FeSi,FeSiAl And FeSiCr Soft Magnetic Composites Fabricated By Surface Nitridation And Oxidation

Soft magnetic composites(SMCs)have been widely used in power,electronic,communication and energy devices due to their high saturation magnetization,high permeability and low loss.With the development of information and communication technology,the requirements for SMCs have grown significantly.To satisfy the application requirements at high frequencies,it is critical to reduce the eddy current loss between the magnetic parlicles using insulation coatings.In this work,nitridation,oxidation,nitridation and oxidation have been used to prepare insulation coatings for FeSi,FeSiAl and FeSiCr powders.SEM,XRD,XPS have been used to characterize the insulation coatings.Detailed growth mechanism of the insulation coatings for different alloy systems have been established based on combined thermodynamic and kinetics analysis.Correlations between the insulation coatings and the magnetic performance of different SMCs have also been revealed.The main results are as follows:The FeSi,FeSiAl and FeSiCr SMCs have been fabricated by surface nitridation and their nitride coatings are composed of a-Fe(N),Fe4N and a-Fe(N),a-Fe(N)and CrN,respectively.Core loss of the nitrided FeSi SMCs is larger than the phosphate-coated ones,since the nitride coating is thin and with low electrical resistivity.The thickness of the coatings is restrained due to the decreased solubility of N induced by the existence of the Si.The nitrided FeSiAl SMCs exhibits large Pcv at high frequencies,since the obtained Fe4N phase is hard and brittle,which tends to induce inner stress,or even cracks during compaction.The nitrided FeSiCr SMCs exhibit improvement of 31.8%in the ?e compared with the phosphate-coated samples because of the thin coatings with high adhesion.The reduction of 24.2%in the Pcv results from the the formation of CrN with relatively large electrical resistivity.The FeSiCr SMCs have been fabricated by surface oxidation.The insulation coatings of surface oxidized FeSiCr powders are composed of Fe2O3,Fe2SiO4,Cr2O3 and SiO2.They exhibit improvement of 32.8%in the ?e and reduction of 24.2%in the Pcv compared with the samples passivated by phosphate-coated samples.The improvement of the magnetic performance is attributed to the oxide coatings,which are thin,uniform,compact and with high electrical resistivity.The FeSi,FeSiAl and FeSiCr SMCs have been fabricated by surface nitridation and oxidation.The as-treated coatings are composed of FeSi(Fe3O4,Fe2O3,Fe2SiO4,SiO2,Fe4N),FeSiAl(Fe3O4,Fe2O3,Fe2SiO4,SiO2,Al2O3,s)and FeSiCr(Fe3O4,Fe2O3,SiO2,Cr2O3,Fe4N).Despite the Pcv of FeSi SMCs is decreased by 19.9%,the ?e is decreased by 65.5%because the oxide coatings obtained at high temperatures are too thick and nonmagnetic.The as-treated FeSiAl SMCs exhibits improvement of 13.3%in ?e compared with nitrided ones due to the formation of Fe3O4 and iron nitride with high Ms.The reduction of 24.9%in Pcv is due to the formation of coatings with high electrical resistivity.Lower ?e is obtained for the as-treated FeSiCr SMCs,because trigonal Cr2O3 forms along the grain boundaries,which results in incoherent interface with iron nitrides and hinders domain movement.While the larger Pcv is resulted from the formation of hard and brittle Cr2O3,which tends to crack during compaction.