Fe-based amorphous soft magnetic composites with SiO2 insulation coatings: A study on coatings thickness, microstructure and magnetic properties

Core-shell structured FeSiBCCr@SiO2 amorphous soft magnetic composites (ASMCs) with tunable insulating layer thicknesses have been designed and fabricated by the powder metallurgy method. The growth process of the coatings obtained via the sol-gel method has been systematically studied based on the analysis of the growth mechanism and the insulating layer thicknesses calculated by a modified method. In the powder-coating process using small quantities of tetraethyl orthosilicate (TEOS) (0.025-0.1 mL/g), a relatively uniform insulating layer is formed on the surface of the powders. Increasing the TEOS concentration further (0.25-1 mL/g) greatly enhances the reaction kinetics of the hydrolysis and condensation reactions. Consequently, the SiO2 shells of the amorphous powder rapidly grow in thickness and also generate aggregates. The evolution of the thickness of the coatings under different growth conditions also corresponds to the variation of the magnetic performance of the ASMCs. The ASMCs with an appropriate SiO2 insulating layer exhibit stability at high frequencies and significantly lower the contribution of eddy current to the total loss. Conversely, a thick insulation layer results in a high fraction of the non-magnetic phase and consequently enhances the hysteresis loss. Thus, an enhanced magnetic performance of the FeSiBCCr ASMCs can be achieved by adjusting the TEOS concentration (0.025-0.1 mL/g) or the thickness of the SiO2 insulating layer.