Mechanism and Effect of the Dilution Gas Flow Rate on Various Fe–Si/SiO2 Soft Magnetic Composites during Fluidised Bed Chemical Vapour Deposition

The effect of the dilution gas flow rate on inorganic oxide insulating layers can improve fluidised bed chemical vapour deposition (FBCVD) in Fe–Si/inorganic-oxide soft magnetic composites and obtain excellent magnetic properties. Herein, Fe–Si/SiO2 composite particles coated via FBCVD and deposited at a 125–350 mL/min Ar-dilution gas flow rate were prepared and sintered into soft magnetic composites. Results demonstrate that SiO2 deposited on the Fe–Si substrate particle surface changed from submicron SiO2 clusters (125 mL/min) to an incomplete SiO2 film, then to a complete SiO2 film, and finally to a porous SiO2 film as the Ar-dilution gas flow rate increased. SiO2 layers began to transform from the amorphous to the beta-cristobalite state with a hexagonal crystal structure between 1149.45 K and 1280.75 K. However, the SiO2 amorphous layers’ crystallisation did not affect the Fe–Si substrate particles’ crystal structure. With the increasing Ar-dilution gas flow rate, the saturation magnetisation of Fe–Si/SiO2 soft magnetic composites initially decreased and then increased. The electrical resistivity increased before 150 mL/min, followed by an increase between 150 and 250 mL/min and then decreased, whereas the total core loss exhibited the opposite trend. These results show that magnetic performance can be promoted by selecting a suitable dilution flow rate.