Computer-Aided Formulation of Magnetic Pastes for Magnetic Components in Power Electronics

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Date
2021-05-25
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Publisher
Virginia Tech
Abstract

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.

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Keywords
magnetic paste, soft magnetic moldable composite (SM2C), finite element method, discrete element method, magnetic components, tailorable permeability
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