Original Research Paper
Core-shell structured Mn-Zn-Fe ferrite/Fe-Si-Cr particles for magnetic composite cores with low loss

https://doi.org/10.1016/j.apt.2018.03.012Get rights and content

Highlights

  • Mn-Zn-Fe ferrite/Fe-Si-Cr core-shell powders were prepared and pressed into cores.

  • Ferrimagnetic Mn-Zn-Fe ferrite layer electrically insulated Fe-Si-Cr cores.

  • Compacted cores exhibited a high saturation flux density.

  • Compacted cores exhibited higher µ′ in the MHz region than that of Fe-Si-Cr cores.

  • Mn-Zn-Fe ferrite layer reduced eddy-current loss of the compacted cores.

Abstract

A Mn-Zn-Fe ferrite layer, several hundred nanometers thick, was deposited on the surface of Fe-3.5Si-4.5Cr (mass%) powder particles as an insulating material by an ultrasonic enhanced ferrite plating method. The compositions of the Mn-Zn-Fe ferrite layer were Mn0.18Zn0.27Fe2.55O4, Mn0.38Zn0.25Fe2.37O4, and Mn0.54Zn0.24Fe2.22O4. The core loss (Pcv) performances of the compacted cores and magnetic properties of the core-shell structured powders were evaluated. All the ferrite-coated cores exhibited a saturation flux density (Bs) in the range of 1.54–1.56 T derived from their soft magnetic metal and ferrite composition. All ferrite-coated cores annealed at 773 K exhibited a constant permeability µ′ in the frequency range up to 50 MHz owing to the insulating effect of the ferrite layer, and the Mn0.54Zn0.24Fe2.22O4 ferrite-coated core exhibited the highest real permeability µ′ of 56 at 50 MHz. The core loss of the Mn-Zn-Fe ferrite-coated Fe-3.5Si-4.5Cr cores was 604–738 kW/m3 at 100 kHz and 50 mT, which was much smaller than that obtained for the Fe-3.5Si-4.5Cr core without a ferrite layer (3617 kW/m3). The eddy-current loss (Pe) of the Mn-Zn-Fe ferrite-coated Fe-3.5Si-4.5Cr cores considerably decreased compared with those of the non-coated Fe-3.5Si-4.5Cr core owing to the insulating properties of the ferrite layer.

Introduction

Soft magnetic materials with a high saturation flux density (Bs) and low total core loss (Pcv) are required in devices such as coils, motors, switching devices, and transformers. Their magnetic properties require improvements in the high frequency region to meet demands for device downsizing and effective electromagnetic energy transfer [1]. Soft magnetic composites (SMCs) consisting of magnetic metal powders coated with electrically insulating materials are a particularly attractive class of materials expected to meet these requirements [1], [2], [3]. Powders can be easily shaped into complex structures by press shaping processes [4], [5], [6]. The electrical resistivity of soft magnetic composites is higher than that of conventional magnetic steel sheets. To improve the energy transfer efficiency at high frequencies (over 1 kHz), insulating materials have an important role in soft magnetic composites [1].

The use of insulating materials can reduce eddy-current loss in the high frequency region [7]. However, insulating layers such as resin generally do not have a magnetic moment and so contribute to a decrease in magnetization and permeability per unit volume [8], [9], [10]. Furthermore, cores are typically annealed at high temperature to reduce their hysteresis loss (Ph) derived from particle size, purity of the metal powder, internal stress of the cores, distortion, and grain boundaries [11], [12]. Insulating layers, such as resin, are damaged by these annealing processes, which leads to an increase in hysteresis loss and eddy-current loss. Therefore, many efforts have been made to improve the magnetic properties of powder cores using more appropriate insulating materials such as epoxy [12], phosphate [13], SiO2 [14], [15], ZrO2 [16], and Al2O3 [9].

In this work, we focused on ferrite materials for the insulating layer [17]. Ferrite is widely used in coils, motors, and transformers owing to its soft magnetic properties and high electrical resistivity. The surfaces of metal powder can be covered with a ferrite layer with the use of an ultrasonic enhanced ferrite plating method [18], [19]. This technique can be used to deposit ferrite layers on the surface of various types of particle, including metals [17], [20], [21], [22], silica [23], [24], and polymer powders [25]. The ultrasonic enhanced ferrite plating method uses sonochemical techniques and ferrite formation is accelerated by ultrasonic irradiation. We previously reported the Ni-Zn-Fe ferrite-coated cores for high frequency (over 100 kHz) applications, and Ni-Zn-Fe ferrite was used as high electrical resistive insulator [17]. Mn-Zn-Fe ferrite was used for the insulating layer in this work since Mn-Zn-Fe ferrite possesses a high real permeability µ′, and a low core loss in the kHz to MHz region, which are suitable properties for the application in such frequency range [26], [27], [28]. It is the reason why we developed the Mn-Zn-Fe ferrite-coated cores in this study. We deposited Mn-Zn-Fe ferrite layers with three different compositions on Fe-3.5Si-4.5Cr (mass%) metal powder to create core-shell structured powders. We evaluated the magnetic properties of the samples through structural and chemical analysis, and measured the core loss performances of the compacted cores up to 300 kHz.

Section snippets

Materials and methods

Fe-3.5Si-4.5Cr (mass%) powder (Purity >99.5%, Nippon Atomized Powders Corporation, Japan) with an average particle size of 10 µm was used in this experiment. Fe-3.5Si-4.5Cr powder possesses a high saturation flux density (approximately 1.7 T) and good chemical stability against rust and corrosion owing to its chromium component [29]. This powder was dispersed in a buffer solution containing CH3COOK (Purity >95.0%, Wako Pure Chemical Industries, Ltd, Japan) maintained at 343 K and pH 10 under

Characterization of the ferrite-coated Fe-3.5Si-4.5Cr powder

Table 1 shows the chemical composition of synthesized Mn-Zn-Fe ferrite measured by ICP-OES. The chemical compositions of the synthesized Mn-Zn-Fe ferrites were slightly different from the prepared composition and the Fe content was higher than that of other metallic components in all samples. That was because iron ferrite (magnetite, Fe3O4) is more easily synthesized than another ferrites, such as Mn-Zn-Fe or Ni-Zn-Fe ferrites, by the ultrasonic enhanced ferrite plating method. The molar ratios

Conclusions

In this work, Mn-Zn-Fe ferrite layers with three different compositions were successfully deposited over the whole surface of Fe-3.5Si-4.5Cr powder particles by an ultrasonic enhanced ferrite plating method. Composite cores were fabricated by compaction to produce a novel type of soft magnetic composite with a high saturation flux density, high permeability, and low core loss. The ferrite layer separated the metal powders electrically, and operated as an effective insulating material. The

Acknowledgments

This work was financially supported by the NGK spark plug Co., Ltd. foundation. We thank Mr. Shinya Awano for analyzing the XRD patterns.

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