Fabrication Of Core-shell Structure Soft Magnetic Composites With High Magnetic Performance By Surface Oxidation And Nitridation

In recent years,soft magnetic composites(SMCs)have been widely used in electrical and electronic applications due to its high saturation magnetization and high electrical resistivity.For high applications,low loss of SMCs is desirable where the insulation coating plays a critical role.Since thermochemical treatment results in the changes of surface microstructures and compositions of metallic materials by diffusing active atoms into matrix under high temperatures,it is regarded as a new coating method with great potential in SMCs field.In this paper,surface oxidation and nitridation methods were adopted to prepare oxides and nitride(Fe4N)shells at the surface of ferromagnetic particles.XRD?SEM?EDS?FTIR methods were used to characterize the shell microstructures and element compositions.Finally,correlations of magnetic properties with oxides and nitride shells have been estabilished.The main conclusions are as follows:1)Fe/(Fe2O3,Fe3O4),FeSi/(Fe203,SiO2)and FeSiAl/(Fe2O3,Si02,A1203)ferromagnetic particles with core-shell structures were prepared.A bilayered film with an inner Fe3O4 layer and an outer hematite Fe2O3 layer was created at the surface of iron powders;for FeSi powdres,a smooth and thin film was formed at 400? while a smooth SiO2 rich film dispersed with Fe2O3 nodules was created at 700?;for FeSiAl powders,a smooth thin film mainly containing Fe2O3 and Al2O3 oxides were obtained.2)Direct current(DC)and alternating current(AC)magnetic propreties are comprehensively compared between surface oxidation and phosphoric passivation.The saturation magnetization values of oxidized Fe,FeSi and FeSiAl powders are 216.8,169.5 and119.1 emu/g respectively.In contrast,the values for their passivated powders are 190.5,173.4 and 118.9 emu/g.For both methods,core loss behaved similarly while permeabilities were varied.The permeability values for Fe,FeSi and FeSiAl SMCs are 116.3,72.5 and 167.4 respectively,while their related passivated samples are 84.8,107 and 139.3)The influence of oxidation temperature on magnetic properites has also been investigated.With the increase of oxidation temperature,the shell thicknesses of Fe and FeSi grow rapidly,thus resulting in a monotonous decline of permeabilities.In contrast,FeSiAl powders obtained a thin film within the range of 300?500? and a stable permeability value around 165 was achieved.A minimum core loss value of 804.8 mW/cm3(50 mT,100 kHz)within 200? was obtained for Fe SMCs;however,the minimum core loss of 616.9 mW/cm3(50 mT,100 kHz)for FeSi SMCs was achieved at 700?.The increase of core loss could be attributed to a thin shell at lower temperature and the Fe2O3 nodules with a much big size at higher temparatue.For FeSiAl SMCs,the minimum core loss of 224.3 mW/cm3(100 mT,50 kHz)was obtained at 500?.4)Fe/Fe4N ferromagnetic particles with core-shell structure were prepared.The powders nitrided at 400 ? exhibited a saturation magnetization value of 225.1 emu/g,which was slightly higher than that of pure iron powders.The electrical resistivity was elevated from 0.57 ?·cm to 1.45 fl·cm after surface nitridation.The magnetic powder core treated at 450? achieved a core loss of 158.4 mW/cm3(10 mT,450 kHz),which was reduced by 40 mW/cm3 compared with the untreated sample,indicating enhanced properties for high frequency band.