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Fe3O4 composite particles
Journal Pre-proofs Research articles Properties of Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe-6.5wt%Si/Fe3O4 composite particles Zigui Luo, Xi'an Fan, Wentao Hu, Fan Luo, Jian Wang, Zhaoyang Wu, Xin Liu, Guangqiang Li, Yawei Li PII: DOI: Reference:
S0304-8853(19)33802-8 https://doi.org/10.1016/j.jmmm.2019.166278 MAGMA 166278
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
1 November 2019 5 December 2019 6 December 2019
Please cite this article as: Z. Luo, X. Fan, W. Hu, F. Luo, J. Wang, Z. Wu, X. Liu, G. Li, Y. Li, Properties of Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe-6.5wt%Si/Fe3O4 composite particles, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.166278
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Properties of Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe-6.5wt%Si/Fe3O4 composite particles Zigui Luoa, c, Xi'an Fana, b, c*, Wentao Hua, c, Fan Luoa, c, Jian Wangd, e, Zhaoyang Wuf, Xin Liud, e, Guangqiang Lia, c, Yawei Lia, b a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China b National-Provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan University of Science and Technology, Wuhan 430081, China c Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China d Guangdong Institute of Materials and Processing, Guangdong Academy of Sciences, Guangzhou, 510650, China e National Engineering Research Center of Powder Metallurgy of Titanium ＆ Rare Metals, Guangdong Academy of Sciences, Guangzhou, 510650, China f Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Anhui University of Technology, Ma’anshan, 243002, China *Corresponding author: Xi'an Fan Address: P. O. Box 185#, School of Materials and Metallurgy, Wuhan University of Science and Technology, 947 Heping Road, Qingshan District, Wuhan, 430081, China. E-mail address: [email protected] / [email protected] 1
Abstract: In this work, Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites (SMCs) have been prepared successfully via ball milling Fe-6.5wt%Si/Fe3O4 composite particles combined with subsequent spark plasma sintering (SPS) process. The formation mechanism of Fe2SiO4/SiO2 insulating layer and effect of Fe3O4 coating content on the microstructure and properties for the Fe-Si SMCs were investigated in detail. It is interesting that high temperature SPS process would lead to the occurrence of redox reactions between Fe3O4 and Si from Fe-Si alloy cores, and resulted in the transformation from Fe3O4 coating layer to Fe2SiO4/SiO2 insulating layer. As a consequence, the novel Fe-Si SMCs exhibited superior performances including high saturation magnetization (190.4 emu/g) and low core loss (9.02 mW/cm3 at 10 kHz) decreased by 89.1 % compared to that of the uncoated Fe-Si SMCs (83.08 mW/cm3 at 10 kHz). The results indicated that the method of preparing insulating layer by the redox reactions is a simple and effective way to synthesize new SMCs with high performance for electro-magnetic energy conversion equipments. Keywords: Soft magnetic composites; Redox reaction; Fe2SiO4/SiO2 insulating layer; Fe3O4 coating layer; Core loss 1. Introduction With the development of high efficiency and energy saving for power electronic equipments such as motor, transformer and inductor, soft magnetic materials used for electro-magnetic energy conversion and transmission, should have the advantages of high saturation magnetic induction and low magnetic loss [1-3]. Soft magnetic alloys such as Fe-Si alloy exhibit superior soft magnetic properties, but there will be a sharp 2
increase in eddy current loss if applied at above 400 Hz [4]. Soft magnetic ferrites possess very low magnetic loss due to their high resistivity, whereas the saturation magnetic induction is only ~25% of that of soft magnetic alloys [5]. Therefore, soft magnetic composites (SMCs), composed of soft magnetic alloy particles and interparticle insulating coatings which can cut off the eddy current between alloy particles, display a increasing attention owing to their excellent performances such as relatively high saturation magnetization, good frequency stability of permeability, and very low eddy current loss at high frequencies [6-10]. In recent years, most of the researches on the insulating coatings are devoted to the use of inorganic insulating materials instead of organic insulators [11]. This is because organic insulators such as epoxy resins exhibit poor heat resistance and will decompose at above 200 °C, which indicates that the internal stresses induced by compaction pressure could not be released completely through a low temperature heat treatment unless annealed at a typical stress-relief temperature (570-775 °C) of Fe [12-15]. As a result that the coercivity would be worsen and the hysteresis loss would rise up. Therefore, inorganic insulating materials with superior chemical/thermal stability such as phosphates [16], SiO2 [17-19] and ferrites [3, 20], have been used as insulating coatings for SMCs. Despite the unsatisfactory insulation of Fe3O4 (~10-2 Ω·cm) [21], some studies have been carried out to coat Fe3O4 on ferromagnetic particles owing to its ferrimagnetism [22] via different coating methods such as chemical surface oxidation and physical mixing [23-25]. Obviously, there will be a large eddy current loss 3
generated in SMCs owing to that Fe3O4 coating cannot effectively block eddy current between magnetic particles especially at high frequencies. Moreover, SiO2 is more stable than Fe3O4 according to Ellingham diagram [26], which implies that SiO2 and Fe can be obtained by high temperature redox reaction between Si and Fe3O4. SiO2 is a kind of ceramic material with excellent electric insulation performance and Fe exhibits the highest saturation magnetization. Based on this, our works aims to minimize the magnetic weakening of coatings on SMCs, while providing excellent interparticle insulation as well as very low eddy current loss [27]. In this paper, nano-sized Fe3O4 particles were coated on the surfaces of Fe-6.5wt%Si alloy particles uniformly via ball milling. And then, The Fe-6.5wt%Si /Fe3O4 composite particles were pressed into compacts by spark plasma sintering which can provide high temperature for the redox reaction. During the sintering process, novel Fe2SiO4/SiO2 insulating layer around the Fe-Si alloy particles was obtained instead of the Fe3O4 coating[28], and a large amount of Fe atoms were generated, in which case the insulating layer can effectively restrict the generation of eddy current between Fe-Si alloy particles, and Fe atoms can maximize magnetic properties such as saturation magnetization. Moreover, the effects of Fe3O4 content on microstructure and magnetic properties are also presented and discussed. 2. Experimental procedure The spherical/quasi-spherical Fe-6.5wt%Si powders with the average particle size of about 53 μm, were purchased from Hunan Ruihua Hi-tech Materials Co., Ltd. The Fe3O4 nanoparticles with the average size of 20 nm, were purchased from 4
Aladdin Chemical Reagent Company. Firstly, the Fe-6.5wt%Si powders and Fe3O4 nano-particles were mixed by ball milling for 8 h with a speed of 150 r/min, in which case agate balls were used and the ball to material ratio was 20:1. The Fe3O4 coating content is 2.5 wt%, 5.0 wt%, 7.5 wt%, 10.0 wt%, 12.5 wt% and 15.0 wt% of the mixture, respectively. Then the raw Fe-6.5wt%Si powders and ball milled Fe-6.5wt%Si/Fe3O4 composite powders were pressed into a cylinder with 20.3 mm diameter by spark plasma sintering (SPS) at 850 °C for 10 min with a uniaxial pressure of 60 MPa under argon atmosphere. The heating rate is 50 °C/min. Finally, all the obtained compacts were annealed at 600 °C for 1 h to eliminate the internal stress educed by the SPS process. The density of the powders was measured by an automatic density analyzer (AccuPyc 1330). The particle size was measured by Laser Particle size analyzer (Mastersizer 2000). The surface morphology and element distribution of the polished surfaces for the samples were characterized by scanning electron microscopy (SEM, Nova400, FEI, America) equipped with an energy dispersive X-ray spectroscopy (EDS, IE350PentaFET-3, Oxford, England). The phases of the samples were analyzed by X-ray diffraction (XRD, D500, Siemens, Germany) at 2θ=10 °-90 ° with a step size of 0.02 ° using Cu Kα radiation. The composition and chemical state of the coatings were investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The DSC curve and TG curve of the sample was investigated using a NETZSCH STA-499C differential scanning calorimeter (DSC) under an argon atmosphere from 100 C to 1500 C with a 15 C/min heating rate. The 5
hysteresis loops and magnetic properties of the samples (2 mm×2 mm×2 mm) were measured using vibrating sample magnetometer (VSM, LakeShore7404). And the resistivity of the samples was measured by four-probe method. The effective permeability calculated by inductance (L) was measured by impedance analyzer (LCR IM3570A988-06, HIOKI, Japan) [17]. The tested ring samples (outer diameter of 20.3 mm and inner diameter of 12.7 mm) are wound with 20 turns of enameled wire. The core loss of the ring samples was obtained by a B-H Analyzer (SY-8219, Iwasaki, Japan) with an applied induction of 20 mT. 3. Results and discussion 3.1. Comparation of the raw Fe-6.5wt%Si SMCs and Fe3O4 coated Fe-Si SMCs Fig. 1 shows the particle size distribution of the Fe-6.5wt%Si powders before and after ball milling with nano-sized Fe3O4. As we can see that the particle size of the uncoated Fe-6.5wt%Si powders is mainly concentrated from ~20 μm to ~100 μm. After ball milling with Fe3O4, a small amount of particles ranging from ~1 μm to ~10 μm generated, which can be attributed to the agglomeration of Fe3O4, and the fragmentation of Fe-6.5wt%Si particles during the ball milling process. As shown in Table 1, the average particle size of the raw Fe-6.5wt%Si powders and Fe3O4 is 52.8 μm and 20 nm, respectively. After ball milling, the Fe-6.5wt%Si/5wt%Fe3O4 composite powders exhibit a relatively low average particle size of 49.5 μm, and lower densities compared to the raw Fe-6.5wt%Si powders. It is worth noting that the Fe-6.5wt%Si/5wt%Fe3O4 composite powders show superior soft magnetic properties such as high saturation magnetization (Ms), low coercivity (Hc) 6
and remanence (Mr). This is because the Fe3O4 coating is ferrimagnetic and can reduce the magnetic dilution. In addition, the chemical/thermal stability of the Fe-6.5wt%Si/5wt% Fe3O4 composite powders has been analysed by DSC curve and TG curve. As shown in Fig. 2, DSC curve shows that there are two endothermic peaks occurred at about 698 °C and 1477 °C, which correspond to the curie point and melting point. A small exothermic peak located at 858 °C can also be found, which should be due to the crystallization of nano-sized Fe3O4 particles. It is interesting that there are two broad exothermic peaks ranging from 1023°C to 1077°C (Peak 1) and 1077 °C to 1230°C (Peak 2), respectively. It implies that there might be some reactions accompanied by exothermic behavior occurred at Temperature 1 and Temperature 2. TG curve shows that the mass varies little with temperature, which indicates that there is no production of gaseous substances with the increase of the temperature. Fig. 3(a-c) shows the SEM images and XRD patterns of the uncoated and coated Fe-6.5wt%Si powders. It is easy to observe that the uncoated Fe-6.5wt%Si powders exhibit a smooth surface with some cracks, and the surface of the coated Fe-6.5wt%Si powders become rough, which is covered by some nano-particles as shown in the enlarged region from Fig. 3(b). Fig. 3(c) shows that the coating layer should be still Fe3O4 after ball milling. After sintering the uncoated and coated Fe-6.5wt%Si powders into compacts, it can be seen from Fig. 3(d, e), that uncoated Fe-Si SMCs exhibit smooth and dense surface morphology with no intergranular void, and the Fe-Si alloy particles are well separated by the coating layer for the coated Fe-Si SMCs. 7
Noting that the coating layer is mainly composed of dark area and grey area as shown in the enlarged region from Fig. 3(e), and corresponding line scanning has also bee carried out, it can be seen that the dark area is mainly Si and O elements, which should be SiO2 [26], the grey area should be a compound composed of Fe, Si and O elements. Fig. 3(f) confirms that the compound is a kind of iron silicate i.e. Fe2SiO4. The characteristic peak of SiO2 is not detected. This should be due to the SiO2 content is below the XRD detection limit. As a consequence, the coating layer has changed from Fe3O4 to Fe2SiO4/SiO2 after the high temperature SPS process. It is suggested that the high temperature redox reactions between Si and Fe3O4 can occur and generate ferromagnetic Fe and Fe2SiO4/SiO2 with good insulation, which would not only enhance magnetic performance, but also reduce eddy current loss for the coated Fe-Si SMCs. And the reactions can be expressed as follows: ① Fe 3 O4 Si Fe2 SiO4 Fe ② Fe 2 SiO4 Si 2 SiO2 2 Fe
(1) (2)
Due to discharge effect between the Fe-Si alloy particles during SPS process, the temperature near the coating layer is higher than the sintering temperature (850 °C). Therefore, the reaction (1) and (2) can occur at Temperature 1 and 2, respectively. As shown in Table 2, the corresponding magnetic properties and core loss of the uncoated and 5.0 wt% Fe3O4 coated Fe-Si SMCs has been tested. It is clear that the coated Fe-Si SMCs also exhibit excellent soft magnetic properties including high Ms (190.4 emu/g) and low Hc (4.0 Oe) owing to the generation of Fe atoms. On the other 8
hand, the core loss (9.02 mW/cm3 at 10 kHz) exhibits a marked decline and decreases by 89.1 % in comparation with that of the uncoated Fe-Si SMCs (83.08 mW/cm3 at 10 kHz), which should be due to the good insulation of Fe2SiO4/SiO2 coatings. 3.2 Effect of Fe3O4 coating content on microstructure and magnetic properties of the Fe-Si SMCs Fig. 4 shows the surface structures of the Fe-6.5wt%Si/Fe3O4 powders coated with different Fe3O4 contents. With increasing the coating content from 2.5 wt% to 15.0 wt%, the surfaces of the Fe-6.5wt%Si/Fe3O4 powders gradually become rough, and the shape becomes irregular especially when the coating content is above 10.0 wt%. This can be understood as a consequence of the uneven aggregation for the nano-sized Fe3O4 on the surfaces of the Fe-6.5wt%Si powders. XRD patterns in Fig. 5(a) shows that there are only Fe(Si) phase and Fe3O4 phase in the Fe-6.5wt%Si/Fe3O4
powders
after
ball
milling.
After
SPS
process,
the
Fe-6.5wt%Si/Fe3O4 powders are sintered into Fe-Si SMCs. Fig. 5(b) presents the corresponding XRD patterns. It can be seen that all the sample contain Fe(Si) phase and Fe2SiO4 phase. When the coating content is 7.5 wt%, a small characteristic peak corresponding to SiO2 (JCPDS Card No. 089-8938) can be observed. Fig. 6 shows the different Fe3O4 content coated Fe-Si SMCs’ backscatter images in which different substances can be distinguished by contrast. When the coating content increases from 2.5 wt% to 7.5 wt%, the Fe3O4 is reacted completely with the diffused Si from Fe-Si alloy cores at Temperature 1, thus the coating layer is mainly Fe2SiO4. However, it can be seen that the 7.5 wt% Fe3O4 coated Fe-Si SMCs exhibit a 9
relatively uneven coating layer. This may be because more SiO2 and Fe are generated due to the enhanced discharge effect as well as the increased temperature which results in more violent reaction (2). With increasing the coating content from 10.0 wt% to 15.0 wt%, there is residual Fe3O4 which has not been reacted occurred. As a consequence, only the reaction (1) occurs and no enough diffused Si can guarantee the reaction (2). Especially when the coating content is above 12.5 wt%, there is only Fe2SiO4 and Fe3O4 remained in the coating layer. In addition, XPS-peak-differenating analysis for the 5.0 wt% and 12.5 wt% Fe3O4 coated Fe-Si SMCs has also been carried out in Fig. 7. As we can see that both the two samples’ Fe2p peaks including Fe3+ peak (713.2 eV), Fe2+ peak (710.4 eV) and Fe0 peak (706.5 eV), are detected. Fig. 7(b) shows the O1s peak which can be divided into four obvious peaks located at 532.6 eV, 531.2 eV, 530.3 eV and 529.8 eV, corresponding to the SiO2, silicate, Fe3+ and Fe2+, respectively. It indicates that the coating layer for the Fe-Si SMCs coated with 5.0 wt% Fe3O4, mainly consists of iron silicate and SiO2. It is worth noting that the O1s peak for the 12.5 wt% Fe3O4 coated Fe-Si SMCs, exhibits a small peak for SiO2, which is obviously lower than that of the 5.0 wt% Fe3O4 coated Fe-Si SMCs. It implies the reduction of SiO2 content. The result is consistent with the corresponding SEM images in Fig. 6(b, f). Fig. 8 presents the effective permeability’s frequency stability for the Fe-Si SMCs coated with different Fe3O4 contents. With increasing the coating content from 2.5 wt% to 15.0 wt%, the effective permeability (5 kHz) gradually decreases from 81 to 36. This is because the increase of Fe3O4 coating content will lead to the increase of 10
nonmagnetic Fe2SiO4 content, and then result in the magnetic dilution as well as increased pinning sites between the Fe-Si magnetic particles, which would block the magnetic domain inversion and magnetic domain wall displacement. On the other hand, the frequency stability becomes better when the coating content increases due to the weakened skin effect caused by eddy current. Good frequency stability of effective permeability indicates that inductors can maintain stable inductance. Fig. 9 shows the hysteresis loops of the Fe-Si SMCs coated with different Fe3O4 contents. The corresponding saturation magnetization (Ms), coercivity (Hc) and resistivity (ρ) have also been presented in Table 3. As we can see that the Ms remains a high value (182.0 emu/g) even if the coating content is 15.0 wt%, which is attributed to the existence of Fe3O4 and generation of Fe atoms. When the coating content increases from 2.5 wt% to 15.0 wt%, the Ms gradually decreases, while the Hc exhibits an opposite trend. This should be due to the magnetic dilution induced by the increase of no magnetic coatings including Fe2SiO4 and SiO2. The no magnetic coatings would act as pinning sites to prevent the displacement of magnetic domains and the reversal of magnetic domains, ultimately leading to the decreased Ms and increased Hc. In addition, the resistivity first increases and then decreases when the coating content increases from 2.5 wt% to 7.5 wt%. According to the microstructure shown in Fig. 6, when the coating content is 7.5 wt%, the excessive Fe atoms generated by reaction (2) connect the Fe-Si alloy particles, and lead to an uneven coating layer. And with increasing the coating content from 7.5 wt% to 12.5 wt%, the resistivity exhibits an enhanced trend, which can be attribute to the increase of the 11
thickness of Fe2SiO4/SiO2 insulating layer. However, when the coating content reaches 15.0 wt%, the resistivity drops abruptly. Generally, excessive coatings by ball milling would result in uneven coating layer for SMCs. And Fe3O4 possesses poor resistivity, its increase in the coating layer may deteriorate the insulation of the Fe-Si SMCs. Except for soft magnetic properties, Core loss is an important parameter to measure SMCs’ performance. The smaller the core loss, the higher the energy transmission efficiency of the device. As we can see from Fig. 10, that the core loss increases with the increase of the frequency. And the increase of applied frequency will lead to dramatic increase for the core loss. It well known that core loss is mainly caused by magnetic lag and eddy current effect. As a result, core loss (Pcv) can be expressed by follows [6]:
Pcv Ph Pe f
HdB
C
( Bfd ) 2
(3)
Where f is the frequency, H is the magnetic field strength, B is the magnetic induction, C is the proportionality constant, d is the thickness of the SMCs and ρ is the resistivity. Hysteresis loss (Ph) generated by the former is related to the area of hysteresis loop and takes the main part of core loss at low frequency. When the applied frequency increases, eddy current effect will be more serious and lead to increased resistance loss i.e. eddy current loss (Pe). Therefore, core loss is mainly influenced by eddy current loss at high frequency. It can be seen that eddy current loss eddy current loss is inversely proportional to resistivity. Accordingly, when the coating content 12
increases from 2.5 wt% to 15.0 wt%, the core loss and resistivity exhibit opposite trends. 4. Conclusions In summary, Fe-Si soft magnetic composites (SMCs) coated with Fe2SiO4/SiO2 insulating
layer
were
prepared
successfully
via
sintering
ball
milled
Fe-6.5wt%Si/Fe3O4 composite particles. We have studied the formation mechanism and performance of the novel Fe-Si SMCs in detail. The effect of Fe3O4 coating content on microstructure and properties for the Fe-Si SMCs has also been investigated systematically. It is worth noting that the Fe3O4 coating layer would be transformed into Fe2SiO4/SiO2 insulating layer by redox reactions with diffused Si from Fe-Si alloy cores at high temperature sintering process. The coated Fe-Si SMCs exhibited high Ms (190.4 emu/g), low Hc (4.0 Oe) and very low Pcv (9.02 mW/cm3 at 10 kHz). The results indicate that the preparation method in this paper can synthesize novel Fe-Si SMCs with high performances, and provides a potential application in FeSi(Al, Cr) SMCs. Acknowledgements This work was supported by the National Natural Science Foundation of China (51674181), the Key Projects of Hubei Provincial Department of Education (D20151103) and Natural Science Foundation of Anhui Province (Grant No. 1908085QE190). References [1] D. Liu, C. Wu, M. Yan, J. Wang, Correlating the microstructure, growth 13
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Figure Captions Fig.1 Particle size distribution of the uncoated and 5 wt% Fe3O4 coated Fe-6.5wt%Si powders.
Fig.2 DSC curve and TG curve of the Fe-6.5wt%Si/5wt% Fe3O4 composite powders.
Fig.3 SEM images of the (a)uncoated and (b)coated Fe-6.5wt%Si powders with a larger image, and (c)corresponding XRD patterns; SEM images of polished surfaces for the (d)uncoated and (e)coated Fe-Si SMCs with EDS analysis, and (f)corresponding XRD patterns.
Fig. 4 SEM images of the Fe-6.5wt%Si/Fe3O4 powders coated with different Fe3O4 contents.
Fig. 5 XRD patterns of the (a)Fe-6.5wt%Si/Fe3O4 powders and (b)corresponding Fe-Si SMCs coated with different Fe3O4 contents.
Fig.6 SEM images of polished surfaces for the Fe-Si SMCs coated with different Fe3O4 contents.
Fig. 7 XPS analysis showing the Fe2p and O1s peaks of the Fe-Si SMCs coated with 5.0 wt% and 12.5 wt%. 18
Fig.8 Effective permeability as a function of frequency for the Fe-Si SMCs coated with different Fe3O4 contents.
Fig. 9 Hysteresis loops with (a) saturation magnetization and (b) coercivity for the Fe-Si SMCs coated with different Fe3O4 contents
Fig. 10 Core loss as a function of frequency for the Fe-Si SMCs coated with different Fe3O4 contents.
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Table 1 Average particle size, density and magnetic properties of Fe-6.5wt%Si, Fe3O4 and Fe-6.5wt%Si/5wt%Fe3O4 powders
Material
Density
Ms
Hc
Mr
(g/cm3)
(emu/g)
(Oe)
(emu/g)
52.8 μm
7.46
195.5
3.2
0.25
20
nm
4.95
59.3
21.0
2.1
49.5 μm
7.12
188.6
5.0
0.32
Particle size
Fe-6.5wt%Si Fe3O4 Fe-6.5wt%Si/5wt%Fe 3O4
Table 2 Magnetic properties and core loss of the uncoated and 5.0 wt% Fe3O4 coated Fe-Si SMCs Core loss (mW/cm3)
Ms
Hc
(emu/g)
(Oe)
5 kHz
10 kHz
20 kHz
uncoated
196.8
2.0
30.64
83.08
224.85
coated
190.4
4.0
2.92
9.02
32.13
Sample
Table 3 Magnetic and electric properties of the Fe-Si SMCs coated with different Fe3O4 coating contents Physical
Fe3O4 coating content
quantity
2.5 wt%
5.0 wt%
7.5 wt%
10.0 wt%
12.5 wt%
15.0 wt%
Ms (emu/g)
191.3
190.4
188.4
183.9
182.8
182.0
Hc (Oe)
3.1
4.0
4.6
5.0
5.7
5.7
31
ρ (μΩ·m)
72.6
112.6
45.1
74.0
126.3
55.6
Highlights 1. Fe2SiO4/SiO2 coated Fe-Si SMCs transformed from Fe-6.5wt%Si/Fe3O4 composites were prepared. 2. The transformation mechanism was revealed in detail. 3. Effect of Fe3O4 coating content on structure and magnetic properties was studied. 4. The novel Fe-Si SMCs exhibited high Ms (190.4 emu/g) and very low Pcv (9.02 mW/cm3).
32
S0304-8853(19)33802-8 https://doi.org/10.1016/j.jmmm.2019.166278 MAGMA 166278
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
1 November 2019 5 December 2019 6 December 2019
Please cite this article as: Z. Luo, X. Fan, W. Hu, F. Luo, J. Wang, Z. Wu, X. Liu, G. Li, Y. Li, Properties of Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe-6.5wt%Si/Fe3O4 composite particles, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.166278
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Properties of Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites prepared by sintering Fe-6.5wt%Si/Fe3O4 composite particles Zigui Luoa, c, Xi'an Fana, b, c*, Wentao Hua, c, Fan Luoa, c, Jian Wangd, e, Zhaoyang Wuf, Xin Liud, e, Guangqiang Lia, c, Yawei Lia, b a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China b National-Provincial Joint Engineering Research Center of High Temperature Materials and Lining Technology, Wuhan University of Science and Technology, Wuhan 430081, China c Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China d Guangdong Institute of Materials and Processing, Guangdong Academy of Sciences, Guangzhou, 510650, China e National Engineering Research Center of Powder Metallurgy of Titanium ＆ Rare Metals, Guangdong Academy of Sciences, Guangzhou, 510650, China f Key Laboratory of Metallurgical Emission Reduction & Resources Recycling, Ministry of Education, Anhui University of Technology, Ma’anshan, 243002, China *Corresponding author: Xi'an Fan Address: P. O. Box 185#, School of Materials and Metallurgy, Wuhan University of Science and Technology, 947 Heping Road, Qingshan District, Wuhan, 430081, China. E-mail address: [email protected] / [email protected] 1
Abstract: In this work, Fe2SiO4/SiO2 coated Fe-Si soft magnetic composites (SMCs) have been prepared successfully via ball milling Fe-6.5wt%Si/Fe3O4 composite particles combined with subsequent spark plasma sintering (SPS) process. The formation mechanism of Fe2SiO4/SiO2 insulating layer and effect of Fe3O4 coating content on the microstructure and properties for the Fe-Si SMCs were investigated in detail. It is interesting that high temperature SPS process would lead to the occurrence of redox reactions between Fe3O4 and Si from Fe-Si alloy cores, and resulted in the transformation from Fe3O4 coating layer to Fe2SiO4/SiO2 insulating layer. As a consequence, the novel Fe-Si SMCs exhibited superior performances including high saturation magnetization (190.4 emu/g) and low core loss (9.02 mW/cm3 at 10 kHz) decreased by 89.1 % compared to that of the uncoated Fe-Si SMCs (83.08 mW/cm3 at 10 kHz). The results indicated that the method of preparing insulating layer by the redox reactions is a simple and effective way to synthesize new SMCs with high performance for electro-magnetic energy conversion equipments. Keywords: Soft magnetic composites; Redox reaction; Fe2SiO4/SiO2 insulating layer; Fe3O4 coating layer; Core loss 1. Introduction With the development of high efficiency and energy saving for power electronic equipments such as motor, transformer and inductor, soft magnetic materials used for electro-magnetic energy conversion and transmission, should have the advantages of high saturation magnetic induction and low magnetic loss [1-3]. Soft magnetic alloys such as Fe-Si alloy exhibit superior soft magnetic properties, but there will be a sharp 2
increase in eddy current loss if applied at above 400 Hz [4]. Soft magnetic ferrites possess very low magnetic loss due to their high resistivity, whereas the saturation magnetic induction is only ~25% of that of soft magnetic alloys [5]. Therefore, soft magnetic composites (SMCs), composed of soft magnetic alloy particles and interparticle insulating coatings which can cut off the eddy current between alloy particles, display a increasing attention owing to their excellent performances such as relatively high saturation magnetization, good frequency stability of permeability, and very low eddy current loss at high frequencies [6-10]. In recent years, most of the researches on the insulating coatings are devoted to the use of inorganic insulating materials instead of organic insulators [11]. This is because organic insulators such as epoxy resins exhibit poor heat resistance and will decompose at above 200 °C, which indicates that the internal stresses induced by compaction pressure could not be released completely through a low temperature heat treatment unless annealed at a typical stress-relief temperature (570-775 °C) of Fe [12-15]. As a result that the coercivity would be worsen and the hysteresis loss would rise up. Therefore, inorganic insulating materials with superior chemical/thermal stability such as phosphates [16], SiO2 [17-19] and ferrites [3, 20], have been used as insulating coatings for SMCs. Despite the unsatisfactory insulation of Fe3O4 (~10-2 Ω·cm) [21], some studies have been carried out to coat Fe3O4 on ferromagnetic particles owing to its ferrimagnetism [22] via different coating methods such as chemical surface oxidation and physical mixing [23-25]. Obviously, there will be a large eddy current loss 3
generated in SMCs owing to that Fe3O4 coating cannot effectively block eddy current between magnetic particles especially at high frequencies. Moreover, SiO2 is more stable than Fe3O4 according to Ellingham diagram [26], which implies that SiO2 and Fe can be obtained by high temperature redox reaction between Si and Fe3O4. SiO2 is a kind of ceramic material with excellent electric insulation performance and Fe exhibits the highest saturation magnetization. Based on this, our works aims to minimize the magnetic weakening of coatings on SMCs, while providing excellent interparticle insulation as well as very low eddy current loss [27]. In this paper, nano-sized Fe3O4 particles were coated on the surfaces of Fe-6.5wt%Si alloy particles uniformly via ball milling. And then, The Fe-6.5wt%Si /Fe3O4 composite particles were pressed into compacts by spark plasma sintering which can provide high temperature for the redox reaction. During the sintering process, novel Fe2SiO4/SiO2 insulating layer around the Fe-Si alloy particles was obtained instead of the Fe3O4 coating[28], and a large amount of Fe atoms were generated, in which case the insulating layer can effectively restrict the generation of eddy current between Fe-Si alloy particles, and Fe atoms can maximize magnetic properties such as saturation magnetization. Moreover, the effects of Fe3O4 content on microstructure and magnetic properties are also presented and discussed. 2. Experimental procedure The spherical/quasi-spherical Fe-6.5wt%Si powders with the average particle size of about 53 μm, were purchased from Hunan Ruihua Hi-tech Materials Co., Ltd. The Fe3O4 nanoparticles with the average size of 20 nm, were purchased from 4
Aladdin Chemical Reagent Company. Firstly, the Fe-6.5wt%Si powders and Fe3O4 nano-particles were mixed by ball milling for 8 h with a speed of 150 r/min, in which case agate balls were used and the ball to material ratio was 20:1. The Fe3O4 coating content is 2.5 wt%, 5.0 wt%, 7.5 wt%, 10.0 wt%, 12.5 wt% and 15.0 wt% of the mixture, respectively. Then the raw Fe-6.5wt%Si powders and ball milled Fe-6.5wt%Si/Fe3O4 composite powders were pressed into a cylinder with 20.3 mm diameter by spark plasma sintering (SPS) at 850 °C for 10 min with a uniaxial pressure of 60 MPa under argon atmosphere. The heating rate is 50 °C/min. Finally, all the obtained compacts were annealed at 600 °C for 1 h to eliminate the internal stress educed by the SPS process. The density of the powders was measured by an automatic density analyzer (AccuPyc 1330). The particle size was measured by Laser Particle size analyzer (Mastersizer 2000). The surface morphology and element distribution of the polished surfaces for the samples were characterized by scanning electron microscopy (SEM, Nova400, FEI, America) equipped with an energy dispersive X-ray spectroscopy (EDS, IE350PentaFET-3, Oxford, England). The phases of the samples were analyzed by X-ray diffraction (XRD, D500, Siemens, Germany) at 2θ=10 °-90 ° with a step size of 0.02 ° using Cu Kα radiation. The composition and chemical state of the coatings were investigated by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI). The DSC curve and TG curve of the sample was investigated using a NETZSCH STA-499C differential scanning calorimeter (DSC) under an argon atmosphere from 100 C to 1500 C with a 15 C/min heating rate. The 5
hysteresis loops and magnetic properties of the samples (2 mm×2 mm×2 mm) were measured using vibrating sample magnetometer (VSM, LakeShore7404). And the resistivity of the samples was measured by four-probe method. The effective permeability calculated by inductance (L) was measured by impedance analyzer (LCR IM3570A988-06, HIOKI, Japan) [17]. The tested ring samples (outer diameter of 20.3 mm and inner diameter of 12.7 mm) are wound with 20 turns of enameled wire. The core loss of the ring samples was obtained by a B-H Analyzer (SY-8219, Iwasaki, Japan) with an applied induction of 20 mT. 3. Results and discussion 3.1. Comparation of the raw Fe-6.5wt%Si SMCs and Fe3O4 coated Fe-Si SMCs Fig. 1 shows the particle size distribution of the Fe-6.5wt%Si powders before and after ball milling with nano-sized Fe3O4. As we can see that the particle size of the uncoated Fe-6.5wt%Si powders is mainly concentrated from ~20 μm to ~100 μm. After ball milling with Fe3O4, a small amount of particles ranging from ~1 μm to ~10 μm generated, which can be attributed to the agglomeration of Fe3O4, and the fragmentation of Fe-6.5wt%Si particles during the ball milling process. As shown in Table 1, the average particle size of the raw Fe-6.5wt%Si powders and Fe3O4 is 52.8 μm and 20 nm, respectively. After ball milling, the Fe-6.5wt%Si/5wt%Fe3O4 composite powders exhibit a relatively low average particle size of 49.5 μm, and lower densities compared to the raw Fe-6.5wt%Si powders. It is worth noting that the Fe-6.5wt%Si/5wt%Fe3O4 composite powders show superior soft magnetic properties such as high saturation magnetization (Ms), low coercivity (Hc) 6
and remanence (Mr). This is because the Fe3O4 coating is ferrimagnetic and can reduce the magnetic dilution. In addition, the chemical/thermal stability of the Fe-6.5wt%Si/5wt% Fe3O4 composite powders has been analysed by DSC curve and TG curve. As shown in Fig. 2, DSC curve shows that there are two endothermic peaks occurred at about 698 °C and 1477 °C, which correspond to the curie point and melting point. A small exothermic peak located at 858 °C can also be found, which should be due to the crystallization of nano-sized Fe3O4 particles. It is interesting that there are two broad exothermic peaks ranging from 1023°C to 1077°C (Peak 1) and 1077 °C to 1230°C (Peak 2), respectively. It implies that there might be some reactions accompanied by exothermic behavior occurred at Temperature 1 and Temperature 2. TG curve shows that the mass varies little with temperature, which indicates that there is no production of gaseous substances with the increase of the temperature. Fig. 3(a-c) shows the SEM images and XRD patterns of the uncoated and coated Fe-6.5wt%Si powders. It is easy to observe that the uncoated Fe-6.5wt%Si powders exhibit a smooth surface with some cracks, and the surface of the coated Fe-6.5wt%Si powders become rough, which is covered by some nano-particles as shown in the enlarged region from Fig. 3(b). Fig. 3(c) shows that the coating layer should be still Fe3O4 after ball milling. After sintering the uncoated and coated Fe-6.5wt%Si powders into compacts, it can be seen from Fig. 3(d, e), that uncoated Fe-Si SMCs exhibit smooth and dense surface morphology with no intergranular void, and the Fe-Si alloy particles are well separated by the coating layer for the coated Fe-Si SMCs. 7
Noting that the coating layer is mainly composed of dark area and grey area as shown in the enlarged region from Fig. 3(e), and corresponding line scanning has also bee carried out, it can be seen that the dark area is mainly Si and O elements, which should be SiO2 [26], the grey area should be a compound composed of Fe, Si and O elements. Fig. 3(f) confirms that the compound is a kind of iron silicate i.e. Fe2SiO4. The characteristic peak of SiO2 is not detected. This should be due to the SiO2 content is below the XRD detection limit. As a consequence, the coating layer has changed from Fe3O4 to Fe2SiO4/SiO2 after the high temperature SPS process. It is suggested that the high temperature redox reactions between Si and Fe3O4 can occur and generate ferromagnetic Fe and Fe2SiO4/SiO2 with good insulation, which would not only enhance magnetic performance, but also reduce eddy current loss for the coated Fe-Si SMCs. And the reactions can be expressed as follows: ① Fe 3 O4 Si Fe2 SiO4 Fe ② Fe 2 SiO4 Si 2 SiO2 2 Fe
(1) (2)
Due to discharge effect between the Fe-Si alloy particles during SPS process, the temperature near the coating layer is higher than the sintering temperature (850 °C). Therefore, the reaction (1) and (2) can occur at Temperature 1 and 2, respectively. As shown in Table 2, the corresponding magnetic properties and core loss of the uncoated and 5.0 wt% Fe3O4 coated Fe-Si SMCs has been tested. It is clear that the coated Fe-Si SMCs also exhibit excellent soft magnetic properties including high Ms (190.4 emu/g) and low Hc (4.0 Oe) owing to the generation of Fe atoms. On the other 8
hand, the core loss (9.02 mW/cm3 at 10 kHz) exhibits a marked decline and decreases by 89.1 % in comparation with that of the uncoated Fe-Si SMCs (83.08 mW/cm3 at 10 kHz), which should be due to the good insulation of Fe2SiO4/SiO2 coatings. 3.2 Effect of Fe3O4 coating content on microstructure and magnetic properties of the Fe-Si SMCs Fig. 4 shows the surface structures of the Fe-6.5wt%Si/Fe3O4 powders coated with different Fe3O4 contents. With increasing the coating content from 2.5 wt% to 15.0 wt%, the surfaces of the Fe-6.5wt%Si/Fe3O4 powders gradually become rough, and the shape becomes irregular especially when the coating content is above 10.0 wt%. This can be understood as a consequence of the uneven aggregation for the nano-sized Fe3O4 on the surfaces of the Fe-6.5wt%Si powders. XRD patterns in Fig. 5(a) shows that there are only Fe(Si) phase and Fe3O4 phase in the Fe-6.5wt%Si/Fe3O4
powders
after
ball
milling.
After
SPS
process,
the
Fe-6.5wt%Si/Fe3O4 powders are sintered into Fe-Si SMCs. Fig. 5(b) presents the corresponding XRD patterns. It can be seen that all the sample contain Fe(Si) phase and Fe2SiO4 phase. When the coating content is 7.5 wt%, a small characteristic peak corresponding to SiO2 (JCPDS Card No. 089-8938) can be observed. Fig. 6 shows the different Fe3O4 content coated Fe-Si SMCs’ backscatter images in which different substances can be distinguished by contrast. When the coating content increases from 2.5 wt% to 7.5 wt%, the Fe3O4 is reacted completely with the diffused Si from Fe-Si alloy cores at Temperature 1, thus the coating layer is mainly Fe2SiO4. However, it can be seen that the 7.5 wt% Fe3O4 coated Fe-Si SMCs exhibit a 9
relatively uneven coating layer. This may be because more SiO2 and Fe are generated due to the enhanced discharge effect as well as the increased temperature which results in more violent reaction (2). With increasing the coating content from 10.0 wt% to 15.0 wt%, there is residual Fe3O4 which has not been reacted occurred. As a consequence, only the reaction (1) occurs and no enough diffused Si can guarantee the reaction (2). Especially when the coating content is above 12.5 wt%, there is only Fe2SiO4 and Fe3O4 remained in the coating layer. In addition, XPS-peak-differenating analysis for the 5.0 wt% and 12.5 wt% Fe3O4 coated Fe-Si SMCs has also been carried out in Fig. 7. As we can see that both the two samples’ Fe2p peaks including Fe3+ peak (713.2 eV), Fe2+ peak (710.4 eV) and Fe0 peak (706.5 eV), are detected. Fig. 7(b) shows the O1s peak which can be divided into four obvious peaks located at 532.6 eV, 531.2 eV, 530.3 eV and 529.8 eV, corresponding to the SiO2, silicate, Fe3+ and Fe2+, respectively. It indicates that the coating layer for the Fe-Si SMCs coated with 5.0 wt% Fe3O4, mainly consists of iron silicate and SiO2. It is worth noting that the O1s peak for the 12.5 wt% Fe3O4 coated Fe-Si SMCs, exhibits a small peak for SiO2, which is obviously lower than that of the 5.0 wt% Fe3O4 coated Fe-Si SMCs. It implies the reduction of SiO2 content. The result is consistent with the corresponding SEM images in Fig. 6(b, f). Fig. 8 presents the effective permeability’s frequency stability for the Fe-Si SMCs coated with different Fe3O4 contents. With increasing the coating content from 2.5 wt% to 15.0 wt%, the effective permeability (5 kHz) gradually decreases from 81 to 36. This is because the increase of Fe3O4 coating content will lead to the increase of 10
nonmagnetic Fe2SiO4 content, and then result in the magnetic dilution as well as increased pinning sites between the Fe-Si magnetic particles, which would block the magnetic domain inversion and magnetic domain wall displacement. On the other hand, the frequency stability becomes better when the coating content increases due to the weakened skin effect caused by eddy current. Good frequency stability of effective permeability indicates that inductors can maintain stable inductance. Fig. 9 shows the hysteresis loops of the Fe-Si SMCs coated with different Fe3O4 contents. The corresponding saturation magnetization (Ms), coercivity (Hc) and resistivity (ρ) have also been presented in Table 3. As we can see that the Ms remains a high value (182.0 emu/g) even if the coating content is 15.0 wt%, which is attributed to the existence of Fe3O4 and generation of Fe atoms. When the coating content increases from 2.5 wt% to 15.0 wt%, the Ms gradually decreases, while the Hc exhibits an opposite trend. This should be due to the magnetic dilution induced by the increase of no magnetic coatings including Fe2SiO4 and SiO2. The no magnetic coatings would act as pinning sites to prevent the displacement of magnetic domains and the reversal of magnetic domains, ultimately leading to the decreased Ms and increased Hc. In addition, the resistivity first increases and then decreases when the coating content increases from 2.5 wt% to 7.5 wt%. According to the microstructure shown in Fig. 6, when the coating content is 7.5 wt%, the excessive Fe atoms generated by reaction (2) connect the Fe-Si alloy particles, and lead to an uneven coating layer. And with increasing the coating content from 7.5 wt% to 12.5 wt%, the resistivity exhibits an enhanced trend, which can be attribute to the increase of the 11
thickness of Fe2SiO4/SiO2 insulating layer. However, when the coating content reaches 15.0 wt%, the resistivity drops abruptly. Generally, excessive coatings by ball milling would result in uneven coating layer for SMCs. And Fe3O4 possesses poor resistivity, its increase in the coating layer may deteriorate the insulation of the Fe-Si SMCs. Except for soft magnetic properties, Core loss is an important parameter to measure SMCs’ performance. The smaller the core loss, the higher the energy transmission efficiency of the device. As we can see from Fig. 10, that the core loss increases with the increase of the frequency. And the increase of applied frequency will lead to dramatic increase for the core loss. It well known that core loss is mainly caused by magnetic lag and eddy current effect. As a result, core loss (Pcv) can be expressed by follows [6]:
Pcv Ph Pe f
HdB
C
( Bfd ) 2
(3)
Where f is the frequency, H is the magnetic field strength, B is the magnetic induction, C is the proportionality constant, d is the thickness of the SMCs and ρ is the resistivity. Hysteresis loss (Ph) generated by the former is related to the area of hysteresis loop and takes the main part of core loss at low frequency. When the applied frequency increases, eddy current effect will be more serious and lead to increased resistance loss i.e. eddy current loss (Pe). Therefore, core loss is mainly influenced by eddy current loss at high frequency. It can be seen that eddy current loss eddy current loss is inversely proportional to resistivity. Accordingly, when the coating content 12
increases from 2.5 wt% to 15.0 wt%, the core loss and resistivity exhibit opposite trends. 4. Conclusions In summary, Fe-Si soft magnetic composites (SMCs) coated with Fe2SiO4/SiO2 insulating
layer
were
prepared
successfully
via
sintering
ball
milled
Fe-6.5wt%Si/Fe3O4 composite particles. We have studied the formation mechanism and performance of the novel Fe-Si SMCs in detail. The effect of Fe3O4 coating content on microstructure and properties for the Fe-Si SMCs has also been investigated systematically. It is worth noting that the Fe3O4 coating layer would be transformed into Fe2SiO4/SiO2 insulating layer by redox reactions with diffused Si from Fe-Si alloy cores at high temperature sintering process. The coated Fe-Si SMCs exhibited high Ms (190.4 emu/g), low Hc (4.0 Oe) and very low Pcv (9.02 mW/cm3 at 10 kHz). The results indicate that the preparation method in this paper can synthesize novel Fe-Si SMCs with high performances, and provides a potential application in FeSi(Al, Cr) SMCs. Acknowledgements This work was supported by the National Natural Science Foundation of China (51674181), the Key Projects of Hubei Provincial Department of Education (D20151103) and Natural Science Foundation of Anhui Province (Grant No. 1908085QE190). References [1] D. Liu, C. Wu, M. Yan, J. Wang, Correlating the microstructure, growth 13
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Figure Captions Fig.1 Particle size distribution of the uncoated and 5 wt% Fe3O4 coated Fe-6.5wt%Si powders.
Fig.2 DSC curve and TG curve of the Fe-6.5wt%Si/5wt% Fe3O4 composite powders.
Fig.3 SEM images of the (a)uncoated and (b)coated Fe-6.5wt%Si powders with a larger image, and (c)corresponding XRD patterns; SEM images of polished surfaces for the (d)uncoated and (e)coated Fe-Si SMCs with EDS analysis, and (f)corresponding XRD patterns.
Fig. 4 SEM images of the Fe-6.5wt%Si/Fe3O4 powders coated with different Fe3O4 contents.
Fig. 5 XRD patterns of the (a)Fe-6.5wt%Si/Fe3O4 powders and (b)corresponding Fe-Si SMCs coated with different Fe3O4 contents.
Fig.6 SEM images of polished surfaces for the Fe-Si SMCs coated with different Fe3O4 contents.
Fig. 7 XPS analysis showing the Fe2p and O1s peaks of the Fe-Si SMCs coated with 5.0 wt% and 12.5 wt%. 18
Fig.8 Effective permeability as a function of frequency for the Fe-Si SMCs coated with different Fe3O4 contents.
Fig. 9 Hysteresis loops with (a) saturation magnetization and (b) coercivity for the Fe-Si SMCs coated with different Fe3O4 contents
Fig. 10 Core loss as a function of frequency for the Fe-Si SMCs coated with different Fe3O4 contents.
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Table 1 Average particle size, density and magnetic properties of Fe-6.5wt%Si, Fe3O4 and Fe-6.5wt%Si/5wt%Fe3O4 powders
Material
Density
Ms
Hc
Mr
(g/cm3)
(emu/g)
(Oe)
(emu/g)
52.8 μm
7.46
195.5
3.2
0.25
20
nm
4.95
59.3
21.0
2.1
49.5 μm
7.12
188.6
5.0
0.32
Particle size
Fe-6.5wt%Si Fe3O4 Fe-6.5wt%Si/5wt%Fe 3O4
Table 2 Magnetic properties and core loss of the uncoated and 5.0 wt% Fe3O4 coated Fe-Si SMCs Core loss (mW/cm3)
Ms
Hc
(emu/g)
(Oe)
5 kHz
10 kHz
20 kHz
uncoated
196.8
2.0
30.64
83.08
224.85
coated
190.4
4.0
2.92
9.02
32.13
Sample
Table 3 Magnetic and electric properties of the Fe-Si SMCs coated with different Fe3O4 coating contents Physical
Fe3O4 coating content
quantity
2.5 wt%
5.0 wt%
7.5 wt%
10.0 wt%
12.5 wt%
15.0 wt%
Ms (emu/g)
191.3
190.4
188.4
183.9
182.8
182.0
Hc (Oe)
3.1
4.0
4.6
5.0
5.7
5.7
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ρ (μΩ·m)
72.6
112.6
45.1
74.0
126.3
55.6
Highlights 1. Fe2SiO4/SiO2 coated Fe-Si SMCs transformed from Fe-6.5wt%Si/Fe3O4 composites were prepared. 2. The transformation mechanism was revealed in detail. 3. Effect of Fe3O4 coating content on structure and magnetic properties was studied. 4. The novel Fe-Si SMCs exhibited high Ms (190.4 emu/g) and very low Pcv (9.02 mW/cm3).
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