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Effect of the addition of Al2O3 nanoparticles on the magnetic properties of Fe soft magnetic composites
Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of the addition of Al2O3 nanoparticles on the magnetic properties of Fe soft magnetic composites Yuandong Peng a,b,n, Junwu Nie b, Wenjun Zhang b, Jian Ma b, Chongxi Bao b, Yang Cao b a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China NBTM New Materials Group Co., Ltd., Ningbo 315191, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 August 2015 Received in revised form 10 September 2015 Accepted 21 September 2015 Available online 25 September 2015
We investigated the effect of the addition of Al2O3 nanoparticles on the permeability and core loss of Fe soft magnetic composites coated with silicone. Fourier transform infra-red spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis revealed that the surface layer of the powder particles consisted of a thin insulating Al2O3 layer with uniform surface coverage. The permeability and core loss of the composite with the Al2O3 addition annealed at 650 °C were excellent. The results indicated that the Al2O3 nanoparticle addition increases the permeability stablility with changing frequency and decreases the core loss over a wide range of frequencies. & 2015 Elsevier B.V. All rights reserved.
Keywords: Soft magnetic composite Nano aluminium oxide Insulation coating Microstructure Magnetic properties
1. Introduction Soft magnetic composites (SMCs) consist of a ferromagnetic metal powder core coated with a thin electrically insulating layer that is pressed into the desired shape and heat cured; the final product is prepared using powder metallurgy methods. SMCs possess unique magnetic properties such as three-dimensional isotropic ferromagnetic behaviour, low eddy current loss, relatively lower total core loss at medium and high frequencies and flexible design, which can be applied in various fields [1,2]. These properties make SMCs best suited for high-frequency applications such as inductance coils, transformer cores, antennas, electrical motors, pulse transformer cores, synchronous electric motors, linear actuators, ABS motors and power-steering torque sensors [1,3]. The materials used for the electrical insulating layer include organics, inorganics and their mixtures [4–7]. Organic coatings are widely applied in SMCs because of their rapid and non-hazardous coating process and largely improved cured-film properties and include epoxides, polyamides, silicone resins and polyvinyl alcohol [7]. An annealing treatment is required to minimize the deleterious effects of cold-work on the magnetic performance of the core material. The thermal treatment temperature for these materials is limited by the thermal resistance of the insulating layer between magnetic particles [8]. Therefore, inorganics, including phosphates n Correspondng author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China.
http://dx.doi.org/10.1016/j.jmmm.2015.09.069 0304-8853/& 2015 Elsevier B.V. All rights reserved.
(zinc/Fe/manganese) and oxides (MgO, SiO2) with high resistance temperatures, are used to increase the heat-treatment temperature [9]. The use of organic or inorganics alone cannot meet the requirements of insulation coatings; thus, the mixtures of organics and inorganics are typically used. However, inorganic insulation remains the focus of numerous studies. Zhong et al. [10] prepared magnetic-Fe-powder-coated NixZn1 xFe2O4 using a chemical coprecipitation method with metal chloride as a raw material. Peng et al. [11] reported SMCs for Fe-powder-coated with Ni–Zn ferrite fine particles prepared using the microwave treating method. Zhao et al. [12] even prepared Fe/Ni–Zn ferrite SMCs with high strength through spark plasma sintering. Taghvaei et al. [13] prepared Febased SMCs with MgO insulation using the sol–gel method whereas Sun et al. [14] prepared Fe-based SMCs with a Mn–Zn ferrite nanoparticle coating using the sol–gel method. Liu et al. [15] investigated the structure and magnetic properties of FeSiAlbased SMCs with an AlN and Al2O3 insulating layer prepared by selective nitridation and oxidation. These coated particles most required organics in the outer layer. Recently, Yaghtin et al. [16] investigated Fe powder coated with Al2O3 using the sol–gel process at room temperature. The insulating layer exhibited high thermal stability. In this study, Al2O3 nanoparticles as insulation were mixed with Fe powder and treated before being coated with a silicone layer. The effects of the Al2O3 nanoparticle addition on the magnetic properties and microstructure of the resulting Fe SMCs were investigated.
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
89
Fig. 1. SEM images of Al2O3 powder nanoparticles at (a) low-magnification and (b) high-magnification.
Fig. 2. Al2O3 nanoparticle size distribution by volume.
2. Experimental procedures 2.1. Procedure Fe powder with a particle size of less than 48 μm (approximately 97.1%) was supplied. The purity of the Fe was above 99% with 0.008% C, 0.005% S, 0.012% P, 0.03% Si, 0.11% Mn and some oxide impurities. One set of powder particles was left untreated; the second set of Fe powder particles were mixed with 1.0% nano Al2O3 and then treated. The two mixtures were modified with KH550 silane coupling agent before being coated with an organic material (silicone), which provided electrical insulation between the particles. The coated powders were dried at 150 °C for 120 min and then sifted with a 100-mesh-sized sieve. Then, 0.6% zinc stearate, a lubricant, was added to the sifted composite powders before compression at 1200 MPa in a die, resulting in a solid ringlike structure with an outer diameter of 18 mm, inner diameter of 12 mm and height of 4.5 mm. The samples were heat treated at 650 °C for 60 min in a H2 atmosphere. 2.2. Materials characterisation The Fe powder and Al2O3 particles were examined using scanning electron microscopy (SEM). The Al2O3 powder particle size was measured using a Nanometre particle size analyser (England Malvern Co.). The insulating layers were characterised using SEM coupled with energy-dispersive X-ray spectroscopy
(EDS) and Fourier transform infra-red spectroscopy (FTIR, Nicolet 6700). After polishing, the compositional distribution of the coated powders and SMC samples were characterised using elemental distribution maps. The AC magnetic properties of the samples were measured using a B–H curve analyser (SY-8232, Japan Iwastu) under 5–100 kHz and B ¼ 20 mT conditions, which are the conditions typically applied in industry.
3. Results and discussion 3.1. Characterisation of powder particles and insulating layer Fig. 1 presents SEM images of the Al2O3 powder nanoparticles. The Al2O3 powder appears fluffy and loose. The average powder particle size was 167 nm, as measured by the Nanometre particle size analyser. Fig. 2 reveals that the particle size follows a normal distribution. Fig. 3 presents typical SEM micrographs of the bare Fe powder particles and those with the Al2O3 addition. The surface of the bare Fe is smooth, as illustrated in Fig. 3(a) and (c). However, as observed in Fig. 3(b) and (d), the surface of the sample with the Al2O3 addition is coarse and densely coated by numerous fine particles. The EDS spectrum of these fine particles reveals that they mainly consist of Fe, Al, O and a small amount of C (Fig. 4). The elemental distribution maps reveal that Al and O are uniformly distributed on the surface of the particles. A comparison of the coverage of Al
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Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
Fig. 3. SEM images of Fe powder: (a, c) with and (b, d) without the addition of Al2O3 nanoparticles.
or O with that of Fe indicates that the Al2O3 layer is thin. Therefore, the EDS analysis demonstrates that the particles were coated by a uniform and thin Al2O3 layer, which indicates that the nanoparticles and Fe powder were well distributed. Al2O3 exhibits beneficial properties such as a high melting point, thermal stability and electrical resistivity. The Al2O3 nanoparticle addition offers good heat resistance, which could increase the thermal treatment temperature of the compact sample. MgO, TiO2, B2O3 and SiO2 nanoparticles have also been applied for the insulation of Fe-based powder particles [17,18]. The nature of the insulating layer was assessed by FTIR analysis. Fig. 5(a)–(c) presents the FTIR spectra of the coated powders mixed with Al2O3 nanoparticles, modified with the KH550 silanecoupling agent and coated with silicone resin, respectively. The bands at 3430 and 1630 cm 1 are assigned to the stretching vibration and bending vibration of O–H and H–OH, respectively. This result indicates the presence of hydroxyl structures on the coated surfaces of the three samples. The bands near 802 and 557 cm 1
are attributed to the Al–O–Al and Al–O bonds, respectively, which implies that the insulation layer consists of Al2O3. After modification with the KH550 silane coupling agent and coating with the silicone resin, the location and intensity of the absorption peaks substantially changed, as observed in Fig. 5(b) and (c). The small peak at 2920 cm 1 is associated with the stretching vibration of the C–H-modified–CH2 group. The peak near 1570 cm 1 represents the vibration of N–H. The bands at 1440, 1380 and 1080 cm 1 correspond to absorption peaks of Si–O–CH2CH3 groups. The absorption peaks at approximately 1260 cm 1 correspond to the vibration of Si–CH3. The bands at 1030 and 1000 cm 1 are due to asymmetric stretching and bending vibrations of Si–O–Si, respectively. From this figure, it can be concluded that after modification, the KH550 layer grafted well onto the surface of the Fe particles coated with Al2O3. After coating with the silicone resin, the surface of the Fe particles formed a stable coated layer.
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
91
Fig. 4. (a) Low-magnification and (b) high-magnification SEM micrographs of Fe powder particles coated with Al2O3 nanoparticles; (c) EDS spectrum of the particle in (b) and EDS elemental distribution maps of (d) Fe, (e) Al and (f) O.
3.2. Magnetic properties and microstructure of SMCs Table 1 lists the effective permeability (μ) and magnetic core losses (Pc) as a function of frequency in the range of 5–100 kHz for samples heated at 650 °C for 60 min. The heat treatment results in a highly-uniform film as well as a low volume fraction of defects and reduces the distortion within particles; thus, the coercive force decreases, and the magnetic permeability increases [9]. Compared with the sample without the addition of Al2O3, the effective permeability of the sample with added Al2O3 exhibits a flat profile within the frequency range of 5–100 kHz, indicating good highfrequency stability and that the insulation layer of Fe interparticles is not destroyed. However, at low frequency (5 and 20 kHz), the effective permeability of the sample with the Al2O3 addition is lower than that without the Al2O3 addition because the insulation
layer of the former is thicker than that of the latter. The permeability of the SMC greatly decreases with the addition of a small amount of the coated non-magnetic phase [19]. At high-frequency (50 and 100 kHz), with the Al2O3 addition, the former has higher permeability. Therefore, the conductor skin effect of the sample with the Al2O3 addition is not distinct in the total range frequency. The heat treatment reduces distortions within the particles as well as decreases the dislocation density and thereby increases the magnetic permeability. The core losses for the sample with the Al2O3 addition were also lower. However, the core losses for both samples increased with increasing frequency. The total losses (Pc) in a core material consist of three types of losses, i.e. the hysteresis loss (Ph), eddy current loss (Ped) and excess loss (Pex) according to the following relation [13]: Pc ¼Ph þPed þ Pex. The excess losses are not well
92
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
(a)
802 557
Transmittance (a.u.)
(b)
1380 1570 1080 1000
(c)
2920
1440 1630 1260
2360
3430 525
1030
802 696 4000
3000
2000
463
1000
Wave number Fig. 5. FTIR spectra of Fe powder after (a) mixing with Al2O3 nanoparticles, (b) modification with the KH550 silane coupling agent and (c) coating with silicone resin.
Table 1 Magnetic properties of SMCs manufactured with and without the addition of Al2O3 powder. Samples
With Al2O3 Without Al2O3
Magnetic properties (B ¼20 mT) Frequency (kHz)
5
20
50
100
μ Pcv (kW m 3) μ Pcv (kW m 3)
120.73 5.68 98.08 1.59
109.61 58.96 98.00 15.77
88.10 310.65 97.47 56.79
64.25 1007.42 97.24 167.28
Fig. 6. SEM micrographs and EDS analysis of SMC sample with Al2O3 nanoparticle addition annealed at 650 °C for 60 min. (a) Low-magnification and (b) high-magnification SEM image and EDS elemental distribution maps of (c) Fe, (d) Al, (e) O, (f) Si.
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
understood and perhaps represent an expression of our ignorance of the system. They are only important at very low induction levels and very high frequencies and can be ignored in power applications [1]. However, some experts consider Pex to be proportional to the frequency squared under certain conditions [20]. Ph is proportional to the frequency and Ped is proportional to the frequency squared. Even Ped is divided into two parts because eddy currents can flow in the core sample at two different scales: the single Fe particle (microscopic eddy currents) and the specimen cross-section (macroscopic eddy currents) [21]. The latter results from imperfect insulation between Fe particles [22]. Therefore, at higher frequency, the core loss rapidly increases. SEM micrographs and EDS analysis of the Fe SMC sample with Al2O3 nanoparticle addition after heat treatment are presented in Fig. 6. Elemental distribution maps were utilized to further characterise the sample after curing. Al and O were clearly distributed in the layer between particles, and the Si was indistinct because it persists at only 30% at this high temperature according to reference [23]. However, the Al, O and Si seemingly distributed inside the Fe particle originates from the polishing solution used during sample preparation, which contains these elements.
4. Conclusion The effect of Al2O3 nanoparticle addition on the permeability and core loss of Fe SMCs coated with silicone was investigated. FTIR, SEM and EDS analyses revealed that the surface of the powder particles consisted of a thin insulating layer composed of Al2O3 with uniform surface coverage. The permeability and core loss of the sample with the Al2O3 addition annealed at 650 °C were excellent. Adding Al2O3 nanoparticles in SMCs increases the permeability stablility with changing frequency and decreases the core loss over a wide range of frequencies.
Acknowledgements This work was funded for independent innovation by the State Key Laboratory of Powder Metallurgy (2014).
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
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[17]
[18]
[19] [20]
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Effect of the addition of Al2O3 nanoparticles on the magnetic properties of Fe soft magnetic composites Yuandong Peng a,b,n, Junwu Nie b, Wenjun Zhang b, Jian Ma b, Chongxi Bao b, Yang Cao b a b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China NBTM New Materials Group Co., Ltd., Ningbo 315191, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 August 2015 Received in revised form 10 September 2015 Accepted 21 September 2015 Available online 25 September 2015
We investigated the effect of the addition of Al2O3 nanoparticles on the permeability and core loss of Fe soft magnetic composites coated with silicone. Fourier transform infra-red spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy analysis revealed that the surface layer of the powder particles consisted of a thin insulating Al2O3 layer with uniform surface coverage. The permeability and core loss of the composite with the Al2O3 addition annealed at 650 °C were excellent. The results indicated that the Al2O3 nanoparticle addition increases the permeability stablility with changing frequency and decreases the core loss over a wide range of frequencies. & 2015 Elsevier B.V. All rights reserved.
Keywords: Soft magnetic composite Nano aluminium oxide Insulation coating Microstructure Magnetic properties
1. Introduction Soft magnetic composites (SMCs) consist of a ferromagnetic metal powder core coated with a thin electrically insulating layer that is pressed into the desired shape and heat cured; the final product is prepared using powder metallurgy methods. SMCs possess unique magnetic properties such as three-dimensional isotropic ferromagnetic behaviour, low eddy current loss, relatively lower total core loss at medium and high frequencies and flexible design, which can be applied in various fields [1,2]. These properties make SMCs best suited for high-frequency applications such as inductance coils, transformer cores, antennas, electrical motors, pulse transformer cores, synchronous electric motors, linear actuators, ABS motors and power-steering torque sensors [1,3]. The materials used for the electrical insulating layer include organics, inorganics and their mixtures [4–7]. Organic coatings are widely applied in SMCs because of their rapid and non-hazardous coating process and largely improved cured-film properties and include epoxides, polyamides, silicone resins and polyvinyl alcohol [7]. An annealing treatment is required to minimize the deleterious effects of cold-work on the magnetic performance of the core material. The thermal treatment temperature for these materials is limited by the thermal resistance of the insulating layer between magnetic particles [8]. Therefore, inorganics, including phosphates n Correspondng author at: State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, PR China.
http://dx.doi.org/10.1016/j.jmmm.2015.09.069 0304-8853/& 2015 Elsevier B.V. All rights reserved.
(zinc/Fe/manganese) and oxides (MgO, SiO2) with high resistance temperatures, are used to increase the heat-treatment temperature [9]. The use of organic or inorganics alone cannot meet the requirements of insulation coatings; thus, the mixtures of organics and inorganics are typically used. However, inorganic insulation remains the focus of numerous studies. Zhong et al. [10] prepared magnetic-Fe-powder-coated NixZn1 xFe2O4 using a chemical coprecipitation method with metal chloride as a raw material. Peng et al. [11] reported SMCs for Fe-powder-coated with Ni–Zn ferrite fine particles prepared using the microwave treating method. Zhao et al. [12] even prepared Fe/Ni–Zn ferrite SMCs with high strength through spark plasma sintering. Taghvaei et al. [13] prepared Febased SMCs with MgO insulation using the sol–gel method whereas Sun et al. [14] prepared Fe-based SMCs with a Mn–Zn ferrite nanoparticle coating using the sol–gel method. Liu et al. [15] investigated the structure and magnetic properties of FeSiAlbased SMCs with an AlN and Al2O3 insulating layer prepared by selective nitridation and oxidation. These coated particles most required organics in the outer layer. Recently, Yaghtin et al. [16] investigated Fe powder coated with Al2O3 using the sol–gel process at room temperature. The insulating layer exhibited high thermal stability. In this study, Al2O3 nanoparticles as insulation were mixed with Fe powder and treated before being coated with a silicone layer. The effects of the Al2O3 nanoparticle addition on the magnetic properties and microstructure of the resulting Fe SMCs were investigated.
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
89
Fig. 1. SEM images of Al2O3 powder nanoparticles at (a) low-magnification and (b) high-magnification.
Fig. 2. Al2O3 nanoparticle size distribution by volume.
2. Experimental procedures 2.1. Procedure Fe powder with a particle size of less than 48 μm (approximately 97.1%) was supplied. The purity of the Fe was above 99% with 0.008% C, 0.005% S, 0.012% P, 0.03% Si, 0.11% Mn and some oxide impurities. One set of powder particles was left untreated; the second set of Fe powder particles were mixed with 1.0% nano Al2O3 and then treated. The two mixtures were modified with KH550 silane coupling agent before being coated with an organic material (silicone), which provided electrical insulation between the particles. The coated powders were dried at 150 °C for 120 min and then sifted with a 100-mesh-sized sieve. Then, 0.6% zinc stearate, a lubricant, was added to the sifted composite powders before compression at 1200 MPa in a die, resulting in a solid ringlike structure with an outer diameter of 18 mm, inner diameter of 12 mm and height of 4.5 mm. The samples were heat treated at 650 °C for 60 min in a H2 atmosphere. 2.2. Materials characterisation The Fe powder and Al2O3 particles were examined using scanning electron microscopy (SEM). The Al2O3 powder particle size was measured using a Nanometre particle size analyser (England Malvern Co.). The insulating layers were characterised using SEM coupled with energy-dispersive X-ray spectroscopy
(EDS) and Fourier transform infra-red spectroscopy (FTIR, Nicolet 6700). After polishing, the compositional distribution of the coated powders and SMC samples were characterised using elemental distribution maps. The AC magnetic properties of the samples were measured using a B–H curve analyser (SY-8232, Japan Iwastu) under 5–100 kHz and B ¼ 20 mT conditions, which are the conditions typically applied in industry.
3. Results and discussion 3.1. Characterisation of powder particles and insulating layer Fig. 1 presents SEM images of the Al2O3 powder nanoparticles. The Al2O3 powder appears fluffy and loose. The average powder particle size was 167 nm, as measured by the Nanometre particle size analyser. Fig. 2 reveals that the particle size follows a normal distribution. Fig. 3 presents typical SEM micrographs of the bare Fe powder particles and those with the Al2O3 addition. The surface of the bare Fe is smooth, as illustrated in Fig. 3(a) and (c). However, as observed in Fig. 3(b) and (d), the surface of the sample with the Al2O3 addition is coarse and densely coated by numerous fine particles. The EDS spectrum of these fine particles reveals that they mainly consist of Fe, Al, O and a small amount of C (Fig. 4). The elemental distribution maps reveal that Al and O are uniformly distributed on the surface of the particles. A comparison of the coverage of Al
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Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
Fig. 3. SEM images of Fe powder: (a, c) with and (b, d) without the addition of Al2O3 nanoparticles.
or O with that of Fe indicates that the Al2O3 layer is thin. Therefore, the EDS analysis demonstrates that the particles were coated by a uniform and thin Al2O3 layer, which indicates that the nanoparticles and Fe powder were well distributed. Al2O3 exhibits beneficial properties such as a high melting point, thermal stability and electrical resistivity. The Al2O3 nanoparticle addition offers good heat resistance, which could increase the thermal treatment temperature of the compact sample. MgO, TiO2, B2O3 and SiO2 nanoparticles have also been applied for the insulation of Fe-based powder particles [17,18]. The nature of the insulating layer was assessed by FTIR analysis. Fig. 5(a)–(c) presents the FTIR spectra of the coated powders mixed with Al2O3 nanoparticles, modified with the KH550 silanecoupling agent and coated with silicone resin, respectively. The bands at 3430 and 1630 cm 1 are assigned to the stretching vibration and bending vibration of O–H and H–OH, respectively. This result indicates the presence of hydroxyl structures on the coated surfaces of the three samples. The bands near 802 and 557 cm 1
are attributed to the Al–O–Al and Al–O bonds, respectively, which implies that the insulation layer consists of Al2O3. After modification with the KH550 silane coupling agent and coating with the silicone resin, the location and intensity of the absorption peaks substantially changed, as observed in Fig. 5(b) and (c). The small peak at 2920 cm 1 is associated with the stretching vibration of the C–H-modified–CH2 group. The peak near 1570 cm 1 represents the vibration of N–H. The bands at 1440, 1380 and 1080 cm 1 correspond to absorption peaks of Si–O–CH2CH3 groups. The absorption peaks at approximately 1260 cm 1 correspond to the vibration of Si–CH3. The bands at 1030 and 1000 cm 1 are due to asymmetric stretching and bending vibrations of Si–O–Si, respectively. From this figure, it can be concluded that after modification, the KH550 layer grafted well onto the surface of the Fe particles coated with Al2O3. After coating with the silicone resin, the surface of the Fe particles formed a stable coated layer.
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
91
Fig. 4. (a) Low-magnification and (b) high-magnification SEM micrographs of Fe powder particles coated with Al2O3 nanoparticles; (c) EDS spectrum of the particle in (b) and EDS elemental distribution maps of (d) Fe, (e) Al and (f) O.
3.2. Magnetic properties and microstructure of SMCs Table 1 lists the effective permeability (μ) and magnetic core losses (Pc) as a function of frequency in the range of 5–100 kHz for samples heated at 650 °C for 60 min. The heat treatment results in a highly-uniform film as well as a low volume fraction of defects and reduces the distortion within particles; thus, the coercive force decreases, and the magnetic permeability increases [9]. Compared with the sample without the addition of Al2O3, the effective permeability of the sample with added Al2O3 exhibits a flat profile within the frequency range of 5–100 kHz, indicating good highfrequency stability and that the insulation layer of Fe interparticles is not destroyed. However, at low frequency (5 and 20 kHz), the effective permeability of the sample with the Al2O3 addition is lower than that without the Al2O3 addition because the insulation
layer of the former is thicker than that of the latter. The permeability of the SMC greatly decreases with the addition of a small amount of the coated non-magnetic phase [19]. At high-frequency (50 and 100 kHz), with the Al2O3 addition, the former has higher permeability. Therefore, the conductor skin effect of the sample with the Al2O3 addition is not distinct in the total range frequency. The heat treatment reduces distortions within the particles as well as decreases the dislocation density and thereby increases the magnetic permeability. The core losses for the sample with the Al2O3 addition were also lower. However, the core losses for both samples increased with increasing frequency. The total losses (Pc) in a core material consist of three types of losses, i.e. the hysteresis loss (Ph), eddy current loss (Ped) and excess loss (Pex) according to the following relation [13]: Pc ¼Ph þPed þ Pex. The excess losses are not well
92
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
(a)
802 557
Transmittance (a.u.)
(b)
1380 1570 1080 1000
(c)
2920
1440 1630 1260
2360
3430 525
1030
802 696 4000
3000
2000
463
1000
Wave number Fig. 5. FTIR spectra of Fe powder after (a) mixing with Al2O3 nanoparticles, (b) modification with the KH550 silane coupling agent and (c) coating with silicone resin.
Table 1 Magnetic properties of SMCs manufactured with and without the addition of Al2O3 powder. Samples
With Al2O3 Without Al2O3
Magnetic properties (B ¼20 mT) Frequency (kHz)
5
20
50
100
μ Pcv (kW m 3) μ Pcv (kW m 3)
120.73 5.68 98.08 1.59
109.61 58.96 98.00 15.77
88.10 310.65 97.47 56.79
64.25 1007.42 97.24 167.28
Fig. 6. SEM micrographs and EDS analysis of SMC sample with Al2O3 nanoparticle addition annealed at 650 °C for 60 min. (a) Low-magnification and (b) high-magnification SEM image and EDS elemental distribution maps of (c) Fe, (d) Al, (e) O, (f) Si.
Y. Peng et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 88–93
understood and perhaps represent an expression of our ignorance of the system. They are only important at very low induction levels and very high frequencies and can be ignored in power applications [1]. However, some experts consider Pex to be proportional to the frequency squared under certain conditions [20]. Ph is proportional to the frequency and Ped is proportional to the frequency squared. Even Ped is divided into two parts because eddy currents can flow in the core sample at two different scales: the single Fe particle (microscopic eddy currents) and the specimen cross-section (macroscopic eddy currents) [21]. The latter results from imperfect insulation between Fe particles [22]. Therefore, at higher frequency, the core loss rapidly increases. SEM micrographs and EDS analysis of the Fe SMC sample with Al2O3 nanoparticle addition after heat treatment are presented in Fig. 6. Elemental distribution maps were utilized to further characterise the sample after curing. Al and O were clearly distributed in the layer between particles, and the Si was indistinct because it persists at only 30% at this high temperature according to reference [23]. However, the Al, O and Si seemingly distributed inside the Fe particle originates from the polishing solution used during sample preparation, which contains these elements.
4. Conclusion The effect of Al2O3 nanoparticle addition on the permeability and core loss of Fe SMCs coated with silicone was investigated. FTIR, SEM and EDS analyses revealed that the surface of the powder particles consisted of a thin insulating layer composed of Al2O3 with uniform surface coverage. The permeability and core loss of the sample with the Al2O3 addition annealed at 650 °C were excellent. Adding Al2O3 nanoparticles in SMCs increases the permeability stablility with changing frequency and decreases the core loss over a wide range of frequencies.
Acknowledgements This work was funded for independent innovation by the State Key Laboratory of Powder Metallurgy (2014).
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