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ZrO2 soft magnetic composites
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Original Research Paper
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Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites
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Kaijie Geng, Yuye Xie, Lili Xu, Biao Yan ⇑
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School of Materials Science and Engineering, Tongji University, Shanghai 201804, China Shanghai Key Laboratory of D&A for Metal-Functional Materials, Shanghai 201804, China
a r t i c l e
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Article history: Received 5 January 2017 Received in revised form 22 March 2017 Accepted 29 April 2017 Available online xxxx Keywords: Soft magnetic composites ZrO2-coated Fe powders Coating process Low core loss
a b s t r a c t Fe particles were coated with ZrO2 nanopowders using mechanical milling method combined with high temperature recovery annealing process. The effect of milling time on particle size, phase structure and magnetic properties of the core-shell structure powders was studied. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) revealed that the surfaces of the composite powders comprised thin and uniform layers of ZrO2 insulating powders after milling. Also, the SEM images showed the morphology of micro-cellular structured compacts with cell-body of Fe particles and indicated that Fe particles were well separated and insulated by thin ZrO2 layers. The Fe/ZrO2 soft magnetic composites displayed much higher electrical resistivity, lower core loss than that of the pure Fe powder cores without ZrO2 layers at medium and high frequencies. The preparation method of ZrO2-insulated Fe powders provides a promising method to reduce the core loss and improve the magnetic properties for soft magnetic composite materials. Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Soft magnetic composites (SMCs) consisting of ferromagnetic particles separated by electrical insulating materials have attracted considerable attention recently due to their high electrical resistivity, three-dimensional isotropic ferromagnetic behavior and relatively low total core loss [1–3]. Generally, core loss in magnetic cores mainly consist of hysteresis loss (Wh), eddy-current loss (We) and residual loss (Wr) [4]. About 9% of electrical energy is lost during electromagnetic transmission and distribution mainly because of eddy-current loss, especially at medium and high frequencies [5]. Therefore, a key challenge to improve energy efficiency for power conversion is to prepare soft magnetic composite with better magnetic properties and lower core loss. The eddy-current loss could be reduced significantly by eliminating the electrical conducting path or increasing the electrical resistivity between Fe particles in magnetic materials [6,7]. And substantial numbers of insulating materials have been applied in SMCs for separating the conductive Fe particles including organic materials [8,9], inorganic materials [10,11] and in-situ passivation coatings [12,13]. Unfortunately, most of the insulating materials
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⇑ Corresponding author at: No. 4800 Cao’an Road, Jiading District, Shanghai 201804, China. Fax: +86 021 69582007. E-mail address: [email protected] (B. Yan).
could not endure high temperature and would be decomposed or evaporated, leading to a sharp deterioration in mechanical and magnetic properties of the magnetic cores during subsequent annealing process. Thus inorganic oxides, such as SiO2, Al2O3 and MgO [14–16], with high thermal stability and electric resistance are alternative insulating materials to decrease core loss in SMCs. ZrO2 is also one of the most important oxide ceramics with high chemical inertness, refractory properties and high dielectric constant. It is capable of electrically insulating iron powders, even after high temperature treatment, to eliminate eddy currents, and at the same time is effective to keep the original magnetic properties. There are several routes to prepare such composite powders. Myagkov et al. [17] synthesized ferromagnetic Fe-ZrO2 nanocomposite thin films by using a thermite reaction between Zr and Fe2O3 layers and found that the synthesized Fe-ZrO2 nanocomposite films possess soft magnetic behavior, high magnetization, high resistivity and good chemical stability. de Resende et al. [18] investigated the formation of Fe-ZrO2 nanocomposite powders by reduction in H2 of a totally stabilized Zr0.9Fe0.1O1.95 solid solution. Protsenko et al. [19] investigated electrodeposition of iron and composite iron-zirconia coatings from a methanesulfonate electrolyte. Zulhijah et al. [20] reported a00 -Fe16N2 phase formation of plasma-synthesized core-shell type a-Fe nanoparticles and the prepared nanoparticles with a thin shell exhibited an enhanced magnetic performance over a-Fe nanoparticles.
http://dx.doi.org/10.1016/j.apt.2017.04.029 0921-8831/Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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the effect of ZrO2 insulating layers on magnetic properties, a pure Fe magnetic powder cores without any insulating materials was also prepared under the same conditions for comparison. The particle size distribution of raw Fe powders and composite powders was measured by laser particle size analyzer (MS2000G, Malvern, England). The phase identification was analyzed by X-ray diffraction (XRD) (DX-2007, Dandong fangyuan Co., Ltd, China) operated at 30 kV and 30 mA using Cu-Ka radiation. The morphology and local chemical homogeneity of composite powders and compacts were examined by scanning electron microscopy (SEM) (Nova NanoSEM 450, FEI, USA) coupled with a energy-dispersive spectroscopy (EDS) (Ultra, EDAX, USA). The density of the SMCs was determined by using Archimedes principle with ethanol as the immersion fluid. The static magnetic properties of the powders were tested at room temperature by vibrating sample magnetometer (VSM) (Quantum Design, USA). Total core loss of the ring-shaped magnetic powder cores was measured at 0.02 T from 1 kHz up to 100 kHz by using a soft magnetic AC measuring instrument (MATS-2010SA/500k, Linkioin, China). The hysteresis loss of the compacts was measured by using a DC B-H loop tracer (MATS-2010SD, Linkioin, China).
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However, most of the reported methods for the production of nanocomposite materials are quite complex in preparation route or applicable only in film form. In recent years mechanical milling has demonstrated the ability to produce composite granular powders. This technique of mechanical milling has some advantages over sol-gel route, chemical reduction, thin film deposition or nitridation since large quantities of material may be produced quickly, efficiently and cheaply [21,22]. Furthermore, the particle size of the constituent materials and the component content can be easily controlled by varying the milling parameters. In this study, a novel soft magnetic composites insulated by ZrO2 coatings were prepared. Our work provided a simple and effective method to prepare high-performance ZrO2-insulated Fe powders. The coating method described here can be achieved by mechanical milling of the mixture of pure Fe powders and ZrO2 nanopowders. Not only the practicability of ZrO2 nanopowders to coat Fe powders was confirmed, the effect of milling time on structure and magnetic properties of the composite powders was also investigated. We show, through a combination of structural identification and chemical analysis cooperated with magnetic property testing, that these composite powders have homogeneous ZrO2 layers which are capable of diminishing eddy-current loss, and possessing enhanced magnetic properties.
3. Results and discussion
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2. Material and methods
3.1. Effect of mechanical milling on particle size evolution and microstructure of the ZrO2-coated Fe powders
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Commercial sphere-shaped Fe powders (purity =99.5%, Tianjiu Metal Material Co., Ltd, China) with an average particle size of 75 µm and ZrO2 powders (purity =99.9%, Jingrui New Material Co., Ltd, China) with an average particle size of 30 nm were used as starting materials. The specific experimental procedures to prepare ZrO2-coated Fe powders and corresponding SMCs were as follows: (a) Fe powders and ZrO2 nanopowders (the content of ZrO2 was 5, 7.5, 10 and 12.5 wt.%, respectively) were pre-mixed in a V-type blender for 15 min, and then the mixture was dry-milled for various times (1, 2, 3, 4, 5 and 6 h, respectively) with a ball-to-powder ratio of 50:1 in a high-energy ball milling machine (1-SL, Qingdao Union Machinery Co., Ltd, China) under argon atmosphere. 304 stainless steel vessel (with a inner diameter of 220 mm) and 304 stainless steel balls (with a diameter of 5 mm) were used in this process; (b) the obtained composite powders were then annealed at 600 °C for 2 h in argon atmosphere to relieve internal stress; (c) the annealed Fe/ZrO2 composite powders were mixed with 1.5 wt.% muscovite (purity =95%, Meryer (Shanghai) Chemical Technology Co., Ltd, China) and then compacted under 1000 MPa to form toroidal-shaped magnetic cores with 22 mm outer diameter, 12 mm inner diameter and 5 mm thickness as shown in Fig. 1; (d) the magnetic powder cores were annealed again at 650 °C for 2 h in flowing argon atmosphere. To investigate
Fig. 2 shows the particle size distribution of Fe powders before and after mechanical coating, and it can be found that the
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Fig. 1. The toroidal-shaped magnetic cores of the Fe/ZrO2 SMCs.
Fig. 2. Particle size distribution of the raw Fe powders (a) and composite powders (with 10 wt.% ZrO2) after 2 h of mechanical milling (b).
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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raw Fe powders and composite powders both displayed good concentration with an average diameter of 77.79 and 74.79 µm, respectively. The SEM images of pure Fe powders and Fe/ZrO2 composite powders processed for different milling times are shown in Fig. 3. Fig. 3(a) is the original Fe powders, Fig. 3(b) and (c) are the SEM images of the Fe powders milled for 2 h and 6 h, respectively. And Fig. 3(d) and (e) are the SEM images of the composite powders milled for 2 h and 6 h, respectively. The particle size of Fe powders was not changed much at 2 h, but slight deformation resulting from ball-particle collisions during milling process can be observed. After milling for 6 h, obvious deformation and particle fragmentation is visible for the Fe powders. Furthermore, the surface of Fe powders displayed a lamellar morphology with slight agglomerations and many cracks. The composite powders at 6 h, however, displayed a different morphology that the cracks are almost not visible and the powders with less fragments are not agglomerated compared with pure Fe powders. This phenomenon should be attributed to the nano-sized ZrO2 powders in the mixture which probably work as process control agents (PCA) similar to benzene during wet mechanical alloying reported in the literature [23,24]. It is generally known that the particle size evolution is given by the competition between cold welding, which tends to increase the particle size and fragmentation, which tends to decrease the particle size [25]. Consequently, the existence of ZrO2 nanopowders will reduce the rate of cold welding by separating Fe powders. In addition, the ZrO2 nanopowder can gradually embed into the cracks of the powder surface during, which results in a rough but no-cracked morphology.
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Fig. 4. X-ray diffraction patterns of raw Fe powders and composite powders (with 10 wt.% ZrO2) processed for various milling times (0 h, 2 h and 6 h, respectively). The inset in figure is the magnified image of the selected region.
Fig. 4 reveals the X-ray diffraction patterns of raw Fe powders and composite powders with 10 wt.% ZrO2 processed for various milling times. As can be seen from the patterns, raw Fe powders
Fig. 3. SEM images of the pure Fe powders milled for (a) 0 h; (b) 2 h; (c) 6 h, and SEM images of composite powders milled for (d) 2 h and (e) 6 h.
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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presented typical bcc-Fe diffraction peaks and composite powders displayed a combination of ZrO2 and Fe peaks as expected. The width of Fe peaks broadened and peak intensity decreased slightly with milling time increased, indicating that plastic deformation induced by mechanical milling could lead to grain refining and stress increasing. During the mechanical alloying, generally, there is solid solution, and the intensity of ZrO2 peaks weakened with increasing milling time is a positive proof of that. In addition, the XRD pattern of the composite powders (milled for 6 h) after annealing is given here. The inset in figure is the magnified image of the selected region. It can be found that no phase transformation for the composite powders was identified during annealing process, the ZrO2 phase still maintained a monoclinic structure with 2h at 28.17°, 31.47° and 34.16°, respectively.
3.2. Morphology characteristics and magnetic properties of the ZrO2coated Fe powders
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Fig. 5 shows the SEM images of raw Fe and ZrO2-coated Fe particles. It can be observed that the surfaces of raw Fe particles, which are spherical and smooth (Fig. 5(a)), tend to be relatively rougher and irregularly shaped after mechanical milling for 2 h (Fig. 5(b)). Fig. 5(e) shows the EDS spectrum for the surface of selected region in Fig. 5(c). The existence of Zr and O elemental peaks proves the formation of ZrO2 layers coating around the surfaces of Fe particles, which further demonstrates that it only takes 2 h to coat the Fe powders uniformly with 10 wt.% ZrO2 nanopowders through mechanical milling. The SEM image of the polished surface of an individual ZrO2-coated Fe particle is shown in Fig. 5
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Fig. 5. SEM images of (a) the raw Fe powders; (b) the composite powders; (c) the individual ZrO2-coated Fe powder; (d) the polished surface of the ZrO2-coated Fe powder, and (e) the EDS spectrum of the selected region in (c).
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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Fig. 6. The dependence of coercive force (HC) on milling time for the composite powders before and after annealing in argon atmosphere.
Fig. 9. SEM images (a) of the polished surface for Fe/ZrO2 composite compacts and (b) of the selected region in (a).
Fig. 7. The hysteresis loops of raw Fe powders and ZrO2-coated Fe powders milled for various times. The inset in figure is the magnified image of the selected region.
Fig. 8. The hysteresis loops of the composite powders (with 10 wt.% ZrO2) milled for 2 h before and after annealing. The inset in figure is the magnified image of the selected region.
(d), revealing that the ZrO2 layer, with a thickness of about 1 µm, displayed a loose morphology. The dependence of coercive force (Hc) on milling time for the composite powders before and after relief annealing is summarized in Fig. 6. The Hc of composite powders increased gradually with milling time because more imperfections and internal stress which work as pinning sites for domain walls to obstruct magnetization process were induced within Fe particles during milling process. After annealing at 600 °C in argon atmosphere for 2 h, the Hc was much lower than that of the as-milled composite powders suggesting that internal stress was released effectively after high temperature heat treatment. Moreover, it is easy to find that the Hc of the composite powders milled for 6 h was doubled compared with raw Fe powders and was incapable of further reducing by high temperature annealing. This is due to the irreversible motion of domain boundaries caused by more defects and strong pinning effect of embedded ZrO2 particles in the Fe surfaces. As shown in Fig. 7, the saturation magnetization (Ms) of composite powders milled for 2 h was slightly lower than that of the raw Fe powders. This phenomenon should be ascribed to the existence of nonmagnetic ZrO2 insulating layer, which led to a decrease of the concentration of the magnetic atoms and a significant decrease in the overall saturation magnetization. Furthermore, the Ms value of the composite powders monotonically decreased with increasing milling time. It is well known that the chemical composition and proximity of the magnetic atoms are the main factors that influence the saturation magnetization process of the materials [26,27]. Consequently, there are several reasons for the decrease in Ms value of the composite powders with increasing milling time in this work. On the one hand, the nano-sized nonmagnetic particles of ZrO2 might be gradually dissolved in the Fe
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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Fig. 10. EDS spectrum of the polished surface for the Fe/ZrO2 composite compacts.
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structure and weaken the magnetic interaction between iron atoms, and consequently cause a decrease in saturation magnetization of the composite powders. On the other hand, high defect densities such as microstrain, microcracks and gaps induced by mechanical milling may also result in a drift apart of the iron atoms, therefore a diminution of magnetic interaction and a decrease of saturation magnetization are predictable. Similar results of increasing milling time leads to a dissolution of nonmagnetic atoms into matrix and an increase of defect densities, and thus cause a decrease of saturation magnetization, were also reported elsewhere [25,28]. Fig. 8 presents the hysteresis loops for the composite powders (with 10 wt.% ZrO2) after milling for 2 h before and after annealing at 600 °C under argon atmosphere for 2 h. It can be found that the annealing process of the milled powders leads to a minor improvement of its saturation magnetization. Before annealing, the Ms and remanent magnetization (Mr) values are 203.78 emu/g and 2.21 emu/g, respectively. After annealing, the Ms and Mr are 205.56 emu/g and 1.62 emu/g, respectively. However, the Hc decreases drastically to from 27.66 Oe (before annealing) to 18.34 Oe (after annealing). This phenomenon should be attributed to the stresses release, which has already been discussed above. 3.3. Microstructure and magnetic properties of the Fe/ZrO2 soft magnetic composites (SMCs) Fig. 9(a) shows the SEM micrograph of the mechanical polished surface for Fe/ZrO2 SMCs (without etching). It can be observed that the grey-white regions were completely surrounded by net-shaped grey-black regions. And it can be found in Fig. 9(b) that the thickness of the cell-wall was about 1–2 µm, and the cell-body bonds well with the cell-wall. The EDS spectrum (Fig. 10) further revealed that grey-white regions were mainly Fe elements. Oxygen and zirconium elements lay mainly in grey-black regions. This phenomenon implied that Fe particles (the grey-white regions) were well separated and insulated by thin ZrO2 layers (the grey-black regions) in the Fe/ZrO2 magnetic powder cores.
Table 1 Density, resistivity and magnetic properties for pure Fe powder cores and composite powder cores with varying ZrO2 content. Samples
Density (g/cm3)
Resistivity (µXcm)
Ms (emu/g)
Hysteresis loss (kJ/m3)
Pure Fe 5 wt.% 7.5 wt.% 10 wt.% 10 wt.% (annealed) 12.5 wt.%
6.94 6.85 6.73 6.63 6.63 6.38
25.47 63.15 128.48 243.15 238.68 251.23
224.15 210.36 207.14 203.42 204.68 191.64
2.51 2.54 2.52 2.54 1.97 2.67
To investigate the effect of inorganic insulating materials on magnetic properties, pure Fe powder cores without ZrO2 powders were also prepared under the same conditions. The density, resistivity and magnetic properties of pure Fe powder cores and composite powder cores are summarized in Table 1. As shown in the table, the resistivity of the cores tends to increase as the ZrO2 content is increased up to 10 wt.%. But with further increasing ZrO2 content, the resistivity shows slight increase, which indicates that excess additions of ZrO2 can’t further improve insulation effect. There is no doubt that the additions of non-magnetic insulating materials should be minimized as long as they can provide sufficient electrical insulation. Therefore, the optimal content of ZrO2 for the composites in this work is 10 wt.%. Hysteresis loss (Wh) was obtained directly from the DC-hysteresis loops in this work. It can be found that all the samples almost possess an approximate value of hysteresis loss (Wh). However, the Wh value of the composite powder cores (with 10 wt.% ZrO2) decreased from 2.54 kJ/m3 (before annealing) to 1.97 kJ/m3 (after annealing). It is known that Wh in SMCs strongly depends on particle size, defect densities, impurities and internal microstrain introduced by mechanical deformation [29]. Therefore, the decrease in Wh of composite cores should be ascribed to the stress relief resulting from the annealing process. The total core loss as a function of frequency for pure Fe powder cores and composite powder cores measured from 1 kHz to 100 kHz
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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Fig. 11. Total core loss as a function of frequency for pure Fe powder cores and composite powder cores with different ZrO2 contents, measured at maximum induction of 0.02 T in the frequency range from 1 kHz to 100 kHz.
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at maximum induction of 0.02 T is illustrated in Fig. 11. Comparing these curves, the following three points can be understood: 1. With increasing frequency, the core loss increases significantly. 2. The ZrO2-coated Fe powder cores exhibit much lower core loss than that of the pure Fe powder cores at high frequency range. 3. The core loss decreases remarkably up to 10 wt.% of the ZrO2 additions and displayed slight change with further increasing ZrO2 content.
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As mentioned above, total core loss in magnetic materials consists of hysteresis loss (Wh), eddy current loss (We), and residual loss (Wr). Among these values, Wr mostly depends on the size and arrangement of magnetic domains, which is minor in metallic material and can be negligible compared with Wh and We. We is primary loss especially in high frequency ranges, which can be expressed by [30]:
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W ¼ W h þ W e ¼ k1 Af þ
k2 B 2 t 2
q
f
2
ð1Þ
where k1 and k2 are proportionality constants, A is the area of the DC-hysteresis loop, B is the maximum flux density, t is the thickness of material, f is the frequency and q is the electrical resistivity. It is clear that Wh in this paper don’t determine the total core loss especially at high frequency because this part for all samples is equivalent and about 100 times smaller than the total core loss (at 50 kHz). We may reasonably conclude that a much lower core loss of the composite powder cores is caused by the reduction of eddy current loss (We). As shown in formula (1), We is proportional to the square of the thickness and inversely proportional to the q of the magnetic materials. Therefore, the decrease in We for the Fe/ZrO2 composite powder cores is caused by their higher electrical resistivity than that of pure Fe magnetic compacts. The micrograph of the polished surface of the composite compacts (with 10 wt.% ZrO2) demonstrates that the electric Fe particles are well separated by thin ZrO2 layers. Thus, most of the eddy currents can be limited within the ZrO2-insulated Fe particles, which is equivalent to diminishing the effective radius of the eddy current to reduce core loss [31]. But incomplete coating for the samples with less amount of ZrO2 (<10 wt.%) might still results in high eddy current loss.
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4. Conclusions
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Several conclusions could be drawn from the results described above:
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1. ZrO2-coated Fe powders and corresponding Fe/ZrO2 soft magnetic composites were successfully prepared in this study. 2. The structure and magnetic properties of composite powders was studied as a function of milling time. The results showed that mechanical milling could coat spherical Fe powders uniformly with ZrO2 nanopowders and induce defects and stress simultaneously. 3. We confirm that the ZrO2 insulating layers, which separate the conductive Fe particles from one to another, could avoid metal-to-metal contact and raise the resistivity of magnetic powder cores significantly. Hence the Fe/ZrO2 SMCs show a much lower core loss and more excellent magnetic properties, which provide a potential possibility to reach high energy conversion efficiency for electric-magnetic switching device.
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Acknowledgments
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This work was supported by the National Key Research and Development Program of the 13th Five-Year Plan of China (grant number 2016YFB1200602-02). Also, we thank for the technical support of Hunan Linkjoin Technology Co., Ltd, Changsha, China.
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Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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Contents lists available at ScienceDirect
Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
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Original Research Paper
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Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites
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Kaijie Geng, Yuye Xie, Lili Xu, Biao Yan ⇑
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School of Materials Science and Engineering, Tongji University, Shanghai 201804, China Shanghai Key Laboratory of D&A for Metal-Functional Materials, Shanghai 201804, China
a r t i c l e
i n f o
Article history: Received 5 January 2017 Received in revised form 22 March 2017 Accepted 29 April 2017 Available online xxxx Keywords: Soft magnetic composites ZrO2-coated Fe powders Coating process Low core loss
a b s t r a c t Fe particles were coated with ZrO2 nanopowders using mechanical milling method combined with high temperature recovery annealing process. The effect of milling time on particle size, phase structure and magnetic properties of the core-shell structure powders was studied. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD) revealed that the surfaces of the composite powders comprised thin and uniform layers of ZrO2 insulating powders after milling. Also, the SEM images showed the morphology of micro-cellular structured compacts with cell-body of Fe particles and indicated that Fe particles were well separated and insulated by thin ZrO2 layers. The Fe/ZrO2 soft magnetic composites displayed much higher electrical resistivity, lower core loss than that of the pure Fe powder cores without ZrO2 layers at medium and high frequencies. The preparation method of ZrO2-insulated Fe powders provides a promising method to reduce the core loss and improve the magnetic properties for soft magnetic composite materials. Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
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1. Introduction
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Soft magnetic composites (SMCs) consisting of ferromagnetic particles separated by electrical insulating materials have attracted considerable attention recently due to their high electrical resistivity, three-dimensional isotropic ferromagnetic behavior and relatively low total core loss [1–3]. Generally, core loss in magnetic cores mainly consist of hysteresis loss (Wh), eddy-current loss (We) and residual loss (Wr) [4]. About 9% of electrical energy is lost during electromagnetic transmission and distribution mainly because of eddy-current loss, especially at medium and high frequencies [5]. Therefore, a key challenge to improve energy efficiency for power conversion is to prepare soft magnetic composite with better magnetic properties and lower core loss. The eddy-current loss could be reduced significantly by eliminating the electrical conducting path or increasing the electrical resistivity between Fe particles in magnetic materials [6,7]. And substantial numbers of insulating materials have been applied in SMCs for separating the conductive Fe particles including organic materials [8,9], inorganic materials [10,11] and in-situ passivation coatings [12,13]. Unfortunately, most of the insulating materials
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⇑ Corresponding author at: No. 4800 Cao’an Road, Jiading District, Shanghai 201804, China. Fax: +86 021 69582007. E-mail address: [email protected] (B. Yan).
could not endure high temperature and would be decomposed or evaporated, leading to a sharp deterioration in mechanical and magnetic properties of the magnetic cores during subsequent annealing process. Thus inorganic oxides, such as SiO2, Al2O3 and MgO [14–16], with high thermal stability and electric resistance are alternative insulating materials to decrease core loss in SMCs. ZrO2 is also one of the most important oxide ceramics with high chemical inertness, refractory properties and high dielectric constant. It is capable of electrically insulating iron powders, even after high temperature treatment, to eliminate eddy currents, and at the same time is effective to keep the original magnetic properties. There are several routes to prepare such composite powders. Myagkov et al. [17] synthesized ferromagnetic Fe-ZrO2 nanocomposite thin films by using a thermite reaction between Zr and Fe2O3 layers and found that the synthesized Fe-ZrO2 nanocomposite films possess soft magnetic behavior, high magnetization, high resistivity and good chemical stability. de Resende et al. [18] investigated the formation of Fe-ZrO2 nanocomposite powders by reduction in H2 of a totally stabilized Zr0.9Fe0.1O1.95 solid solution. Protsenko et al. [19] investigated electrodeposition of iron and composite iron-zirconia coatings from a methanesulfonate electrolyte. Zulhijah et al. [20] reported a00 -Fe16N2 phase formation of plasma-synthesized core-shell type a-Fe nanoparticles and the prepared nanoparticles with a thin shell exhibited an enhanced magnetic performance over a-Fe nanoparticles.
http://dx.doi.org/10.1016/j.apt.2017.04.029 0921-8831/Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: K. Geng et al., Structure and magnetic properties of ZrO2-coated Fe powders and Fe/ZrO2 soft magnetic composites, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.04.029
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the effect of ZrO2 insulating layers on magnetic properties, a pure Fe magnetic powder cores without any insulating materials was also prepared under the same conditions for comparison. The particle size distribution of raw Fe powders and composite powders was measured by laser particle size analyzer (MS2000G, Malvern, England). The phase identification was analyzed by X-ray diffraction (XRD) (DX-2007, Dandong fangyuan Co., Ltd, China) operated at 30 kV and 30 mA using Cu-Ka radiation. The morphology and local chemical homogeneity of composite powders and compacts were examined by scanning electron microscopy (SEM) (Nova NanoSEM 450, FEI, USA) coupled with a energy-dispersive spectroscopy (EDS) (Ultra, EDAX, USA). The density of the SMCs was determined by using Archimedes principle with ethanol as the immersion fluid. The static magnetic properties of the powders were tested at room temperature by vibrating sample magnetometer (VSM) (Quantum Design, USA). Total core loss of the ring-shaped magnetic powder cores was measured at 0.02 T from 1 kHz up to 100 kHz by using a soft magnetic AC measuring instrument (MATS-2010SA/500k, Linkioin, China). The hysteresis loss of the compacts was measured by using a DC B-H loop tracer (MATS-2010SD, Linkioin, China).
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However, most of the reported methods for the production of nanocomposite materials are quite complex in preparation route or applicable only in film form. In recent years mechanical milling has demonstrated the ability to produce composite granular powders. This technique of mechanical milling has some advantages over sol-gel route, chemical reduction, thin film deposition or nitridation since large quantities of material may be produced quickly, efficiently and cheaply [21,22]. Furthermore, the particle size of the constituent materials and the component content can be easily controlled by varying the milling parameters. In this study, a novel soft magnetic composites insulated by ZrO2 coatings were prepared. Our work provided a simple and effective method to prepare high-performance ZrO2-insulated Fe powders. The coating method described here can be achieved by mechanical milling of the mixture of pure Fe powders and ZrO2 nanopowders. Not only the practicability of ZrO2 nanopowders to coat Fe powders was confirmed, the effect of milling time on structure and magnetic properties of the composite powders was also investigated. We show, through a combination of structural identification and chemical analysis cooperated with magnetic property testing, that these composite powders have homogeneous ZrO2 layers which are capable of diminishing eddy-current loss, and possessing enhanced magnetic properties.
3. Results and discussion
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2. Material and methods
3.1. Effect of mechanical milling on particle size evolution and microstructure of the ZrO2-coated Fe powders
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Commercial sphere-shaped Fe powders (purity =99.5%, Tianjiu Metal Material Co., Ltd, China) with an average particle size of 75 µm and ZrO2 powders (purity =99.9%, Jingrui New Material Co., Ltd, China) with an average particle size of 30 nm were used as starting materials. The specific experimental procedures to prepare ZrO2-coated Fe powders and corresponding SMCs were as follows: (a) Fe powders and ZrO2 nanopowders (the content of ZrO2 was 5, 7.5, 10 and 12.5 wt.%, respectively) were pre-mixed in a V-type blender for 15 min, and then the mixture was dry-milled for various times (1, 2, 3, 4, 5 and 6 h, respectively) with a ball-to-powder ratio of 50:1 in a high-energy ball milling machine (1-SL, Qingdao Union Machinery Co., Ltd, China) under argon atmosphere. 304 stainless steel vessel (with a inner diameter of 220 mm) and 304 stainless steel balls (with a diameter of 5 mm) were used in this process; (b) the obtained composite powders were then annealed at 600 °C for 2 h in argon atmosphere to relieve internal stress; (c) the annealed Fe/ZrO2 composite powders were mixed with 1.5 wt.% muscovite (purity =95%, Meryer (Shanghai) Chemical Technology Co., Ltd, China) and then compacted under 1000 MPa to form toroidal-shaped magnetic cores with 22 mm outer diameter, 12 mm inner diameter and 5 mm thickness as shown in Fig. 1; (d) the magnetic powder cores were annealed again at 650 °C for 2 h in flowing argon atmosphere. To investigate
Fig. 2 shows the particle size distribution of Fe powders before and after mechanical coating, and it can be found that the
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Fig. 1. The toroidal-shaped magnetic cores of the Fe/ZrO2 SMCs.
Fig. 2. Particle size distribution of the raw Fe powders (a) and composite powders (with 10 wt.% ZrO2) after 2 h of mechanical milling (b).
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raw Fe powders and composite powders both displayed good concentration with an average diameter of 77.79 and 74.79 µm, respectively. The SEM images of pure Fe powders and Fe/ZrO2 composite powders processed for different milling times are shown in Fig. 3. Fig. 3(a) is the original Fe powders, Fig. 3(b) and (c) are the SEM images of the Fe powders milled for 2 h and 6 h, respectively. And Fig. 3(d) and (e) are the SEM images of the composite powders milled for 2 h and 6 h, respectively. The particle size of Fe powders was not changed much at 2 h, but slight deformation resulting from ball-particle collisions during milling process can be observed. After milling for 6 h, obvious deformation and particle fragmentation is visible for the Fe powders. Furthermore, the surface of Fe powders displayed a lamellar morphology with slight agglomerations and many cracks. The composite powders at 6 h, however, displayed a different morphology that the cracks are almost not visible and the powders with less fragments are not agglomerated compared with pure Fe powders. This phenomenon should be attributed to the nano-sized ZrO2 powders in the mixture which probably work as process control agents (PCA) similar to benzene during wet mechanical alloying reported in the literature [23,24]. It is generally known that the particle size evolution is given by the competition between cold welding, which tends to increase the particle size and fragmentation, which tends to decrease the particle size [25]. Consequently, the existence of ZrO2 nanopowders will reduce the rate of cold welding by separating Fe powders. In addition, the ZrO2 nanopowder can gradually embed into the cracks of the powder surface during, which results in a rough but no-cracked morphology.
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Fig. 4. X-ray diffraction patterns of raw Fe powders and composite powders (with 10 wt.% ZrO2) processed for various milling times (0 h, 2 h and 6 h, respectively). The inset in figure is the magnified image of the selected region.
Fig. 4 reveals the X-ray diffraction patterns of raw Fe powders and composite powders with 10 wt.% ZrO2 processed for various milling times. As can be seen from the patterns, raw Fe powders
Fig. 3. SEM images of the pure Fe powders milled for (a) 0 h; (b) 2 h; (c) 6 h, and SEM images of composite powders milled for (d) 2 h and (e) 6 h.
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presented typical bcc-Fe diffraction peaks and composite powders displayed a combination of ZrO2 and Fe peaks as expected. The width of Fe peaks broadened and peak intensity decreased slightly with milling time increased, indicating that plastic deformation induced by mechanical milling could lead to grain refining and stress increasing. During the mechanical alloying, generally, there is solid solution, and the intensity of ZrO2 peaks weakened with increasing milling time is a positive proof of that. In addition, the XRD pattern of the composite powders (milled for 6 h) after annealing is given here. The inset in figure is the magnified image of the selected region. It can be found that no phase transformation for the composite powders was identified during annealing process, the ZrO2 phase still maintained a monoclinic structure with 2h at 28.17°, 31.47° and 34.16°, respectively.
3.2. Morphology characteristics and magnetic properties of the ZrO2coated Fe powders
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Fig. 5 shows the SEM images of raw Fe and ZrO2-coated Fe particles. It can be observed that the surfaces of raw Fe particles, which are spherical and smooth (Fig. 5(a)), tend to be relatively rougher and irregularly shaped after mechanical milling for 2 h (Fig. 5(b)). Fig. 5(e) shows the EDS spectrum for the surface of selected region in Fig. 5(c). The existence of Zr and O elemental peaks proves the formation of ZrO2 layers coating around the surfaces of Fe particles, which further demonstrates that it only takes 2 h to coat the Fe powders uniformly with 10 wt.% ZrO2 nanopowders through mechanical milling. The SEM image of the polished surface of an individual ZrO2-coated Fe particle is shown in Fig. 5
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Fig. 5. SEM images of (a) the raw Fe powders; (b) the composite powders; (c) the individual ZrO2-coated Fe powder; (d) the polished surface of the ZrO2-coated Fe powder, and (e) the EDS spectrum of the selected region in (c).
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Fig. 6. The dependence of coercive force (HC) on milling time for the composite powders before and after annealing in argon atmosphere.
Fig. 9. SEM images (a) of the polished surface for Fe/ZrO2 composite compacts and (b) of the selected region in (a).
Fig. 7. The hysteresis loops of raw Fe powders and ZrO2-coated Fe powders milled for various times. The inset in figure is the magnified image of the selected region.
Fig. 8. The hysteresis loops of the composite powders (with 10 wt.% ZrO2) milled for 2 h before and after annealing. The inset in figure is the magnified image of the selected region.
(d), revealing that the ZrO2 layer, with a thickness of about 1 µm, displayed a loose morphology. The dependence of coercive force (Hc) on milling time for the composite powders before and after relief annealing is summarized in Fig. 6. The Hc of composite powders increased gradually with milling time because more imperfections and internal stress which work as pinning sites for domain walls to obstruct magnetization process were induced within Fe particles during milling process. After annealing at 600 °C in argon atmosphere for 2 h, the Hc was much lower than that of the as-milled composite powders suggesting that internal stress was released effectively after high temperature heat treatment. Moreover, it is easy to find that the Hc of the composite powders milled for 6 h was doubled compared with raw Fe powders and was incapable of further reducing by high temperature annealing. This is due to the irreversible motion of domain boundaries caused by more defects and strong pinning effect of embedded ZrO2 particles in the Fe surfaces. As shown in Fig. 7, the saturation magnetization (Ms) of composite powders milled for 2 h was slightly lower than that of the raw Fe powders. This phenomenon should be ascribed to the existence of nonmagnetic ZrO2 insulating layer, which led to a decrease of the concentration of the magnetic atoms and a significant decrease in the overall saturation magnetization. Furthermore, the Ms value of the composite powders monotonically decreased with increasing milling time. It is well known that the chemical composition and proximity of the magnetic atoms are the main factors that influence the saturation magnetization process of the materials [26,27]. Consequently, there are several reasons for the decrease in Ms value of the composite powders with increasing milling time in this work. On the one hand, the nano-sized nonmagnetic particles of ZrO2 might be gradually dissolved in the Fe
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Fig. 10. EDS spectrum of the polished surface for the Fe/ZrO2 composite compacts.
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structure and weaken the magnetic interaction between iron atoms, and consequently cause a decrease in saturation magnetization of the composite powders. On the other hand, high defect densities such as microstrain, microcracks and gaps induced by mechanical milling may also result in a drift apart of the iron atoms, therefore a diminution of magnetic interaction and a decrease of saturation magnetization are predictable. Similar results of increasing milling time leads to a dissolution of nonmagnetic atoms into matrix and an increase of defect densities, and thus cause a decrease of saturation magnetization, were also reported elsewhere [25,28]. Fig. 8 presents the hysteresis loops for the composite powders (with 10 wt.% ZrO2) after milling for 2 h before and after annealing at 600 °C under argon atmosphere for 2 h. It can be found that the annealing process of the milled powders leads to a minor improvement of its saturation magnetization. Before annealing, the Ms and remanent magnetization (Mr) values are 203.78 emu/g and 2.21 emu/g, respectively. After annealing, the Ms and Mr are 205.56 emu/g and 1.62 emu/g, respectively. However, the Hc decreases drastically to from 27.66 Oe (before annealing) to 18.34 Oe (after annealing). This phenomenon should be attributed to the stresses release, which has already been discussed above. 3.3. Microstructure and magnetic properties of the Fe/ZrO2 soft magnetic composites (SMCs) Fig. 9(a) shows the SEM micrograph of the mechanical polished surface for Fe/ZrO2 SMCs (without etching). It can be observed that the grey-white regions were completely surrounded by net-shaped grey-black regions. And it can be found in Fig. 9(b) that the thickness of the cell-wall was about 1–2 µm, and the cell-body bonds well with the cell-wall. The EDS spectrum (Fig. 10) further revealed that grey-white regions were mainly Fe elements. Oxygen and zirconium elements lay mainly in grey-black regions. This phenomenon implied that Fe particles (the grey-white regions) were well separated and insulated by thin ZrO2 layers (the grey-black regions) in the Fe/ZrO2 magnetic powder cores.
Table 1 Density, resistivity and magnetic properties for pure Fe powder cores and composite powder cores with varying ZrO2 content. Samples
Density (g/cm3)
Resistivity (µXcm)
Ms (emu/g)
Hysteresis loss (kJ/m3)
Pure Fe 5 wt.% 7.5 wt.% 10 wt.% 10 wt.% (annealed) 12.5 wt.%
6.94 6.85 6.73 6.63 6.63 6.38
25.47 63.15 128.48 243.15 238.68 251.23
224.15 210.36 207.14 203.42 204.68 191.64
2.51 2.54 2.52 2.54 1.97 2.67
To investigate the effect of inorganic insulating materials on magnetic properties, pure Fe powder cores without ZrO2 powders were also prepared under the same conditions. The density, resistivity and magnetic properties of pure Fe powder cores and composite powder cores are summarized in Table 1. As shown in the table, the resistivity of the cores tends to increase as the ZrO2 content is increased up to 10 wt.%. But with further increasing ZrO2 content, the resistivity shows slight increase, which indicates that excess additions of ZrO2 can’t further improve insulation effect. There is no doubt that the additions of non-magnetic insulating materials should be minimized as long as they can provide sufficient electrical insulation. Therefore, the optimal content of ZrO2 for the composites in this work is 10 wt.%. Hysteresis loss (Wh) was obtained directly from the DC-hysteresis loops in this work. It can be found that all the samples almost possess an approximate value of hysteresis loss (Wh). However, the Wh value of the composite powder cores (with 10 wt.% ZrO2) decreased from 2.54 kJ/m3 (before annealing) to 1.97 kJ/m3 (after annealing). It is known that Wh in SMCs strongly depends on particle size, defect densities, impurities and internal microstrain introduced by mechanical deformation [29]. Therefore, the decrease in Wh of composite cores should be ascribed to the stress relief resulting from the annealing process. The total core loss as a function of frequency for pure Fe powder cores and composite powder cores measured from 1 kHz to 100 kHz
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Fig. 11. Total core loss as a function of frequency for pure Fe powder cores and composite powder cores with different ZrO2 contents, measured at maximum induction of 0.02 T in the frequency range from 1 kHz to 100 kHz.
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at maximum induction of 0.02 T is illustrated in Fig. 11. Comparing these curves, the following three points can be understood: 1. With increasing frequency, the core loss increases significantly. 2. The ZrO2-coated Fe powder cores exhibit much lower core loss than that of the pure Fe powder cores at high frequency range. 3. The core loss decreases remarkably up to 10 wt.% of the ZrO2 additions and displayed slight change with further increasing ZrO2 content.
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As mentioned above, total core loss in magnetic materials consists of hysteresis loss (Wh), eddy current loss (We), and residual loss (Wr). Among these values, Wr mostly depends on the size and arrangement of magnetic domains, which is minor in metallic material and can be negligible compared with Wh and We. We is primary loss especially in high frequency ranges, which can be expressed by [30]:
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W ¼ W h þ W e ¼ k1 Af þ
k2 B 2 t 2
q
f
2
ð1Þ
where k1 and k2 are proportionality constants, A is the area of the DC-hysteresis loop, B is the maximum flux density, t is the thickness of material, f is the frequency and q is the electrical resistivity. It is clear that Wh in this paper don’t determine the total core loss especially at high frequency because this part for all samples is equivalent and about 100 times smaller than the total core loss (at 50 kHz). We may reasonably conclude that a much lower core loss of the composite powder cores is caused by the reduction of eddy current loss (We). As shown in formula (1), We is proportional to the square of the thickness and inversely proportional to the q of the magnetic materials. Therefore, the decrease in We for the Fe/ZrO2 composite powder cores is caused by their higher electrical resistivity than that of pure Fe magnetic compacts. The micrograph of the polished surface of the composite compacts (with 10 wt.% ZrO2) demonstrates that the electric Fe particles are well separated by thin ZrO2 layers. Thus, most of the eddy currents can be limited within the ZrO2-insulated Fe particles, which is equivalent to diminishing the effective radius of the eddy current to reduce core loss [31]. But incomplete coating for the samples with less amount of ZrO2 (<10 wt.%) might still results in high eddy current loss.
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4. Conclusions
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Several conclusions could be drawn from the results described above:
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1. ZrO2-coated Fe powders and corresponding Fe/ZrO2 soft magnetic composites were successfully prepared in this study. 2. The structure and magnetic properties of composite powders was studied as a function of milling time. The results showed that mechanical milling could coat spherical Fe powders uniformly with ZrO2 nanopowders and induce defects and stress simultaneously. 3. We confirm that the ZrO2 insulating layers, which separate the conductive Fe particles from one to another, could avoid metal-to-metal contact and raise the resistivity of magnetic powder cores significantly. Hence the Fe/ZrO2 SMCs show a much lower core loss and more excellent magnetic properties, which provide a potential possibility to reach high energy conversion efficiency for electric-magnetic switching device.
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Acknowledgments
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This work was supported by the National Key Research and Development Program of the 13th Five-Year Plan of China (grant number 2016YFB1200602-02). Also, we thank for the technical support of Hunan Linkjoin Technology Co., Ltd, Changsha, China.
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