In-Situ Al2O3-Coated FeSiAl Powders for High-Frequency Soft Magnetic Composites with Enhanced Electrical and Thermal Performance

: FeSiAl soft magnetic composites (SMCs) suffer from a trade-off among permeability retention, eddy-current-loss suppression, and thermal accumulation under high-frequency operation. In this work, an in-situ α-Al 2 O 3 insulating layer was constructed on FeSiAl powder surfaces via an AlCl 3 –NH 3 ·H 2 O co-precipitation process followed by high-temperature annealing. The precipitation time was systematically adjusted to regulate the continuity and thickness of the insulating layer. The optimized sample prepared with a precipitation time of 35 min exhibited a balanced performance, with a saturation magnetization (Ms) of 154.7 emu g −1 , an effective permeability (μ) of 91.2, a peak quality factor (Q) of 83.6, and a core loss (P cv ) of 122.5 kW m −3 at 10 mT and 500 kHz. Meanwhile, the resistivity increased with increasing precipitation time, indicating enhanced inter-particle insulation. Loss separation analysis shows that the Al 2 O 3 layer effectively suppresses inter-particle eddy-current paths, while excessive coating thickness leads to magnetic dilution and reduced permeability. First-principles calculations reveal stable interfacial bonding and charge redistribution at the FeSiAl/α-Al 2 O 3 interface, supporting the proposed interfacial barrier mechanism for electron transport. Multiphysics simulations further suggest that the coated composite can exhibit a lower steady-state temperature than the uncoated sample under identical conditions. This work does not aim to propose a new coating system, but rather provides a precipitation-time-controlled interface engineering strategy for balancing electrical insulation, magnetic performance, and high-frequency loss in FeSiAl-based SMCs.