Magnetic properties of Fe soft magnetic composites with a double-insulating layer comprising Fe3O4 and silicone resin
Introduction
Soft magnetic composites (SMCs), which comprise soft magnetic metal powder and insulators, have attracted considerable attention from researchers because their magnetic properties determine the energy efficiency of various important components such as stators of electric vehicle motors, inductors for controlling magnetic circuits, and iron cores of transformers [1], [2], [3], [4]. The core loss of SMCs is one of the most important magnetic properties for achieving energy savings. A reduction in the core loss results in an enhancement of the energy efficiency of the various components.
Core loss consists of hysteresis and eddy current losses [2], [3]. Hysteresis loss mainly depends on the magnetic properties of the soft magnetic metal powder [5], [6], [7]. This implies that the hysteresis loss can be lowered by removing as many impurities and inclusions in the metals as possible [2], [7]. Eddy current loss is strongly dependent on the electrical insulation between the metal particles. The introduction of an annealing step is unavoidable to remove the strains generated inside the metal powders while compacting them to make the SMC core [8], [9]. Thus, electrical insulation must be maintained up to the annealing temperature [8], [9], [10], [11]. Insulation is achieved by coating organic [12], [13] or/and inorganic [10], [14] materials on the metal powders. Instead of low-heat-resistance organic materials [15], inorganic materials, such as phosphate [16], [17], Al2O3 [10], SiO2 [18], and MgO [19] are adopted to provide high heat resistance to coated insulators. However, the complexity of wet coating processes and the weak adhesion of the inorganic coating to the metal powder should be considered [20]. In addition, it is difficult to accurately control the thickness of coatings using the coating techniques [19], [20]. Therefore, there are few studies on the influence of the insulation thickness on the magnetic properties of SMCs.
A previous study reported that an Fe3O4 insulating layer was formed on the surface of Fe powder by high-temperature oxidation in a closed reaction system [21]. The thickness of the Fe3O4 layer can be controlled by changing the amount of oxygen gas supplied to the closed system. The effect of Fe3O4 thickness on the magnetic properties of the Fe SMC core was not investigated. In addition, Fe powder were oxidized in a stacked state in the previous study [21]. The surface of the stacked powder bed preferentially reacted with oxygen. This implies that it is difficult to increase the bed height and oxidize a large amount of Fe powder. In this study, a rotating closed reaction system was proposed for the high-temperature oxidation of a large amount of Fe powder. The rotation of the reactor caused each Fe powder to react uniformly with oxygen gas. Utilizing the closed system, the thickness of the Fe3O4 insulating layer can be changed by controlling the amount of oxygen gas supplied to the rotating closed reaction system. The effect of the Fe3O4 thickness on the magnetic properties of the SMC cores was investigated. It was found that the core loss decreased with the increasing thickness of Fe3O4 insulating layer, but most of the core loss comprised eddy current loss. This is because of the low electrical resistivity of Fe3O4, 10−2 Ω∙cm [22]. To provide sufficient insulation resistance, a silicone resin with a high specific resistivity (>1013 Ω∙cm) was coated on the Fe3O4 insulating layer to form a double-insulating structure. The magnetic properties of Fe SMC cores with this double-insulating layer were investigated.
Section snippets
Formation of the insulation layer
A rotating closed oxidation system was used to coat an oxide layer on the Fe powder (Fig. 1(a)). The rotation of the inclined Inconel chamber efficiently mixed the Fe powder homogeneously, thus increasing the contact area between the Fe powder and oxygen gas. This system could be used to treat a large amount of Fe powder (ABC 100.30, Höganäs). The average particle size of Fe powder is 100 µm (Fig. 1(b)). The inclination angle was 10° and the rotation speed was 5 rpm. As an anti-sintering agent,
Formation of the oxide layer
Fig. 2 shows the change in the oxygen content of the Fe powder depending on the amount of oxygen gas supplied. The solid circle in Fig. 2 indicates the oxygen content in the Fe powder before the oxidation reaction, which is 623 ppm. On adding MgO, 827 ppm of oxygen was released. The increase in the oxygen content of the Fe powder per 1 cc/g of supplied oxygen gas was calculated to be 1350 ppm. The dashed line in Fig. 2 shows the calculated oxygen content for the three oxygen sources. The
Conclusion
A rotating closed oxidation system was developed to form a Fe3O4 oxide layer on the surface of a large amount of Fe powder. Measurement of oxygen contents in the oxidized Fe powder showed that the entire oxygen gas supplied to the closed system was consumed to oxidize the Fe powder. The thickness of Fe3O4 oxide layer was controlled within the range of 30–110 nm with oxygen volume-to-Fe powder mass ratios of 1–4 cc/g. The core loss decreases from 400.9 W/kg to 185.5 W/kg as the thickness of the
CRediT authorship contribution statement
J.Y. Byun: Conceptualization, Methodology, Validation, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition, K.D. Choi: Conceptualization, Methodology, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing Preparation, Visualization, S.Y. Lee: Formal analysis, Investigation, Data Curation, J.S. Hwang: Formal analysis, Writing - Original Draft, Writing - Review & Editing Preparation, S. Yang: Formal
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This study was supported financially by the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (No. 2020M3H4A3105643).
References (25)
- et al.
Soft magnetic composite materials (SMCs
), J. Mater. Process. Technol.
(2007) - et al.
Soft magnetic composites: recent advancements in the technology
Met. Powder Rep.
(2017) - et al.
Evolution of phosphate coatings during high-temperature annealing and its influence on the Fe and FeSiAl soft magnetic composites
J. Alloy. Compd.
(2015) - et al.
Enveloping Fe–12% Al atomized powders in selectively oxidized insulating films for soft magnetic composite (SMC) cores
J. Alloy. Compd.
(2021) - et al.
Effect of heat treatment on magnetic properties of iron-based soft magnetic composites with Al2O3 insulation coating produced by sol–gel method
J. Alloy. Compd.
(2013) - et al.
Effect of annealing on magnetic properties of Fe/Fe3O4 soft magnetic composites prepared by in-situ oxidation and hydrogen reduction methods
J. Alloy. Compd.
(2019) - et al.
Annealing effects on magnetic properties of silicone-coated iron-based soft magnetic composites
J. Magn. Magn. Mater.
(2012) - et al.
Magnetic and structural properties of iron phosphate–phenolic soft magnetic composites
J. Magn. Magn. Mater.
(2009) - et al.
Phosphoric acid addition effect on the microstructure and magnetic properties of iron-based soft magnetic composites
J. Magn. Magn. Mater.
(2018) - et al.
Properties of FeSiAl-based soft magnetic composites with AlN/Al2O3 and hybrid phosphate–silane insulation coatings
J. Alloy. Compd.
(2018)
Effect of annealing on magnetic properties of iron-based soft magnetic composites with iron oxide insulator
J. Magn. Magn. Mater.
Electronic conductivity and transition point of magnetite (‘Fe3O4′)
Physica
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