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

https://doi.org/10.1016/j.jallcom.2022.168255Get rights and content

Highlights

  • Rotating closed reaction system for homogeneously oxidizing Fe powders is proposed

  • The closed system can control the thickness of Fe3O4 oxide insulation layer and investigate effect of the Fe3O4 thickness on magnetic properties of cores.

  • Double-insulating layer (Fe3O4 and silicone resin) drastically reduced core loss.

Abstract

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.

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).

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