Fabrication of Fe-4.5 wt% Si core-shell soft magnetic composite (SMC) via milling assisted pressureless sintering method for high-frequency application

Abstract

Fe-4.5Si-based soft magnetic composite (SMC) cores suitable for high-frequency applications were studied using ball milling and furnace sintering. Through ball milling, Fe-4.5Si powder was coated with Mg(OH)2 nanoparticles to form a uniform insulation layer, introducing a novel method to optimize the furnace sintering process by adding H3BO3 and SiO2 to the insulation layer. Mg(OH)2 nanoparticles effectively coated on Fe-4.5Si powder through ball milling formed MgO and Mg2SiO4 double insulation layers after heat treatment, which provided stable insulation layer and minimized core loss in high-frequency regions up to 1 MHz. Furthermore, applying a 7% MgO–B2O3–SiO2 insulation for sintering at 1100 °C resulted in an increase in permeability to 68, starting from its initial value of 28 at 100 kHz, while porosity decreased from 18.1% to 9.3%, minimizing the degradation of the insulation layer. After sintering, phase analysis of the MgO–B2O3–SiO2 insulation revealed the presence of Mg2B2O5, MgSiO3, and a liquid phase, indicating densification of the cores owing to the diffusion reaction and liquid phase sintering among MgO–B2O3–SiO2. Therefore, the ball-milling and furnace sintering processes shown in this study provide an effective solution for increasing density of Fe-4.5Si powder-based SMC cores to develop magnetic cores suitable for high-frequency applications.