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

Area of Science:

• Materials Science
• Powder Metallurgy
• Electromagnetism

Background:

• Soft magnetic powders are crucial for power inductors operating at high frequencies (100 kHz-1 MHz).
• Understanding the relationship between powder microstructure and magnetic performance is key for optimizing inductor design.
• Existing materials face trade-offs between permeability, core loss, and environmental durability.

Purpose of the Study:

• To systematically benchmark four single soft magnetic powders: 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 magnetic loss.
• To provide a microstructure-informed selection map for single-powder formulations in molded power inductors.

Main Methods:

• Preparation of toroidal compacts at 200 MPa from four distinct soft magnetic powders.
• Characterization of magnetic properties: initial permeability (μi), core loss (Pcv(f)), and loss partitioning (hysteresis loss coefficient Kh, eddy-current loss coefficient Ke).
• Microstructural analysis using Electron Backscatter Diffraction (EBSD) for microstrain metrics (Kernel Average Misorientation - KAM, grain boundary fractions) and corrosion testing (5 wt% NaCl, 35 °C, 24 h).

Main Results:

• FeSiCr exhibited the highest initial permeability (μi) due to coarse particle size and lower intragranular pinning, despite lower compaction density.
• Hysteresis loss (Kh) dominated the loss spectra; fine, silica-insulated powders (RIP/CIP) were most effective at suppressing eddy-current loss (Ke).
• High coercivity and hysteresis loss in CIP/RIP correlated with dense, deformation-induced subgrain networks (higher KAM), while FeSiCr showed lower KAM.
• Corrosion resistance ranked FeSiCr ≳ CIP ≈ RIP ≫ CIP-P, linked to passivation and shell properties.
• Inductance retention at 15 A ranked CIP (67.9%) > RIP (55.7%) > CIP-P (48.8%) > FeSiCr (33.2%).

Conclusions:

• Particle size and shell chemistry are primary drivers for eddy-current loss (Ke).
• KAM-indexed substructure dictates hysteresis loss (Kh) and DC-bias performance.
• A framework is established for rational trade-offs between magnetic permeability, core loss, inductance retention, and environmental durability based on powder microstructure.