Study On Alumina Coated Iron-based Soft Magnetic Composites Prepared By One-pot Synthesis Method

Soft magnetic composites（SMCs）are usually prepared by powder metallurgy（PM）consisting of insulation coating,compaction and heat treatment.These materials have good performance such as low eddy current loss,three-dimensional isotropic ferromagnetic behavior and flexible design,which are widely used in different fields.As the most important step,in this paper,insulation coating of Al₂O₃ was investigated,and the main research content is as follows:（1）The effect of Al₂O₃ insulating layer on the magnetic properties of iron-based soft magnetic composites by one-pot synthesis method was investigated.The effect of surfactant on one-pot synthesis process was also studied.The main controlling factors of the experiment were the reaction concentration and the reaction temperature.The microstructure of composites were investigated by scanning electron microscopy（SEM）,Fourier transform infrared spectroscopy（FTIR）,energy-dispersive and X-ray spectroscopy（EDS）.The reaction model diagram of the coated Fe powder in an air-tight stainless steel reactor with the polytetrafluorethylene inner was established.It revealed that the surface of reduced iron powder was coated with Al₂O₃ insulating layer when prepared with different Al（NO₃）₃ concentration.Besides,different reaction temperatures had less difference in coating effect.In the composites with Al₂O₃insulating layer,the Al₂O₃ insulating layer enhanced the frequency stability of permeability compared with the pure reduced iron compact.At the same time,the core losses had been reduced to some extent.Surfactants were beneficial to the optimization of the coating effect and further reduced the loss,and the specific mechanism still needs further study.（2）The effects of heat treatment and lubricant on the magnetic properties of Al₂O₃ coated iron-based soft magnetic composites by one-pot synthesis method was investigated.Compared with pure reduced Fe compact within the frequency range of 5-100 kHz after heat treatment,the coated Fe compacts show lower loss core,especially lower eddy current loss.The experiment results indicated that using lubricant in the coated iron-based compacts presented higher permeability,higher resistivity,lower core loss over the wide frequency range compared with those without using lubricant after heat treatment at 773 K.The above effects would be reduced with the increase of the thickness of the alumina coating.As the heat treatment temperature raised from 573 K to 773 K and even higher 873 K,zinc stearate continued to decompose,and its decomposition product carbon reacted with the iron oxide of the thinner coating.As a result,the frequency stability of the permeability decreased especially at high frequencies,the resistivity decreased significantly,and the eddy current loss increased significantly.When the alumina insulation layer became thick,the effects of zinc stearate and its leaving carbon after decomposition on the properties of the pressed magnet decreased.Finally,based on the research results,a model diagram of the coating layer changing with heat treatment temperature was established.