Trans. Nonferrous Met. Soc. China 30(2020) 3067-3077

Preparation and characterization of carbonyl iron soft magnetic composites with magnesioferrite insulating coating layer

Shi-geng LI^1,2, Ru-tie LIU¹, Xiang XIONG¹

1. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China;

2. Department of Materials and Chemistry Engineering, Pingxiang University, Pingxiang 337000, China

Received 24 October 2019; accepted 28 September 2020

Abstract: Spherical carbonyl iron (Fe) powders were coated with magnesioferrite (MgFe₂O₄) insulating coating layer and then mixed with epoxy-modified silicone resin (ESR). Soft magnetic composites (SMCs) were fabricated by compaction of the coated powders and annealing treatment. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS) revealed that the MgFe₂O₄ layer was coated on the surface of the iron powders. The magnetic properties of SMCs were determined using a vibrating sample magnetometer and an auto testing system for magnetic materials. The results showed that the SMCs prepared at 800 MPa and 550 °C exhibited a significant core loss of 167.5 W/kg at 100 kHz and 50 mT.

Key words: soft magnetic composites; in-situ oxidation preparation; magnesioferrite insulating coating layer; annealing treatment; magnetic properties

1 Introduction

Soft magnetic composites (SMCs) are widely applied in electrical reactors, transformers, filters, frequency modulation chokes and power supply owing to their excellent soft magnetic properties including high saturation magnetization, low coercivity and low core loss [1-3]. According to the application requirements, SMCs are designed and synthesized by smartly selecting metal magnetic particles and insulating coating materials [4,5]. The soft magnetic particles are generally pure Fe powders, Fe-Si alloys, Fe-Co alloys, Fe-Ni alloys, Fe-Si-Al alloys, Fe-Si-Ni alloys, Fe-Ni-Mo alloys, Fe-Si-B-P alloys and so on [6,7], each of which shows unique electromagnetic properties. Among these properties, it is critical to achieve  low core loss to improve energy efficiency for applications in the medium and high frequency fields. Core loss mainly consists of hysteresis loss and eddy current loss. The eddy current loss is a significant component of the core loss at high frequency and can be greatly reduced by increasing the electrical resistivity of the SMCs. Lots of types of insulating materials have been used as the coating layer around the magnetic powders for increasing the electrical resistivity of the SMCs [8].

Generally, insulating layers are classified into the organic or inorganic coating. Organic materials including phenol-formaldehyde resin [9] and epoxy silicon resin [10] have been used to increase the electrical resistivity and enhance the mechanical strength of the SMCs. Inorganic materials such as oxides (e.g. Al₂O₃, SiO₂ and MgO) [11-14] and phosphates [15-17] are non-ferromagnetic, which reduce the saturation magnetization and improve the coercivity of the SMCs. Nevertheless, ferrite has unique properties such as high electrical resistivity and low eddy current loss, which should be an appropriate insulating coating for the soft magnetic composites without any remarkable decrease in magnetic properties [18]. Furthermore, ferrite is commonly applied at higher and wider frequencies due to the low eddy current loss [19,20]. The spinel-type ferrite magnetic materials with  the general formula of M²⁺(Fe³⁺)₂O₄ (M²⁺=Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, etc) have become an important area in nanoscience and nanotechnology, because of their huge applications in various fields such as biomedicine, magnetic materials, electronic materials, and photocatalyst [21-23]. Among various spinel ferrites, magnesium ferrite  (MgFe₂O₄) has received great attention in the area of magnetic storage devices, microwave, and electronic devices because of its high magnetic permeability and high electrical resistance [24].

In this study, we have carefully fabricated uniform MgFe₂O₄ layer on the surface of Fe powder by in-situ oxidation. To the best of our knowledge, no studies have been reported on the use of MgFe₂O₄ as an insulating layer for carbonyl iron soft magnetic composites. The obtained coating powders were pressed and heat-treated to obtain the required SMCs. The characteristics of Fe/MgFe₂O₄ coated powders and microstructures of the SMCs after annealing treatment and magnetic properties were investigated. Moreover, the SMCs exhibited stable amplitude permeability and low core loss.

2 Experimental

2.1 Preparation of coated powders by in-situ oxidation method

The carbonyl iron powders with an average particle size of about 5.5 μm were supplied by Jiangxi Yuean Superfine Metal Co., Ltd. Magnesium chloride and ammonium hydroxide were from the Sinopharm Chemical Reagent Co., Ltd. MgFe₂O₄ insulating coating on the surface of Fe powder particles was produced by in-situ oxidation method. 100 g carbonyl iron powders were mixed with 300 mL different concentrations of NH₃·H₂O (1.5, 2.5, 3.5 wt.%) with the addition of 2.03 g MgCl₂·6H₂O_. The mixture was annealed at 180 °C for 1 h in an autoclave of 500 mL to complete in-situ oxidation reaction. The in-situ oxidized powders were then washed with deionized water and ethanol three times, prior to mixing with epoxy-modified silicone resin (ESR) (2 wt.%) dissolved in dimethyl benzene. The mixture was stirred at 80 °C until complete evaporation of the solution.

2.2 Composite preparation

The toroidal cores with an outer diameter of 26.1 mm, an inner diameter of 18.1 mm and a height of 4.5 mm were fabricated at a cold pressure of 800 MPa and room temperature. Then, the compacted cores were annealed at various temperatures (500-600 °C) for 1 h under Ar atmosphere to reduce the internal stress caused by pressing. The schematic diagram of the fabrication process of the Fe/MgFe₂O₄ soft magnetic composites is shown in Fig. 1.

2.3 Material characterization

Fig. 1 Schematic diagram of fabrication process of Fe/MgFe₂O₄ soft magnetic composites

The surface morphology of the oxidized Fe powders was observed by the transmission electron microscopy (TEM, JEM-2100F). The morphologies of the raw, insulating layer on the surface of Fe powders and annealed SMCs samples were examined using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy X-ray analysis (EDS, Hitachi SU8010). The phase identification of the Fe powders and the oxidized powders were analyzed by X-ray diffractometry (XRD, Advance D8) using Cu K_α radiation. The chemical state of the coating was analyzed by an X-ray photoelectron spectrometer (XPS, ESCALAB250Xi). Fourier transform infrared spectra (FT-IR) (Bruker VERTEX 70) were recorded in the wavenumber range from 400 to 4000 cm^-1 at room temperature to investigate the composition of the insulating layer. Raman studies were performed in a LABRAM Aramis Raman microscopy system from Jobin Yvon France in the wavenumber range from 100 to 1000 cm^-1 with a 532 nm laser. The thermal properties were examined by thermogravimetry and differential scanning calorimetry (TG-DSC, NETZSCH STA 449C), in which the samples were heated from room temperature to 700 °C under Ar atmosphere in alumina crucibles with a heating rate of 10 °C/min. The hysteresis loops were measured at room temperature by a vibrating sample magnetometer (VSM, Lake-shore 7400-s). The amplitude permeability and core loss of the SMCs were measured with a maximum applied magnetic induction of 50 mT over the frequency range from 10 to 170 kHz by an auto testing system (SY8258B-H/m) for magnetic materials.

3 Results and discussion

3.1 Microstructures

The typical TEM image of the oxidized Fe powders is shown in Fig. 2. The external gray coating layer was coated on the surface of the internal black part. The black part of Fe is confirmed to be coated with MgFe₂O₄. The generation mechanism of MgFe₂O₄ will be discussed in the following section.

Figures 3(a) and (b) show the morphologies of both the raw Fe particles and Fe powders oxidized with 2.5 wt.% NH₃·H₂O, respectively. Compared with the raw powders, the surface of the oxidized Fe powders was rougher, manifesting the formation of a coating layer after in-situ oxidation. SEM micrograph and EDS analysis of the coated powders after being polished are presented in Figs. 3(c-e). It can be obviously seen that the Fe powders were coated with a thin surface layer. The selected area 1 of the interior consists of iron and gold, while the selected area 2 of the coating layer obviously consists of magnesium, oxygen, iron and gold. Therefore, the MgFe₂O₄ layer has been formed. With respect to the analyzed depth, the comparison among the intensities of magnesium and iron peaks further confirms that the MgFe₂O₄ layer is thin. Gold appears in the EDS analysis as a result of the sample sprayed with gold.

Fig. 2 TEM image of Fe/MgFe₂O₄ powders prepared with 2.5 wt.% NH₃·H₂O

The SEM image and EDS element distribution maps of the Fe powders coated with an insulating layer oxidized with 2.5 wt.% NH₃·H₂O are shown in Fig. 4. Apparently, the MgFe₂O₄ layer was completely and uniformly covered on the surface of the Fe powders, which is the key to ensure the relatively low eddy current loss of the SMCs. Moreover, the elemental distribution maps revealed that Mg, O and Fe were uniformly distributed on the surface of the Fe powder particles. It can be inferred that each particle was coated by a uniform and thin insulating layer.

3.2 XRD patterns

XRD analysis has been implemented to identify the phase composition of the coating. Figure 5 shows the XRD patterns of the raw Fe powders and those oxidized with different NH₃·H₂O concentrations (1.5, 2.5, 3.5 wt.%). The diffraction peaks at 2θ values of 44.70°, 65.03° and 82.36° were assigned to the (110), (200) and (211) planes of Fe (JCPDS No. 87-0721). Additional characteristic peaks appeared at 2θ values of 35.39° and 62.56° for the surface-oxidized Fe powders, corresponding to the (311) and (440) planes of the MgFe₂O₄ (JCPDS No. 88-1935) [21,25]. No peak of any other phase was detected. The intensity ratio between the MgFe₂O₄ and Fe main peaks increases with increasing ammonia concentration, indicating the formation of thicker MgFe₂O₄.

Fig. 3 SEM images of raw (a) and oxidized (b) Fe powders, EDS spectrum of polished oxidized Fe powders (c) with EDS analysis of selected areas 1 (d) and 2 (e) in (c)

Fig. 4 SEM image (a) and EDS elemental distribution maps (b-d) of Fe/MgFe₂O₄ composite powders

Fig. 5 XRD patterns of raw Fe powders and those oxidized with different NH₃·H₂O concentrations

3.3 XPS spectra

In order to further investigate the oxidation state of the coating layer, the XPS test was performed for the oxidized powders. Figure 6 shows the XPS spectra of the oxidized Fe powders after Ar⁺ sputtering for 1000 s. Detailed XPS spectra of the Mg 1s and the Fe 2p peaks are shown in Figs. 6(a) and (b), respectively. The Mg 1s peak located at around 1303.3 eV is observed in the XPS spectrum which corresponds to Mg²⁺. Two main peaks, Fe 2p_1/2 and Fe 2p_3/2 located at around 724.3 and 710.9 eV are observed in the XPS spectra, respectively. Additionally, a weak satellite peak at ~719 eV is observed in the XPS spectrum which corresponds to Fe³⁺ [26-28]. No peak corresponding to the metallic Fe is observed, suggesting the formation of a complete oxide coating layer. Figure 6(c) shows a survey spectrum of the oxidized powders, indicating the existence of magnesium, iron and oxygen. Experimental results of XPS measurement further demonstrate the oxidation layer composition of MgFe₂O₄.

3.4 FT-IR and Raman spectra

In addition, the FT-IR spectrum in the range  of 400-4000 cm^-1 was used to characterize the oxidized Fe powders, as shown in Fig. 7. The broad absorption bands due to the —OH vibration are observed at 3420, 1634, 1396 and 1030 cm^-1, due to adsorbed water on the surface. The spectrum of the oxidized Fe powders reveals the absorption band at 592 cm^-1, which is assigned to the Fe—O stretching band. The small absorption band at around 448 cm^-1 may be assigned to the Mg—O stretching vibration mode [29-31], which is attributed to the existence of MgFe₂O₄, confirming that the MgFe₂O₄ insulating layer was successfully coated on the surface of Fe powders.

Fig. 6 XPS spectra of Mg 1s (a), Fe 2p (b) and survey XPS spectrum (c) of Fe powders oxidized with 2.5 wt.% NH₃·H₂O

Fig. 7 FT-IR spectrum of oxidized Fe powders prepared with 2.5 wt.% NH₃·H₂O

Fig. 8 Raman spectrum of Fe/MgFe₂O₄ composites

Raman spectrum of Fe/MgFe₂O₄ composites was collected in the range of 100-1000 cm^-1, as shown in Fig. 8. The recorded spectrum of the MgFe₂O₄ shows five expected Raman active modes E_g at 285 and 400 cm^-1 and 3F_2g at 220, 487 and 598 cm^-1 [32-34]. For the division of peaks, the literature is inconsistent. The Raman bands observed at 220, 285 and 487 cm^-1 could be attributed to the F_2g(1), E_g and F_2g(2) Raman modes involving motion of the Fe³⁺ and those at 400 and 598 cm^-1 could be assigned to the E_g and F_2g(3) Raman modes involving motion of the Mg²⁺, respectively.

Based on the above-mentioned results, the in-situ oxidation mechanism can be deduced. The reactions for the formation of MgFe₂O₄ can be expressed in the following equations. According to the Pourbaix diagram [35], Fe reacts with H₂O to form Fe(OH)₂ and is further oxidized to be [Fe(OH)_n]^-(n-3) (n=2, 3) by the small amount of dissolved O₂ in solution. Mg(OH)₂ will form quickly because Mg²⁺ is rendered in an ammonia solution. Finally, the newly formed Mg(OH)₂ will react with hydroxy complexes [Fe(OH)_n]^-(n-3) to form ferromagnetic MgFe₂O₄ [36]:

Fe+2H₂O→Fe(OH)₂+H₂↑                           (1)

Fe(OH)₂+OH^-+O₂+H₂O→[Fe(OH)_n]^-(n-3)  (n=2, 3)    (2)

Mg²⁺+2OH^-→Mg(OH)₂                                    (3)

Mg(OH)₂+[Fe(OH)_n]^-(n-3)→MgFe₂O₄                    (4)

3.5 TG-DSC curves

Aiming to study the thermal stability of the coating, TG-DSC analysis was executed by heating the MgFe₂O₄/ESR-coated Fe powders in the temperature range from 30 to 700 °C on a NETZSCH STA 449C, under Ar atmosphere in alumina crucibles with a heating rate of 10 °C/min. An apparently decrease in the mass was observed at about 200 °C due to the thermal degradation of the ESR layer in Fig. 9. The ESR layer decomposes slowly with the temperature increasing. When mass fraction of ESR in the SMCs is 2%, the mass loss of SMCs powders is merely 1.12% at 700 °C.

Fig. 9 TG-DSC curves of Fe powders coated with MgFe₂O₄/ESR

All the above results revealed that the Fe powders were coated with MgFe₂O₄. Then, the ESR was coated on the oxidized Fe particles constructing inorganic-organic hybrid insulating layer. The outstanding and unique MgFe₂O₄ insulating layer on the surface of Fe powders is the key to manufacture SMCs with the excellent property.

3.6 Hysteresis loops

The hysteresis loops of the Fe powders and those undergoing different reaction concentrations were measured by VSM. Figure 10 shows the saturated magnetization (M_s) as a function of the magnetic field strength ranging from -1600 to 1600 kA/m. The saturated magnetization (M_s) values decrease gradually from 1654 to 1600 A/m with increasing ammonia concentration, which can be explained by the insulating layer of MgFe₂O₄ with lower M_s [37]. These M_s values of the oxidized Fe powders bring into correspondence with the raw Fe powders (1677 A/m) as shown in the inset in Fig. 10. This is attributed to the formation of the ferromagnetic MgFe₂O₄ coating by the in situ oxidation method for the prevention of magnetic dilution.

3.7 Microstructures after heat treatment

Fig. 10 Hysteresis loops for raw Fe powders and those oxidized with different NH₃·H₂O concentrations

The cross-section SEM images and the element distributions of the samples oxidized with 2.5 wt.% NH₃·H₂O after annealing treatment at 500 °C for 1 h in the Ar atmosphere are shown in Fig. 11. The complete and uniform insulation layer is still coated on the surface of the Fe particles after the annealing treatment. The gold appears in the EDS analysis due to the sample sprayed with gold. The large particles are pure Fe and do not contain other elements (Figs. 11(a₁, a₂); coating layer (dark part) of the particles contains Fe, O, Mg and Si, (Fig. 11(b₁, b₂)). Because of the thin MgFe₂O₄ layer, the intensities of the Mg are significantly weaker than those of Fe.

Fig. 11 Cross-section SEM images (a₁, b₁) and element distributions (a₂, b₂) of samples oxidized with 2.5 wt.% NH₃·H₂O after annealing treatment at 500 °C for 1 h in Ar atmosphere

3.8 Effect annealing temperature on magnetic properties of SMCs

Figure 12 shows the amplitude permeability (μ_a) dependence on the frequency for Fe SMCs annealed at different temperatures, and oxidized with different ammonia concentrations and annealed at 550 °C. It can be seen that all the Fe SMCs exhibit good frequency stability of μ_a from 10 kHz to 170 kHz. The μ_a of the Fe SMCs increases slightly when the annealing temperature is raised from 500 to 550 °C. This phenomenon can be explained that annealing treatment can reduce the distortions within the particles, lower the residual stress, and improve the heat expansion coefficient of iron particles. Clearly, when the annealing temperature increases from 550 to 600 °C, the μ_a decreases slightly due to the decomposition of the organic insulating layer to form pores.

Figure 13 shows core loss (P_s) as a function of frequency for Fe SMCs annealed at different temperatures and oxidized with different NH₃·H₂O concentrations after annealing at 550 °C. The P_s of the SMCs mainly consists of the hysteresis loss P_h and eddy current loss P_e, given by the following equation [20,38]:

          (5)

where f is the frequency, H is the magnetic field intensity, B is the flux density, C is proportionality constant, d is the thickness of the material and ρ is the resistivity of the material. Hence, with an increase in frequency, P_s grows rapidly. P_e can be divided into two different components: in-particle, which originates from eddy current loss inside the particle, and inter-particle, which is caused by insulating between the particles [1]. When the insulating layer of the sample is not damaged, the effect of annealing treatment on the eddy current loss is quite small. The core loss measured at 100 kHz and 50 mT decreases from 190.3 to 167.5 W/kg as the annealing temperature changes from 500 to 550 °C, as shown in Fig. 13(b). The internal stress is not completely eliminated at 500 °C, which causes the hysteresis loss to be high. When the annealing temperature ranges from 550 to 600 °C, the core loss decreases slightly from 167.5 to 162.1 W/kg. It can be explained by the internal stress which is completely eliminated at 550 °C. This can reduce the hysteresis loss. Therefore, 550 °C is the most suitable annealing temperature for the Fe SMCs. Figure 13(d) shows the core loss of the samples annealed at 550 °C with NH₃·H₂O concentration ranging from 1.5 to 3.5 wt.%. The minimum core loss of 167.5 W/kg measured at  100 kHz is achieved with NH₃·H₂O concentration of 2.5 wt.%. When NH₃·H₂O concentration is 1.5 wt.%, the very thin MgFe₂O₄ insulating layer is insufficient to provide large electrical resistivity for the SMCs, which results in large eddy current loss. Based on the above discussion, it can be indicated that the optimal magnetic performance can be obtained at 550 °C with NH₃·H₂O concentration of 2.5 wt.%.

Fig. 12 Amplitude permeability (μ_a) dependence on frequency of Fe SMCs prepared with NH₃·H₂O concentrations of 1.5 wt.% (a), 2.5 wt.% (b), 3.5 wt.% (c) and annealed at different temperatures, and of those prepared with different ammonia concentrations at annealing temperature of 550 °C (d)

Fig. 13 Core loss-frequency curves of Fe SMCs prepared with different ammonia concentrations of 1.5 wt.% (a), 2.5 wt.% (b), 3.5 wt.% (c) and annealed at different temperatures, and of those and prepared with different ammonia concentrations at annealing temperature of 550 °C (d)

4 Conclusions

(1) Soft magnetic composites containing Fe powders coated with MgFe₂O₄ insulating layer were prepared via in-situ oxidation method. The TEM analysis shows that a thin and uniform layer was coated on the surface of the Fe powders. The SEM, EDS, XRD, XPS, FT-IR and Raman analysis results indicated that the chemical composition of the coating layer is MgFe₂O₄. The MgFe₂O₄ layer has effectively enhanced the magnetic properties of Fe SMCs with significant low core loss.

(2) The core loss of 167.5 W/kg at 100 kHz and 50 mT is obtained for the SMCs made of Fe powders in-situ oxidized with NH₃·H₂O concentration of 2.5 wt.% after annealing at 550 °C. This study provides a promising approach to achieve low core loss for other Fe-based SMCs.

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铁酸镁绝缘包覆羰基铁粉软磁复合材料的制备与性能表征

李石庚^1,2，刘如铁¹，熊 翔¹

1. 中南大学 粉末冶金国家重点实验室，长沙 410083；

2. 萍乡学院 材料与化学工程学院，萍乡 337000

摘  要：球形羰基铁粉颗粒经铁酸镁(MgFe₂O₄)绝缘包覆后，与改性环氧有机硅树脂(ESR)均匀混合，通过压制成型和退火处理制得软磁复合材料。TEM、SEM、EDS、XRD和XPS分析结果表明，MgFe₂O₄层包覆在铁粉颗粒表面。采用振动样品磁强计和磁性材料自动测试系统对样品进行磁性能测试。经800 MPa压制和550 °C退火处理后软磁复合材料在100 kHz和50 mT时的磁芯损耗为167.5 W/kg。

关键词：软磁复合材料；原位氧化制备；铁酸镁绝缘涂层；退火处理；磁性能

(Edited by Wei-ping CHEN)

Foundation item: Project (2016YFB0700302) supported by the National Key Research and Development Program of China; Projects (51862030, 51563020) supported by the National Natural Science Foundation of China

Corresponding author: Ru-tie LIU; Tel: +86-13974870967; E-mail: llrrtt@csu.edu.cn

DOI: 10.1016/S1003-6326(20)65443-7