High-Performance FeSiAl Soft Magnetic Composites with Polyethyleneimine/Silicone Resin Double Organic Insulation Layers for 50 kHz–1 MHz Frequency Power Applications

Despite their advantages of light weight, complete coating and good formability, the poor high-temperature stability of organic materials limits their use as insulation coatings for soft magnetic powders. Therefore, a polyethyleneimine (PEI)/silicone resin (SR) double organic insulation layer was exploited to release internal stress and maintain insulation stability during the high-temperature annealing process, which achieved low core losses (P_cv) and high effective permeability (μ_e). Cross-sectional scanning transition images indicate that the PEI/SR layer undergoes the following evolution during annealing at 500–900°C: coating → densification → crack → rupture, implying thermal stability up to 700°C. Because of the integrity of double inorganic shells and the weak pin on magnetic domain movement, FeSiAl@PEI/SR cores annealed at 800°C deliver remarkable AC soft magnetic performance at 50 and 100 kHz, that is, low P_cv of 61.6 mW/cm³ (50 mT, 100 kHz), 121.2 mW/cm³ (100 mT, 50 kHz) and 307.3 mW/cm³ (100 mT, 100 kHz), high μ_e of 55.6 up to 1 MHz and 140°C, and DC bias of 51.5% at 100 Oe. Eddy current loss reaches the minimum value at 700°C and deteriorates at 800°C, and hysteresis loss continues to decrease with increasing annealing temperature, confirming the evolution of the insulation layer. Moreover, cores annealed at 700°C deliver μ_e of 57.6 at 1 MHz and P_cv of 278 mW/cm³ (20 mT, 1 MHz), exhibiting great potential for 1 MHz frequency power applications.