Magnetic components such as inductors and transformers are generally the largest circuit elements in switch-mode power systems for controlling and processing electrical energy. To meet the demands of higher conversion efficiency and power density, there is a growing need to simplify the process of fabricating magnetics for better integration with other power electronics components. The potential benefits of additive manufacturing (AM), or more commonly known as three-dimensional (3D) printing technologies, include shorter lead times, mass customization, reduced parts count, more complex shapes, less material waste, and lower life-cycle energy usage—all of which are needed for manufacturing power magnetics. In this work, an AM technology for fabricating and integrating magnetic components, including the design of manufacturing methodology and the development of the feedstock material, was investigated. A process flow chart of additive manufacturing functional multi-material parts was developed and applied for the fabrication of magnetic components. One of the barriers preventing the application of 3D-printing in power magnetics manufacturing is the lack of compatible and efficient magnetic materials for the printer's feedstock. In this work, several magnetic-filled-benzocyclobutene (BCB) pastes curable below 250 degree C were formulated for a commercial multi-material extrusion-based 3D-printer to form the core part. Two magnetic fillers were used: round-shaped particles of permalloy, and flake-shaped particles of Metglas 2750M. To guide the formulation, 3D finite-element models of the composite, consisting of periodic unit cells of magnetic particles and flakes in the polymer-matrix, was constructed. Ansoft Maxwell was used to simulate magnetic properties of the composite. Based on the simulation results, the pastes consisted of 10 wt% of BCB and 90 wt% of magnetic fillers—the latter containing varying amounts of Metglas from 0 to 12.5 wt%. All the pastes displayed shear thinning behavior and were shown to be compatible with the AM platform. However, the viscoelastic behavior of the pastes did not exhibit solid-like behavior, instead requiring layer-by-layer drying to form a thick structure during printing. The key properties of the cured magnetic pastes were characterized. For example, bulk DC electrical resistivity approached 107 Ω⋅cm, and the relative permeability increased with Metglas addition, reaching a value of 26 at 12.5 wt%. However, the core loss data at 1 MHz and 5 MHz showed that the addition of Metglas flakes also increased core loss density. To demonstrate the feasibility of fabricating magnetic components via 3D-printing, several inductors of differing structural complexities (planar, toroid, and constant-flux inductors) were designed. An AM process for fabricating magnetic components by using as-prepared magnetic paste and a commercial nanosilver paste was developed and optimized. The properties of as-fabricated magnetic components, including inductance and DC winding resistance, were characterized to prove the feasibility of fabricating magnetic components via 3D-printing. The microstructures of the 3D-printed magnetic components were characterized by Scanning-electron-microscope (SEM). Results indicate that both the winding and core magnetic properties could be improved by adjusting the formulation and flow characteristics of the feed paste, by fine-tuning printer parameters (e.g., motor speed, extrusion rate, and nozzle size), and by updating the curing profile in the post-process. The main contributions of this study are listed below: 1. Developed a process flow chart for additive manufacturing of functional multi-material components. This methodology can be used as a general reference in any other research area targeting the utilization of AM technology. 2. Designed, formulated and characterized low-temperature curable magnetic pastes. The pastes are physically compatible with the additive manufacturing platform and have applications in the area of power electronics integration. 3. Provided an enhanced understanding of the core-loss mechanisms of soft magnetic materials and soft magnetic composites at high frequency applications. / Ph. D.

Magnetic components are necessary for switch-mode power electronics converters, but they are often the bulkiest and heaviest in the system. Novel magnetic designs with intricate structures lead to the size reduction of power electronics converters but pose challenges to the fabrication process and material availability. Because of their low-temperature and pressure-less process-ability, magnetic pastes would be the material of choice to make magnetic cores with complex geometries. However, most magnetic pastes reported in the literature suffer from low relative permeability (µr < 26) due to the low magnetic fraction limited by viscosity. The conventional approach of developing magnetic pastes involves experimental iterations with trial-and-error efforts to determine the optimal compositions. To shorten the development cycle and take advantage of the computational power in the current age, this work focuses on exploring, validating, and demonstrating a computer-aided methodology to correlate material's processing, microstructure, and property to guide the development of magnetic pastes. The discrete element method (DEM) simulation was explored to create materials' microstructure and the finite element method (FEM) simulation was utilized to study the magnetic permeability based on the microstructure created by DEM or taken from an actual material sample. The combination of DEM and FEM provided the linkage among processing-microstructure-property relations. Then, the methodology was verified and demonstrated by improving a starting formulation. The formulation was modeled with DEM based on multiple variables, e.g., particle shape, size, size distribution, mixing ratio, gap, gap distribution, magnetic volume fraction, etc. The optimal mixing ratio of different powders to achieve the maximum magnetic fraction was determined by DEM. Experimental results confirmed the predicted optimal mixing ratio. To further take advantage of the computational tools, the magnetic permeability of the magnetic pastes was computed by FEM based on the DEM-generated microstructures. The effects of powder mixing ratio and magnetic volume fraction on the magnetic permeability were studied, respectively. Compared with the experimental values, the microstructure-based FEM simulations could predict the magnetic permeability of the formulations with varied powder mixing ratios or magnetic volume fractions with an average error of only 10 %. Another critical aspect of employing magnetic pastes for magnetic components in power electronics is capable of tailoring their magnetic permeability to meet different design needs. The methodology was further verified and demonstrated by guiding the selection of composition parameters for tailorable magnetic permeability of a starting formulation with flaky particles. An FEM model was constructed from a microstructural image and varied parameters were explored (particle permeability, matrix permeability, particle volume fraction, etc.) to tailor the magnetic permeability. To verify the simulated results, a set of magnetic pastes with various volume fractions of flakes was prepared experimentally and characterized for their permeability. Comparing the simulated and measured permeability, the error was found to be less than 10 %. Last, the guideline was demonstrated to predict a material composition to achieve a target relative permeability of 30. From the predicted composition, the magnetic paste was prepared and characterized. The error between experimental permeability and the target was only 5 %. With the guideline, one can formulate magnetic pastes with tailorable permeability with minimal experimental effort and select the composition parameters to achieve a target permeability. After developing a series of magnetic pastes with tailorable permeability and a maximum value of 35, the feasibility of making magnetic components with magnetic pastes was demonstrated. The commonly used magnetic cores – C-core, E-core, toroid core, bar core, and plate core were fabricated by a low-temperature (< 200 °C) and pressure-less molding process. Several innovative magnetic components with intricate core structures were also fabricated to demonstrate the shape-forming flexibility. The magnetic paste can also be used as the feedstock for paste-extrusion-based additive manufacturing, which further enhances the shape-forming capability. For demonstration, a multi-permeability core was fabricated by 3D printing the magnetic pastes with tailored permeability. The feasibility of making high-performance magnetic components by additive manufacturing or low-temperature pressure-less molding of magnetic pastes opens the door to power electronics researchers to explore more innovative magnetic designs to further improve the efficiency and power density of the power electronics converters. / Doctor of Philosophy / Magnetic components are necessary for switch-mode power electronics converters, but they are often the bulkiest and heaviest in the system. To reduce the size of the power converters, it is crucial to reduce the size of magnetic components by employing innovative magnetic designs. However, the complicated geometries of the novel magnetic designs pose challenges to the availability of material feedstock and the fabrication process. Magnetic pastes would be the material of choice to make magnetic components with intricate structures because of their flexibility in shape-forming with low-temperature and pressure-less processes. However, most magnetic pastes reported in the literature suffer from low magnetic permeability due to the low magnetic fraction limited by viscosity. The conventional approach of developing magnetic pastes involves experimental trial-and-error efforts to determine the optimal compositions. To shorten the development cycle and take advantage of computational power in the current age, this project focuses on exploring, validating, and demonstrating a computer-aided way to correlate material's processing, microstructure, and property relations to guide material development. The numerical simulations were explored to generate the microstructures and study the properties. With the guidance provided by computer simulations, a series of magnetic pastes with tailorable permeability was developed. Several novel magnetic components were fabricated with the as-developed magnetic pastes via molding or additive manufacturing to demonstrate the shape-forming flexibility.