Fabrication And Magnetic Properties Of Amorphous/Inorganic Oxide Soft Composite Powder Cores

Soft magnetic composites（SMCs）,which are widely used in electromagnetic applications such as inductors,transformers and electric motors,have attracted considerable attention owing to their excellent soft magnetic properties.Despite their rapid development inspired by the continuously increasing industrial needs,the inevitable energy losses,particularly the eddy current loss,at high frequencies hinder their applications.Therefore,in order to mitigate the energy loss associated with eddy currents,the key to tackle such issue is to fabricate compact and uniform insulation coating layers with appropriate thickness,high electrical resistivity on the surface of magnetic powders.Organic coatings,most of which,including epoxy,phenolic and silicon resins,are physical coating process and cause the uniformity in coating are poor.Besides,organic coatings normally exhibit low thermal stability,may decompose or even burn during annealing.In this case,the annealing temperature is relatively low and obstructs the complete stress-relieve during compaction,which leads to significantly reduced magnetic properties.Phosphate coatings were produced using phosphoric acid directly reacting with the alloy elements of the magnetic powders,which has been widely used owing to the simple preparation procedure and high electrical resistivity.However,when the annealing temperature exceeded 500°C,the electrical resistivity of the core was significantly reduced owning to the crystallization or decomposition of the phosphate coating layer.Against this backdrop,inorganic oxides with high electrical resistivity and thermal stability be used as insulation coating layer in this study.The water atomization FeSiBCCr amorphous powders has been selected to prepare the amorphous magnetic powder cores（AMPCs）.Novel TiO₂ coating layer has been prepared by sol-gel method and adjust coating process parameters to optimize the soft magnetic properties of the AMPCs.Besides,the influences of solvent concentration and annealing temperature on the magnetic properties of the AMPCs were also investigated.The main results of the work are as follow:（1）The core-shell structured FeSiBCCr/TiO₂ amorphous magnetic powder cores（AMPCs）with excellent comprehensive magnetic properties were successfully prepared using spherical amorphous powders with high quality insulating layer.The performances of the AMPCs can be well tuned by changing the thickness of the insulation coating layer,which can be adjusted by controlling the concentration of tetrabutyl titanate（TBOT）in the hydrolysis and condensation reaction system.The surface of the powders turned from initially smooth into rough upon the increases in the TBOT concentration,from 0.05 to 0.3 ml/g.The AMPCs with TBOT concentrations of 0.15 ml/g exhibited excellent comprehensive soft magnetic properties,including the highest quality factor of 104.5 at 560 kHz and lowest core loss of 265 mW/cm³ at f=100 kHz and B_m=0.05 T.It also maintained a relatively high permeability of 67 up to 5 MHz and superior DC-bias permeability of 68%at a bias field of100 Oe.This study demonstrated a modified method to develop a novel insulation coating layer to reduce the core loss and improve the magnetic properties of AMPCs.（2）The magnetic performance of AMPCs can be further improved by reduce the appearance of non-magnetic aggregates and regulating the annealing temperature.The growth process of TiO₂ coating layer on the powder surface is directly affected by the water concentration in the reaction system.When the concentration of H₂O in the range of 0.01 to0.02 ml/g,the coating layer grew with heterogeneous nucleation,and the coating layer was relatively smooth and uniform.With further increases the concentration of H₂O to 0.03 ml/g,the growth of the coating layer maintained at the phase of heterogeneous nucleation and homogeneous nucleation,the excessive titanium oligomers in the system would make the coating layer grow rapidly,and the thickness of the layer will increase continuously.The results show that upon annealing at the optimum temperature of 480°C,the AMPCs with H₂O concentration of 0.02 ml/g exhibit high permeability of 81.5 with a high frequency stability up to 10 MHz,high quality factor of 102 at 530 kHz,low core loss of 42 mW/cm³ at500 kHz for B_m=0.01 T.