Effects Of SiO₂-based Insulation Coating Process On The Soft Magnetic Properties Of FeSiCr Powder Cores

With the rapid development of electronic communication industry,the molding inductor has become a hot research topic in recent years,which can meet the requirements of miniaturization,integration,multi-function and high power.FeSiCr alloy powder is one of the ideal materials for molding inductor due to its excellent saturation magnetization,DC-bias and mechanical properties,but low resistivity limits the application of soft magnetic composites(SMCs)at high frequencies.Since the insulation coating processes of magnetic powder cores are also applicable to molding inductors,FeSiCr alloy powders was used as the research object in this paper,and SiO₂ was used as coating material because of its high resistivity.FeSiCr SMCs were prepared by physical doping and sol-gel dumping method,respectively.The soft magnetic comprehensive properties were optimized by adjusting the insulation coating parameters.Besides,the filling and growth mechanism of SiO₂ during insulation coating process were also investigated,which are expected to be used in molding inductors.The magnetic powder cores prepared by 0.5 wt.%hydrophobic SiO₂ composite organic resin had better soft magnetic performance after annealing.The effective permeability and core loss of FeSiCr SMCs were 33.06 and 623.53 k W/m³ at 0.05 T and 100 k Hz,respectively;the quality factor was 60.0 at 1 MHz.The insulation coating of FeSiCr magnetic powders with coupling-modified hydrophilic SiO₂ composite organic resin was studied,with increase of SiO₂addition,the density,magnetization saturation and permeability of FeSiCr SMCs decreased,but specific resistance,quality factor and DC-bias property increased.Additionally,thermal stability of insulation layer was also improved.In the insulation coating process,hydrophobic SiO₂ is preferentially distributed in the interstices of magnetic powder particles,and hydrophilic SiO₂ modified by KH-550 can be uniformly distributed on the surface of magnetic powders,and the contribution of the latter to the coating effect is better than the former.cores are suitable for higher frequency applications.Therefore,FeSiCr SMCs prepared by coupling modified hydrophilic SiO₂ composite organic resin coating process are suitable for higher frequency applications.By using Sol-gel preparation of SiO₂ for magnetic powders insulation coating,the growth process of SiO₂ insulating layer can be tuned by controlling the ammonia concentration.With the increase of ammonia concentration,the saturation magnetization and effective permeability gradually decreased,while the specific resistance,quality factor and DC-bias property are increased.Within the reaction time of 0.5 h,SiO₂ grew with heterogeneous nucleation when the ammonia concentration was 0.08?0.16 ml/g,and SiO₂ coating layer was relatively molding and uniform.With a high ammonia concentration of 0.20?0.24 ml/g,SiO₂ grew in both heterogeneous and homogeneous nucleation,and SiO₂ particles gather and grow..With ammonia concentration of 0.12 ml/g,the thickness of SiO₂ insulating layer was about 30?60nm,and the annealed SMCs had good soft magnetic comprehensive performance,and the effective permeability and core loss were 45.52 and 418.32 k W/m³ at 0.05 T and 100 k Hz,respectively;the quality factor was 76.5 at 1 MHz.Based on this result,the thickness of SiO₂insulation layer increases by lengthening the reaction time,the thickness of SiO₂ insulation layer was about 135 nm when the reaction time was 3.5 h.FeSiCr SMCs had better soft magnetic comprehensive properties when the reaction time was 2.5 h,the effective permeability and core loss were 37.79 and 484.57 k W/m³,respectively;the quality factor was 84 at 1 MHz.On the basis of 0.12 ml/g ammonia concentration as catalyst,the pre-addition of hydrochloric acid can eliminate the steric hindrance effect and accelerate the TEOS hydrolysis rate.Adding a small amount of hydrochloric acid can increase the coating layer thickness while ensuring the uniformity,but the excessive amount of hydrochloric acid will lead to serious agglomeration of SiO₂ particles,and the reaction between hydrochloric acid and Fe element will lead to the deterioration of magnetic properties.With the increase of hydrochloric acid addition,the saturation magnetization,density and effective permeability of FeSiCr SMCs decreased,while the specific resistance,core loss and DC-bias properties increased.The FeSiCr SMCs prepared by adding 0.5 ml hydrochloric acid had good soft magnetic properties after annealing,and the effective permeability and loss were 36.89 and 500.72 k W/m³ at 0.05 T and100 k Hz,respectively;the quality factor was 78.0 at 1 MHz.The results also showed that annealing treatment can effectively released internal stress of SMCs,which lead to the effective permeability increase,and the coercivity and hysteresis loss reduce.The hysteresis loss is the main core loss in the frequency range of 20?200 k Hz for the water atomized FeSiCr alloy powder system used in this paper.