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 Bs of 1.29 T and Ps 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.