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SiO2 nanoparticles have been introduced into epoxy-modified silicone resin (ESR) via two routes including direct addition (DA) and in-situ growth (ISG). Growth mechanism of the SiO2 nanoparticles has been revealed where chemical bonding forms between the SiO2 and the epoxy, ester and hydroxyl groups of the ESR during ISG. The obtained hybrid resins have been used as the matrix in the fabrication of Fe soft magnetic composites (SMCs). Effects of the SiO2 incorporation method and its content on the magnetic, electrical and mechanical properties of the SMCs have been investigated. Optimized performance of the SMC with large effective permeability (107.9), reduced core loss (1220 mW/cm3) and enhanced mechanical strength (2.95 MPa) can be achieved by introduction of 34 wt% SiO2 nanoparticles into the ESR via ISG. The formation of chemical bonding between the SiO2 nanoparticles and the ESR also gives rise to improved thermal stability for complete stress relaxation of the SMCs.

Phosphate layers were prepared as the insulation coating for Fe and FeSiAl soft magnetic composites (SMCs) and mechanisms of the phosphating process have been investigated. Effects of high-temperature annealing on the phosphate coating and the magnetic properties of the SMCs have also been revealed. For Fe SMCs, the insulating coating mainly consists of iron phosphate after surface passivation. It degrades into iron phosphide after annealing at relatively high temperatures (600 °C), leading to decreased electrical resistivity and increased eddy current for deteriorated magnetic properties of Fe SMCs. For the FeSiAl SMCs, the phosphate coating mainly contains Al(PO 3 ) 3 , which converts into oxides such as Al 2 O 3 and P 2 O 5 after annealing. These oxides possess high electrical resistivity, and continue to serve as the insulation coating for the composites. Consequently, the resistivity of the FeSiAl SMCs increases with the annealing temperature, which is beneficial to decrease the eddy current loss for enhanced magnetic performance.