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Magnetic properties of iron-based soft magnetic composites with SiO2 coating obtained by reverse microemulsion method
Journal of Magnetism and Magnetic Materials 381 (2015) 451–456
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of iron-based soft magnetic composites with SiO2 coating obtained by reverse microemulsion method Shen Wu a,b, Aizhi Sun a,b,n, Zhenwen Lu a, Chuan Cheng a, Xuexu Gao b a b
School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 February 2014 Received in revised form 29 December 2014 Accepted 12 January 2015 Available online 13 January 2015
In this work, iron-based soft magnetic composites coated with the amorphous SiO2 layer have been fabricated by utilizing tetraethoxysilane in the reverse microemulsion method, and then the effects of addition amount of SiO2 and annealing temperature on the magnetic properties were investigated. The results show that the surface of iron powders contains a thin amorphous SiO2 insulation layer, which effectively decreases the magnetic loss of synthesized magnets. The magnetic loss of coated samples decreased by 87.8% as compared with that of uncoated samples at 150 kHz. Magnetic measurements show that the sample with 1.25 wt% SiO2 has an acceptable real part and minimum imaginary part of permeability in comparison with other samples. Also, the annealing treatment increased the initial permeability, the maximum permeability and the magnetic induction and decreased the coercivity with increasing temperature in the range 300–600 °C. The results of the loss separation imply that the annealed SMCs have a higher hysteresis loss coefficient (k2) and lower eddy current loss coefficient (k3) as compared with the pure iron compacts after the same heat treatment due to the preservation of the SiO2 layer. & 2015 Elsevier B.V. All rights reserved.
Keywords: Soft magnetic composites Annealing treatment Permeability Loss separation
1. Introduction With growing requirements of miniaturization for powder transformers, computers and inductors, soft magnetic composites (SMCs) with high flux density, permeability and low magnetic loss have a huge market for various applications [1–3]. Usually, SMCs are produced by the powder metallurgy method from ferromagnetic particles coated with a thin electrically insulating layer. In the forming procedure of SMCs, the dislocation density and imperfections would be introduced to the particles by cold work, leading to impede the movement of domain walls and increase the hysteresis loss of produced SMCs [4,5]. In order to reduce the hysteresis loss, a specific heat treatment for stress-relief is often conducted after the compaction. It is generally known that the typical stress-relief temperature for pure iron is between 570 °C and 775 °C [6], and most of the organic coatings would be decomposed under this temperature range. Therefore, many studies have been focused on finding the inorganic coatings with high thermal resistance such as phosphate, MgO and Al2O3 [7–9]. In recent years, silica as the coating agent has been used to n Corresponding author at: School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Fax: 86 10 623 33375. E-mail address: [email protected] (A. Sun).
http://dx.doi.org/10.1016/j.jmmm.2015.01.030 0304-8853/& 2015 Elsevier B.V. All rights reserved.
prepare magnetic nanocomposites, and these silica-coated magnetic nanoparticles have many advanced characteristics in the microstructure, magnetic properties and thermal stability. Silica coated iron submicrometre laminates have been investigated by Zhao et al., which shows a latent use in high frequency applications with a maximum operating frequency up to 50 MHz [10]. In addition, Yang et al. investigated the structure and magnetic properties of Fe/SiO2 SMCs by controlled hydrolysation of tetraethyl orthosilicate (TEOS), reporting that the core loss was only 3.5 W kg 1 when measured at a frequency of 50 Hz and an induction level of 1 T due to the preservation of the SiO2 layer [11]. However, the key factors such as the coating amount, thickness of silica coats and annealing temperature effects are still missing in the above documented works. Thus, it is reasonable and necessary to do more fundamental investigations about the factors determining the performances of produced SMCs. On the other hand, among various SiO2 coating techniques, the reverse microemulsion method has aroused much scientific interest because it can produce homogeneous and narrow distributed nanoparticles and achieve in situ surface modification of nanoparticles [12,13]. In this study, in order to increase the annealing temperature and obtain homogeneous thin insulating layer, a new kind of SMCs coated with amorphous SiO2 insulating layer with high thermal stability was produced by the reverse microemulsion method from tetraethoxysilane (TEOS). With an aim to expand the applications
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of soft magnetic composite materials in the near future, we attempt to clarify the effects of SiO2 content and annealing temperature on their magnetic properties. Furthermore, different components of the core loss factor were calculated for the asprepared and annealed SMCs.
2. Experimental details 2.1. Materials Iron powder (supplied by Licheng Co., Ltd.) with a particle size o150 μm (Fig. 1(a)) was used as the basic ferromagnetic material. Octyl phenol ethoxylates (OP-10, 99%, Merck), n-butanol (99%, Aldrich), ammonia (NH3, 26% aq., Aldrich) and cyclohexane (99%, Merck) were used for synthesis of reverse microemulsion. Tetraethoxysilane (TEOS, 99%, Merck) was used as a source for SiO2.
Fig. 1. SEM image of (a) and (b) original iron powders; (c) EDS analysis of box in (b); (d) cross-sectional photograph of insulated iron powders; (e) Fe–SiO2 powders and (f) EDS analysis of box in (e).
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2.2. Composite fabrication The detailed experimental procedure to prepare the SiO2 coated compacts are as follows: (a) 100 g of iron particle powders were washed by using acetone solution for three times to clean the surface of iron particles; (b) 67.6 mL of cyclohexane and 15.5 mL of OP-10 (surfactants), 17.6 mL of n-butanol (oil phase) and 0.4 mL of ammonia (water phase) were mixed in an Erlenmeyer flask, and then the mixture was stirred with an electrical stirrer for 1 h at room temperature; (c) the cleaned iron powder and a predetermined amount of TEOS (1.9 ml, 2.8 ml, 3.7 ml, 4.6 ml and 5.5 ml) were slowly added to the microemulsion, and then the powders were rapidly stirred for 3 h keeping the pH-values at 9. For adjusting the pH-value, ammonia was added to the microemulsion; (d) after stirring, used 25 vol% distilled water and 75 vol% acetone solution as demulsifier, the lower layer suspension was filtered, and then washed with distilled water several times, finally dried under vacuum for 2 h at 50 °C to obtain SiO2 coated iron particles; (e) the SiO2 coated iron powders were pressed at 500 MPa, and then the produced compacts were annealed in nitrogen at 300, 500 and 600 °C for 1 h.
Fig. 2. XRD patterns of the pure SiO2 powders and the SiO2 coated powders.
2.3. Characterization The SiO2 insulating layer was characterized by scanning electron microscopy (SEM, ZEISS EVO 18, Germany) coupled with energy dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (Philips APD-10 diffractometer using Cu–Ka source). The maximum permeability, magnetic induction and coercivity of synthesized samples were measured by a B-H curve analyzer (MATS2010SD, China). The complex permeability and magnetic loss of the toroid samples were measured by an AC performance tester (NIM-3000, China, 400 Hz–500 kHz) at a saturation flux density Bm ¼ 100 mT.
3. Results and discussion 3.1. Insulating layer characterization The representative SEM images of pure and the coated iron powder are shown in Fig. 1. Compared with the pure iron particles (Fig. 1(b)), the coated iron powder (Fig. 1(e)) exhibits fewer voids. Cross-sectional SEM photograph of insulation layer is shown in Fig. 1(d), and the observation results confirm that a coating layer exists around the iron particle, where the thickness of the layer is 0.5–1 μm. Figs. 1(c) and 1(f) show the EDS analysis of the original and the coated iron powders, respectively. As shown in Fig. 1(f), the average atomic ratio of Si:O in the coating layer is nearly 1:2, indicating that the composition of coating layer is silicon oxide. The presence of iron, silicon and oxygen elemental peak in Fig. 1 (f) proves the formation of SiO2 insulting layer coating around the surface of iron particles. With respect to the thickness of coating layer, the comparison between the intensity of silicon, oxygen and under laying iron peaks can reveal the existence of thin insulating layer. Therefore, a uniform and thin SiO2 coating layer is obtained on the iron powder surface. Fig. 2 displays the XRD patterns of as-received pure SiO2 powders (a) and the SiO2 coated iron powders (b). According to JCPDS Card no. 51-1379, the synthesized powder has an amorphous structure, which indicates that the coated powder can obtain a pure amorphous SiO2 phase via the reverse microemulsion method. Compared with the pure SiO2 particle, the XRD pattern of the coated iron powders exhibits the characteristics of an amorphous phase. According to the above analysis, the amorphous phase in the coated powders is believed to be amorphous SiO2,
Fig. 3. Real part (a) and imaginary part (b) of permeability of samples with different coating contents as a function of frequency.
which is formed in the hydrolysis process of TEOS. 3.2. Magnetic characteristics Fig. 3 shows the real part (μ′) and imaginary part (μ′′) of permeability of samples with different coating contents as a function of frequency. From Fig. 3(a), the real part of permeability decreases
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frequencies. At high frequencies, the SiO2 coated powders have the lower effective particle size and the relatively higher resistivity, which results in a lower eddy current of the tested sample. Fewer eddy currents increase the frequency stability and magnetic permeability. This point can explain why the coated samples possess the excellent frequency characteristics as compared with the uncoated samples. Since the natural brittleness of silica can decrease dramatically the compressibility of iron powders and especially for the large thickness coated sample, the sample's density is lower under the same compaction pressure by increasing the amount of SiO2. As the sample's density decreases, the volume fraction of magnetic material decreases and consequently the permeability reduces. On the other hand, the appearance of air gap and voids results in the higher resistivity by decreasing the density of sample. The increase in resistivity reduces the eddy current loss and thus contributes to reduce the total core loss. Theoretically, to obtain the maximum magnetic permeability, the amount of added insulation and iron particle should be minimized and maximized, respectively. Unfortunately, there is a high probability that the insulation coating will be damaged during pressure molding (Fig. 4). Therefore, it is necessary to find an appropriate balance between achieving high enough permeability and minimizing the magnetic energy loss. As shown in Fig. 3(b), the imaginary part of permeability continuously increases with the frequency, and the sample coated with 1.25 wt% SiO2 has the lowest imaginary part of permeability. The imaginary part of permeability arises due to the lag between the magnetization and applied alternating field. From both Fig. 3 (a) and (b), it can be concluded that the optimum content of SiO2 attaining the acceptable permeability and the minimum imaginary part of permeability is 1.25 wt%. Fig. 5 shows the magnetic loss versus frequency of the SiO2 coated and uncoated compacts. As seen in Fig. 5, the magnetic loss of coated samples exhibits a lower value. At 150 kHz, the SiO2 insulated sample decreases by 87.8%, as compared with the uncoated sample under an induction level of 100 mT. The lower magnetic loss for the SiO2 coated sample is attributed to its lower effective particle size and the SiO2 insulating layer could enhance the electrical resistivity, leading to decreasing in the eddy current loss [16]. Fig. 6 depicts the effect of different annealing temperatures on the DC performance of SiO2 coated samples in a Hm ¼5000 A m 1 driving field. The annealing treatment can increase the initial permeability, maximum permeability, magnetic induction and decreases the coercivity. The heat treatment can reduce the
Fig. 4. SEM micrographs for different samples compacted at 500 MPa: (a) the uncoated sample, (b) the 0.5 wt% SiO2 coated sample and (c) the 1.25 wt% SiO2 coated sample.
with increasing SiO2 content at low frequencies. As we know, the real part of permeability strongly depends on the density, nonmagnetic phase, number of pores, magnetic anisotropy and crystal anisotropy [14,15]. It can be seen in Fig. 4(a)–(c) that the iron particles are separated by a continuous and thin SiO2 layer, and the thickness of insulating layer gradually increases with increasing SiO2 content. The presence of non-magnetic SiO2 between two magnetic powders acts the similar function of an air gap so that there is a demagnetizing field in the entire magnetic circuit, resulting in decreasing the magnetic permeability at low
Fig. 5. Magnetic loss versus frequency for uncoated and Fe–1.25 wt% SiO2 compacted (Bm ¼ 100 mT).
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Table 1 The hysteresis and eddy current loss coefficients of the as-prepared and annealed compacts at different annealing temperatures.
Fig. 6. Effect of annealing treatment at different temperatures on the DC performance (Hm ¼5000 A m 1).
distortions within the particles and lower the dislocation density [17], therefore, the Fe–SiO2 SMCs exhibit a good DC performance after annealing treatment. To further explore the energy-loss mechanism, the energy loss per cycle versus frequency at an induction level of 100 mT was studied by the loss separation method. The magnetic loss of a core can be separated into three types including hysteresis loss, eddy current loss and residual loss [18]. The total loss can be attained by summing the above mentioned parts, therefore
tan δtot = k1/f + k2 + k 3 f + k 4 (f )
(1)
where k1, k2, k3 and k4 are the winding loss, hysteresis loss, eddy current loss and residual loss coefficient, respectively. Residual loss is a combination of relaxation and resonant loss, and these losses are only important at very low induction levels and very high frequencies, which can be ignored in power applications [19,20]. Fig. 7 depicts the effect of annealing temperature on the core loss factor for the SiO2 coated and uncoated samples. To calculate the core loss factor, the winding loss factor which is prevailing at very low frequencies can be subtracted from the total loss factor [21]. The results show that the core loss factor exhibits nearly a linear relation with frequency for both uncoated and SiO2 coated iron particles. Owing to this fact and linear relationship between eddy current loss factor and frequency (Eq. (1)), the slope of each line may indicate the eddy current loss coefficient (k3) and the
SMCs
Hysteresis loss coefficient k2
Eddy current loss coefficient k3
Fe–SiO2, without annealing Fe–SiO2, annealed at 300 °C Fe–SiO2, annealed at 500 °C Fe–SiO2, annealed at 600 °C Pure Fe, annealed at 600 °C
0.10625
0.00038
0.09516
0.00151
0.09342
0.00357
0.09156
0.00435
0.08963
0.00751
hysteresis loss coefficient (k2) of core loss can acquire when the extrapolation lines reach to zero frequency Table 1 lists the hysteresis and eddy current loss coefficients of as-prepared and annealed compacts at different annealing temperatures. According to Table 1, the hysteresis loss coefficient is relatively lower for the annealed compacts, and its value gradually decreases with increasing annealing temperature, which can be explained by the stress relieving and decreasing dislocation density by the annealing process. From Table 1, the annealed pure iron compacts exhibit a lower hysteresis loss coefficient compared to the SMCs annealed at the same temperature. The existence of the silica insulation as a non-magnetic phase increases the internal stray field and consequently decreases the hysteresis loss coefficient. The results are in good agreement with the permeability variations at very low frequencies (Fig. 3(a)). On the contrary, the eddy current loss coefficient enhancement increases with increase of the annealing temperature. The annealing treatment can decrease the particles distortion and consequently decrease the electrical resistivity, which leads to increased eddy loss coefficient of the SMCs. At higher frequency, the core loss is dominated by the eddy loss contributions, this can be explained why the core loss factor is increased with the frequency. Due to high thermal stability of silica insulation, the silica insulation remains intact after heat treatment and composites with this insulation have greater electrical resistivity. Therefore, the coated SMCs exhibit a lower eddy current loss coefficient in comparison with the pure iron compacts at 600 °C. Through comprehensively considering the above analysis, the SiO2 coated iron powders exhibit the insulating characteristics, which are believed to prevent the eddy current loss of the compacted samples.
4. Conclusions In this study, we have successfully fabricated Fe–SiO2 soft magnetic composites via the reverse microemulsion method. Furthermore, the effects of annealing treatment and the SiO2 content on the magnetic properties were investigated. The results led to the following conclusions:
Fig. 7. The variations of the core loss factor with frequency for the as-prepared and annealed samples.
1. SEM, EDS analysis and XRD indicated that the pure iron powders were uniformly and entirely covered with SiO2 insulating layer, which validly decreased the magnetic loss of synthesized samples. 2. Silica content has a great impact on the magnetic properties of SMCs. Results showed that the sample coated with 1.25 wt% SiO2 has an acceptable real part of permeability and minimum imaginary part of permeability in comparison with other samples. 3. Annealing treatment increased the initial permeability, the
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maximum permeability and the magnetic induction, and decreased the coercivity in the temperature range of 300–600 °C. 4. The lower value of eddy current coefficient and higher hysteresis loss coefficient of the annealed SMCs in comparison with the pure iron compacts were attributed to the preservation of the SiO2 insulation.
[9]
[10]
[11]
Acknowledgments This work was supported by State Key Lab of Advanced Metals and Materials, University of Science and Technology Beijing, under Grant no. 2012-Z04.
References [1] E. Bayramli, O. Golgelioglu, H.B. Ertan, Powder metal development for electrical motor applications, J. Mater. Process. Technol. 161 (2005) 83–88. [2] Y.X. Pang, S.N.B. Hodgson, J. Koniarek, B. Weglinski, The influence of the dielectric on the properties of dielectromagnetic soft magnetic composites. Investigations with silica and silica hybrid sol–gel derived model dielectric, J. Magn. Magn. Mater. 310 (2007) 83–91. [3] H. Shokrollahi, K. Janghorban, Soft magnetic composite materials (SMCs), J. Mater. Process. Technol. 189 (2007) 1–12. [4] H. Shokrollahi, K. Janghorban, Different annealing treatments for improvement of magnetic and electrical properties of soft magnetic composites, J. Magn. Magn. Mater. 317 (2007) 61–67. [5] P.H. Zhou, J.L. Xie, Y.Q. Liu, L.J. Deng, Composition dependence of microstructure, magnetic and microwave properties in ball-milled FeSiB nanocrystalline flakes, J. Magn. Magn. Mater. 320 (2008) 3390–3393. [6] I. Hemmati, H.R. Madaah Hosseini, A. Kianvash, The correlations between processing parameters and magnetic properties of an iron–resin soft magnetic composite, J. Magn. Magn. Mater. 305 (2006) 147–151. [7] A.H. Taghvaei, H. Shokrollahi, K. Janghorban, Properties of iron-based soft magnetic composite with iron phosphate–silane insulation coating, J. Alloys Compd. 481 (2009) 681–686. [8] A.H. Taghvaei, A. Ebrahimi, M. Ghaffari, K. Janghorban, Magnetic properties of
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties of iron-based soft magnetic composites with SiO2 coating obtained by reverse microemulsion method Shen Wu a,b, Aizhi Sun a,b,n, Zhenwen Lu a, Chuan Cheng a, Xuexu Gao b a b
School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 February 2014 Received in revised form 29 December 2014 Accepted 12 January 2015 Available online 13 January 2015
In this work, iron-based soft magnetic composites coated with the amorphous SiO2 layer have been fabricated by utilizing tetraethoxysilane in the reverse microemulsion method, and then the effects of addition amount of SiO2 and annealing temperature on the magnetic properties were investigated. The results show that the surface of iron powders contains a thin amorphous SiO2 insulation layer, which effectively decreases the magnetic loss of synthesized magnets. The magnetic loss of coated samples decreased by 87.8% as compared with that of uncoated samples at 150 kHz. Magnetic measurements show that the sample with 1.25 wt% SiO2 has an acceptable real part and minimum imaginary part of permeability in comparison with other samples. Also, the annealing treatment increased the initial permeability, the maximum permeability and the magnetic induction and decreased the coercivity with increasing temperature in the range 300–600 °C. The results of the loss separation imply that the annealed SMCs have a higher hysteresis loss coefficient (k2) and lower eddy current loss coefficient (k3) as compared with the pure iron compacts after the same heat treatment due to the preservation of the SiO2 layer. & 2015 Elsevier B.V. All rights reserved.
Keywords: Soft magnetic composites Annealing treatment Permeability Loss separation
1. Introduction With growing requirements of miniaturization for powder transformers, computers and inductors, soft magnetic composites (SMCs) with high flux density, permeability and low magnetic loss have a huge market for various applications [1–3]. Usually, SMCs are produced by the powder metallurgy method from ferromagnetic particles coated with a thin electrically insulating layer. In the forming procedure of SMCs, the dislocation density and imperfections would be introduced to the particles by cold work, leading to impede the movement of domain walls and increase the hysteresis loss of produced SMCs [4,5]. In order to reduce the hysteresis loss, a specific heat treatment for stress-relief is often conducted after the compaction. It is generally known that the typical stress-relief temperature for pure iron is between 570 °C and 775 °C [6], and most of the organic coatings would be decomposed under this temperature range. Therefore, many studies have been focused on finding the inorganic coatings with high thermal resistance such as phosphate, MgO and Al2O3 [7–9]. In recent years, silica as the coating agent has been used to n Corresponding author at: School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Fax: 86 10 623 33375. E-mail address: [email protected] (A. Sun).
http://dx.doi.org/10.1016/j.jmmm.2015.01.030 0304-8853/& 2015 Elsevier B.V. All rights reserved.
prepare magnetic nanocomposites, and these silica-coated magnetic nanoparticles have many advanced characteristics in the microstructure, magnetic properties and thermal stability. Silica coated iron submicrometre laminates have been investigated by Zhao et al., which shows a latent use in high frequency applications with a maximum operating frequency up to 50 MHz [10]. In addition, Yang et al. investigated the structure and magnetic properties of Fe/SiO2 SMCs by controlled hydrolysation of tetraethyl orthosilicate (TEOS), reporting that the core loss was only 3.5 W kg 1 when measured at a frequency of 50 Hz and an induction level of 1 T due to the preservation of the SiO2 layer [11]. However, the key factors such as the coating amount, thickness of silica coats and annealing temperature effects are still missing in the above documented works. Thus, it is reasonable and necessary to do more fundamental investigations about the factors determining the performances of produced SMCs. On the other hand, among various SiO2 coating techniques, the reverse microemulsion method has aroused much scientific interest because it can produce homogeneous and narrow distributed nanoparticles and achieve in situ surface modification of nanoparticles [12,13]. In this study, in order to increase the annealing temperature and obtain homogeneous thin insulating layer, a new kind of SMCs coated with amorphous SiO2 insulating layer with high thermal stability was produced by the reverse microemulsion method from tetraethoxysilane (TEOS). With an aim to expand the applications
452
S. Wu et al. / Journal of Magnetism and Magnetic Materials 381 (2015) 451–456
of soft magnetic composite materials in the near future, we attempt to clarify the effects of SiO2 content and annealing temperature on their magnetic properties. Furthermore, different components of the core loss factor were calculated for the asprepared and annealed SMCs.
2. Experimental details 2.1. Materials Iron powder (supplied by Licheng Co., Ltd.) with a particle size o150 μm (Fig. 1(a)) was used as the basic ferromagnetic material. Octyl phenol ethoxylates (OP-10, 99%, Merck), n-butanol (99%, Aldrich), ammonia (NH3, 26% aq., Aldrich) and cyclohexane (99%, Merck) were used for synthesis of reverse microemulsion. Tetraethoxysilane (TEOS, 99%, Merck) was used as a source for SiO2.
Fig. 1. SEM image of (a) and (b) original iron powders; (c) EDS analysis of box in (b); (d) cross-sectional photograph of insulated iron powders; (e) Fe–SiO2 powders and (f) EDS analysis of box in (e).
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2.2. Composite fabrication The detailed experimental procedure to prepare the SiO2 coated compacts are as follows: (a) 100 g of iron particle powders were washed by using acetone solution for three times to clean the surface of iron particles; (b) 67.6 mL of cyclohexane and 15.5 mL of OP-10 (surfactants), 17.6 mL of n-butanol (oil phase) and 0.4 mL of ammonia (water phase) were mixed in an Erlenmeyer flask, and then the mixture was stirred with an electrical stirrer for 1 h at room temperature; (c) the cleaned iron powder and a predetermined amount of TEOS (1.9 ml, 2.8 ml, 3.7 ml, 4.6 ml and 5.5 ml) were slowly added to the microemulsion, and then the powders were rapidly stirred for 3 h keeping the pH-values at 9. For adjusting the pH-value, ammonia was added to the microemulsion; (d) after stirring, used 25 vol% distilled water and 75 vol% acetone solution as demulsifier, the lower layer suspension was filtered, and then washed with distilled water several times, finally dried under vacuum for 2 h at 50 °C to obtain SiO2 coated iron particles; (e) the SiO2 coated iron powders were pressed at 500 MPa, and then the produced compacts were annealed in nitrogen at 300, 500 and 600 °C for 1 h.
Fig. 2. XRD patterns of the pure SiO2 powders and the SiO2 coated powders.
2.3. Characterization The SiO2 insulating layer was characterized by scanning electron microscopy (SEM, ZEISS EVO 18, Germany) coupled with energy dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (Philips APD-10 diffractometer using Cu–Ka source). The maximum permeability, magnetic induction and coercivity of synthesized samples were measured by a B-H curve analyzer (MATS2010SD, China). The complex permeability and magnetic loss of the toroid samples were measured by an AC performance tester (NIM-3000, China, 400 Hz–500 kHz) at a saturation flux density Bm ¼ 100 mT.
3. Results and discussion 3.1. Insulating layer characterization The representative SEM images of pure and the coated iron powder are shown in Fig. 1. Compared with the pure iron particles (Fig. 1(b)), the coated iron powder (Fig. 1(e)) exhibits fewer voids. Cross-sectional SEM photograph of insulation layer is shown in Fig. 1(d), and the observation results confirm that a coating layer exists around the iron particle, where the thickness of the layer is 0.5–1 μm. Figs. 1(c) and 1(f) show the EDS analysis of the original and the coated iron powders, respectively. As shown in Fig. 1(f), the average atomic ratio of Si:O in the coating layer is nearly 1:2, indicating that the composition of coating layer is silicon oxide. The presence of iron, silicon and oxygen elemental peak in Fig. 1 (f) proves the formation of SiO2 insulting layer coating around the surface of iron particles. With respect to the thickness of coating layer, the comparison between the intensity of silicon, oxygen and under laying iron peaks can reveal the existence of thin insulating layer. Therefore, a uniform and thin SiO2 coating layer is obtained on the iron powder surface. Fig. 2 displays the XRD patterns of as-received pure SiO2 powders (a) and the SiO2 coated iron powders (b). According to JCPDS Card no. 51-1379, the synthesized powder has an amorphous structure, which indicates that the coated powder can obtain a pure amorphous SiO2 phase via the reverse microemulsion method. Compared with the pure SiO2 particle, the XRD pattern of the coated iron powders exhibits the characteristics of an amorphous phase. According to the above analysis, the amorphous phase in the coated powders is believed to be amorphous SiO2,
Fig. 3. Real part (a) and imaginary part (b) of permeability of samples with different coating contents as a function of frequency.
which is formed in the hydrolysis process of TEOS. 3.2. Magnetic characteristics Fig. 3 shows the real part (μ′) and imaginary part (μ′′) of permeability of samples with different coating contents as a function of frequency. From Fig. 3(a), the real part of permeability decreases
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frequencies. At high frequencies, the SiO2 coated powders have the lower effective particle size and the relatively higher resistivity, which results in a lower eddy current of the tested sample. Fewer eddy currents increase the frequency stability and magnetic permeability. This point can explain why the coated samples possess the excellent frequency characteristics as compared with the uncoated samples. Since the natural brittleness of silica can decrease dramatically the compressibility of iron powders and especially for the large thickness coated sample, the sample's density is lower under the same compaction pressure by increasing the amount of SiO2. As the sample's density decreases, the volume fraction of magnetic material decreases and consequently the permeability reduces. On the other hand, the appearance of air gap and voids results in the higher resistivity by decreasing the density of sample. The increase in resistivity reduces the eddy current loss and thus contributes to reduce the total core loss. Theoretically, to obtain the maximum magnetic permeability, the amount of added insulation and iron particle should be minimized and maximized, respectively. Unfortunately, there is a high probability that the insulation coating will be damaged during pressure molding (Fig. 4). Therefore, it is necessary to find an appropriate balance between achieving high enough permeability and minimizing the magnetic energy loss. As shown in Fig. 3(b), the imaginary part of permeability continuously increases with the frequency, and the sample coated with 1.25 wt% SiO2 has the lowest imaginary part of permeability. The imaginary part of permeability arises due to the lag between the magnetization and applied alternating field. From both Fig. 3 (a) and (b), it can be concluded that the optimum content of SiO2 attaining the acceptable permeability and the minimum imaginary part of permeability is 1.25 wt%. Fig. 5 shows the magnetic loss versus frequency of the SiO2 coated and uncoated compacts. As seen in Fig. 5, the magnetic loss of coated samples exhibits a lower value. At 150 kHz, the SiO2 insulated sample decreases by 87.8%, as compared with the uncoated sample under an induction level of 100 mT. The lower magnetic loss for the SiO2 coated sample is attributed to its lower effective particle size and the SiO2 insulating layer could enhance the electrical resistivity, leading to decreasing in the eddy current loss [16]. Fig. 6 depicts the effect of different annealing temperatures on the DC performance of SiO2 coated samples in a Hm ¼5000 A m 1 driving field. The annealing treatment can increase the initial permeability, maximum permeability, magnetic induction and decreases the coercivity. The heat treatment can reduce the
Fig. 4. SEM micrographs for different samples compacted at 500 MPa: (a) the uncoated sample, (b) the 0.5 wt% SiO2 coated sample and (c) the 1.25 wt% SiO2 coated sample.
with increasing SiO2 content at low frequencies. As we know, the real part of permeability strongly depends on the density, nonmagnetic phase, number of pores, magnetic anisotropy and crystal anisotropy [14,15]. It can be seen in Fig. 4(a)–(c) that the iron particles are separated by a continuous and thin SiO2 layer, and the thickness of insulating layer gradually increases with increasing SiO2 content. The presence of non-magnetic SiO2 between two magnetic powders acts the similar function of an air gap so that there is a demagnetizing field in the entire magnetic circuit, resulting in decreasing the magnetic permeability at low
Fig. 5. Magnetic loss versus frequency for uncoated and Fe–1.25 wt% SiO2 compacted (Bm ¼ 100 mT).
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Table 1 The hysteresis and eddy current loss coefficients of the as-prepared and annealed compacts at different annealing temperatures.
Fig. 6. Effect of annealing treatment at different temperatures on the DC performance (Hm ¼5000 A m 1).
distortions within the particles and lower the dislocation density [17], therefore, the Fe–SiO2 SMCs exhibit a good DC performance after annealing treatment. To further explore the energy-loss mechanism, the energy loss per cycle versus frequency at an induction level of 100 mT was studied by the loss separation method. The magnetic loss of a core can be separated into three types including hysteresis loss, eddy current loss and residual loss [18]. The total loss can be attained by summing the above mentioned parts, therefore
tan δtot = k1/f + k2 + k 3 f + k 4 (f )
(1)
where k1, k2, k3 and k4 are the winding loss, hysteresis loss, eddy current loss and residual loss coefficient, respectively. Residual loss is a combination of relaxation and resonant loss, and these losses are only important at very low induction levels and very high frequencies, which can be ignored in power applications [19,20]. Fig. 7 depicts the effect of annealing temperature on the core loss factor for the SiO2 coated and uncoated samples. To calculate the core loss factor, the winding loss factor which is prevailing at very low frequencies can be subtracted from the total loss factor [21]. The results show that the core loss factor exhibits nearly a linear relation with frequency for both uncoated and SiO2 coated iron particles. Owing to this fact and linear relationship between eddy current loss factor and frequency (Eq. (1)), the slope of each line may indicate the eddy current loss coefficient (k3) and the
SMCs
Hysteresis loss coefficient k2
Eddy current loss coefficient k3
Fe–SiO2, without annealing Fe–SiO2, annealed at 300 °C Fe–SiO2, annealed at 500 °C Fe–SiO2, annealed at 600 °C Pure Fe, annealed at 600 °C
0.10625
0.00038
0.09516
0.00151
0.09342
0.00357
0.09156
0.00435
0.08963
0.00751
hysteresis loss coefficient (k2) of core loss can acquire when the extrapolation lines reach to zero frequency Table 1 lists the hysteresis and eddy current loss coefficients of as-prepared and annealed compacts at different annealing temperatures. According to Table 1, the hysteresis loss coefficient is relatively lower for the annealed compacts, and its value gradually decreases with increasing annealing temperature, which can be explained by the stress relieving and decreasing dislocation density by the annealing process. From Table 1, the annealed pure iron compacts exhibit a lower hysteresis loss coefficient compared to the SMCs annealed at the same temperature. The existence of the silica insulation as a non-magnetic phase increases the internal stray field and consequently decreases the hysteresis loss coefficient. The results are in good agreement with the permeability variations at very low frequencies (Fig. 3(a)). On the contrary, the eddy current loss coefficient enhancement increases with increase of the annealing temperature. The annealing treatment can decrease the particles distortion and consequently decrease the electrical resistivity, which leads to increased eddy loss coefficient of the SMCs. At higher frequency, the core loss is dominated by the eddy loss contributions, this can be explained why the core loss factor is increased with the frequency. Due to high thermal stability of silica insulation, the silica insulation remains intact after heat treatment and composites with this insulation have greater electrical resistivity. Therefore, the coated SMCs exhibit a lower eddy current loss coefficient in comparison with the pure iron compacts at 600 °C. Through comprehensively considering the above analysis, the SiO2 coated iron powders exhibit the insulating characteristics, which are believed to prevent the eddy current loss of the compacted samples.
4. Conclusions In this study, we have successfully fabricated Fe–SiO2 soft magnetic composites via the reverse microemulsion method. Furthermore, the effects of annealing treatment and the SiO2 content on the magnetic properties were investigated. The results led to the following conclusions:
Fig. 7. The variations of the core loss factor with frequency for the as-prepared and annealed samples.
1. SEM, EDS analysis and XRD indicated that the pure iron powders were uniformly and entirely covered with SiO2 insulating layer, which validly decreased the magnetic loss of synthesized samples. 2. Silica content has a great impact on the magnetic properties of SMCs. Results showed that the sample coated with 1.25 wt% SiO2 has an acceptable real part of permeability and minimum imaginary part of permeability in comparison with other samples. 3. Annealing treatment increased the initial permeability, the
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maximum permeability and the magnetic induction, and decreased the coercivity in the temperature range of 300–600 °C. 4. The lower value of eddy current coefficient and higher hysteresis loss coefficient of the annealed SMCs in comparison with the pure iron compacts were attributed to the preservation of the SiO2 insulation.
[9]
[10]
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Acknowledgments This work was supported by State Key Lab of Advanced Metals and Materials, University of Science and Technology Beijing, under Grant no. 2012-Z04.
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