| Metallic soft magnetic powder core is a new kind of composite,mainly composed of magnetic metallic particles,insulating substance and distributed air gaps.Metallic magnetic particles provide high saturation magnetization and Curie temperature.Both the insulating substance and distributed air gaps lead to low eddy-current loss and good anti-saturation capability.But the nonmagnetic insulating substance and air gap also cause the increase of the effective demagnetizing field and thus result in the deterioration of the permeability and the hysteresis loss.In this thesis,the Fe-Si powder cores were prepared firstly by using different resins and the silica nanoparticle as the basic insulating agents and the auxiliary one,respectively.The influence of the resin type on the magnetic properties of the Fe-Si powder core was studied and the optimum resin was selected for the subsequent preparation of the composite core.Then the Mn-Zn ferrite nanoparticles were prepared with different parameters via the chemical co-precipitation method.And the Mn-Zn ferrite/carbonyl-iron composite powder cores were prepared through insulating the carbonyl-iron powder with ferrite nanoparticles and the selected optimum resin.The influences of the processing parameters of the ferrite on the ferrite's phase structure and the magnetic properties of the composite core were investigated and the parameters of the ferrite with high permeability were selected for the subsequent research accordingly.Finally,the Mn-Zn ferrite/Fe-Si composite powder cores were prepared through insulating the Fe-Si powder with the selected optimum resin and ferrite.The influences of the ferrite insulation content and the compaction pressure on the magnetic properties of the composite core were studied and the loss-separation fitting was performed.The research findings are listed as follows.(1)The Fe-Si powder core prepared with epoxy resin(A)had magnetic properties much better than those of the industrial product with the same specification.The sample with 60?permeability had a good magnetic performance.Its percent permeability was as high as 79.16%at 100 Oe and its core loss was as low as 501.33m W/cm~3 at 50k Hz/100m T.Besides,it also possessed a high tensile strength and the relatively highest density and powder flow rate.(2)For the preparation of Mn-Zn ferrite,increasing the p H value and the reaction temperature and prolonging the reaction time can improve the crystallization quality of the ferrite.However,a decrease in the average grain size of the ferrite was found when the reaction time is too long.(3)The Mn-Zn ferrite/carbonyl-iron composite powder core based on the Mn-Zn ferrite nanoparticles prepared at the p H value of 13 and at the reaction temperature of95°C for 1 h possessed the relatively highest permeability.(4)Insulating with a moderate quantity of ferrite was found to improve the effective permeability and the core loss,while the inadequate insulation and the excessive insulation with ferrite led to the rapid growth of the eddy-current loss at high frequency and the increase of the hysteresis loss,respectively.Appropriate increase of the compaction pressure is beneficial to increase the core's effective permeability and decrease the core loss.The effective demagnetizing field determined by the core's density and the magnetic nature of the ferrite,the resistivity determined by the core's density and the high resistivity of the ferrite and the internal stress determined by the compaction pressure were proposed to be the main causes of the variation of the core's magnetic performance.(5)Loss separation fitting revealed that the hysteresis loss is much higher than the eddy-current loss for the core measured at the frequency lower than 120k Hz.This suggested that improving the effective permeability and the loss of the powder core in low-frequency range should focus on the means of reducing the effective demagnetizing field within the core. |
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