In-situ formation of Fe3O4 and ZrO2 coated Fe-based soft magnetic composites by hydrothermal method

Author’s Accepted Manuscript In-situ formation of Fe3O4 and ZrO2 coated Febased soft Magnetic Composites by hydrothermal method Wangchang Li, Zhaojia Wang, Yao Ying, Jing Yu, Jingwu Zheng, Liang Qiao, Shenglei Che www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)33163-8 https://doi.org/10.1016/j.ceramint.2018.11.058 CERI20032

To appear in: Ceramics International Received date: 27 September 2018 Revised date: 6 November 2018 Accepted date: 9 November 2018 Cite this article as: Wangchang Li, Zhaojia Wang, Yao Ying, Jing Yu, Jingwu Zheng, Liang Qiao and Shenglei Che, In-situ formation of Fe3O4 and ZrO2 coated Fe-based soft Magnetic Composites by hydrothermal method, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.11.058 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

In-situ formation of Fe3O4 and ZrO2 coated Fe-based soft Magnetic Composites by hydrothermal method

Wangchang Lia,b, Zhaojia Wanga,b, Yao Yinga,b, Jing Yua,b, Jingwu Zhenga,b, Liang Qiaoa,b, Shenglei Chea,b

College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, People’s Republic of

a

China

Research Center of Magnetic and Electronic Materials, Zhejiang University of Technology, Hangzhou 310014, People’s

b

Republic of China

Abstract

In this paper, the soft magnetic composite was prepared using a powder metallurgy method with the Fe3O4 and ZrO2 co-coated iron powder which was synthesized though the in-situ hydrothermal method. Energy dispersive spectroscopy (EDS) coupled with X-ray photoelectron spectroscopy (XPS) revealed that the Fe3O4 and ZrO2 were uniformly coated on the surface of iron powders. The addition of ZrO2 in the coating effectively improves the properties of the soft magnetic composite. ZrO2 can further improve the insulating properties, and enhance the thermostability of the coating simultaneously which will lead to resistivity increase and eddy current loss decrease. By controlling the additive amount of zirconium cation and annealing temperature, high magnetic induction of 1.002 T (measured at 8000 A/m), relatively high permeability of 83 (measured at 100 kHz), and low total core loss of 255.2 W/kg (measured at 100 kHz and 50 mT) were achieved. The Fe@ZrO 2&Fe3O4 SMCs show high electrical resistivity and saturation induction, which may be widely used in high speed electric motor, transformer, inductance and switching power supply.

Keywords：ZrO2; in-situ surface oxidation; hydrothermal method; SMCs

1. Introduction Soft magnetic Composite (SMC) is an important kind of soft magnetic materials which has

advantages including magnetic homogeneity, high permeability, low coercivity, high Curie temperature and low loss [1]. A SMC can be understood as a bulk multi-phase system where at least one dielectric phase coats a soft magnetic material originally in powder form [2].It is prepared following the process of insulation coating of metal magnetic powder, high pressure forming and annealing. Therefore, soft magnetic composites have the potential to be widely used in the field of transformer, inductance and electrical machinery and have attracted the attention of academia and industry. Silicon steel sheet, a typical and widely used soft magnetic material, is limited to

low frequency of merely several hundred

hertz due to the high eddy current loss, while the ferrite core requires much higher frequency (generally over hundreds kilohertz). Metal soft magnetic composite materials can make up for these two defects because of its unique loss characteristics. It can be used in the frequency range from several hundred to hundreds of kilohertz [3]. The advantages not only expand the applicable range of electrical motors, but also foreshadow its huge potential in high frequency inductance [4, 5]. According to the application requirements, soft magnetic composites are designed and synthesized by smartly selecting metal magnetic particles and insulating coating materials [6]. These two factors determine the magnetic properties such as permeability, saturation magnetization and coercivity. In the research of soft magnetic composites, it is necessary to consider different kinds of metal magnetic particles, insulating coating and their synthesis methods to balance the relationship of various properties, such as permeability, saturation, loss and mechanical strength [7]. The soft magnetic particles are generally pure iron powder, Fe-Si alloy (high resistivity), Fe-Si-Al alloy, Fe-Co alloys (high magnetic saturation strength), Fe-Ni alloys, Fe-Ni-Mo alloys and so on [8], each of which shows unique electromagnetic properties. Amorphous and nanocrystalline materials developed rapidly in recent years can also be employed in soft magnetic composites [9, 10]. The soft magnetic composites coated with thermosetting organic polymers such as epoxy resin, acrylic resin, polyester, polyurethane and their mixtures were widely utilized for their advantage of easy formation, but lack of thermal stability [1]. It can not be annealed at high temperature, resulting in the low magnetic properties with magnetic saturation intensity and maximum permeability of the soft magnetic composite to be only 0.64T and 225 [11]. On the contrary, the inorganic coating such as FePO4 [12, 13], MgO [14], SiO2 [15], Al2O3 [16], etc. can be annealed at high temperatures. Silica is a typical coating material used in soft magnetic composites [17, 18] with the power loss of only 3.5W/kg at 50Hz and 1T [15]. However, the non-magnetic elements introduced in the coating would lead to the decrease of saturation magnetization of the material. Hence, recently soft magnetic ferrite is considered to be insulating coating material to enhance the permeability and saturation. Compared with nonmagnetic material coated iron powder, the saturation magnetization and permeability of ferrite coated magnetic composite remains at a high level [19]. However, the ferrite Fe3O4 are decomposed to FeO at 466 ℃ which sharply increases the eddy current loss [20, 21]. Mn–Zn [22] and Ni–Zn ferrite [23] coating are formed at high sintering temperature up to 1000 ℃, making it difficult to coat uniformly on the magnetic particles. The preparation of ferrites coating with several-nanometer thickness is critical and many methods have been explored, such as evaporation condensation [24], high energy mechanical ball milling [25], physical grinding [26], and hydrothermal method [27]. Hydrothermal method is reacted in a closed

autoclave at high temperature and high pressure which has been widely applied in the surface coating reaction. ZrO2 is a kind of metallic oxide with high melting point and high boiling point [28]. It also has the advantages of high hardness, high strength, strong toughness and chemical corrosion resistance [29]. Because of its high mechanical toughness, thermal expansion coefficient closi to many metals, it is an excellent coating layer to form the isolation network in soft magnetic composites, reducing the eddy current loss between soft magnetic particles [30]. By mixing Fe3O4 and ZrO2 with coating materials, it is possible to combine the superior thermal stability and magnetic properties to fabricate the advanced soft magnetic composite. In this paper, on the basis of the hydrothermal formation Fe 3O4 coating experiment, zirconium source was introduced, followed by depositing nanometer ZrO2 particles uniformly on the surface of iron powder. For mixing thin coating layer of Fe3O4 and ZrO2, the resistivity and temperature stability of the composite were improved with little effect on the magnetic properties by the addition of Zr 2+. Hence, the high Bm, low eddy current loss and the reduced hysteresis loss were also obtained. 2. Experimental methods 2.1. Preparation of the coated powders with a hydrothermal co-precipitation method The reduced iron powder was supplied by Jilin Huaxing Powder Metallurgy Technology Co. Ltd with average particle size about 75μm. Zirconium oxychloride hydrate (ZrOCl 2·8H2O) was used as the starting material. In a typical procedure, a certain amount of ZrOCl2·8H2O was dissolved in 40 mL of the deionized water. Then, the solution was transferred into a 50mL Teflon-lined stainless-steel autoclave. Small amount of yttrium nitrate was added to introduce Y 3+ to stable tetragonal phase and cubic phase ZrO2. Meanwhile, NH3·H2O was added to make pH of precursor solution about 10. The autoclave was sealed and maintained at 150 ℃ for 1h. After hydrothermal treatment, the autoclave was allowed to cool naturally to room temperature, the powder washed several times with deionized water and absolute ethyl alcohol, after about 30min dry at 50 ℃, composites made up by reduced iron powder and ZrO2 coating was obtained. The experimental schematic diagram is shown in Fig. 1. A lubricant (0.2 wt% zinc stearate) was mixed uniformly to the composite powders. Then the composite powders were compressed at 1200 MPa into a ring-like size with outer diameter of 12.7 mm, inner diameter of 7.6 mm, and height of 3.4 mm. Finally, all the cores were treated at different temperatures from 400 ℃ to 600 ℃ for 1 h in N2 atmosphere. 2.2. Material characterization The particle morphologies of the powder were detected by scanning electron microscopy (SEM, Hitachi SU1510). The morphology and elements distribution of reaction particles and thermally treated SMC samples were examined using X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XI) and field emission scanning electron microscope (FE-SEM, FEI Nova Nano SEM 450, USA) coupled with energy-dispersive spectroscopy (EDS), respectively. The resistivity was analyzed by 4-point probe system (RTS-8, Guangzhou Four – probe Technology Co., Ltd, China). The initial permeability was

measured by LCR meter (Agilent E4980A) from 1kHz to 100kHz, while the core loss of the SMC was determined by a B-H analyzer (SY-8218, IWATSU, with less than 0.5% tolerance) at a magnetic excitation level of different excited magnetic fields within the same frequency range. The DC magnetic property was recorded using a DC B-H tracer (SK-110, Metron with less than 0.5% tolerance) under the maximum applied magnetic field of 8000A/m. 3. Results and discussion 3.1. Characterization of Fe@ZrO2&Fe3O4 composite powder The possible reactions that occurred in the aqueous starting batch (1st step) and were processed hydrothermally by heating in an autoclave (2nd step). The whole process is described as follows: 1st step：

ZrOCl2 + H 2 O → ZrO(OH)2 + 2Cl -； Y(NO 3 ) 3 + H 2 O → YO(OH) + 3NO-3； 2nd step：

ZrO(OH)2 → ZrO2 + H 2 O 2YO(OH) → Y2 O 3 + H 2 O The hydrolysis of ZrOCl2 and Y(NO3)3 occurred after dissolved in water. NH3·H2O was added to make pH≈10, and then hydrothermally reacted in autoclave at 150 ℃ for 1h. The nanometer ZrO2 particles were deposited on the surface of iron powder by ZrO(OH) 2 decomposition. The following in situ oxidation reactions occurred on the surface of reduced iron powder at the same time：

Fe + H 2 O → FeO + H 2 ↑ 3FeO + H 2 O → Fe3 O 4 + H 2 ↑

Fig.1. The schematic formation process for Fe@Fe3O4&ZrO2 particles.

The SEM image of the composite powders with different reaction on the surface of pure reduced Fe particles is shown in Fig.2. Compared with the smooth surface of the pure reduced Fe powder in Fig.2(a), the surface of the coated powder with additive reactant shows that the different coating particles layer were formed in hydrothermal process, as shown in Fig.2(b) and Fig.2(c). Fig.2(b) depicts that the coarse particles were deposited on the surface of iron powder by adjusting the pH at around 10 with only adding ammonia solution in the autoclave during hydrothermal reaction process. However, the coating agents obtained with 0.8mol/L addition of Zr 2+ had more integrated and uniform particles than those only in alkaline environment.

Fig.2. SEM images of (a) pure reduced iron powders, (b) only adding ammonia reaction iron powders (c) 0.8mol/L addition of Zr2+ reaction iron powders.

The XPS spectra of Fe@Fe3O4&ZrO2 SMC with heated at different temperatures is shown in Fig.3(a). Detailed XPS results on the region of Fe2p and ZrO2 are presented in Fig.3(b)-(d), respectively. The spectrum in Fig. 3(b) is fitted with component at a binding energy value of 710.5eV, which corresponds to Fe3O4. The peak centered at 529.8eV of O1s in Fig.3(c) also supports the presence of Fe3O4. Thus, it is confirmed the existence of Fe 3O4 on the surface of reduced iron particles before and after heating treatment. The minor peak centered at 181.8eV can be assigned to small portion of ZrO2 revealed in Fig. 3(d). However, with the heating temperature rising to 600 ℃, the peak corresponding to ZrO2 at 181.8eV in Fig. 3(d) grows sharper due to the enhanced crystallinity.

Fig.3. XPS (a)survey spectra, (b) Fe3O4 spectra, (c)O1s spectra, (d)ZrO2 spectra of the Fe@ Fe3O4&ZrO2 core-shell particles prepared with added Zr2+ contents of 0.008mol after different heat treatment temperature. 3.2. Microstructure and magnetic properties of SMC composites Fig.4 presents the cross-section SEM images and the element distribution of Fe3O4&ZrO2 coated iron-based SMCs annealed at 400 ℃. It can be seen from Fig. 4(b-e) that O and Zr are uniformly distributed in the boundary of the composites, and that Fe is distributed both in the particles and boundary, which indicates that Fe3O4 and ZrO2 nanoparticles are uniformly coated on the surface of the iron powder. ZrO2 can improve the Fe3O4 insulating property and bonding strength of the compact, which is vital to ensure the relatively low eddy current loss.

Fig 4. (a)Cross-section SEM images of Fe3O4&ZrO2 coated iron-based SMCs and (b)all elemental distribution maps, single elemental distribution mapping of (c)Fe, (d)O, (e)Zr, of the selected area at annealed temperature of 400 ℃. Fig.5 shows the effect of the content of Zr2+ on the effective permeability at different annealing temperatures. It can be obviously seen from Fig. 5 that the initial permeability of each group is slightly

lower than that of the sample without Zr2+ because ZrO2 is a nonmagnetic substance. However, the effective permeability of the sample without Zr2+ decreased significantly with frequency more than that of other samples doped with Zr2+, indicating the frequency stability of the Zr2+-containing sample. It is noteworthy that the permeability of the composite decreases slightly as the Zr2+ increases from 0.2mol/L to 0.4mol/L, but the permeability increased slightly when the concentration increased to 0.8mol/L. Fe3O4 helps improve the magnetic properties of materials.[21] For the same reason, saturation induction and maximum permeability exhibit similar trends to that of permeability, as seen from Table 1 and Fig. 6. However, ZrO2[30] has better insulation than Fe3O4, so the resistivity shows the opposite trend. The permeability exhibits different decreasing trend with the increasing frequency as shown in Fig.5(b-d). The phenomenon is attributed to the remaining Fe3O4 insulation coating which is not destroyed. When the heating treatment temperature rises to 600 ℃, the coating layer of Fe3O4 was completely decomposed, and the permeability decreases dramatically.

Fig.5. Effective permeability of the composite powder cores with (a) no annealed, (b) 400 ℃ annealed, (c) 500 ℃ annealed, (d) 600 ℃ annealed.

Table 1 Electrical resistivity (ρ), maximum magnetic induction (Bm), maximum permeability (μm) and core loss (Ps) of Fe@Fe3O4&ZrO2 SMCs fabricated with different zirconium ion concentration (c(Zr 2+)) and annealing temperature(T). c(Zr2+)/mol·L-1

ρ/μΩ·m

Bm@8000A/m/T

μm

0

48.9

1.25

0.2

81.2

0.4

Ps@50mT/W·kg-1 50kHz

100kHz

238

198.5

617.4

1.14

221

95.5

290.8

108.9

1.00

169

86.5

255.2

0.6

62.0

1.21

268

108.4

332.1

0.8

57.7

1.22

321

127.5

384.2

0

11.9

1.33

359

504.5

1558.2

0.2

47.2

1.15

339

130.4

395.4

0.4

53.5

1.08

256

108.3

331.6

0.6

23.9

1.25

313

176.9

530.0

0.8

22.9

1.21

317

177.2

534.2

8.1

1.21

218

-

-

400 ℃ annealed

500 ℃ annealed

600 ℃ annealed 0.4

Fig.6. Electrical resistivity (ρ), maximum magnetic induction intensity (Bm) of (a) different addition of Zr2+ after 400 ℃ annealed, (b) different annealed temperature of 0.4 mol/L Zr2+ addition.

Table 1 and Fig.6 presents the effect of the fabricating parameters on the electrical resistivity (ρ), maximum magnetic induction (Bm) ， maximum permeability (μm) and core loss (Ps) of the Fe@Fe3O4&ZrO2 SMCs. The total core loss obviously drops with the increase of Zr 2+ concentration, corroborating the increase of resistivity. It can be seen that ZrO2 has an obvious effect on the enhancement of the insulation as mentioned above. At the same time, the maximum magnetic induction intensity (Bm) of the sample (measured at 8000A/m) is still above 1T, and the magnetic properties are still at a high level. However, the electrical resistivity sharply decreased as the temperature of annealing

treatment increased from 400 ℃ to 600 ℃ due to the decomposition of the Fe3O4 coating layer. However, during densification, the particles plastically deformed and created dislocations and residual stress, which impeded the movement of the magnetic domain walls.[27] The hysteresis loss increases, and thus, the core loss increases. This drawback can be partially avoided via calcination, which provides a low-volume fraction of defects, reduces the distortion within the particles, lowers down the dislocation density, and thereby decreases the total loss.[28] However, calcination at temperatures above 400 ℃ will lead to the decomposition of the coating, which as a result makes the coating incomplete, affecting the insulation between iron powder and the stability of permeability when the frequency increases. Table 1 and Fig.6 shows that the optimum preparation requires 0.4mol/L addition of zirconium source and the annealing temperature is 400 ℃.

Fig. 7. The total core loss of the composite powder cores with (a) different added Zr 2+ contents at 50 mT and 100 kHz after 400 ℃ annealed and (b)different annealed temperature at 50 mT and 0.4 mol/L Zr2+ addition

The total loss of soft magnetic composite consists of hysteresis loss, eddy current loss and residual loss.[31] The hysteresis loss is determined by the magnetic properties of the material and increases with the increase of non-magnetic material. The eddy current loss roots in the eddy current formed for the connection of iron powder, hence increasing the insulation of the coating layer and reducing the connection between the iron powder. These in turn can effectively reduce the eddy current loss.[32] In this experiment, the coating layer is composed of two kinds of materials, Fe3O4 and ZrO2. Fe3O4 can not only give insulating coating but also hold the high saturation and permeability for its magnetic properties. However, due to the serious failure of Fe3O4 in the coating layer after heated at 600 ℃, the coating is incomplete. ZrO2 can further improve the insulating properties, which simultaneously enhance the thermal stability of the coating. The eddy current loss and the associated total loss is the lowest when the concentration of zirconium ion is 0.4mol/L under different annealing conditions as shown in Fig.7(a), which is relevant to the resistivity revealed in Table 1. However, the total loss grows up from 150 to 300 W/kg when the calcination temperature increases to 600 ℃ as shown in the Fig.7(b) for destroying the insulating

coating. High pressure compaction causes dislocation and internal stress in the material, resulting in the high coercivity and high hysteresis loss of the material [26]. Annealing is needed to eliminate internal stress to increase permeability and reduce coercivity. Zinc stearate as demoulding lubricant decomposed at 200 ℃ was added in the pressing process, which will reduce the insulation oxide ZrO 2 and Fe3O4. This may lead to the contact of the iron powder and the slightly increase of the total loss. The optimum fabricating parameters is 0.4mol/L of zirconium source and annealing temperature of 400 ℃, as revealed from the Fig.7.

Fig.8. Separation of the total loss into the eddy current, loss hysteresis loss residual loss and of the composite powder cores with (a) no annealed, (b) 400 ℃ annealed and (c) 500 ℃ annealed.

As previously mentioned, hysteresis loss, eddy current loss and residual loss constitute the total core loss. According to Steinmetz Law [33] and correction by Kollar [34] et al, the total loss can be expressed using the following formula: P  Chyst  B  f  Cec  B 2  f 2  Cexc  B1.5  f 1.5

In this formula, B is the excited magnetic field in the test, f is for the test frequency, and Chyst, Cec, Cexc are the coefficient of hysteresis loss, eddy current loss and residual loss, respectively. By measuring the total loss under different excited magnetic fields and different frequencies, various coefficients including Chyst, Cec, Cexc and α in the formula can be calculated though the numerical fitting method, and the specific formula of loss separation can also be derived. Using the above method, as shown in Fig.8, the total loss is separated into three parts: hysteresis loss (red curved surface), eddy

current loss (green curved surface) and residual loss (blue curved surface) and the values of coefficients ( Chyst, Cec, Cexc and α ) are obtained in Table 2.

Table 2 The coefficients of Chyst, Cec, Cexc and α of the SMC sample heat treated at various temperatures. Chyst

Cec

Cexc

α

Not annealed

353.8

1.916

0.2578

1.965

400 ℃

393.6

6.871

0.0403

2.052

500 ℃

392.2

9.302

1.829

2.058

Heating

temperature

It can be seen from Fig.8(a-c) and Table 2 that with the increase of heating temperature, the values of Chyst and α increase lightly, which assumes the hysteresis loss (red curved surface) remains stable, but the eddy current loss increases greatly derived from the coefficients of Cexc grows rapidly. Since the exponential coefficient of B and f in eddy current loss parts in the formula is 2, the influence of Cexc growth on the total loss is the most drastic. The change of residual loss is negligible and can be ignored when compared to hysteresis loss and eddy current loss. This conclusion also verifies the previous analysis of the total core loss. When the heating temperature is increased to 500 ℃, Fe3O4 in the coating layer starts to decompose, making coating layer incomplete, and the eddy current loss increases obviously after the interconnection between the particles. To sum up, 400 ℃ is the best heating treatment temperature. However, combined with the loss data of 400 ℃ annealed condition in Table 1, the total loss of the sample with Zr2+ added is much lower than that of the unadded sample. The addition of Zr2+ increases the thermal stability of coating layer, enhances the insulation between iron particles, which will

greatly reduce the eddy current loss.

Conclusion In this study, Fe@Fe3O4&ZrO2 soft magnetic composites with a high resistivity, high magnetic induction and low core loss were successfully fabricated through a hydrothermal method. The SEM and XPS analysis showed that a thin and insulating layer was formed on the powder surface. Fe3O4 and ZrO2 nanoparticles were uniformly coated on the surface of the iron powder. The optimized fabricating parameters are determined: 0.004 mol of zirconium source and annealing temperature of 400 ℃. The addition of ZrO2 increases the insulation and thermostability of the coating layer, which will greatly reduce the eddy current loss and substantially increase resistivity. The composite core with 0.004 mol Zr2+ after being annealed at 400 ℃ displayed the lowest total core loss of 255.2W/kg (measured at 100 kHz and 50mT), relatively high permeability of 83 (measured at 100 kHz), and

magnetic induction of 1.002T (measured at 8000A/m). The magnetic characteristics are stable and good in a wide range of frequency.

Acknowledgments This work are supported by National Nature Science Foundation of China through Grant [No.51602283] and the Zhejiang Provincial Natural Science Foundation of China through Grant [No. LY18E020016].

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