Evolution of the insulation matrix and influences on the magnetic performance of Fe soft magnetic composites during annealing

• Thermal oxidation as a new method to fabricate insulation coating for SMCs.
• Sole ferromagnetic Fe3O4 achieved as the inorganic coating for excellent magnetic performance.
• Evolution of the insulation matrix during annealing investigated.
• Annealing effects on the properties of SMCs revealed and correlated to the matrix evolution.

Fe soft magnetic composites (SMCs) containing Fe powders embedded in Fe3O4/epoxy-modified silicone resin (ESR) matrix have been fabricated via thermal oxidation of Fe powders with H2O under alkalic conditions, prior to mixing with ESR. The ferromagnetic Fe3O4 coating effectively reduces magnetic dilution, while high electrical resistivity of both Fe3O4 and ESR provides satisfactory insulation of the SMCs, giving rise to simultaneous high permeability and low core loss. Evolution of the Fe3O4/ESR matrix during the annealing process has been investigated, which is correlated to changes in the magnetic properties of the SMCs. The ESR starts to decompose at around 382 °C, leaving reducing groups which then convert the Fe3O4 coating into Fe with increased annealing temperature to 466 °C. Further raised annealing temperature to 570 °C leads to the reduction of Fe3O4 into FeO. Such evolution provides in-sight information on choosing appropriate annealing conditions to avoid decomposition of the Fe3O4/ESR matrix and can be expanded to other Fe-based SMCs.