Fabrication Of SiO₂-coated Fe-based Soft Magnetic Composites With Low Loss At High Frequency

Fe-based soft magnetic composites (SMCs) were widely used in motor, transformer and sensor. In this investigation, silica insulation coatings were synthesized on the surface of ferromagnetic powder with tetraethyl orthosilicate (TEOS) as precursor and (3-Aminopropyl) triethe-oxysilane (APTES) as coupling agent. Fe-based soft magnetic composites were fabricated with the silica coated powder, which could contribute to increasing the operating frequency via decreasing Ac magnetic loss at the same time of maintaining high permeability and saturation magnetization. Microstructure of the powder and composites were characterized with XRD,FTIR,XPS,SEM,EDS and MFM. Magnetic properties of coated powders and bulk materials were tested by VSM and B-H analyzer.The results show that there is a flocculent amorphous silica layer on the surface of CIP,Fe and Fe85Si9.6Al5.4powders via sol-gel method. Magnetization of the powder decreases slightly after being coated.Compared with uncoated materials, electrical resistivity of Fe/SiO2SMCs increases remarkably and iron loss decreases significantly with operating frequency increasing. The saturation magnetization and coercivity of Fe/SiO2SMCs,1.10T and125A·m-1, respectively.Electrical resistivity of SMCs fabricated from CIP/SiO2and Fe/SiO2powder mixture are apparently higher than that of Fe/SiO2SMCs. With increasing the content of CIP/SiO2, the resistivity increase, iron loss and magnetization decrease, and coercivity increase slightly. The SMCs have low iron loss, good frequency stability when the mass ratio of CIP/SiO2:Fe/SiO2is20:80,and when operating frequency is100kHz,iron loss is19.08W/kg. The Fe85Si9.6Al5.4/SiO2SMCs have good frequency stability, and when operating frequency is200kHz,iron loss is8.4W/kg.Density, saturation magnetization and permeability of SMCs increase while resistivity decreases and iron loss increases with compacting pressure increasing. Enhancement of the heat treatment temperature helps reduce internal stress coming from pressing process and increase permeability, but excessive temperature increases the iron loss. The optimum compacting pressure and heat treatment temperature for Fe/SiO2SMCs are800MPa and600℃,and those for Fe85Si9.6Al5.4SiO2SMCs are1800MPa and700℃,respectively.APTES can improve the affinity of TEOS with the surface of CIP, Fe and Fe85Si9.6A15.4particles, and is beneficial to the formation of continuous uniform SiO2layer on the surface of particles. With the increase of TEOS content, thickness of SiO2layer increases and saturation magnetization decreases, and it is the same with APTES. Heat treatment of coated powder is conducive to the elimination of unreacted organics and the hydrolysis reaction of TEOS and APTES, which improve the integrity, homogenization and relative density of SiO2layer.