Improved magnetic properties of iron-based soft magnetic composites with a double phosphate-SiO2 shells structure

Herein a double shells structure was proposed to overcome the drawbacks such as poor heat resistance, incomplete insulation and high core loss coating of iron-based soft magnetic composites (SMCs) with single shell structure. The surface of the iron powder was coated with a double shells structure composed of an inner phosphate coating and an outer SiO₂ coating through phosphating and hydrolysis successively. Subsequently, the effects of different SiO₂ addition amount on the microstructure of iron powder and magnetic properties of SMCs were studied. The introduction of SiO₂ in the double shells structure inhibited the decomposition and failure of phosphating layer after being annealed at a high temperature. The iron powder coated with the phosphate-SiO₂ insulating layer was still effective after annealing in a N₂ atmosphere at 570 °C, achieving the purpose of eliminating residual stress and improving magnetic properties. The optimal process parameters were set at 0.2 wt% phosphoric acid and 0.5 wt% tetraethyl orthosilicate to fabricate the phosphate-SiO₂ double shells. The iron-based SMCs presented excellent magnetic properties with B_s of 1.29 T and P_s of 169.2 W/kg (measured at 1 T and 1 kHz). In addition, the core loss of SMCs introduced with SiO₂ is 83% lower than that of SMCs produced by the phosphating process. This paper provides a feasible method for improving the magnetic properties of SMCs.