Magnetic properties of Fe soft magnetic composites with a double-insulating layer comprising Fe₃O₄ and silicone resin

In this study, we suggest a rotating closed reaction system for oxidizing Fe powder at 500 ℃. This rotation leads to the homogeneous oxidation of large amounts of Fe powder. The analysis of the oxygen content in the oxidized Fe powder shows that all oxygen gas supplied to the closed system is consumed for the oxidation of Fe powder. This indicates that the thickness of the Fe₃O₄ layer formed by oxidation increases proportionally with the amount of oxygen gas supplied to the reactor. It was observed that the loss of the toroidal core decreased as the thickness of the Fe₃O₄ layer increased. The change in hysteresis loss was negligible compared with the change in eddy current loss. The eddy current loss decreased from 364.2 W/kg to 139 W/kg (@ 1 T, 400 Hz) as the Fe₃O₄ layer thickened from 30 to 110 nm. Because the Fe₃O₄ layer did not provide sufficient insulation, another approach was attempted to coat the silicone resin (>10¹³ Ω∙cm) with a specific resistivity approximately 10¹⁵ times higher than that of Fe₃O₄ on the Fe₃O₄ layer. To observe this double-layer structure, a thermally evaporated Ni film was coated on top of the double-insulating layer. The energy-dispersive X-ray spectroscopy analysis of a cross-sectional transmission electron microscopy image showed that the Ni film successfully protected the silicone resin layer from the ion-beam irradiated during FIB sampling, making it possible to observe the double-layer structure. When a ∼30 nm thick silicone resin layer was coated on the ∼30 nm thick Fe₃O₄ layer, the eddy current loss was abruptly reduced from 364.2 W/kg to 8.6 W/kg (@ 1 T, 400 Hz). It was found that the silicone resin layer with high electrical resistivity contributed significantly in reducing eddy current loss, and a resin layer several of tens nanometers thick was sufficient to insulate Fe powder from each other.