Augmenting magnetic properties: Ectopic reconstruction of insulation layer in soft magnetic composites via coating-doping method

The doping of carbonyl iron powders (CIPs) with small particle sizes and high ferromagnetism can effectively enhance the magnetic conductivity of soft magnetic composites (SMCs). However, the poor fluidity and the direct contact between CIPs will increase the effective eddy current size, resulting in increased core loss. To address the negative effects, the ectopic reconstruction of CaCO3 insulation layer in CIPs was realized by heat treatment. A composite powder of Fe–Si–Cr matrix/CaCO3 insulation layer + CIPs was fabricated, exhibiting a unique core–shell heterostructure. Furthermore, the effect of the doping amount of CIPs on the permeability optimisation and core-loss reduction in the SMCs was investigated. Findings demonstrated that simultaneous execution of insulation coating and CIPs doping can promote the ectopic reconstruction and nucleation of the CaCO3 insulation layer on CIPs surfaces during heat treatment. This coating-doping method is expected to prevent the direct contact between different magnetic powder particles, potentially mitigating the magnetic dilution effect caused by the non-magnetic phase. The SMCs exhibited best soft magnetic performance at 15 wt% doping level, where the saturation magnetisation peaked at 185.4 emu/g, permeability increased to 38.6 and the core loss was minimised to 466.0 kW/m3 (at 30 mT and 200 kHz). Compared with conventional doping techniques, the proposed method enables CIPs doping within the insulation layer and enhances the intergranular insulation between the Fe–Si–Cr soft magnetic powders and CIPs, improving the SMCs performance.