/(m K) in Co3Sn2S2. Unlike their magnetoelectric counterparts, the Nernst coefficient and the Nernst conductivity exhibit a sign change as a function of temperature. These experimental results strengthen the theoretical framework, that the anomalous Hall and Nernst effect probe different states. Furthermore, an additional contribution at the magnetic phase transition can be identified using the Mott relation. This contribution indicates that the anomalous Nernst conductivity can be enhanced by magnetic fluctuations near the magnetic ordering temperature. It appears to be a generic contribution in ferromagnetic materials and it is so far not accounted for by this particular formulation of the Mott relation. This subtle feature is uncovered when comparing the large magnetoelectric and magnetothermal transport response in the magnetic Weyl semimetal Co3Sn2S2.