Simultaneous improvements of effective magnetic permeability, core losses and temperature characteristics of Fe-Si soft magnetic composites induced by annealing treatment

In this paper, silicone-resin coating Fe-Si soft magnetic composites (SMCs) with high magnetic permeability, low core losses and good temperature/frequency stability were successfully fabricated. Microstructural characterizations suggest that the thin, continuous and heat-resistant silicone-resin coatings are constructed surrounding magnetic particles. The effective permeability of SMCs is significantly improved to 139.7 and 153.6 when the stress relief annealing temperatures increase to 700 °C and 800 °C, and meanwhile their core losses are obviously reduced to 657 mW/cm³ and 632 mW/cm³ (0.1 T/50 kHz), respectively. Loss-separation analyses confirm that hysteresis losses predominate in the total core losses within quite wide frequency ranges that depend on annealing temperatures, and higher temperature annealing can more effectively decrease hysteresis losses due to larger residual stress relaxation, which contributes to the simultaneous improvements of permeability and core losses. However, for the SMCs annealed at 800 °C, due to partially decomposed silicone-resin coatings, the strong eddy-current effects at high frequency region will cause sharply increased core losses and slightly decreased frequency stability of permeability. Noticeably, the temperature-dependent behaviors of core losses and permeability of Fe-Si SMCs can be also effectively tuned by optimizing annealing treatment.