Fabrication And Investigation On Fe-based Soft Magnetic Composites With High B_s And Low Loss

Soft magnetic composites (SMCs) have attracted extensive interest due to their excellent properties, including high saturation magnetization, low loss and 3D isotropic ferromagnetic behavior. Phosphate coating is usually used for the surface passivation of the Fe powders due to its good electrical resistivity and adhesiveness to the iron powder. The non-ferromagnetic phosphate coating, however, tends to decrease the overall saturation magnetization and permeability of the SMCs. Moreover, poor thermal stability of the phosphate limits the post-annealing temperature as a result of decomposition and significantly decreased electrical resistivity of the iron phosphate at elevated temperatures. Consequently, it is essential to fabricate SMCs with both high flux density and low loss. In the present work, new insulation coating methods have been used to grow ferromagnetic iron oxide layers as the insulation coating for the iron powders, which avoids the effect of magnetic dilution. Moreover, the high electrical resistivity and thermostability of iron oxide layers effectively reduces the core loss. Main results of the work are as follows:A series of SMCs containing iron oxide coatings have been prepared by bluing technique (including acidic bluing and alkaline bluing). The reaction principle involves oxidation of Fe with H2O into Fe3O4 and partial ?-Fe2O3. The ferromagnetic Fe3O4 improves the magnetic flux density of the SMCs, and the core loss can be reduced due to the high electrical resistivity and thermostability of Fe3O4 and ?-Fe2O3. With acidic bluing coating, SMCs with optimized effective permeability of 67.7 and core loss of 785.1 mW/cm3 (50 mT,100 kHz) are obtained, while alkaline bluing gives rise to permeability of 97.7 and core loss of 771.3 mW/cm3 (50 mT,100 kHz).Surface oxidation of the Fe powders has been achieved via reaction with H2O and O2 at elevated temperatures. Mechanism of the oxidation process has been investigated where coating layer consisting of Fe3O4 and/or y-Fe2O3 grows depending on the oxidation temperature and time. The Fe3O4 coating partially converts into y-Fe2O3 with increased oxidation temperature until 250?. The thickness of y-Fe2O3 layer increases with increased oxidation time. The existence of ferromagnetic Fe3O4 and y-Fe2O3 gives rise to larger magnetic flux density of the surface oxidized iron powders (214.1 emu/g) than that of the phosphate-coated sample (193.5 emu/g). Excellent effective permeability of 89.5 and significantly reduced core loss of 688.7 mW/cm3 is obtained for the SMC made of Fe powders oxidized at 250? for 30 min.Hydrothermal method has been used to produce Fe3O4 insulation coating in the absence of O2. Evolution of the Fe3O4/silicone resin matrix during the annealing process has been investigated to provide in-sight information on choosing appropriate annealing conditions. The silicone resin starts to decompose at around 382?, leaving reducing groups which then convert the Fe3O4 coating into Fe with increased annealing temperature to 466?. Further raised annealing temperature to 570? leads to the reduction of Fe3O4 into FeO. Such evolution is correlated to changes in the magnetic performance of the SMCs after annealing. Effective permeability increases with increasing annealing temperature, and when the annealing temperature is further increased to 550?, the permeability becomes unstable against frequency. Moreover, core loss increases slightly when annealing temperature increases from 450? to 500?. The overall core loss then increases significantly due to the deterioration of the insulation matrix after annealing at 550?.