Microstructure And Magnetic Performance Of FeSiAl Soft Magnetic Composites With High Permeability And Low Core Loss

Soft magnetic composites(SMCs)containing metallic magnetic powders surrounded by insulation coatings are fabricated by powder metallurgy process and are widely used in applications such as transformer coil,resonant inductor and electric motor core.Despite their rapid development urged by the ever-rising requirements from the electric and electronic industry,the inevitable loss especially the eddy current loss refrains their applications at high frequencies.For the SMCs,the key to tackle such issue is to fabricate uniform insulation layers with small thickness,high electrical resistivity and strong adhesion to the magnetic powders.Although traditional organic coatings provide satisfactory adhesion and flexibility,most of them including the epoxy,phenolic and silicon resins exhibit poor resistance against high temperature annealing and will decompose after annealing above 200 ?.This limits the temperature used for the post-annealing process and hinders complete relaxation of the internal stress.Surface passivation by phosphoric acid in the formation of phosphate coatings is the most commonly used method.It has,however,been reported that heat treatment leads to significantly reduced electrical resistivity of the phosphate due to its crystallization and/or decomposition at relatively high annealing temperatures.Oxide insulation coating with high resistivity and thermal stability serves as an excellent alternative to reduce the core loss.In this study,FeSiAl soft magnetic alloy with low magnetocrystalline anisotropy,high permeability and low electrical resistivity has been selected to prepare the SMCs.New phosphate-alumina coating has been prepared by sol-gel method.Nitric acid in-situ passivation has been used to fabricate compact insulation coatings.Besides,FeSiAl powders have been surface oxidized by NaNO3 solution with varied concentration and pH values.The main results of the work are as follows:(1)Al2O3 insulation coating has been prepared by sol-gel process.New phosphate-alumina hybrid coating has been fabricated due to its satisfactory adhesion to the powders combined with high electrical resistivity and thermal stability.Composition and evolution of different insulation coatings after high temperature annealing has been studied to explain the changes of magnetic performance for the FeSiAl SMCs.(2)Nitric acid in-situ oxidation has been used to fabricate insulation coatings of the FeSiAl SMCs.Detailed high-resolution transmission electron microscopy(TEM)and energy dispersive X-ray spectroscopy(EDS)mapping analysis provide direct and comprehensive evidence of the thickness,microstructure and elemental distribution of the oxidation layers,based on which the evolution and growth mechanism of the coatings with varied concentrations have been revealed.The origin of pitting corrosion has been explained based on the point defect model.Correlating the magnetic performance and evolution of the coating layers under different conditions has been revealed.(3)FeSiAl powders have been surface passivated by NaNO3 solution with varied concentration and pH values.The thickness,microstructure and composition of the passivation layer have been investigated.Correlating the growth mechanism and evolution during the annealing process of the passive films reveals the enhanced magnetic properties of FeSiAl SMCs.Thermodynamic analysis displays the growth mechanism of the insulation coating under different passivation conditions.