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Microstructure, formation mechanism and magnetic properties of Fe1.82Si0.18@Al2O3 soft magnetic composites
Journal of Magnetism and Magnetic Materials 493 (2020) 165744
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Research articles
Microstructure, formation mechanism and magnetic properties of Fe1.82Si0.18@Al2O3 soft magnetic composites
T
⁎
Fan Luoa,c, Xi'an Fana,b,c, , Zigui Luoa,c, Wentao Hua,c, Jian Wangd,e, Zhaoyang Wuf, Guangqiang Lia,c, Yawei Lia,b, Xin Liud,e a
State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 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 b
A R T I C LE I N FO
A B S T R A C T
Keywords: Soft magnetic composites FeSiAl@Fe3O4 Spark plasma sintering Fe1.82Si0.18@Al2O3 Core loss
Fe1.82Si0.18@Al2O3 soft magnetic composites (SMCs) have been fabricated by NaOH oxidation combined with subsequent spark plasma sintering (SPS). The microstructure and generation mechanism of Fe3O4 coatings on the surface of FeSiAl powders have been investigated systematically. Effects of oxidation time on the magnetic properties also have been revealed. In addition, the displacement reaction between Fe3O4 and Al during SPS process results in the formation of Al2O3 insulation layer, leading to a remarkable variation of the magnetic performances of SMCs. The increased NaOH oxidation time had positive influences on the electrical resistivity, frequency stability of permeability and core loss. The saturation magnetization first decreases then increases with the increase of oxidation time. These above results indicate that the fabrication method in this paper creates a way to form Fe3O4 nanoparticles and Al2O3 insulation layer, which enhance the magnetic performance of FeSiAl SMCs and will facilitate the design of efficient magnets in the future.
1. Introduction With the development of science and technology, the market of miniaturization, low loss, high power devices have become increasingly demanding, and higher requirements are placed on the performance of metal soft magnetic powder cores. Metal soft magnetic powder core is a soft magnetic material, which is a kind of magnetic core produced by special process using powders made of metal or alloy soft magnetic materials. Among the varieties of metal soft magnetic powder cores, FeSiAl soft magnetic composite (SMCs) has relatively good performance, high saturation magnetic induction, low loss and relatively low cost, which has been widely used in pulse fly back transformers and energy storage, filter inductors, power transformer cores, power factor correction circuit filters, etc. [1]. However, the high frequency eddy current losses of ferromagnetic powder cores limit their application in the high frequency field. By insulating the surface of the FeSiAl powder, the electrical contact between the powder particles is blocked, which
brings about the decrease of the eddy current loss. A great deal of effort has been made to coat SMCs with organic, inorganic and organic-inorganic combined insulation materials in this critical area [2–4]. But the organic coating has poor high temperature annealing resistance (above 200 °C) [5,6], and the inorganic coating (such as SiO2, A12O3, MgO, etc.) [7–9] have unsatisfactory adhesion and are prone to cracking during compaction. Recently, another method is to react directly with the alloy of the magnetic powder in situ to produce a surface insulating coating. Taghvaei et al. [10] investigated the magnetic and structural properties of iron phosphate-aminosilane soft magnetic composites, which got a thin layer of iron phosphate. Dong et al. [11] fabricated insulation coatings of the FeSiAl SMCs by nitric acid oxidation and also acquired several different coatings. Luo et al. [12] obtained the SiO2-Fe2SiO4-SiO2 coating with high resistivity by water oxidation of Fe-Si powders, which effectively reduced the core loss. However, the in-situ oxidation of FeSiAl SMCs by NaOH has been rarely studied so far. It is worth investigating the generation mechanism of the
⁎ Corresponding author at: 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] (X. Fan).
https://doi.org/10.1016/j.jmmm.2019.165744 Received 19 June 2019; Received in revised form 9 August 2019; Accepted 24 August 2019 Available online 26 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 493 (2020) 165744
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small diffraction peaks at 2θ angles of 30.21° (2 0 0), 35.53° (1 0 3), 56.96° (2 3 1) and 62.61° (2 2 4), corresponding to Fe3O4 phase (JCPDS Card No: 019–0629). In other words, FeSiAl oxidized by NaOH forms a nano-scale Fe3O4 coating. And it is worth figuring out the formation mechanism of Fe3O4. Fig. 3 exhibits the schematic illustration of the chemical coating process. Thermodynamically, both Al and Si in the FeSiAl alloy particles can be oxidized by NaOH following the reactions (1) and (2). The [Al (OH)4]− and (SiO3)2− are formed at the beginning of the reaction, which are washed away by distilled water. As the reaction proceeds, the content of Al and Si gradually decreases. The remaining large amount of Fe occur oxygen-absorbing corrosion [15]. Fe loses electrons to become Fe2+, and then reacts with OH− to form a precipitate, during the stirring process at 60 °C, it is gradually oxidized by oxygen in the air to finally form Fe3O4 [16,17]. The possible reactions during NaOH oxidation can be summarized as follows [18–21]:
coatings and magnetic performances of the FeSiAl SMCs. In the work, the FeSiAl alloy particles were prepared via NaOH oxidation with varied reaction time, which obtained successfully a nano-scale Fe3O4 coating. And then synthetized Fe1.82Si0.18 based SMCs coated with continuous and well-distributed Al2O3 insulating layer by spark plasma sintering (SPS). The microstructure and formation principle of Fe3O4 coatings have been investigated systematically. And the substitution reaction during SPS process and magnetic properties of SMCs also were revealed. 2. Experimental 2.1. Fabrication of FeSiAl@Fe3O4 composite particles by NaOH oxidation The commercial FeSiAl alloy powers with a composition of Al 5.5 wt %, Si 9.5 wt%, and Fe 85 wt% alloy powders were bought from Hunan Tian Jiu metal material Co., Ltd, which were used as raw materials in the experiment. The FeSiAl alloy particles were dispersed in 10 wt% NaOH solution. Under mechanical stirring, the solid–liquid system was heated to 60 °C in water bath for 1 h, 2 h, 3 h and 4 h, respectively. The oxidized powders were washed five times with distilled water, and then the acquired powders were vacuum dried for 24 h at 60 °C.
2A l+ 2OH− + 6H2 O → 2[Al(OH )4]− + 3H2
ΔGmθ = −873.2 kJ/mol (1)
Si + 2OH− + H2 O → (SiO3 )2− + 2H2
ΔGmθ = −460.3 kJ/mol
Fe − 2e− → F e2+ ΔGmθ = −78.87 kJ/mol
ΔGmθ = −175.44 kJ/mol
2.2. Formation of Fe1.82Si0.18@Al2O3 SMCs
F e2+ + 2OH− → F e(OH )2
The FeSiAl@Fe3O4 composite particles were sintered at 1000 °C for 20 min by SPS process. And the acquired composite compacts were cut into rings with outer diameter 20.3 mm and inner diameter 12.7 mm. The pure FeSiAl alloy was also fabricated under the same conditions for the purpose of studying the influence of the NaOH oxidation on the magnetic performances.
6F e(OH )2 + O2 → 2F e3 O4 + 6H2 O
ΔGmθ = −513.64 kJ/mol
(2) (3) (4) (5)
Fig. 4 illustrates the cross-section SEM photograph and corresponding EDS element surface scanning results of FeSiAl compacts oxidized by 10 wt% NaOH with 2 h. It is clear that the FeSiAl particles are well separated by coating layer (Fig. 4(a)), which is continuous and well-distributed. Fig. 4(b) displays the cross-sectional image, which can be seen that the gray and black area show unique core-shell structure. As shown in Fig. 4(d-f) of the corresponding EDS mapping. The gray area consists of Fe and Si element, and the black area mainly is comprised of Al and O. The coating layer is no longer Fe3O4, which implies a material change during the high temperature sintering process. In order to figure out the phase of the coating, the XRD of FeSiAl compacts has been analysed as shown in Fig. 5. And the XRD result of the pure FeSiAl compact exhibits three mainly diffraction peaks with 2θ angles at 45.186°(1 1 0), 65.701°(2 0 0) and 83.217°(2 1 1) conforming to Al0.3Fe3Si0.7 phase. And the three peaks 2θ = 44.961°(0 1 1), 65.470°(0 0 2) and 82.948°(1 1 2) of the oxidized FeSiAl compacts accord with the standard peaks of Fe1.82Si0.18 phase, which shift to the left by about 0.2° compared to that of the pure FeSiAl compact. Note that the diffraction peak of the aluminoxy compound is not detected. For a deeper determining the specific chemical composition of the insulation layer, the XPS analysis of the oxidized compact was implemented as shown in Fig. 6. The Fe2p spectrum (Fig. 6(a)) for the coating obtained by 10 wt% NaOH after SPS includes Fe0 (707.1 eV), Fe2+ (710.2 eV) and Fe3+ (711.9 eV) peaks [22], which confirms the presence of the Fe3O4 as indicated by the XRD data (Fig. 2(b)). And Fig. 6(b) displays the Al spectrum where both Al0 (72.1 eV) and Al3+ (73.9 eV) peaks [23] are detected. Combined with the peak at 531.4 eV of O1s spectrum (Fig. 6(c)), it is confirmed the existence of Al2O3 [11,23,24]. Fig. 6(c) also reveals a Si4+ peak at 533.7 eV, which implies that there are a little SiO2 in the composite compacts [25]. In conclusion, the insulation layers obtained by 10 wt% NaOH after SPS are composed of large amounts of Al2O3, trace a small amount of remaining Fe3O4 and SiO2. Based the above analysis, the solid state reaction of oxidized composite powders during the SPS process can be expressed as [12,26,27]:
2.3. Characterizations The surface microstructure and the components uniformity of the powers were investigated by scanning electron microscopy (SEM) (Nova400, FEI, America) equipped with an energy dispersive X-ray spectroscopy (EDS) (IE350PentaFET-3, Oxford, England) [13]. The phase compositions of the composite compacts were investigated by Xray diffraction (XRD) (D500, Siemens, Germany) using Cu Kα radiation. The composition and chemical state of the compacts were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The hysteresis loops of SMCs were calculated using vibrating sample magnetometer (VSM) (JDAW-2000D, jilin University, China). The effective permeability (μeff) was acquired through inductance (L) computed by impedance analyzer [14] (LCR IM3570A988-06, HIOKI, Japan). And the resistivity was obtained by a four-probe method. The core loss was analysed by a B-H Analyzer (SY-8219, Iwasaki, Japan). 3. Results and discussion Fig. 1 displays the SEM microstructures of FeSiAl particles oxidized by 10 wt% NaOH with different times. It can be seen that there is smooth and clean on the surface of the original FeSiAl particles (Fig. 1(a)). Differently, the FeSiAl particles oxidized by NaOH solution exhibit a rough surface, which are covered by a uniform layer of nanoscale particles as shown in Fig. 1(b-e). And with the increase of oxidation time, the number and size of nanoparticles continuously increase. To ascertain the specific composition of the nanoscale particles on the surface of oxidized FeSiAl alloy particles, the EDS analysis (Point& ID) and XRD experiments (Fig. 2) were carried out. Fig. 2(a) shows that the nanoscale particles consist of Fe and O element. And the atomic number ratio of Fe to O is close to 3:4, which suggests that the nanoparticles are Fe3O4. What's more, It can be seen that apart from the same characteristic peaks of Al0.3Fe3Si0.7 phase, Fig. 2(b) shows four 2
8Al + 3Fe3 O4 → 4Al2 O3 + 9F e ΔGm = −3010.24 kJ/mol
(6)
Fe + Si → Fe1.82 Si 0.18
(7)
2Si + Fe3 O4 → 2SiO2 + 3F e ΔGm = −712.71 kJ/mol
(8)
Journal of Magnetism and Magnetic Materials 493 (2020) 165744
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Fig. 1. SEM micrographs of the original FeSiAl particles (a) and the FeSiAl composite particles prepared by 10 wt% NaOH with different period of oxidation time: (b) 1 h, (c) 2 h, (d) 3 h and (e) 4 h.
original structure has been destroyed, the insulation layer has not broken. And the formed Al2O3 prepared has high resistivity, which can play an important role in blocking magnetic losses. Fig. 7 depicts the hysteresis loops of Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with various oxidation time. It can be seen that as the oxidation reaction time increases, the saturation magnetization (Ms) first decreases and then increases, eventually reaching 125.14 emu/g, which is higher than that of the original FeSiAl. In the initial oxidation stage, the generated Fe3O4 (80 ± 0.5 emu/g) has a lower Ms than FeSiAl (124.29 emu/g), and the converted Al2O3 in the SPS process is a non-magnetic phase, thereby weakening the magnetic properties. With the oxidation time increases, the Al content in FeSiAl is gradually consumed, resulting in the relative content of Fe-Si (185 ~ 202 emu/g) [29] increasing, and finally improves the Ms of
As the temperature rises, Al atoms begin to spread from the core of FeSiAl particles and contact with Fe3O4 on the surface of FeSiAl particles to form the Al-Fe3O4 interfaces. With the further increase of temperature, the Eq. (6) takes place to generate new Al2O3 and Fe. Under the push of the concentration gradient, Al atoms continuously diffuse to the interface, prompting the reaction to proceed, and eventually forming a uniform Al2O3 insulating layer. Besides, the newly formed Fe and Si will form Fe1.82Si0.18 solid solution, which was confirmed by the XRD pattern (Fig. 5). And a small amount of Si reacts with the remaining Fe3O4 to form SiO2 as shown in reaction (8). The XPS spectra (Fig. 6(c)) also verified the existence of SiO2. In a word, FeSiAl@Fe3O4 core–shell composite powders can be transformed into Fe1.82Si0.18@Al2O3 composites after high temperature sintering, accompanied by small amount of Fe3O4 and SiO2 [28]. Although the
Fig. 2. EDS analysis (a) of nanoscale particles and XRD patterns (b) of FeSiAl composite particles prepared by 10 wt% NaOH with various period of oxidation time. 3
Journal of Magnetism and Magnetic Materials 493 (2020) 165744
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Fig. 3. The schematic formation process for FeSiAl@Fe3O4 composites.
Fe1.82Si0.18@Al2O3 SMCs with different oxidation times are shown in Fig. 9. It can be clearly seen that as the frequency increases, the core loss of the SMCs increases. However, the total core loss of the Fe1.82Si0.18@Al2O3 SMCs exhibited lower values than the original FeSiAl compacts. Additionally, with the oxidation time increases, the total loss of the Fe1.82Si0.18@Al2O3 SMCs gradually decreases. It exhibits minimum total core loss of 80.96 W/kg ( f =50 kHz) when the oxidation time of 4 h, reduced by 19.5% compared with the original FeSiAl SMCs, which indicates the increase of the oxidation time is conducive to reduce the PT values. Traditionally, the total core loss of the SMCs in AC applications is comprised of three components: hysteresis loss (Physt), eddy current loss (Ped) and residual loss (Pr). Since the residual loss is quite small, which can be ignored at a low frequency, so the core loss can be expressed by the following relation Eq. (9) [32,33]:
Fe1.82Si0.18@Al2O3 composite compacts. In addition, it is noteworthy that oxidized SMCs are more easily saturated, which is of great significance to the practical application of materials. The variations of effective permeability (μeff) with frequency for FeSiAl SMCs prepared by NaOH with different oxidation time are shown in Fig. 8. It can be seen the values of μeff for the Fe1.82Si0.18@Al2O3 SMCs are lower than the original FeSiAl compact at low frequencies, which comes down to the non-magnetic Al2O3 and SiO2 phases in the SMCs. The non-magnetic Al2O3 and SiO2 increase the distance between the magnetic particles, leading to the reduction of the effective permeability [30]. On the contrary, the Fe1.82Si0.18@Al2O3 SMCs shows favourable frequency stability compared to the pure FeSiAl SMCs. This is because the uniform Al2O3 insulating layer can effectively diminish the eddy current loss, thereby only small skin effect appeared in the Fe1.82Si0.18@Al2O3 SMCs [31]. Besides, the values of μeff for the Fe1.82Si0.18@Al2O3 SMCs exceed the original FeSiAl compact at high frequencies. Consequently, the prepared FeSiAl compacts have an extensive frequency range of applications. The total core loss (PT) versus frequency ( f ) of the
P ≈ Ph yst + Ped =
C (Bƒd )2 +ƒ ρ
∮ HdB
(9)
where C denotes constant, B refers to the magnetic flux density, H is the
Fig. 4. SEM (a) Overall pattern (backscatter pattern), (b) EDS mapping of cross-sectional pattern with (c) Fe, (d) Si, (e) O and (f) Al elements for the composite compact oxidized by 10 wt% NaOH for 2 h. 4
Journal of Magnetism and Magnetic Materials 493 (2020) 165744
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Fig. 7. Hysteresis loops of Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with different period of oxidation time.
Fig. 5. XRD patterns of FeSiAl composite compacts prepared by 10 wt% NaOH with different period of oxidation time.
magnetic field strength, ρ represents the resistivity, ƒ means the frequency, and d expresses the thickness of compact. The eddy current loss of for SMCs can be divided into two parts: a part root in the loss inside the particle (intra-particle), and the other part originates from flowing current in the entire current of the compact (inter-particle). Accordingly, the eddy current loss can be expressed as following [34,35]:
Ped (t ) =
r2 re2 1 ⎛ dB 2 + kintra FeSiAl ⎟⎞ ⎛ ⎞ ⎜k inter 10 ⎝ ρFeSiAl ⎠ ⎝ dt ⎠ ρc
(10)
where re represents the effective radius of composite compacts, ρc means the compacts resistivity, rFeSiAl denotes the radius of FeSiAl particles, ρFeSiAl refers to the resistivity of individual FeSiAl particles and B sands for the magnetic flux density. Additionally, kinter and kintra (0 ~ 1) defines the contribution of the eddy current loss in Fe1.82Si0.18@Al2O3 and FeSiAl SMCs, respectively. The sum of kinter and kintra is 1. For composite compacts with a perfect intergranular insulation, the entire eddy current is limited in the FeSiAl particles, hence kinter = 0 and kintra = 1. Conversely, when the effect of particles can be neglected, kinter = 1 and kintra = 0. Hence, the total eddy current loss of original FeSiAl and Fe1.82Si0.18@Al2O3 SMCs can be showed as following two equations:
P′ (t ) =
2 2 1 rFeSiAl ⎛ dB ⎞ 10 ρFeSiAl ⎝ dt ⎠
P″ (t ) =
r2 re21 1 ⎛ dB 2 + kintra e2 ⎟⎞ ⎛ ⎞ ⎜k inter 10 ⎝ ρc 2 ⎠ ⎝ dt ⎠ ρc1
Fig. 8. Effective permeability with frequency for Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with different period of oxidation time.
and Fe1.82Si0.18@Al2O3 SMCs, the total eddy current loss of original FeSiAl compact can be expressed by a similar formula with (12) and expressed as:
(11)
′ (t ) = PFeSiAl
(12)
r2 re2 1 ⎛ dB 2 + kintra FeSiAl ⎟⎞ ⎛ ⎞ ⎜k inter 10 ⎝ ρFeSiAl ⎠ ⎝ dt ⎠ ρF eSiAl
(13)
It can be seen from the Table 1 that the ρc1 of Fe1.82Si0.18@Al2O3
For visually comparing the total eddy current loss of original FeSiAl
Fig. 6. XPS spectra with (a) Fe2p, (b) Al2p and (c) O1s peaks acquired from the FeSiAl compact oxidized by 10 wt% NaOH with 2 h. 5
Journal of Magnetism and Magnetic Materials 493 (2020) 165744
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[2]
[3]
[4]
[5]
[6]
[7]
[8]
Fig. 9. Total losses of the Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with different period of oxidation time measured at 20 mT in frequency range 5–50 kHz.
[9]
[10]
Table 1 Resistivity of original FeSiAl compact and oxidized FeSiAl composite compacts. Parameters
[11]
FeSiAl oxidation times (h) [12]
ρ (mΩ·cm)
0
1
2
3
4
0.08
0.43
0.68
1.17
1.42
[13]
[14]
SMCs are larger than that of original FeSiAl compact ( ρc1 > ρFeSiAl ), which is due to the existence of insulating Al2O3 layer. Although the room temperature resistivity of Fe1.82Si0.18 alloy is about 0.001 mΩ·cm [36], lower than 0.08 mΩ·cm of FeSiAl ( ρc 2 < ρFeSiAl ), rFeSiAl is much larger than re2 , which differ by three or four orders of magnitude. As a result, the total eddy current loss of Fe1.82Si0.18@Al2O3 SMCs below that of FeSiAl.
[15]
[16] [17]
[18]
4. Conclusions
[19]
In this paper, Fe1.82Si0.18@Al2O3 soft magnetic composites were prepared by a combination of NaOH oxidation and spark plasma sintering. The microstructure and formation mechanism of Fe3O4 coating were systematically studied. It also reveals the effect of oxidation time on magnetism. Besides, during the SPS process, the substitution reaction of Fe3O4 with Al leads to the formation of an Al2O3 insulating layer, which causes a significant improvement in the magnetic properties of the SMCs. The prolongation of NaOH oxidation time has a positive effect on resistivity, magnetic permeability frequency stability and core loss. And the saturation magnetization tends to decrease first and then increase with an increase in oxidation time. The above results display that the fabrication method of this study can acquired Fe3O4 nanoparticles and Al2O3 insulating layer, which improves the magnetic properties of FeSiAl SMCs.
[20] [21] [22]
[23]
[24]
[25] [26]
[27]
Acknowledgements [28]
The work was supported by the National Natural Science Foundation of China (51674181), Key Project of Hubei Provincial Department of Education (D20151103) and Natural Science Foundation of Anhui Province (1908085QE190).
[29]
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Solid State Chem. 231 (2015) 152–158. B. Yang, Z. Wu, Z. Zou, R. Yu, High-performance Fe/SiO2 soft magnetic composites
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Mater. Des. 30 (10) (2009) 3989–3995. [35] W. Jian, L. Xin, M. Jian, M. Xinhua, F. Xi’An, L. Zigui, The influence of doping Ti on the microstructure and magnetic performances of Fe-6.5Si soft magnetic composites, J. Alloys Compd. 766 (2018) 769–774. [36] J.B. Rausch, F.X. Kayser, Elastic constants and electrical resistivity of Fe3Si, J. Appl. Phys. 48 (2) (1977) 487–493.
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Research articles
Microstructure, formation mechanism and magnetic properties of Fe1.82Si0.18@Al2O3 soft magnetic composites
T
⁎
Fan Luoa,c, Xi'an Fana,b,c, , Zigui Luoa,c, Wentao Hua,c, Jian Wangd,e, Zhaoyang Wuf, Guangqiang Lia,c, Yawei Lia,b, Xin Liud,e a
State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China 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 b
A R T I C LE I N FO
A B S T R A C T
Keywords: Soft magnetic composites FeSiAl@Fe3O4 Spark plasma sintering Fe1.82Si0.18@Al2O3 Core loss
Fe1.82Si0.18@Al2O3 soft magnetic composites (SMCs) have been fabricated by NaOH oxidation combined with subsequent spark plasma sintering (SPS). The microstructure and generation mechanism of Fe3O4 coatings on the surface of FeSiAl powders have been investigated systematically. Effects of oxidation time on the magnetic properties also have been revealed. In addition, the displacement reaction between Fe3O4 and Al during SPS process results in the formation of Al2O3 insulation layer, leading to a remarkable variation of the magnetic performances of SMCs. The increased NaOH oxidation time had positive influences on the electrical resistivity, frequency stability of permeability and core loss. The saturation magnetization first decreases then increases with the increase of oxidation time. These above results indicate that the fabrication method in this paper creates a way to form Fe3O4 nanoparticles and Al2O3 insulation layer, which enhance the magnetic performance of FeSiAl SMCs and will facilitate the design of efficient magnets in the future.
1. Introduction With the development of science and technology, the market of miniaturization, low loss, high power devices have become increasingly demanding, and higher requirements are placed on the performance of metal soft magnetic powder cores. Metal soft magnetic powder core is a soft magnetic material, which is a kind of magnetic core produced by special process using powders made of metal or alloy soft magnetic materials. Among the varieties of metal soft magnetic powder cores, FeSiAl soft magnetic composite (SMCs) has relatively good performance, high saturation magnetic induction, low loss and relatively low cost, which has been widely used in pulse fly back transformers and energy storage, filter inductors, power transformer cores, power factor correction circuit filters, etc. [1]. However, the high frequency eddy current losses of ferromagnetic powder cores limit their application in the high frequency field. By insulating the surface of the FeSiAl powder, the electrical contact between the powder particles is blocked, which
brings about the decrease of the eddy current loss. A great deal of effort has been made to coat SMCs with organic, inorganic and organic-inorganic combined insulation materials in this critical area [2–4]. But the organic coating has poor high temperature annealing resistance (above 200 °C) [5,6], and the inorganic coating (such as SiO2, A12O3, MgO, etc.) [7–9] have unsatisfactory adhesion and are prone to cracking during compaction. Recently, another method is to react directly with the alloy of the magnetic powder in situ to produce a surface insulating coating. Taghvaei et al. [10] investigated the magnetic and structural properties of iron phosphate-aminosilane soft magnetic composites, which got a thin layer of iron phosphate. Dong et al. [11] fabricated insulation coatings of the FeSiAl SMCs by nitric acid oxidation and also acquired several different coatings. Luo et al. [12] obtained the SiO2-Fe2SiO4-SiO2 coating with high resistivity by water oxidation of Fe-Si powders, which effectively reduced the core loss. However, the in-situ oxidation of FeSiAl SMCs by NaOH has been rarely studied so far. It is worth investigating the generation mechanism of the
⁎ Corresponding author at: 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] (X. Fan).
https://doi.org/10.1016/j.jmmm.2019.165744 Received 19 June 2019; Received in revised form 9 August 2019; Accepted 24 August 2019 Available online 26 August 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
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small diffraction peaks at 2θ angles of 30.21° (2 0 0), 35.53° (1 0 3), 56.96° (2 3 1) and 62.61° (2 2 4), corresponding to Fe3O4 phase (JCPDS Card No: 019–0629). In other words, FeSiAl oxidized by NaOH forms a nano-scale Fe3O4 coating. And it is worth figuring out the formation mechanism of Fe3O4. Fig. 3 exhibits the schematic illustration of the chemical coating process. Thermodynamically, both Al and Si in the FeSiAl alloy particles can be oxidized by NaOH following the reactions (1) and (2). The [Al (OH)4]− and (SiO3)2− are formed at the beginning of the reaction, which are washed away by distilled water. As the reaction proceeds, the content of Al and Si gradually decreases. The remaining large amount of Fe occur oxygen-absorbing corrosion [15]. Fe loses electrons to become Fe2+, and then reacts with OH− to form a precipitate, during the stirring process at 60 °C, it is gradually oxidized by oxygen in the air to finally form Fe3O4 [16,17]. The possible reactions during NaOH oxidation can be summarized as follows [18–21]:
coatings and magnetic performances of the FeSiAl SMCs. In the work, the FeSiAl alloy particles were prepared via NaOH oxidation with varied reaction time, which obtained successfully a nano-scale Fe3O4 coating. And then synthetized Fe1.82Si0.18 based SMCs coated with continuous and well-distributed Al2O3 insulating layer by spark plasma sintering (SPS). The microstructure and formation principle of Fe3O4 coatings have been investigated systematically. And the substitution reaction during SPS process and magnetic properties of SMCs also were revealed. 2. Experimental 2.1. Fabrication of FeSiAl@Fe3O4 composite particles by NaOH oxidation The commercial FeSiAl alloy powers with a composition of Al 5.5 wt %, Si 9.5 wt%, and Fe 85 wt% alloy powders were bought from Hunan Tian Jiu metal material Co., Ltd, which were used as raw materials in the experiment. The FeSiAl alloy particles were dispersed in 10 wt% NaOH solution. Under mechanical stirring, the solid–liquid system was heated to 60 °C in water bath for 1 h, 2 h, 3 h and 4 h, respectively. The oxidized powders were washed five times with distilled water, and then the acquired powders were vacuum dried for 24 h at 60 °C.
2A l+ 2OH− + 6H2 O → 2[Al(OH )4]− + 3H2
ΔGmθ = −873.2 kJ/mol (1)
Si + 2OH− + H2 O → (SiO3 )2− + 2H2
ΔGmθ = −460.3 kJ/mol
Fe − 2e− → F e2+ ΔGmθ = −78.87 kJ/mol
ΔGmθ = −175.44 kJ/mol
2.2. Formation of Fe1.82Si0.18@Al2O3 SMCs
F e2+ + 2OH− → F e(OH )2
The FeSiAl@Fe3O4 composite particles were sintered at 1000 °C for 20 min by SPS process. And the acquired composite compacts were cut into rings with outer diameter 20.3 mm and inner diameter 12.7 mm. The pure FeSiAl alloy was also fabricated under the same conditions for the purpose of studying the influence of the NaOH oxidation on the magnetic performances.
6F e(OH )2 + O2 → 2F e3 O4 + 6H2 O
ΔGmθ = −513.64 kJ/mol
(2) (3) (4) (5)
Fig. 4 illustrates the cross-section SEM photograph and corresponding EDS element surface scanning results of FeSiAl compacts oxidized by 10 wt% NaOH with 2 h. It is clear that the FeSiAl particles are well separated by coating layer (Fig. 4(a)), which is continuous and well-distributed. Fig. 4(b) displays the cross-sectional image, which can be seen that the gray and black area show unique core-shell structure. As shown in Fig. 4(d-f) of the corresponding EDS mapping. The gray area consists of Fe and Si element, and the black area mainly is comprised of Al and O. The coating layer is no longer Fe3O4, which implies a material change during the high temperature sintering process. In order to figure out the phase of the coating, the XRD of FeSiAl compacts has been analysed as shown in Fig. 5. And the XRD result of the pure FeSiAl compact exhibits three mainly diffraction peaks with 2θ angles at 45.186°(1 1 0), 65.701°(2 0 0) and 83.217°(2 1 1) conforming to Al0.3Fe3Si0.7 phase. And the three peaks 2θ = 44.961°(0 1 1), 65.470°(0 0 2) and 82.948°(1 1 2) of the oxidized FeSiAl compacts accord with the standard peaks of Fe1.82Si0.18 phase, which shift to the left by about 0.2° compared to that of the pure FeSiAl compact. Note that the diffraction peak of the aluminoxy compound is not detected. For a deeper determining the specific chemical composition of the insulation layer, the XPS analysis of the oxidized compact was implemented as shown in Fig. 6. The Fe2p spectrum (Fig. 6(a)) for the coating obtained by 10 wt% NaOH after SPS includes Fe0 (707.1 eV), Fe2+ (710.2 eV) and Fe3+ (711.9 eV) peaks [22], which confirms the presence of the Fe3O4 as indicated by the XRD data (Fig. 2(b)). And Fig. 6(b) displays the Al spectrum where both Al0 (72.1 eV) and Al3+ (73.9 eV) peaks [23] are detected. Combined with the peak at 531.4 eV of O1s spectrum (Fig. 6(c)), it is confirmed the existence of Al2O3 [11,23,24]. Fig. 6(c) also reveals a Si4+ peak at 533.7 eV, which implies that there are a little SiO2 in the composite compacts [25]. In conclusion, the insulation layers obtained by 10 wt% NaOH after SPS are composed of large amounts of Al2O3, trace a small amount of remaining Fe3O4 and SiO2. Based the above analysis, the solid state reaction of oxidized composite powders during the SPS process can be expressed as [12,26,27]:
2.3. Characterizations The surface microstructure and the components uniformity of the powers were investigated by scanning electron microscopy (SEM) (Nova400, FEI, America) equipped with an energy dispersive X-ray spectroscopy (EDS) (IE350PentaFET-3, Oxford, England) [13]. The phase compositions of the composite compacts were investigated by Xray diffraction (XRD) (D500, Siemens, Germany) using Cu Kα radiation. The composition and chemical state of the compacts were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The hysteresis loops of SMCs were calculated using vibrating sample magnetometer (VSM) (JDAW-2000D, jilin University, China). The effective permeability (μeff) was acquired through inductance (L) computed by impedance analyzer [14] (LCR IM3570A988-06, HIOKI, Japan). And the resistivity was obtained by a four-probe method. The core loss was analysed by a B-H Analyzer (SY-8219, Iwasaki, Japan). 3. Results and discussion Fig. 1 displays the SEM microstructures of FeSiAl particles oxidized by 10 wt% NaOH with different times. It can be seen that there is smooth and clean on the surface of the original FeSiAl particles (Fig. 1(a)). Differently, the FeSiAl particles oxidized by NaOH solution exhibit a rough surface, which are covered by a uniform layer of nanoscale particles as shown in Fig. 1(b-e). And with the increase of oxidation time, the number and size of nanoparticles continuously increase. To ascertain the specific composition of the nanoscale particles on the surface of oxidized FeSiAl alloy particles, the EDS analysis (Point& ID) and XRD experiments (Fig. 2) were carried out. Fig. 2(a) shows that the nanoscale particles consist of Fe and O element. And the atomic number ratio of Fe to O is close to 3:4, which suggests that the nanoparticles are Fe3O4. What's more, It can be seen that apart from the same characteristic peaks of Al0.3Fe3Si0.7 phase, Fig. 2(b) shows four 2
8Al + 3Fe3 O4 → 4Al2 O3 + 9F e ΔGm = −3010.24 kJ/mol
(6)
Fe + Si → Fe1.82 Si 0.18
(7)
2Si + Fe3 O4 → 2SiO2 + 3F e ΔGm = −712.71 kJ/mol
(8)
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Fig. 1. SEM micrographs of the original FeSiAl particles (a) and the FeSiAl composite particles prepared by 10 wt% NaOH with different period of oxidation time: (b) 1 h, (c) 2 h, (d) 3 h and (e) 4 h.
original structure has been destroyed, the insulation layer has not broken. And the formed Al2O3 prepared has high resistivity, which can play an important role in blocking magnetic losses. Fig. 7 depicts the hysteresis loops of Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with various oxidation time. It can be seen that as the oxidation reaction time increases, the saturation magnetization (Ms) first decreases and then increases, eventually reaching 125.14 emu/g, which is higher than that of the original FeSiAl. In the initial oxidation stage, the generated Fe3O4 (80 ± 0.5 emu/g) has a lower Ms than FeSiAl (124.29 emu/g), and the converted Al2O3 in the SPS process is a non-magnetic phase, thereby weakening the magnetic properties. With the oxidation time increases, the Al content in FeSiAl is gradually consumed, resulting in the relative content of Fe-Si (185 ~ 202 emu/g) [29] increasing, and finally improves the Ms of
As the temperature rises, Al atoms begin to spread from the core of FeSiAl particles and contact with Fe3O4 on the surface of FeSiAl particles to form the Al-Fe3O4 interfaces. With the further increase of temperature, the Eq. (6) takes place to generate new Al2O3 and Fe. Under the push of the concentration gradient, Al atoms continuously diffuse to the interface, prompting the reaction to proceed, and eventually forming a uniform Al2O3 insulating layer. Besides, the newly formed Fe and Si will form Fe1.82Si0.18 solid solution, which was confirmed by the XRD pattern (Fig. 5). And a small amount of Si reacts with the remaining Fe3O4 to form SiO2 as shown in reaction (8). The XPS spectra (Fig. 6(c)) also verified the existence of SiO2. In a word, FeSiAl@Fe3O4 core–shell composite powders can be transformed into Fe1.82Si0.18@Al2O3 composites after high temperature sintering, accompanied by small amount of Fe3O4 and SiO2 [28]. Although the
Fig. 2. EDS analysis (a) of nanoscale particles and XRD patterns (b) of FeSiAl composite particles prepared by 10 wt% NaOH with various period of oxidation time. 3
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Fig. 3. The schematic formation process for FeSiAl@Fe3O4 composites.
Fe1.82Si0.18@Al2O3 SMCs with different oxidation times are shown in Fig. 9. It can be clearly seen that as the frequency increases, the core loss of the SMCs increases. However, the total core loss of the Fe1.82Si0.18@Al2O3 SMCs exhibited lower values than the original FeSiAl compacts. Additionally, with the oxidation time increases, the total loss of the Fe1.82Si0.18@Al2O3 SMCs gradually decreases. It exhibits minimum total core loss of 80.96 W/kg ( f =50 kHz) when the oxidation time of 4 h, reduced by 19.5% compared with the original FeSiAl SMCs, which indicates the increase of the oxidation time is conducive to reduce the PT values. Traditionally, the total core loss of the SMCs in AC applications is comprised of three components: hysteresis loss (Physt), eddy current loss (Ped) and residual loss (Pr). Since the residual loss is quite small, which can be ignored at a low frequency, so the core loss can be expressed by the following relation Eq. (9) [32,33]:
Fe1.82Si0.18@Al2O3 composite compacts. In addition, it is noteworthy that oxidized SMCs are more easily saturated, which is of great significance to the practical application of materials. The variations of effective permeability (μeff) with frequency for FeSiAl SMCs prepared by NaOH with different oxidation time are shown in Fig. 8. It can be seen the values of μeff for the Fe1.82Si0.18@Al2O3 SMCs are lower than the original FeSiAl compact at low frequencies, which comes down to the non-magnetic Al2O3 and SiO2 phases in the SMCs. The non-magnetic Al2O3 and SiO2 increase the distance between the magnetic particles, leading to the reduction of the effective permeability [30]. On the contrary, the Fe1.82Si0.18@Al2O3 SMCs shows favourable frequency stability compared to the pure FeSiAl SMCs. This is because the uniform Al2O3 insulating layer can effectively diminish the eddy current loss, thereby only small skin effect appeared in the Fe1.82Si0.18@Al2O3 SMCs [31]. Besides, the values of μeff for the Fe1.82Si0.18@Al2O3 SMCs exceed the original FeSiAl compact at high frequencies. Consequently, the prepared FeSiAl compacts have an extensive frequency range of applications. The total core loss (PT) versus frequency ( f ) of the
P ≈ Ph yst + Ped =
C (Bƒd )2 +ƒ ρ
∮ HdB
(9)
where C denotes constant, B refers to the magnetic flux density, H is the
Fig. 4. SEM (a) Overall pattern (backscatter pattern), (b) EDS mapping of cross-sectional pattern with (c) Fe, (d) Si, (e) O and (f) Al elements for the composite compact oxidized by 10 wt% NaOH for 2 h. 4
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Fig. 7. Hysteresis loops of Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with different period of oxidation time.
Fig. 5. XRD patterns of FeSiAl composite compacts prepared by 10 wt% NaOH with different period of oxidation time.
magnetic field strength, ρ represents the resistivity, ƒ means the frequency, and d expresses the thickness of compact. The eddy current loss of for SMCs can be divided into two parts: a part root in the loss inside the particle (intra-particle), and the other part originates from flowing current in the entire current of the compact (inter-particle). Accordingly, the eddy current loss can be expressed as following [34,35]:
Ped (t ) =
r2 re2 1 ⎛ dB 2 + kintra FeSiAl ⎟⎞ ⎛ ⎞ ⎜k inter 10 ⎝ ρFeSiAl ⎠ ⎝ dt ⎠ ρc
(10)
where re represents the effective radius of composite compacts, ρc means the compacts resistivity, rFeSiAl denotes the radius of FeSiAl particles, ρFeSiAl refers to the resistivity of individual FeSiAl particles and B sands for the magnetic flux density. Additionally, kinter and kintra (0 ~ 1) defines the contribution of the eddy current loss in Fe1.82Si0.18@Al2O3 and FeSiAl SMCs, respectively. The sum of kinter and kintra is 1. For composite compacts with a perfect intergranular insulation, the entire eddy current is limited in the FeSiAl particles, hence kinter = 0 and kintra = 1. Conversely, when the effect of particles can be neglected, kinter = 1 and kintra = 0. Hence, the total eddy current loss of original FeSiAl and Fe1.82Si0.18@Al2O3 SMCs can be showed as following two equations:
P′ (t ) =
2 2 1 rFeSiAl ⎛ dB ⎞ 10 ρFeSiAl ⎝ dt ⎠
P″ (t ) =
r2 re21 1 ⎛ dB 2 + kintra e2 ⎟⎞ ⎛ ⎞ ⎜k inter 10 ⎝ ρc 2 ⎠ ⎝ dt ⎠ ρc1
Fig. 8. Effective permeability with frequency for Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with different period of oxidation time.
and Fe1.82Si0.18@Al2O3 SMCs, the total eddy current loss of original FeSiAl compact can be expressed by a similar formula with (12) and expressed as:
(11)
′ (t ) = PFeSiAl
(12)
r2 re2 1 ⎛ dB 2 + kintra FeSiAl ⎟⎞ ⎛ ⎞ ⎜k inter 10 ⎝ ρFeSiAl ⎠ ⎝ dt ⎠ ρF eSiAl
(13)
It can be seen from the Table 1 that the ρc1 of Fe1.82Si0.18@Al2O3
For visually comparing the total eddy current loss of original FeSiAl
Fig. 6. XPS spectra with (a) Fe2p, (b) Al2p and (c) O1s peaks acquired from the FeSiAl compact oxidized by 10 wt% NaOH with 2 h. 5
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[2]
[3]
[4]
[5]
[6]
[7]
[8]
Fig. 9. Total losses of the Fe1.82Si0.18@Al2O3 SMCs prepared by 10 wt% NaOH with different period of oxidation time measured at 20 mT in frequency range 5–50 kHz.
[9]
[10]
Table 1 Resistivity of original FeSiAl compact and oxidized FeSiAl composite compacts. Parameters
[11]
FeSiAl oxidation times (h) [12]
ρ (mΩ·cm)
0
1
2
3
4
0.08
0.43
0.68
1.17
1.42
[13]
[14]
SMCs are larger than that of original FeSiAl compact ( ρc1 > ρFeSiAl ), which is due to the existence of insulating Al2O3 layer. Although the room temperature resistivity of Fe1.82Si0.18 alloy is about 0.001 mΩ·cm [36], lower than 0.08 mΩ·cm of FeSiAl ( ρc 2 < ρFeSiAl ), rFeSiAl is much larger than re2 , which differ by three or four orders of magnitude. As a result, the total eddy current loss of Fe1.82Si0.18@Al2O3 SMCs below that of FeSiAl.
[15]
[16] [17]
[18]
4. Conclusions
[19]
In this paper, Fe1.82Si0.18@Al2O3 soft magnetic composites were prepared by a combination of NaOH oxidation and spark plasma sintering. The microstructure and formation mechanism of Fe3O4 coating were systematically studied. It also reveals the effect of oxidation time on magnetism. Besides, during the SPS process, the substitution reaction of Fe3O4 with Al leads to the formation of an Al2O3 insulating layer, which causes a significant improvement in the magnetic properties of the SMCs. The prolongation of NaOH oxidation time has a positive effect on resistivity, magnetic permeability frequency stability and core loss. And the saturation magnetization tends to decrease first and then increase with an increase in oxidation time. The above results display that the fabrication method of this study can acquired Fe3O4 nanoparticles and Al2O3 insulating layer, which improves the magnetic properties of FeSiAl SMCs.
[20] [21] [22]
[23]
[24]
[25] [26]
[27]
Acknowledgements [28]
The work was supported by the National Natural Science Foundation of China (51674181), Key Project of Hubei Provincial Department of Education (D20151103) and Natural Science Foundation of Anhui Province (1908085QE190).
[29]
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