Effect of Al2(SO4)3·18H2O loading on high-frequency magnetic properties of FeSiCr@Al2O3 soft magnetic composites

Abstract

With the advancement of wide-bandgap semiconductors, power electronic systems are evolving toward higher frequencies, posing dual challenges of eddy current loss and magnetic dilution for FeSiCr soft magnetic composites (SMCs). Different from most reported Al2O3 coating routes relying on intrinsic Al inside FeSiAl matrix, this work constructs an Al2O3 insulating layer in‐situ on Al‐free FeSiCr powder surfaces via liquid‐phase coating and thermal decomposition using Al2(SO4)3·18H2O as the external Al precursor, yielding core‐shell structured FeSiCr@ Al2O3 SMCs. The precursor undergoes stepwise thermal decomposition to form amorphous/nanocrystalline Al2O3, accompanied by selective oxidation of the matrix surface, which builds a tightly bonded composite interface layer without altering the α‐Fe crystal structure of the magnetic cores. As the precursor loading increases, the non‐magnetic Al2O3 phase slightly reduces saturation magnetization while coercivity remains nearly unchanged, confirming that the coating reaction is confined to particle surfaces. Although effective permeability decreases due to magnetic dilution, excellent frequency stability is maintained over a broad range of 0–10 MHz, attributed to the enhanced resistivity provided by Al2O3 barriers and the shifted magnetic relaxation frequency toward higher frequencies. Both quality factor and DC bias performance improve with increasing coating content. Core loss exhibits a non‐monotonic trend, first decreasing and then increasing, with an optimal loading of 3 wt%, at which the best synergy among high‐frequency permeability, loss, and thermal stability is achieved. At the optimal precursor loading of 3.0 wt%, the composite magnetic core has a density of 6.08 g·cm−3, an insulation resistance of 2.08 GΩ at 50 V, a saturation magnetization of 181.16 emu·g−1, and a minimum core loss of 280 mW·cm−3 at 20 mT and 500 kHz, with good thermal stability from 25 °C–120 °C. This work offers a facile route to design insulating layers for high‐performance soft‐magnetic composites.