Optimization of sol-gel prepared SiO2 coating and FeSiCr@SiO2 soft magnetic composites based on critical ammonia concentration

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The SiO2 insulating layer was prepared on the FeSiCr powders successfully by one-step dumping sol-gel method and ammonia was used as the catalyst. It is found that the growth of SiO2 layer can be tuned by the ammonia concentration. Within the reaction time of 0.5 h, SiO2 grows with heterogeneous nucleation when the ammonia concentration is 0.08–0.16 ml/g, and SiO2 coating layer was relatively uniform. With a high ammonia concentration of 0.20–0.24 ml/g, SiO2 grows with both heterogeneous and homogeneous nucleation, and SiO2 particles gathered and grew. The saturation magnetization and effective permeability of FeSiCr@SiO2 SMCs gradually decrease with the increase of ammonia concentration, while the specific resistance, quality factor and DC-bias property increase. With the ammonia concentration of 0.12 ml/g, the thickness of SiO2 insulating layer was about 30–60 nm. The annealed FeSiCr@SiO2 soft magnetic composite shows good soft magnetic performance with effective permeability and core loss of 45.52 and 418.32 kW/m3 at 0.05 T and 100 kHz, respectively, and the quality factor of 76.5 at 1 MHz.