Intergranular insulating reduced iron powder-carbonyl iron powder/SiO₂-Al₂O₃ soft magnetic composites with high saturation magnetic flux density and low core loss

The soft magnetic composites (SMCs) with high saturation magnetic flux density (B_s) and low core loss (P_s) have great potential in power electronics. In this work, the insulating SiO₂-Al₂O₃ layer was prepared by a hydro-thermal method to coat on the surface of the carbonyl iron powders (CIPs). Then, the coated CIPs were heterogeneously distributed in the reduced iron powders (RIPs) to fabricate the intergranular insulated SMCs. The scanning electron microscope (SEM) images and elements analysis confirmed that the surface of the CIPs was covered by a thin insulating layer forming a core-shell structure, and the Si-O-Al bond was detected by FTIR spectra. The resulting samples were annealed at 450 °C in N₂ atmosphere to eliminate the residual stress and improve magnetic properties. The annealed. SMCs containing 25 wt% CIPs@SiO₂-Al₂O₃ have optimum magnetic properties with a low P_s of 106.9 W/kg and a highly approximate B_s of 1.28 T (measured at 500 Hz). Compared with the conventional SMCs, the addition of the coated CIPs can suppress the adverse effect of the nonmagnetic coating layer on magnetic properties; meanwhile, the coated CIPs distributed in the RIPs can reduce the P_s. This work provides a promising approach to the development of SMCs with low hysteresis loss and optimum magnetic properties at low frequency.