Synthesis of well-insulated Fe–Si–Al soft magnetic composites via a silane-assisted organic/inorganic composite coating route

https://doi.org/10.1016/j.jpcs.2020.109841Get rights and content

Highlights

  • Organic/inorganic insulation route for preparing Fe–Si–Al soft magnetic composites.

  • Possible mechanisms proposed regarding chemical reaction in interface area.

  • Effective permeability stable at high frequencies by optimizing insulation content.

Abstract

In this study, an organic/inorganic composite coating method based on silane, silicon oil, and tetraethyl orthosilicate was developed to prepare Fe–Si–Al soft magnetic composites (SMCs). The insulation reaction processes may have involved the hydroxylation of magnetic powder and condensation of silanol groups. Analyses of the microstructure and high frequency stability of the effective permeability of the SMCs demonstrated the formation of a high-quality silicon oxide coating layer on the surfaces of the Fe–Si–Al particles. The effects of the insulation content on the density, resistivity, and magnetic properties were studied for the well-insulated Fe–Si–Al SMCs, and the underlying physical mechanisms were clarified. The results showed that increasing the insulation content reduced the effective permeability and the eddy current loss, but increased the DC-bias performance and hysteresis loss. The enhancement of the effective demagnetizing field indicated by the decrease in the density of the SMCs may have been responsible for the enhanced hysteresis. In addition, the increase in the resistivity of the SMCs may have contributed to the decrease in the eddy current.

Introduction

Compared with traditional laminated steels, soft magnetic composites (SMCs) possess the following advantages: good anti-saturation capability, low core loss at medium and high frequencies, high saturation magnetic induction, and three-dimensional isotropic ferromagnetic behavior, thereby allowing their widespread applications in power supply devices [[1], [2], [3], [4]]. At present, power transmission and distribution systems work at high frequencies and high power levels, which require much better magnetic performance from SMCs. Many studies have aimed to develop techniques to meet these requirements [5,6]. Improving the quality of the insulation layer on SMCs is regarded as an effective technique because an insulation layer comprising high-resistivity materials will keep the magnetic particles isolated from each other [7], thereby breaking both the magnetic circuit and eddy current path to enhance the effective demagnetizing field and reduce the eddy current within the SMCs. The frequency stability of the effective permeability, DC-bias performance, and core loss at medium or high frequencies would also be improved for SMCs.

It should be noted that the organic/inorganic composite coating technique has been applied to improve the quality of the insulation layer because it may overcome the problems due to the poor thermal stability of the organic coating [[8], [9], [10], [11], [12]] and the characteristic easy peeling of the inorganic coating [13,14]. Luo et al. combined epoxy-modified silicone resin with SiO2 and Fe3O4 as coatings for Fe SMCs to achieve high permeability and low loss [15]. However, the interface incompatibility of the organic/inorganic insulating substances and the nonuniform distribution of the insulating oxide nanoparticles remain difficult problems that need to be addressed. These problems may be solved by using a coupling agent such as silane and silicon oxide, which can be produced in situ with the modified Stöber method in a low cost and controllable manner [[16], [17], [18], [19]]. Taghvaei et al. investigated the effect of a silane coupling agent (3-aminopropyltriethoxy silane) on the magnetic properties of iron-phenolic SMCs at operating frequencies between 50 Hz and 1000 kHz, and found that adding the silane coupling agent greatly decreased the loss factor and imaginary part of the permeability [17]. Zhao et al. prepared sub-micrometer laminated Fe/SiO2 SMCs via the controlled hydrolysis of tetraethyl orthosilicate (TEOS), and found that the silica coating effectively reduced the high frequency eddy current loss and raised the maximum operating frequency to about 50 MHz [18]. Despite these positive findings, few studies have reported the development of high quality insulation using the modified Stöber method, which can be indicated by the stable permeability of SMCs at high frequencies. Thus, there is an urgent need to systematically study SMCs insulated with a silane coupling agent and silicon oxide using the modified Stöber method, and to understand the underlying physical mechanisms responsible for the relationships between the microstructure and the magnetic and electrical properties.

In the present study, water-soluble silicon oil and TEOS were used to insulate Fe–Si–Al powder with the aid of a silane coupling agent. The water-soluble silicon oil was employed as an organic coating due to its abundance of hydroxyl groups and the production of silicon oxide after high temperature decomposition. TEOS was used to form an inorganic silicon oxide coating with the modified Stöber method. Silane was specifically selected as a coupling agent to increase the amount of hydroxyl groups and to provide more active sites for TEOS hydrolysis. The silicon oxide content within the insulation layer was controlled by precisely adjusting the amount of TEOS, which eventually affected the quality of the insulation layer and the density of the SMCs. The corresponding variations were analyzed in the magnetic properties, density, and resistivity of the prepared Fe–Si–Al SMCs. Loss separation was also performed to acquire a deeper understanding of the physical mechanisms responsible for the magnetic properties of the SMCs.

Section snippets

Raw materials

Atomized spherical Fe–Si–Al (85 wt% Fe-9.6 wt% Si-5.4 wt% Al) powder, water-soluble silicone oil (204A), TEOS, silane coupling agent (KH550), ammonium hydroxide, and absolute ethanol were purchased from commercial suppliers. Deionized water was prepared in the laboratory.

Insulation of Fe–Si–Al powders

Fe–Si–Al powder was poured into ethanol solutions of 204A and KH550. The mixtures were mechanically stirred at room temperature for 20 min, before drying at 80 °C for 1 h so the Fe–Si–Al powder was coated with a modified

Morphology and microstructure

Fig. 1(a) shows an SEM image illustrating the overall morphology of the raw Fe–Si–Al powder. Spherical particles were observed with sizes ranging from several to tens of microns. Fig. 1(b) and (c) compare the morphology of the raw and insulated Fe–Si–Al particles. Clearly, the scaly surface on the Fe–Si–Al particle was covered by a continuous fluffy layer. The insulation layer coating may have comprised hydrogen-bonded silicone oil and silicon oxide, as shown in Fig. 1(d).

The possible reaction

Conclusions

In this study, a silane-assisted organic/inorganic composite insulating method was developed to prepare Fe–Si–Al/silicon oxide SMCs, and the insulation reaction mechanism was investigated. The interface compatibility of organic and inorganic insulating substances as well as the uniformity of the insulating oxide nanoparticles improved significantly. High-quality silicon oxide insulation coating layers were obtained on the surfaces of the Fe–Si–Al particles in all of the samples and the stable

CRediT authorship contribution statement

W.W. Guan: Formal analysis, Investigation, Writing - original draft. X.Y. Shi: Formal analysis, Investigation. T.T. Xu: Formal analysis. K. Wan: Investigation. B.W. Zhang: Formal analysis. W. Liu: Conceptualization, Writing - review & editing, Funding acquisition. H.L. Su: Conceptualization, Resources, Project administration, Funding acquisition. Z.Q. Zou: Resources, Funding acquisition. Y.W. Du: Resources, Funding acquisition.

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 by the Fundamental Research Funds for the Central Universities (No. JZ2020HGQB0219), Huaian Transformation Project of Sci-tech Achievement (No. HA201907), Grant Project of Shenzhen Microgate Technology Co. Ltd (2017–2020), and Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University (2018–2019).

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