Journal Article

Microstructure (EBSD-KAM)-Informed Selection of Single-Powder Soft Magnetics for Molded Inductors

Wu, Kun-Yang, Huang, Hung-Shang, Tien, Chun-Wei, Hsiang, Hsing-I, Yang, Chang-Ting, Huang, Yu-Fang

2025

Overview

This study systematically benchmarks the performance of four single soft magnetic powders—water-atomized Fe–Si–Cr (FeSiCr), silica-coated reduced iron powder (RIP), silica-coated carbonyl iron powder (CIP), and phosphate-coated CIP (CIP-P)—to establish quantitative relationships between powder attributes, deformation substructure, and high-frequency loss for molded power inductors (100 kHz–1 MHz). We prepared toroidal compacts at 200 MPa and characterized them by initial permeability (μi), core-loss (Pcv(f)), partitioning (Pcv(f) = Khf + Kef2, Kh, Ke: hysteresis and eddy-current loss coefficients), and EBSD (electron backscatter diffraction)-derived microstrain metrics (Kernel Average Misorientation, KAM; low-/high-angle grain-boundary fractions). Corrosion robustness was assessed using a 5 wt% NaCl, 35 °C, 24 h salt-spray protocol. Our findings reveal that FeSiCr achieves the highest μi across the frequency band, despite its lowest compaction density. This is attributed to its coarse particle size (D50 ≈ 18 µm) and the resulting lower intragranular pinning. The loss spectra are dominated by hysteresis over this frequency range, with FeSiCr exhibiting the largest Kh, while the fine, silica-insulated Fe powders (RIP/CIP) most effectively suppress Ke. EBSD analysis shows that the high coercivity and hysteresis loss in CIP (and, to a lesser extent, RIP) are correlated with dense, deformation-induced subgrain networks, as evidenced by higher mean KAM and a lower low-angle grain boundary fraction. In contrast, FeSiCr exhibits the lowest KAM, with strain confined primarily to particle contact regions. Corrosion testing ranked durability as FeSiCr ≳ CIP ≈ RIP ≫ CIP-P, which is consistent with the Cr-rich passivation of FeSiCr and the superior barrier properties of the SiO2 shells compared to low-dose phosphate. At 15 A, inductance retention ranks CIP (67.9%) > RIP (55.7%) > CIP-P (48.8%) > FeSiCr (33.2%), tracking a rise in effective anisotropy and—for FeSiCr—lower Ms that precipitate earlier roll-off. Collectively, these results provide a microstructure-informed selection map for single-powder formulations. We demonstrate that particle size and shell chemistry are the primary factors governing eddy currents (Ke), while the KAM-indexed substructure dictates hysteresis loss (Kh) and DC-bias superposition characteristics. This framework enables rational trade-offs between magnetic permeability, core loss, and environmental durability.

• Publisher: MDPI AG,Multidisciplinary Digital Publishing Institute (MDPI)
• Subject: Analysis / Anisotropy / Atomizing / Bias / Carbonyl powders / Compacts / Core loss / Corrosion / Corrosion and anti-corrosives / Current loss / Deformation / Density / Durability / Eddy current testing / Electron backscatter diffraction / Frequencies / Frequency ranges / Grain boundaries / Inductance / Inductors / Injection molding / Iron / Magnetic permeability / Magnetic properties / Metal powders / Microstructure / Particle size / Permeability / Phosphates / Pinning / Powders / Raw materials / Scanning electron microscopy / Silica / Silicon dioxide