Structure Control And Electromagnetic Performance Optimization Of Fe-Si Based Soft Magnetic Composites

Fe-Si based soft magnetic composites（SMCs）have superior properties such as relatively high saturation magnetization,low magnetic loss at high frequencies and good DC bias ability.They are used as magnetic cores for power inductor and can be widely applied in the fields including domestic appliances,energy storage power stations,photovoltaic inverters and new energy vehicles.In order to meet the high-frequency,high-efficiency and energy-saving for the above power electronic equipments,aiming to the problem that it is difficult to obtain good magneic properties and low high-frequency magnetic loss concurrently for the inorganic insulation coated Fe-Si based SMCs,this paper gradually regulated the uniformity,intrinsic insulation and magnetic dilution of the coating layer in the Fe-Si based SMCs.The transformation from various metal oxide coatings with low resistance to insulating layer with high resistance was achieced by high temperature in situ reactions,and then a magnetic coating with excellent insulation was obtained.Finally,the prepared Fe-Si based SMCs had both superior magnetic properties and very low magnetic loss,which provided a new idea and scheme for the preparation of high-performance SMCs.The main research results as as follows:（1）Fe-Si/MnO-SiO₂ SMCs were prepared by mechanical ball milling MnO₂combined with high temperature sintering.The transformation from MnO₂ coating to MnO-SiO₂ composite insulating layer was achieved.Then the coating layer with excellent insulation was obtained owing to the uniformity combined with the intrinsic insulation,thereby resulting in the greatly reduced high-frequency magnetic loss.The insulation transformation mechanism of MnO₂ coating,the effects of coating content and ball milling process parameters on the insulation structure and electromagnetic properties of Fe-Si based SMCs were systematically investigated.The results showed that the uniform MnO₂ coating can be obtained on the surface of Fe-Si alloy powder by mechanical ball milling.It will react with Si from the Fe-Si alloy cores at about780.1℃to form MnO and SiO₂.Meanwhile the discharge effect promotes the local high temperature between the Fe-Si alloy cores to reach about 985.8℃,the products are further combined to form 2MnO·SiO₂,finally rsulting in the formation of MnO-SiO₂ composite insulating layer.When the MnO₂ coating content increased,the MnO-SiO₂ composite insulating layer gradually thickened,leading to the gradually reduced magnetic loss,but the content of in-situ Fe mixed in the insulating layer increased,thus the decrease of magnetic loss was obviously limited when the MnO₂coating content was higher than 5.0 wt%.On the other hand,compared with stainless steel ball milling,agate ball milling is more conducive to the uniform coating of MnO₂on the surface of Fe-Si alloy powder.In addition,increasing ball milling time can effectively improve the uniformity and density of MnO-SiO₂ composite insulating layer,and the magnetic loss can be further reduced.When the ball milling time reached 20 h,the saturation magnetization was 180.5 emu/g,the effective permeability at 1000 k Hz was maintained at 60,the resistivity was as high as 359.6 mΩ·cm,and the magnetic loss was as low as 352.6 m W/cm³（50 m T/100 k Hz）.（2）Fe-Si/（Fe₂SiO₄/SiO₂）SMCs were prepared by mechanical ball milling Fe_xO_ycombined with high temperature sintering.The transformation from Fe_xO_y coating to Fe₂SiO₄/SiO₂ insulating layer was achieved.The problem that the insulation is difficult to be compatible with weak magnetic dilution for coating layer was partly solved,and then the magnetic properties were significantly enhanced.The insulation transformation mechanism of Fe₃O₄,γ-Fe₂O₃ andα-Fe₂O₃ coatings,and the effect of coating content on the insulation structure and electromagnetic properties of Fe-Si based SMCs were systematically investigated.The results showed that the insulation transformation mechanisms of the Fe_xO_y coatings were similar,but the mian difference lies in the difference of insulation transformation temperature.During the high temperature sintering process,a series of solid-phase interface reactions will occur between the Si from Fe-Si alloy cores and the Fe_xO_y coating to produce Fe₂SiO₄ and Fe,even SiO₂ and more Fe,but the Si content in Fe-Si alloy cores will be reduced.When the Fe_xO_y coating content increased,the saturation magnetization and effective permeability gradually decreased,but the resistivity first increased and then decreased,and the magnetic loss first decreased and then increased.Compared with theγ-Fe₂O₃coating,the transformation temperatures of both the Fe₃O4 andα-Fe₂O₃ coating to the composite insulating layer are lower.Therefore,the former was transformed into Fe₂SiO₄ insulating layer,and the latter two were transformed into Fe₂SiO₄/SiO₂composite insulating layer mixed with more Fe.When the coating content was the same as 5.0 wt%,the former had the lowest magnetic loss（23.34 m W/cm³,20 m T/50k Hz）,but the latter two had better soft magnetic properties,the saturation magnetization of theα-Fe₂O₃ coated sample was as high as 192.5 emu/g.（3）Fe-Si/（Fe₂SiO₄/SiO₂）SMCs were prepared by water oxidation combined with high-temperature sintering.The transformation from oxide coating to Fe₂SiO₄/SiO₂insulating layer was achieved.Then the compatibility of insulation and weak magnetic dilution for coating layer was realized.Thereby,the magnetic loss was markedly reduced and the magnetic properties was enhanced concurrently.The mechanisms of water oxidation and insulation transformation for the oxide coating,as well as the effects of sintering temperature and oxidation time on the microstructure and electromagnetic properties of Fe-Si based SMCs were systematically investigated.The results showed that the uniform Fe₃O₄-Si oxide coating prepared by water oxidation will occur insulation transformation at about 1095.9℃,thereby resulting in the formation of Fe₂SiO₄ insulating layer mixed with Fe.When the temperature at the solid interface reaches about 1180.2℃,Fe₂SiO₄ will further react with Si from the Fe-Si alloy cores to form the SiO₂ insulating layer mixed with more Fe.Thus,when the sintering temperature increased,the saturation magnetization and effective permeability gradually increased and the magnetic loss exhibited a raised trend.With the increase of the oxidation time,the thickness of the Fe₂SiO₄ insulating layer gradually increased,but the Si content in the Fe-Si alloy core was reduced.Therefore,both the saturation magnetization and effective permeability gradually decreased,and the magnetic loss first decreased and then increased.The sample with water oxidation time of 2 h exhibited relatively better electromagnetic performance,such as high saturation magnetization（186.1 emu/g）and effective permeability（62）,high resistivity（480.2 mΩ·cm）and very low magnetic loss（263.0 m W/cm³,50 m T/100 k Hz）.