Formation mechanism and magnetic performance of Fe-Si soft magnetic composites coated with MnO-SiO2 composite coatings

Abstract Fe-Si soft magnetic composites (SMCs) coated with MnO-SiO2 composite coatings were fabricated via sintering the ball milled Fe-Si/MnO2 composite powders. The transformation mechanism from MnO2 coating to MnO-SiO2 composite coatings was studied. The effect of MnO2 content on the microstructure and magnetic properties of Fe-Si soft magnetic composites has also been investigated systematically. The analysis results indicated that MnO2 would be reduced to MnO by Si in the Fe-Si particles at 780.1 ℃. When the temperature reached 985.8 ℃, the generated MnO and SiO2 would further combine to form 2MnO·SiO2, finally forming the MnO-SiO2 composite coatings. With the increase of the MnO2 content from 0.0 wt% to 10.0 wt%, the MnO-SiO2 composite coatings became thicker, thus the saturation magnetization and permeability decreased gradually. However, exorbitant MnO2 content above 5.0 wt% would result in excessive in-situ Fe existed in the composite coatings and obvious reduction of Si content in the Fe-Si particles due to the above solid-state reactions. Therefore, the increasing trend of the resistivity was weakened. And the 5.0 wt% MnO2 coated Fe-Si SMCs exhibited superior comprehensive performance such as high saturation magnetization (186.5 A·m2/kg), good permeability (71), low total core loss (362.6 mW/cm3 at 50 kHz).