| Fe-Si-Al soft magnetic powder core with certain shape can be obtained by pre-annealing,insulating,pressing and annealing soft magnetic Fe-Si-Al powders.This material has been widely used in power electronic devices,because it possesses low magnetic loss and is free of precious metals.However,its low saturation magnetization,poor DC-bias performance and strong high-frequency eddy-current loss severely restrict its practical application.In order to solve these problems,this thesis attempts to improve the saturation magnetization and DC-bias performance of soft magnetic powder core by adjusting the composition of Fe-Si-Al,and suppress the high-frequency eddy-current of soft magnetic powder core by changing the shape of Fe-Si-Al magnetic powder into thin sheet.Firstly,the same batch of Fe-Si-Al powder was selected and divided into 9 groups.After ball milling,the samples were pre-annealed and optimal annealing temperature has been chosen according to low magnetic loss and excellent comprehensive magnetic properties.Then,according to the ternary phase diagram of magnetic properties of Fe-Si-Al alloy,8 different compositions were selected.Corresponding master alloys were melted,coarsely crushed,ball milled and pre-annealed.Powder cores were also prepared by insulating,pressing and annealing above Fe-Si-Al powders,and the influence of Fe-Si-Al compositions on the magnetic properties of the powder core was studied.Finally,the flaky Fe-Si-Al magnetic powders were prepared by high-energy ball milling with standard Sendust alloy as raw material.Corresponding powder cores were obtained after pre-annealing,insulating,pressing and annealing of flaky Fe-Si-Al powders,and the influence of morphology on the magnetic properties of the powder core was discussed.The results showed that:(1)When the pre-annealing temperature is640 ~oC,the powder core has the highest density and volume resistivity,low loss and good DC-bias performance.(2)The hardness of the alloy increases with the increase of silicon content;The saturation magnetization increases with the increase of iron content and some samples exhibit obviously higher saturation magnetization than that of the standard Sendust alloy.The powder cores composed of non-standard Fe-Si-Al alloy own better DC-bias performance,but higher core loss.This may be ascribed to the increase of magnetocrystalline anisotropy constant and magnetostriction coefficient.By testing and separating the loss under the condition of 100 k Hz-800 k Hz and 20 m T,we can find that the maximum frequency of crossover point of hysteresis loss and eddy-current loss can reach 300 k Hz.(3)Flaky Fe-Si-Al powders were prepared by high energy ball-milling and the particle size is about 30?m.Compared with irregular shape and spherical Fe-Si-Al magnetic powder,the saturation magnetization of annealed flaky Fe-Si-Al magnetic powder,which has been ball-milled for 9 h,is the highest,and the reason may be the decrease of lattice constant;The eddy-current loss of flaky Fe-Si-Al magnetic powder core is much lower,which can be explained as:skin depth is reduced and eddy current has been significantly suppressed,thus high-frequency eddy-current loss is weakened. |
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