Effects of axial pressure on the evolution of core–shell heterogeneous structures and magnetic properties of Fe–Si soft magnetic powder cores during hot-press sintering

Abstract

Silicon dioxide (SiO₂) has attracted much attention as an ideal coating material for iron (Fe)-based soft magnetic powder cores (SMPCs). However, maintaining the integrity and uniformity of Fe-based/SiO₂ core–shell heterostructures is still a challenge. The evolution mechanism of core–shell heterostructures determines the performance of Fe-based SMPCs. Herein, the evolution of the core–shell structures and heterogeneous interfaces of Fe–Si@SiO₂ SMPCs with axial pressure and the influence of the evolution on the SMPCs performance were investigated. The results show that in the axial pressure range of 10–15 kN, the core–shell heterostructures were gradually integrated, whereas the SiO₂ insulation coatings underwent an amorphous-to-crystalline transformation. At axial pressure above 16 kN, the Fe–Si powder melted partially, and the core–shell heterostructure collapsed due to overheating, caused by the gradient temperature field during the hot-press sintering. When the core–shell heterostructure was intact, the Fe–Si@SiO₂ SMPCs showed a permeability of over 38 with a wide and stable frequency range of 100–300 kHz, a saturation magnetisation of 231.7 emu g⁻¹, resistivity of 0.8 mΩ cm and total loss of 704.7 kW m⁻³ at 10 mT and 100 kHz. When the core–shell heterostructure was destroyed, the resistivity dropped dramatically and the loss increased to 765.0 and 897.4 kW m⁻³. These results show the relationship between the core–shell heterostructure of Fe–Si@SiO₂ SMPCs, axial pressure and magnetic properties, which would be vital in achieving high power density, high efficiency and miniaturisation in SMPCs.