Fabrication and growth mechanism of iron oxide insulation matrix for Fe soft magnetic composites with high permeability and low core loss

Soft magnetic composites (SMCs) containing Fe powders embedded in insulating matrix have been fabricated via alkaline bluing as a new method. Effects of reaction time on the magnetic properties of the SMCs have been investigated. Mechanism of the oxidation process has also been revealed, which involves initial oxidation of Fe into Fe2+ (Na2FeO2) and Fe3+ (Na2Fe2O4) by NaNO2 and NaNO3 followed by hydrolysis of Fe2+ and Fe3+ into Fe3O4. Hydrolysis of the excessive Fe3+ leads to the formation of Fe2O3 which reduced the effective permeability of the SMCs. To form pure Fe3O4 coating, the reaction process has been improved to suppress the Fe3+ hydrolysis. The obtained ferromagnetic Fe3O4 coating effectively reduces magnetic dilution, while its high electrical resistivity allows satisfactory insulation between the Fe powders, giving rise to simultaneous high effective permeability (97.7) and low core loss (771.3 mW/cm3) of the SMC. •Alkaline bluing as a new method to fabricate insulation coatings surrounding magnetic powders.•Effects of reaction time on the magnetic performance of the SMCs investigated.•Growth mechanism of the coatings revealed based on which the oxidation process has been improved.•Ferromagnetic Fe3O4 coating achieved for improved magnetic properties of the SMCs.