Abstract Dynamic electrical resistance during resistance spot welding has been quantitatively modeled and analyzed in this work. A determination of dynamic resistance is necessary for predicting the transport processes and monitoring the weld quality during resistance spot welding. In this study, dynamic resistance is obtained by taking the sum of temperature dependent bulk resistance of the workpieces and contact resistances at the faying surface and electrode-workpiece interface within an effective area corresponding to the electrode tip where welding current primarily flows. A contact resistance is composed of constriction and film resistances, which are functions of hardness, temperature, electrode force, electrical resistivity and surface condition. Unsteady, axisymmetric transport of mass, momentum, energy, species, and magnetic field intensity with a mushy-zone phase change in workpieces and temperature, and magnetic fields in electrodes during resistance spot welding, are systematically investigated. Electromagnetic force, joule heat, heat generation at the electrode-workpiece interface and faying surface between workpieces, different properties between phase, and geometries of electrodes are taken into account. The predicted nugget thickness and dynamic resistance versus time show quite good agreement with available experimental data. Excluding expulsion, the dynamic resistance curve can be divided into four stages. A rapid decrease of dynamic resistance in stage 1 is attributed to decreases in film resistances at the faying surface and electrode-workpiece interface. In stage 2, the increase in dynamic resistance results from the primary increase of bulk resistance in the workpieces and an increase of the sum of contact resistances at the faying surface and electrode-workpiece interface. Dynamic resistance in stage 3 decreases, because increasing rate of bulk resistance in the workpieces and contact resistances decrease. In stage 4 decrease of dynamic resistance is mainly due to the formation of the molten nugget at the faying surface. The molten nugget is found to occur in stage 4 rather than stage 2 or 3 as qualitatively proposed in the literature. The effects of different parameters on the dynamic resistence curve are also presented. Besides, electromagnetic force effect on velocity field of molten nugget was proven to be crucial. Higher current, smaller magnetic diffusivity and decreasing the radius of electrode tip will lead to high current density around the corner between electrode and workpiece. Sometimes the corner of electrode and surface of workpieces will be melted due to local high current density.

The use of electrostatic transduction has enabled high-Q miniaturized mechanical resonators made of non-piezoelectric material that vibrate at high and ultra high frequencies. However, this transduction technique suffers from large values of motional resistance associated with the technique, limiting its use for interfacing to standard 50 RF circuits. Piezoelectric transduction has advantages over the electrostatic method because of its comparable to 50 motional resistance. However, the technique requires use of thin film piezoelectric materials with the demonstrated Qs that are much lower than their corresponding non-piezoelectric resonators. This research proposes use of electrodynamic transduction, reports analytic and experimental studies on electrodynamic transduction for RF application, highlights the method’s advantages, and lists the contributions. The use of Lorentz-force transduction for RF micromechanical filters proposed in this work is pursued by experimentally evaluating the transduction technique implemented for microfabricated designs. By fabricating single and coupled microresonators in a few different fabrication technologies, including CMOS35, the performance of the Lorentz-force driven microresonators is studied. Using a laser vibrometer, the actual performance, including the displacement and velocity of the moving points of the microstructures’ surfaces, are measured. The mode shapes and resonance specifications of the microstructures in air and vacuum derived by laser vibrometer provide data for characterizing the employed Lorentz-force transduction technique. Furthermore, the results from the electrical measurements are compared to the micromechanical resonators’ frequency response obtained from the mechanical measurements by laser vibrometer. The significantly low values of motional resistance computed for the differently fabricated designs demonstrate the advantage of Lorentz-force transduction for RF filter applications. Should a device similar in size be driven electrostatically, the motional resistance would be multiple orders of magnitude higher. This research reports the experimental results obtained by examining a Lorentz- force transduction application for developing RF micromechanical filters. The results demonstrate the Lorentz-force transduction’s advantages over other transduction methods used for RF μ-mechanical filters. Compared to electrostatic transduction, the Lorentz-force method provides greater electromechanical coupling, multiple orders of magnitude lower motional resistance, the independence of the filter center frequency from the bias voltage, higher power handling, and no requirement for bias lines, which decreases the work in microfabrication. Unlike piezoelectric transduction, the electrodynamic technique requires no piezoelectric material. Use of non-piezoelectric materials provides more flexibility for resonator material in the IC-compatible fabrications. Power handling in electrodynamic transduction has fewer limitations than other transduction techniques because the higher power needed in electrostatic or piezoelectric methods requires a higher voltage, which is limited by the breakdown voltage. The higher power in Lorentz-force-based transduction demands a larger current. The larger current produces heat that is removable by applying an appropriate cooling technique.

Bien que les propriétés de l'antimatière soit largement testées dans le secteur faible, fort et électromagnétique, la question de la gravitation pour ces objets reste ouverte. En observant la chute libre d'atomes d'anti-hydrogène, l'expérience AEGIS prévoit de mesurer pour lapremière fois l'accéleration gravitationelle pour l'antimatière. Le dispositif prévu pour cette mesure se compose de trois réseaux en transmission et d'un détecteur possèdant une haute résolution spatiale. Le travail de cette thèse se concentre sur la caractérisation d'un tel dispositif avec une source d'ions de basse énergie, permettant de mesurer simultanément les champs électriques et magnétiques environnant. La sensibilité atteinte en font un dispositif compétitif avec les meilleurs appareils disponibles. Une attention particulière est également portée sur l'implémentation de l'expérience avec des réseaux de pas plus fin, afin de mettre en évidence l'interférence quantique des ions. Les effets qui pourraient réduire le contraste des franges d'interférences sont passés en revue et discutés. Enfin, la réalisation d'une ligne de faisceau capable de sélectionner des particules de basse énergie et l'intégration d'un détecteur silicium dédié à la détection d'antiprotons est détaillée dans le but de réaliser pour la première fois l'interférence de particules d'antimatière / Although numerous experiments investigate the properties of antimatter in the weak, strong and electromagnetic sector, the gravitational interaction for these objects remains an open question. By observing the free-fall of antihydrogen atoms, the AEGIS experiment plans toperform the first measurement of the gravitational acceleration on antimatter. The device envisioned for such an experiment consists in a set of three transmission gratings with a pitch of few micrometers, associated with a high resolution imaging detector. This thesis focuses on testing such a device with a low-energy ion beam, enabling one to measure simultaneously the magnitude of surrounding electric and magnetic fields. The sensitivity achieved makes such an apparatus competitive with state-of-the-art fieldmeters. An effort is moreover initiated to perform the same experiment with smaller grating periodicities since it could reveal the quantum interference of the ions. In order to probe the quantum behavior of protons, the effects which might destroy the interference pattern are reviewed and discussed. We finally detail the implementation of a velocity selector and a silicon detector dedicated to low energy antiprotons. These two components are indeed a prerequisite to perform for the first time theinterference of antimatter particles

A statistical investigation of the relationship between Lorentz force and Bursty Bulk Flow event (BBF) spatial location in the magnetotail is undertaken. Data is obtained in situ by the ESA Cluster II mission during the period July to October 2004. Firstly, a short introduction to BBFs and the Cluster mission is presented. Secondly, the curlometer method for determining Current densities in the Inner Central Plasma Sheet and its approximations are discussed. The curlometer method uses magnetic field density data from the Fluxgate Magnetometer (FGM) instrument and plasma velocities are obtained by the Hot Ion Analyzer (HIA) instrument. The satellite separation at the time of the measurement in the year 2004 was on the order of 1000 km. Results of the investigation are inconclusive. A few possible sources of error and reference material are mentioned.

Micromirror scanners are the most significant of the micro-optical actuator elements with applications in portable digital displays, automotive head-up displays, barcode scanners, optical switches and scanning optical devices in the health care arena for external scanning diagnostics and in vivo scanning diagnostics. Recent development in microscanning technology has seen a shift from conventional electrostatic actuation to electromagnetic actuation mechanisms with major advantages in the ability to produce large scan angles with low voltages, remote actuation, the absence of the pull-in failure mode and the acceptable electrical safety compared to their electrostatic counterparts. Although attempts have been made to employ silicon substrate based MEMS deposition techniques for magnetic materials, the quality and performance of the magnets are poor compared to commercial magnets. In this project, we have developed novel low-cost single and dual-axis polymer hard magnetic micromirror scanners with large scan angles and low power consumption by employing the hybrid fabrication technique of squeegee coating to combine the flexibility of polydimethylsiloxane (PDMS) and the superior magnetic performance of fine particle isotropic NdFeB micropowders. PCB coils produce the Lorentz force required to actuate the mirror for scanning applications. The problem of high surface roughness, low radius of curvature and the magnetic field interaction between the gimbal frame and the mirror have been solved by a part PDMS-part composite fabrication process. Optimum magnetic, electrical and time dependent parameters have been characterized for the high performance operating conditions of the micromirror scanner. The experimental results have been demonstrated to verify the large scan angle actuation of the micromirror scanners at low power consumption.

This thesis presents a novel study on Second Generation High Temperature Superconductors, which covers their electromagnetic characteristics and AC loss analysis. Lorentz Force Electrical Impedance Tomography (LFEIT) is one of the most promising hybrid diagnostic scanners with burgeoning potential for biological imaging, particularly in the detection of cancer and internal haemorrhages. The author tried a novel combination of superconducting magnets together with the LFEIT system. The reason is that superconducting magnets can generate a magnetic field with high intensity and homogeneity, which could significantly enhance the electrical signal induced from a sample, thus improving the Signal-to-Noise Ratio (SNR). The author developed four magnet designs for the LEFIT system using the Finite Element Method (FEM) package, COMSOL Multiphysics, and found that a Superconducting Halbach Array magnet can achieve all the requirements (magnetic field properties, geometry, portability, etc.) for the LFEIT system. The optimization study of the superconducting Halbach Array magnet has been carried out on the FEM platform of COMSOL Multiphysics, with 2D models using H-formulation based on B-dependent critical current density and bulk approximation. Optimization focused on the location of the coils, as well as the geometry and number of coils on the premise of maintaining the total amount of superconducting material used in the design. The optimization results showed that the Halbach Array configuration based superconducting magnet is able to generate a magnetic field with an intensity of over 1 Tesla and improved homogeneity. In order to efficiently predict the optimization performance, mathematical formulas were developed for these optimization parameters to determine the intensity and homogeneity of the magnetic field. The mathematical model for the LFEIT system was built based on the theory of the magneto-acousto-electric effect. Then the basic imaging of the electrical signal was developed using Matlab. The magnetic field properties of the magnet design were imported into the LFEIT model. The LFEIT model simulated two samples located in three different magnetic fields with varying magnetic strength and homogeneity. Even if there are no actual alternating currents involved in the DC superconducting magnets mentioned above, they have power dissipation during normal operation (e.g. magnet ramping) and under different background fields. This problem generally goes under the category of “AC loss”. Therefore, the AC loss characteristics of HTS tapes and coils are still fundamentally important for HTS magnet designs, even if they are normally operating in DC conditions. This thesis starts with the AC loss study of HTS tapes. The investigation and comparison of AC losses on Surround Copper Stabilizer (SCS) Tape and Stabilizer-free (SF) Tape have been carried out, which includes AC loss measurement using the electrical method, as well as the real geometry and multi-layer HTS tape simulation using the 2D H formulation by COMSOL Multiphysics. Hysteresis AC losses in the superconducting layer, and eddy current AC losses in the copper stabilizer, silver overlayer and substrate were concerned in this investigation. The measured AC losses were compared to the AC losses from the simulation, using 3 cases of different AC frequency: 10 Hz, 100 Hz, and 1000 Hz. The frequency dependence of AC losses from Stabilizer free Tape and Copper Stabilizer Tape were compared and analysed. A comprehensive AC loss study of a circular HTS coil has been fulfilled. The AC losses from a circular double pancake coil were measured using the electrical method. A 2D axisymmetric H-formulation model using FEM package COMSOL has been established, which was able to make consistency with the real circular coil used in the experiment. To model a circular HTS coil, a 2D axisymmetric model provided better accuracy than a general 2D model, and was also more efficient than a 3D model. Three scenarios were analysed: (1) AC transport current and DC magnetic field, (2) DC transport current and AC magnetic field, (3) AC transport current and AC magnetic field. The angular dependence analysis on the coil under the magnetic field with the different orientation angle  was carried out for all three scenarios. For scenario (3), the effect of the relative phase difference ∆ between the AC current and the AC field on the total AC loss of the coil was investigated. To summarise, a current/field/angle/phase dependent AC loss (I, B, , ∆) study of circular HTS coil has been carried out, which could potentially benefit the future design and research of HTS AC systems. The AC losses of horizontally parallel HTS tapes have been investigated. The AC losses of the middle and end tape of three parallel tapes have been measured using the electrical method, and compared to those of an individual isolated tape. The effect of the interaction between tapes on AC losses has been analysed, and compared with finite element method (FEM) simulations using the 2D H formulation implemented in COMSOL Multiphysics. The electromagnetic induction around the three parallel tapes was monitored using COMSOL simulation. The electromagnetic induction and AC losses generated by a conventional three turn coil were simulated as well, and then compared to the case of three parallel tapes with the same AC transport current. The analysis demonstrated that HTS parallel tapes could be potentially used in wireless power transfer systems, which could have lower total AC losses than conventional HTS coils. By using FEM simulations, cases of increasing number of parallel tapes was considered, and the normalised ratio between the total average AC losses per tape and the AC losses of an individual single tape have been calculated for different gap distances. A new parameter is proposed, Ns, a turning point the for number of tapes, to divide Stage 1 and Stage 2 for the AC loss study of horizontally parallel tapes. For Stage 1, N < Ns, the total average losses per tape increased with the increasing number of tapes. For Stage 2, N > Ns, the total average losses per tape started to decrease with the increasing number of tapes. The analysis demonstrates that horizontally parallel HTS tapes could be potentially used in superconducting devices like HTS transformers, which could retain or even reduce the total average AC losses per tape with large numbers of parallel tapes.

<p> Continuous Casting (CC) is an essential process in the steel industry to transform molten steel into solid product. This process begins with primary cooling (PC) where the molten steel is cooled, and the initial solidification begins. It is important to monitor the process of PC as defects such as thinning of the shell in the mold can lead to breakouts. Key parameters in PC are the mold design, casting condition, and steel composition. In the research conducted, key parameters for PC are investigated to analyze the impact on flow formation and solidification. To optimize mold design, angular taper to the narrow face can be employed to accommodate for any shell shrinkage. Utilizing computational fluid dynamics, a range of mold taper is simulated per the developed solidification model with defined temperature-dependent material properties. When simulated without a taper, significant air gap formation in the corners of the mold is visible due to thermal shrinkage of the shell. This air gap decreases the cooling rate due to the shell’s lack of contact with the cooling mold wall. A parametric study of mold taper ranging from no taper to 3° as well as change in casting conditions, superheat and casting speed, are conducted to analyze the impact of taper with respect to the casting conditions. Per the conditions applied, angular taper between 1° and 2° resulted into reduction of undercooling and overcooling in the corner of the mold which is subjected to cooling from the broad face and narrow face of the cool mold wall. The turbulent flow in the mold region was found to drastically influences the quality of steel produced during continuous casting. The flow itself can lead to surface defects or slag entrainment based on the formation. A high surface wave due to turbulence of the injected melt lead to fluctuations and the instability compromised the quality of the steel produced as well as entrained the slag. To regulate the flow, electromagnetic forces can be applied in the mold, dampening the local turbulent flow. As the electrically conductive molten steel interacts with the induced magnetic field, it reduced the velocity of the steel jet released from the ports of the submerged entry nozzle. Per the simulation-based study conducted increasing the EMBr strength from 2975G to 4350G reduced the peak surface wave height by 59.47% and volume of flux rate of decrease by 4.25%. Additionally, increasing the SEN depth from 110 mm to 350 mm increased the average wave height by 19% and volume of flux rate of decrease by 2.6%. Lastly, increasing the mold width from 1.067 m to 1.50m increased average wave height by 8.71% and volume of flux rate of decrease by 0.9%. </p>

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Im Rahmen der vorliegenden Arbeit konnten Einflüsse magnetischer Felder, insbesondere mit hohen Gradienten der magnetischen Flussdichte, auf Korrosionsprozesse am Beispiel Eisen quantifiziert und deren Wirkungsmechanismus erklärt werden. Als ein besonders in technisch relevanten gering konzentrierten sauren wässrigen Lösungen bedeutsamer Effekt wurde eine sekundäre Wirkung der Feldgradientenkraft über den Mechanismus der Wahrung der Ladungsneutralität auf den pH-Wert an der Elektrodenoberfläche identifiziert. Somit konnte ein signifikanter Magnetfeldeinfluss auf die formal ladungstransferkontrollierte Korrosionsreaktion nachgewiesen werden. Um die komplexen Korrosionsvorgänge an mehrphasigen NdFeB-Magneten mit paramagnetischer intergranularer Nd-reicher Phase aufklären zu können, war zunächst eine vertiefte Analyse der freien und anodischen Korrosionsreaktionen des Neodyms notwendig. Die dabei gewonnenen Erkenntnisse erlauben den Magnetfeldeinfluss bei der Korrosion aufmagnetisierter NdFeB-Magnete zu verstehen.