Preparation And Properties Of Fe/Fe₃O₄ Soft Magnetic Composites With Core-shell Structure

The development of miniaturization,high-efficiency,energy-conservation,and high-speed of modern electronic components require that soft magnetic materials with high saturation magnetization,high permeability,and low loss.Therefore,it is vitally important to develop the soft magnetic composites（SMCs）with high-performance to satisfy the requirements mentioned above.In order to solve the two key problems of low magnetic properties and high magnetic loss of soft magnetic composites,the soft magnetic composites with core-shell structure were prepared by in-situ oxidation method.The formation of high-resistivity ferrimagnetic layer reduces eddy-current loss and maintains high magnetic properties.Meanwhile,the thickness and microstructure of the SMCs were adjusted by controlling the reaction conditions and process parameters to obtain SMCs with excellent comprehensive magnetic properties.Fe/Fe₃O₄core-shell structure SMCs were synthesized by in-situ oxidation method,the shell thickness control method and its effect on magnetic properties was investigated,and the oxidation kinetic mechanism was also discussed.The in-situ oxidation of iron shows a linear kinetic law,and the oxidation rate is mainly controlled by the interface reaction.In-situ oxidation rate k₁=4.08×10^-7mg·cm^-2·s^-1,and the reaction order n=0.93,which achive the accurate control of the shell thickness.The formation of Fe₃O₄shell will decrease eddy-current loss for its high resistivity but reduce the permeability and saturation magnetization for its ferrimagnetic characteristics,appropriate shell thickness is the key factor to obtain a good performance of composites.The magnetic loss decreased dramatically due to the existence of a dense insulation layer and increased slightly owing to the presence of cracks in the insulation layer with the shell thickness increase further.The optimum magnetic properties of the composites were obtained for the composites with about 0.17μm shell thickness,which exhibits a high saturation magnetization of 232.1 emu/g,a high permeability of 55 and a low magnetic loss of 30.5 W/kg at 20 m T and 100 k Hz.The effect of reaction temperature,oxygen flow rate,zinc stearate content,compaction pressure,and annealing temperature on the microstructure and magnetic properties of SMCs were investigated systematically on the basis of the optimum shell thickness.With the improvement of oxygen flow rate and reaction temperature,the saturation magnetization,permeability and magnetic loss of the Fe-based SMCs increase;Due to the introduction of non-magnetic phase,the permeability of the SMCs decreases with the increase of zinc stearate content,while the magnetic loss increases with the increase of zinc stearate content;The enhancement of the compaction pressure causes the permeability to increase first and then decrease,while the magnetic loss shows the opposite law to the permeability.Therefore,appropriate compaction pressure is beneficial to improve the magnetic properties;and the residual stress of the composites can be released more sufficient by increasing the annealing temperature,However,Fe₃O₄will decompose into Fe O when the annealing temperature reaches to 570°C,thus decreasing the magnetic properties of Fe-based soft magnetic composites.In conclusion,the optimal process parameters are as follows:reaction temperature is 60°C,oxygen flow rate is 150 sccm,the content of zinc stearate is 1.0 wt%,compaction pressure is 800 MPa,and annealing temperature is 500°C.And this study provides a certain theoretical guidance for the preparation of excellent comprehensive magnetic properties of SMCs.