Effect of Packing Fraction on Magnetic Properties of the Fe-Si-Al Powder Cores by Coarse Powder and Fine Powder Mixing

Fe-Si-Al of soft magnetic materials are known as high permeability alloys, and these are used as dust cores. In general, Fe-Si-Al powders, what is called "Sendust", have poor compressibility because of their hardness, therefore it is difficult to increase the packing fraction of their core. We researched the effect of packing fraction on magnetic properties in order to mix the powders which have different mean particle size. In case of mixing the coarse powders and fine powders, it is assumed that fine particles penetrate into the opening gaps which are located in close packing of coarse particles. They are called 6-, 4- and 3-configuration. The sizes of opening gap in close packing are 0.414, 0.225 and 0.154 of coarse particle size, respectively. Fe-9.5Si-5.5Al alloy powders were prepared by gas atomization process. Mean particle size of coarse powders are fixed about 40 mu m and fine powders are prepared the size by classifier. Coarse powders and fine powders were mixed to the optional ratio. Relative permeability and core loss up to 1 MHz were measured. Packing fraction of core was improved more than 80 % by optimization of the mixing ratio of fine powders. As a result, relative permeability was increased with the packing fraction.