<p dir="ltr">The most predominantly used rotating electric machines today are the radial and axial flux varieties (denoted RFM and AFM, respectively). There is another category of machines called the transverse flux machines (denoted TFMs) which are best suitable for high torque low speed applications such as in wind energy conversion systems, ship propulsion systems, and other direct drive applications. In this work, a design methodology based on a magnetic equivalent circuit (MEC) model for a three-phase stacked transverse flux machine is presented. Using this MEC model, an optimization-based design paradigm is created. Finite element analysis is used to validate a design obtained from the proposed algorithm. </p>

<p>In recent years, switch-mode power electronic converters have gained considerable popularity</p> <p>because of their compact size and high switching frequencies. This makes them</p> <p>suitable for power processing in various applications, including photovoltaic systems and</p> <p>electric vehicles. However, their high switching frequency capabilities have a drawback. A</p> <p>high-frequency common-mode voltage coupled with the switching of the power converters</p> <p>excites the parasitic capacitances of the system. It leads to the flow of common-mode current.</p> <p>Since the common-mode current flows through an unintended path, it can potentially</p> <p>interfere with the performance of system components. Passive filters can be used to mitigate</p> <p>common-mode currents. Using a common-mode inductor in conjunction with strategically</p> <p>placed capacitors makes it possible to limit the flow of common-mode current.</p> <p><br></p> <p>As part of this work, passive mitigation of common-mode current will be investigated in</p> <p>a variable frequency drive system. In this regard, the process of designing a three-phase ac</p> <p>common-mode inductor is explained. As a first step, a mitigation strategy is proposed and</p> <p>described. Next, the issue of self-capacitance of the inductor is discussed. Afterwards, the</p> <p>ac common-mode inductor is designed using a multi-objective optimization-based approach.</p> <p>Following this are the design results, concluding the dissertation.</p>

<p dir="ltr">A novel computational electromagnetic (CEM) solver using potential-based formulations and discrete exterior calculus (DEC) is proposed. The proposed solver consists of two parts: the DEC A-Phi solver and the DEC F-Psi solver. A and Phi are the magnetic vector potential and electric scalar potential of the electromagnetic (EM) field, respectively; F and Psi are the electric vector potential and magnetic scalar potential, respectively. The two solvers are dual to each other, and most research is carried out with respect to the DEC A-Phi solver.</p><p dir="ltr">Systematical approach for constructing the DEC A-Phi matrix equations is provided in this thesis, including the construction of incidence matrices, Hodge star operators and different boundary conditions. The DEC A-Phi solver is proved to be broadband stable from DC to optics, while classical CEM solvers suffer from stability issues at low frequencies (also known as the low-frequency breakdown). The proposed solver is ideal for broadband and multi-scale analysis, which is of great importance in modern industry.</p><p dir="ltr">To empower the proposed solver with the ability to solve industry problems with large number of unknowns, iterative solvers are preferred. The error-minimization mechanism buried in iterative solvers allows user to control the effect of numerical error accumulation to the solution vector. Proper preconditioners are almost always needed to accelerate the convergence of iterative solvers in large scale problems. In this thesis, preconditioning schemes for the proposed solver are studied.</p><p dir="ltr">In the DEC A-Phi solver, current sources can be applied easily, but it is difficult to implement voltage sources. To incorporate voltage sources in the potential-based solver, the DEC F-Psi solver is proposed. The DEC A-Phi and F-Psi solvers are dual formulations to each other, and the construction of the F-Psi solver can be generalized from the A-Phi solver straightforward.</p>

In several applications of casting, dendritic microstructure is not desirable as it results in poor mechanical properties. Enhancing fluid flow in the mushy zone by stirring is one of the means to suppress this dendritic growth. Strong fluid flow detaches the dendrites formed at the solid-liquid interface and carries them into the mould to form slurry. When this slurry solidifies, the microstructure is characterized by globular, non-dendritic primary phase particles, separated and enclosed by a near-eutectic lower-melting secondary phase. This property represents a great potential for further processing in semisolid forming (SSF) by various techniques such as pressure die casting and forging. Among all currently available methods, linear electromagnetic (EM) stirring is considered as one of the most suitable routes for large scale production of semisolid feed stock. One of the biggest advantages of EM stirring is that the stirring intensity and direction can be modulated externally and in a non-intrusive manner. With this viewpoint, the primary objective of the present research is to investigate rheocasting using linear electromagnetic stirring. A systematic development of a linear electromagnetic stirrer (LEMS) is the subject of the first part of the thesis. The LEMS consists of a set of six primary coils displaced in space. These coils are excited by a three-phase power supply to produce an axially travelling magnetic field. The metal to be stirred is placed in the annular space of the stirrer. The travelling field induces secondary current in the molten metal. The current and magnetic fields interact to generate a net mechanical force in the metal, commonly termed as the Lorentz force. The molten metal is stirred under the influence of this force. Two prototype stirrers, one for low melting alloys and the other for aluminium alloys are developed. The stirrers are characterized by measuring forces on low melting point alloy and on solid aluminum cylinders placed inside the annular space of the stirrer. As an outcome of these tests, a non-intrusive method of detecting stirring of liquid metal is developed. The development of a rheocasting mould for the LEMS forms the second part of the work presented in the thesis. The mould design and cooling arrangement are such that solidification in the mould is primarily unidirectional. Heat from the solidifying metal is extracted at the bottom of the mould, so that the axisymmetric EM stirring effectively shears the dendrites formed at the solid-liquid interface. The outer surface of the mould is cooled with water or air exiting from 64 jets, each of 4 mm diameter. Such an arrangement provides a high heat transfer coefficient and a wide range of cooling rate in the metal ranging from 0.01 to 10 K/s. Temperature is measured at various depths in the solidifying melt and at other key locations in the mould to assess the various heat transfer mechanisms. The results from the rheocasting experiments using the above mould and LEMS are presented in the third and final part of the thesis. Such studies are required for understanding the solidification process in presence of electromagnetic stirring and for highlighting the important issues connecting solidification, fluid flow, dendrite fragmentation and the resulting microstructure. A series of experiments are performed with A356 (Al-7Si-0.3Mg) alloy. Experiments are conducted with various combinations of operating parameters, and the resulting microstructures and cooling curves at various locations are examined. The key process parameters are stirring current, cooling rate, pouring temperature, and stirring current frequency. The parametric studies also include the case without EM stirring in which liquid aluminium is poured into the rheocast mould without powering the LEMS. It is found that stirring at high currents produces non-dendritic microstructures at all locations of the billet. For lower currents, however, dendritic microstructures are observed in regions outside the zone of active stirring. Stirring also enhances heat loss from the exposed top surface, leading to solid front advancement from the top as well. Without EM stirring, microstructures are found to be dendritic everywhere. The percentage of primary α-Al phase and its number density are found to increase with stirring intensity. With a decrease in cooling rate with air as the coolant, the average grain size of primary α-Al phase increases. Excitation frequency is found to be an important parameter, with lower frequencies generating a more uniform force field distribution, and higher frequencies enhancing induction heating. At higher frequencies, the effect of higher induction heating results in the formation of larger and coarser primary phase grains. This phenomenon has led to the development of a one-step process for rheocasting and heat treatment of billets.

<p>Passive chipless wireless sensors have gained great attention in Radio Frequency Identification (RFID) applications, inventory tracking, and structural health monitoring, as they offer a prospective low-cost, scalable alternative to the state-of-the-art active sensors. While the popularity and demand for chipless sensors are on the rise, their applications are limited to low-noise environments and their caliber as low-cost, scalable devices has not been explored to a successful degree in challenging domains, such as precision agriculture, healthcare, and food packaging. Size, cost of materials, and complexity of the manufacturing process are the main obstacles to progress in the large-scale production of chipless sensors for practical applications. </p> <p><br></p> <p>Conventional manufacturing processes, such as photolithography, are costly, cumbersome, and time intensive. While additive manufacturing techniques, such as printing technologies, have circumvented some of these challenges, printing techniques require costly inks and complex post-processing steps, such as drying and sintering, which limit their large-scale utilization. To overcome these challenges, this dissertation focuses on investigating the possibility of utilizing laser processing of conventional metalized films and polymers to develop cost-effective chipless sensors. This Scalable Laser Assisted Manufacturing (SLAM) process offers a platform for large-scale roll-to-roll production of high-resolution sensors for precision agriculture, healthcare, and food packaging applications. </p> <p><br></p> <p>In this pursuit, the first study explores combining the SLAM process with 3D printing to develop a miniaturized, biodegradable, chipless sensor for soil moisture monitoring. In the second study, the SLAM process is further explored in the development of metalized stickers for healthcare applications focusing on urine bag management and early risk detection of urinary tract infections. In the third study, the capability of the SLAM process to form moisture-sensitive metal nanoparticles as a co-product of metal patterning is harnessed to develop a chipless humidity sensor. The SLAM process is further expanded in the fourth study by functionalizing metalized films with stimuli-responsive polymers to achieve specificity in detecting unique biomarkers of food spoilage. The SLAM platform described in this work opens up new avenues toward processing metalized fabric for the future of wearable electronics and implementing multi-functional sensors for precision agriculture.</p> <p>  </p>

<p>Similar to thermodynamic phase transitions in matter, readily apparent changes in optical response arise in the transition from isotropic to anisotropic optical phases. Treating the anisotropy of the dielectric permittivity as a control parameter, which changes continuously from zero to a nonzero finite value at the transition, in this work we describe the resulting effect on light propagation. </p> <p><br></p> <p>We begin by investigating a simple case of the manifestations of such optical transition in lossy media. In the presence of loss, isotropic materials do not support Brewster phenomenon, however, if one changes the anisotropy continuously, the exact zero in the reflection at the Brewster incidence angle is recovered. Next, in the case of uniaxial anisotropy, we uncover dramatic changes in far-field thermal radiation induced by the transitions between metal, dielectric, and hyperbolic optical regimes that can be observed in the same material. We demonstrate that continuous evolution between different ''phases'' in the electromagnetic response imprints a characteristic signature in the far-field thermal emission. Finally, we show that the evolution of the optical anisotropy from uniaxial to biaxial symmetry brings qualitatively new optical modes which are different from the conventional propagating and evanescent fields. These emergent ''ghost'' waves offer a unique way to control mode interactions in optical systems. Our work uncovers the connection between the macroscopic properties of the optical materials and the transitions between different regimes of the electromagnetic response in these media. At last, we propose a range of potential applications of the resulting phenomena, from perfect absorption in lossy media to thermal radiation and optical sensing.</p>

<p>  </p> <p>  </p> <p>Concentrator photovoltaics have demonstrated greater solar energy production efficiency than previous solar electric technologies.  However, recent research reveals that heat management is a significant difficulty in CPV systems, and if left unaddressed, it can have a severe influence on system efficiency and lifetime. Traditional CPV cooling relies on active methods such as forced air convection, or liquid cooling, which might lead to an extremely large parasitic power use. In addition, the moving parts of a cooling system result in a shorter lifespan and higher maintenance expenses. </p> <p><br></p> <p>CPV systems can boost their efficiency and lifespan by adopting passive cooling solutions. This work employed radiative cooling and natural convection to construct an efficient and cost-effective cooling system. The excess heat of a solar cell can be dispersed into space via electromagnetic waves via radiative cooling. Due to the fact that the radiative cooling power is related to the difference between the fourth powers of the solar cell and the ambient temperature, much greater cooling powers can be obtained at higher temperatures. Heat pipes were installed to act as a heat pump by transferring excessive heat from solar cells within a system to the exterior, where it can be dissipated via natural air cooling and thermal radiation. Experiments conducted in this study demonstrate that a temperature reduction of 21 ◦C was accomplished through radiative cooling and natural convection, resulting in an increase of 64 mV, or 17% in the open-circuit voltage of a GaSb solar cell.</p>

<p>Plasmonics with the capability to harness electromagnetic waves at a nanoscale can be utilized for multitude of applications in ultra-compact miniature optical devices. Plasmonic metasurfaces which are artificially designed sub-wavelength structures have gained unprecedented interest in being able to engineer and effectively modulate the amplitude and phase of the incident wave. Introducing randomness to such plasmonic metasurfaces can also advance possibilities for extraordinary wave manipulation. Hence, by exploiting the plasmonic response of the ordered and disordered metasurfaces, we can design high performance devices for nanoscale optics.</p> <p>Aiming to provide a holistic solution to the current device limitations and bio-compatibility, my research focuses on non-toxic and environment-friendly coloration using plasmonic disordered metasurfaces. These structures generate a broad range of long-lasting colors in reflection that can be applied to real-life artistic or technological applications with a spatial resolution on the order of 0.3 mm or less. Moreover, my research also deals with the possibility of even concentrating energy in the smallest phase-space volume in optics in the form of coherent radiation through designing nanolasers. The study of carrier dynamics and photophysics of the gain media can be extremely beneficial towards the practicability of these lasers. This work elucidates the evolution of different competing mechanisms for coherent lasing. The dynamic study and experimental demonstration of these devices and respective materials can therefore provide a novel aspect to fundamental and applied research.</p>

<p dir="ltr">High-power microwave (HPM) devices are generally vacuum-based devices that transform electron beam energy into microwaves with peak powers above 100 MW from 1-300 GHz. Solid-state HPM devices provide more compactness and greater reliability while consuming less power. Nonlinear transmission lines (NLTLs) provide a solid-state alternative to HPM generation by sharpening the input pulses from a pulse forming network to create output oscillations.</p><p dir="ltr">The first section of this dissertation evaluates and explores the feasibility of using nonlinear composites containing ferroelectric (e.g., Ba_2/3Sr_1/3TiO₃, BST) and/or ferromagnetic (e.g., Ni_1/2Zn_1/2Fe₂O₄, NZF) inclusions in a linear polymer host (polydimethylsiloxane, PDMS) to tune NLTL properties for HPM applications. Appropriately modelling and designing NLTLs using nonlinear composites require accurately characterizing their linear and nonlinear electromagnetic properties. We first studied the electromagnetic properties of the composites using theoretical, numerical, and experimental approaches. Incorporating these composite models and characterizations into NLTL simulations will be discussed.</p><p dir="ltr">Vacuum-based HPM devices, such as magnetrons and crossed-field amplifiers, generally operate in the space-charge-limited region, which corresponds to the maximum current possible for insertion into the device. This motivated studying the space-charge-limited current and electron flow in a two-dimensional (2D) planar diode with various crossed-magnetic fields using particle-in-cell (PIC) simulations. For non-magnetically insulated diodes (electrons emitted from the cathode can reach the anode), analytical and/or semi-empirical solutions are derived for electrons with nonzero monoenergetic initial velocity that agree well with PIC simulations. For magnetically insulated conditions, we developed new metrics using simulations and analytic theories to assess electron cycloidal and Brillouin flow to understand the implications of increasing injection current for 2D diodes. These analyses provide details on the operation of these devices at high currents, particularly virtual cathode operation, that may elucidate behavior near their limits of operation.</p>

<p>This study, motivated by biomedical applications such as drug delivery and adsorption, is aimed at describing magneto- and dielectro-phoretic systems via dimensional analysis to quantitatively assess the relative contribution of hydrodynamics, electromagnetism, and particle dynamics. Magnetophoresis and dielectrophoresis, phenomena of magnetic and dielectric particle transports, respectively, have been used in various applications requiring selective collecting or separating magnetic particles, especially in microfluidic systems.</p> <p>A multiphysics computational model for a magnetophoretic system was developed to assess magnetophoretic characteristics. The magnetically induced mobility of the magnetic particles was simulated for a range of parameters relevant in biomedical applications, including the particle and fluid properties, fluid velocity, and geometries of the particle, flow channel, and magnet. With the help of dimensional analysis, dimensionless numbers were introduced to reduce the number of parameters characterizing the transport of the particles suspended in an electrically non-conducting fluid exposed to an external magnetic field. As a result, 14 relevant variables determining the particle capture were reduced to only 3 dimensionless numbers describing the magnetophoretic system. The results from multiphysics models supported this analysis, suggesting a scaling law. The functional relationship among the dimensionless numbers resulted in prediction curves to assess the particle capture. The performance of the magnetophoretic system predicted with the dimensional analysis was verified in comparison with the available experimental data. In addition, the dimensionless numbers introduced here were compared with established numbers in magnetohydrodynamics (MHD).</p> <p>These theoretical and parametrical analyses of the magnetophoretic system were applied to the novel magnetic filter proposed to capture the drug-loaded small magnetic particles (MPs) from the bloodstream during the Intra-Arterial Chemotherapy (IAC). The IAC is a preferred treatment for unresectable hepatocellular carcinoma (HCC), the primary liver cancer. In the IAC procedure, chemotherapeutic agents, e.g. doxorubicin (Dox), are administered via a catheter placed in an artery supplying the tumor. The effectiveness of the IAC, however, is limited due to the passage of excessive chemotherapy agents to the blood circulation after their effect on the tumor, causing systemic toxicity. To remove the excessive drugs, the endovascular filtration devices have been developed. The proposed magnetic filtration device could be deployed from a catheter placed in the hepatic vein or inferior vena cava (IVC) to remove the excessive Dox from the bloodstream. The Ferumoxytol approved by the FDA is one of the types of the ultrasmall superparamagnetic iron oxide (USPIO) particles. The excessive Dox-coated USPIO can be filtered by a magnetic catheter-based device generating an external magnetic field. The filter utilizing magnetic fields is a promising method for therapeutic applications since an influence of magnetic field reaches comparatively wide ranges and magnetic fields do not affect biological tissues. To optimize the design, efficacy, and performance of the proposed magnetic filtration device, numerical models were developed based on the proposed dimensionless numbers characterizing drug transport and binding. Drug adsorption can be optimized by modifying magnetic field distribution and device configuration. To enhance the filtering up to 70-80 % of the excessive drug, multi-stage filters were developed by optimizing magnet configuration and flow patterns. By decreasing the concentration of toxins in the cardiovascular system, the drug dosage can be increased while reducing side effects, thus improving the effectiveness of the IAC treatment.</p> <p>In addition, new dimensionless numbers for dielectrophoresis analogous to magnetophoresis were introduced for a range of applications. The proposed dimensionless numbers for dielectrophoresis were evaluated for several conditions and compared with the previously established numbers in electrohydrodynamics (EHD). </p> <p>This study provides a promising framework for analyzing and predicting performance of various magneto- and dielectro-phoretic systems for a range of applications, particularly in biomedicine such as –drug filtering, targeted drug delivery, or small particle separation–, thus providing a reliable methodology for predicting particle manipulation. </p>