Effect of Particle Size Distribution on the Magnetic ...

Effect of Particle Size Distribution on the Magnetic Properties of Fe-Si-Al Powder Core H. J. Liu, H. L. Su, W. B. Geng, Z. G. Sun, T. T. Song, X. C. Tong, Z. Q. Zou, Y. C. Wu & Y. W. Du Journal of Superconductivity and Novel Magnetism Incorporating Novel Magnetism ISSN 1557-1939 Volume 29 Number 2 J Supercond Nov Magn (2016) 29:463-468 DOI 10.1007/s10948-015-3282-4

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Author's personal copy J Supercond Nov Magn (2016) 29:463–468 DOI 10.1007/s10948-015-3282-4

ORIGINAL PAPER

Effect of Particle Size Distribution on the Magnetic Properties of Fe-Si-Al Powder Core H. J. Liu1 · H. L. Su1,2,3 · W. B. Geng2 · Z. G. Sun2 · T. T. Song1 · X. C. Tong3 · Z. Q. Zou3 · Y. C. Wu1 · Y. W. Du4

Received: 27 October 2015 / Accepted: 7 November 2015 / Published online: 16 November 2015 © Springer Science+Business Media New York 2015

Abstract The Fe-Si-Al soft magnetic powder cores with five particle size distributions were prepared. The microstructure study revealed that the cores had a fairly compacted structure with a uniform insulation layer of the phosphate forming on the surface of the magnetic particles. The particle size distribution was found to have the great influence on the core’s magnetic properties. The increase of the percentage of the small particles results in the decrease of the effective permeability, the improvement of the DCbias performance, and the deterioration of the core loss. The effects of the distributed air gap and the demagnetization field on the core’s magnetizing process were believed to

be the underlying physical origins. The core losses at the frequencies lower and higher than 150 kHz were found to be mainly determined by the hysteresis loss and the eddycurrent loss, respectively. The good magnetic performances of the Fe-Si-Al powder cores with the effective permeability of about 55–60 were finally achieved as follows: the percent permeability at 100 Oe is up to 52.3 %, and the lowest core loss at 50 kHz/1000 Gs is 270 mW cm−3 . Keywords Soft magnetic powder core · Fe-Si-Al alloy · Particle size distribution · Magnetic properties

1 Introduction H. L. Su

[email protected] Y. C. Wu

[email protected] 1

School of Materials Science and Engineering and Anhui Provincial Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei 230009, People’s Republic of China

2

Institute of Magnetic Material Engineering and Technology, Anhui ShouWen High-Tech Material Co., Ltd., Suzhou 234000, People’s Republic of China

3

Huaian Engineering Research Center of Soft Magnetic Powder Cores and Devices, Jiangsu Red Magnetic Materials Incorporation, Xuyi 211700, People’s Republic of China

4

National Laboratory of Solid State Microstructure, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China

As an important part of soft magnetic composites, soft magnetic powder cores have been widely used in many electromagnetic devices, such as the reactor, the choke, the filter, the inverter, etc., due to their particular magnetic and electrical properties. Soft magnetic powder cores are fabricated by the powder metallurgy technology [1–3]. Compared with traditional soft magnetic alloys and ferrites, soft magnetic powder cores exhibit many distinct advantages. Firstly, the magnetic powder cores usually have a magnetic flux density much higher than those of the ferrites which makes them more compatible to the miniaturization of the devices [4, 5]. As to the microstructure, the magnetic powder cores are generally comprised of ferromagnetic iron or iron-based alloy powder particles individually coated with some kinds of non-ferromagnetic insulating materials. The insulating materials between the ferromagnetic particles increase the core’s resistivity greatly, which effectively restrains the increase of the core’s eddy-current loss with the frequency

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and thus makes the cores, compared with the soft magnetic alloys, have a great potential in the applications at the frequencies ranging from several kHz to hundreds of kHz [6, 7]. Besides, these non-ferromagnetic insulating materials can also pin the domain walls during the magnetizing process. This enhances the core’s anti-saturation capability. Thus, the magnetic powder cores possess the DC-bias performance much better than those of the soft magnetic alloys and ferrites without air gaps, which makes them more suitable for the high-current applications [3]. The processing parameters, including the powder composition, the type and the content of insulating material, the compaction pressure, the annealing temperature and time, etc., have been reported to influence the performance of soft magnetic powder cores greatly [2, 8–10]. But only few researchers focused their studies on the particle size distribution which is usually fixed in most studies of the powder core. In fact, the magnetic properties also have a great dependence on the variation of the magnetic particle size. As we all know, the decrease of the particle size usually produces more distributed air gap within the core and results in the lower permeability and the higher resistivity [11]. Accordingly, the core’s hysteresis loss will increase, while the core’s eddy-current loss will show an opposite variation. However, the moderate increase of the proportion of small particles, for some particle size distributions, can improve the core’s density and reduce the air gaps among the ferromagnetic particles effectively. In this situation, the core’s permeability, resistivity, and loss will show the opposite variations. Obviously, the appropriate particle size distribution is the key parameter for magnetic powder cores to obtain excellent magnetic properties [12]. In this paper, the influences of the particle size distribution on the effective permeability (μe ), the DC-bias performance, and the loss for the Fe-Si-Al powder core were investigated in detail and good magnetic properties were achieved finally.

2 Experimental Method The mechanically crushed Fe-Si-Al powders with the particle size < 75 μm were commercially purchased and sieved into four groups with different size ranges, namely ≤ 38 μm (group 1), 38–45 μm (group 2), 45–53 μm (group 3), and ≥ 53 μm (group 4), by using the standard sieves with different meshes. To get the raw powders with five size distributions, both the percentages of groups 2 and 3 were fixed to be 25 wt%, the one of group 1 was increased from 15 to 35 wt%, and the one of group 4 was decreased from 35 to 15 wt% accordingly. Then, the powders of four groups with different percentages were mixed before the insulation. Phosphate insulation coating was applied in this work. The mixed powders were firstly passivated in phosphoric

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acid solution, containing 9 wt% phosphoric acid, 2 wt% boric acid, and 18 wt% warm water, for 40 min and dried at 65 ◦ C for 20 min. The 8 wt% phenol formaldehyde resin and 25 wt% alcohol were added into the powders and stirred to dryness. The insulated powders were then blended with 3 wt% zinc stearate and compacted into the toroidal cores with the outer diameter of 26.92 mm, the inner diameter of 14.73 mm, and the height of 11.18 mm at 1822 MPa. The cores were finally annealed in nitrogen atmosphere at 720 ◦ C for 30 min. The crystalline structure of the samples was characterized by an X-ray diffractometer (XRD; D/max 2500 V). The sample’s morphology and element distribution were characterized by a scanning electron microscope (SEM; JSM-6490LV) equipped with an Oxford Instrument INCA energy dispersive spectrometer (EDS). The core’s μe at the frequency (f ) ranging from 10 to 1000 kHz and the percent permeability (%μe ) at 10 kHz under the DC magnetizing field ranging from 50 to 200 Oe were measured by an LCR meter (Wayne Kerr 3260B). The core losses were measured at 50 kHz at the maximum magnetic flux density Bm of 100– 1500 Gs and at 1–200 kHz at the Bm of 1000 Gs by a power loss tester (Clarke-Hess 2335), respectively. All magnetic measurements were carried out at room temperature.

3 Results and Discussion Figure 1 shows the SEM images of the Fe-Si-Al raw powders of groups 1–4. It can be seen that the sieved particles of each group have a relatively uniform size. Figure 2a, b shows the typical SEM images of the raw powder particles and the ones after being insulated. A layer of the insulating material is clearly coated on the surface of the Fe-Si-Al particle after the passivating treatment. Figure 3 shows a representative SEM image and the corresponding EDS spatial elemental mapping of the cross-section of the annealed Fe-Si-Al powder core. The core has a fairly compacted structure. The magnetic particles are mainly composed of three elements of Fe, Si, and Al, while the intervals between the particles have the elemental signals of P and O and these two elements distribute uniformly and continuously around the Fe-Si-Al particles. This conforms to the abovementioned SEM analysis of the insulated particles and indicates that a uniform insulation layer of the phosphate was formed on the surface of the magnetic particles and the layer was not broken after the compaction treatment [13–15]. Figure 4 shows the typical XRD patterns of the raw powder, the insulated powder, and the annealed powder core. The patterns of three samples are almost identical. Except for the diffraction peaks from the Fe-Si-Al alloy, no any other peak can be found in the patterns of the insulated powder and the core annealed at 720 ◦ C. This implies that

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Fig. 1 SEM images of the Fe-Si-Al raw powders of a group 1, ≤ 38 μm; b group 2, 38–45 μm; c group 3, 45–53 μm; and d group 4, ≥ 53 μm

the amount of the insulating substances is too small to be detected by the XRD, which ensures that the cores possess a moderate permeability [16, 17]. Figure 5 gives the μe of the cores with different particle size distributions as functions of the frequency. The μe for all the cores with different particle size distributions only varies within a small range with the frequency increasing from 10 to 1000 kHz. This indicates that these powder cores possess a good frequency stability of the effective permeability. But for any frequency within the measuring range, the core’s μe decreases obviously from about 60 to about 55 with the percentage of group 1 particles increasing from 15 to 35 wt%. The increase of the percentage of small magnetic particles introduces more distributed air gaps into the powder cores. These air gaps impede the shift of the domain wall in the magnetizing process and thus cause the decrease of the μe [17]. Figure 6 presents the DC-bias performance of the cores with different particle size distributions. It is obvious that the %μe of all the cores decreases with the applied DC

Fig. 2 Typical SEM images of a the raw powder particles and b the insulated particles

magnetizing field. This is due to the core approaching the magnetic saturation gradually with the increase of the applied magnetizing field. For any applied DC magnetizing field, the core’s %μe keeps improving with the percentage of group 1 particles increasing from 15 to 35 wt%. Hereinto, the %μe at 100 Oe increases from 49.3 to 52.3 % with the percentage of group 1 particles increasing from 15 to 35 wt%. This indicates that the powder core prepared in this work possess a fairly good DC-bias performance. Clearly, the core with more small particles possesses the better DC-bias performance. This can be attributed to two main factors. The one is that more small particles introduce more distributed air gaps into the powder cores, as mentioned above, and thus impede the shift of the domain wall in the magnetizing process. The other is that smaller magnetic particle possesses higher demagnetization field and the effective magnetizing field for the core with more small particles is weakened accordingly. Both factors make the core with more small particles be magnetized more difficultly and thus possess the better DC-bias performance. Based on


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Fig. 3 Representative a cross-section SEM image and the corresponding EDS spatial elemental mapping of b Fe, c Si, d Al, e P, and f O for the annealed Fe-Si-Al powder core

the above analyses, it can be concluded that decreasing the core’s effective permeability is beneficial to improve the DC-bias performance.

Figure 7 shows the dependence of the loss for the core with different particle size distributions on the measuring frequency. The loss for all the cores exhibits an increasing

Fig. 4 Typical XRD patterns of the raw powder, the insulated powder, and the annealed powder core

Fig. 5 Effective permeability μe of the cores with the percentage of group 1 particles ranging from 15 to 35 wt% as functions of the frequency

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Fig. 6 Percent permeability %μe of the cores with the percentage of group 1 particles ranging from 15 to 35 wt% as functions of the DC magnetizing field

tendency with the frequency increasing. This is generally due to the increase of both the hysteresis loss and the eddycurrent loss with the frequency. Comparing the loss of the core with different particle size distributions, it can be found that the frequency of 150 kHz is a special boundary. The core loss at the frequency lower than 150 kHz increases with the percentage of group 1 particles increasing from 15 to 35 wt%, while the differences in the loss at 150 and 200 kHz for the cores with different particle size distributions are not remarkable. As mentioned above, small particles introduce more distributed air gaps into the powder cores and thus pin the domain wall in the magnetizing process. Therefore, the core with more small particles usually has the higher hysteresis loss. But on the other hand, particles with smaller

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Fig. 8 Dependence of the loss at 50 kHz for the cores with the percentage of group 1 particles ranging from 15 to 35 wt% on the maximum magnetic flux density Bm

size also possess lower eddy-current loss. Clearly, the influences of the particle size on the hysteresis loss and the eddy-current loss are opposite. According to the dependence of the loss on the percentage of group 1 particles shown in Fig. 7, it can be deduced that the hysteresis loss is dominant within the frequency range lower than 150 kHz. Because the eddy-current loss increases more rapidly than the hysteresis loss with the frequency increasing, it is believed that the influence of the eddy-current loss enhances to be equal to, or even overwhelm, that of the hysteresis loss gradually when the frequency increases from 150 to 200 kHz and thus the core loss does not have the distinct dependence on the particle size [4]. Figure 8 shows the dependence of the loss at 50 kHz for the core with different particle size distributions on the Bm . It can be seen that the core loss increases with both the percentage of group 1 particles and the Bm . As discussed above, the core loss at 50 kHz is mainly determined by the hysteresis loss. Therefore, the variation of the core loss can be understood easily because more small particles can enhance the hysteresis and the hysteresis loss is proportional to the Bm . In this work, the relatively lowest loss at 50 kHz/1000 Gs, namely 270 mW cm−3 , was achieved in the core possessing 15 wt% group 1 particles.

4 Conclusion

Fig. 7 Dependence of the loss for the cores with the percentage of group 1 particles ranging from 15 to 35 wt% on the measuring frequency

The Fe-Si-Al powder cores with five particle size distributions were prepared in this paper. The influence of the particle size of the Fe-Si-Al powder on the core’s magnetic properties was investigated. It was found that the increase of the percentage of the small particles results in the decrease of the effective permeability and the improvement of the


DC-bias performance. The increases of the amount of the distributed air gaps within the core and the demagnetization field of the magnetic particles, which makes the core be magnetized more difficultly, were believed to be the physical origins of these phenomena. With the increase of the percentage of the small particles, the core loss at the frequency lower than 150 kHz keeps increasing, while those at 150 and 200 kHz do not have distinct differences. The increase of the hysteresis loss was used to explain the deterioration of the loss at low frequencies. And the enhancement of the influence of the eddy-current loss was believed to mainly determine the variation of the loss at the frequency higher than 150 kHz. Good magnetic properties, namely the %μe (100 Oe) of up to 52.3 % and the lowest loss (50 kHz/1000 Gs) of 270 mW cm−3 , were finally achieved in the Fe-Si-Al powder cores. Acknowledgments This work was supported by the Natural Science Foundation of Education Department of Anhui Province (KJ2012A227). The authors are grateful to Prof. Renjie Chen in Ningbo Institute of Industrial Technology of CAS for the support in the measurements of the core’s microstructure and magnetic properties.

References 1. Lee, T.K., Kim, G.H., Choi, G.B., Jeong, I.B., Kim, K.Y., Jang, P.W.: Mater. Sci. For. 534–536, 1321–1324 (2007) 2. Dhokey, N.B., Patil, S., Dhandare, S., Bandal, V.S.: Electron. Mater. Lett. 10, 591–596 (2014)

J Supercond Nov Magn (2016) 29:463–468 3. Streˇcková, M., Füzer, J., Kobera, L., Brus, J., Fáberová, M., Bureˇs, ´ Girman, V., Hadraba, R., Kollár, P., Lauda, M., Medvecký, L., H., Bat’ková, M., Bat’ko, I.: Mater. Chem. Phys. 147, 649–660 (2014) 4. Liu, Y.P., Yi, Y.D., Shao, W., Shao, Y.F.: J. Magn. Magn. Mater. 330, 119–133 (2013) 5. Tajima, S., Hattori, T., Kondon, M., Kishimoto, H., Sugiyama, M., Kikko, T.: IEEE Trans. Magn. 41, 3280–3282 (2005) 6. Shokrollahi, H., Janghorban, K.: J. Mater. Process. Tech. 189, 1– 12 (2007) 7. Yamada, S., Otsuki, E.: J. Magn. Soc. Japan 23, 640–642 (1999) 8. Ding, W., Jiang, L.T., Li, B.Q., Chen, G.Q., Tian, S.F., Wu, G.H.: J. Supercond. Nov. Magn. 27, 239–245 (2014) 9. Berthault, A., Rousselle, D., Zerah, G.: J. Magn. Magn. Mater. 112, 477–480 (1992) 10. Kim, Y.B., Jang, D.H., Seok, H.K., Kim, K.Y.: Mater. Sci. Eng. A 449–451, 389–393 (2007) 11. Zhao, Y.W., Zhang, X.K., Xiao, J.Q.: Adv. Mater. 17, 915–918 (2005) 12. Wu, L.Z., Ding, J., Jiang, H.B., Chen, L.F., Ong, C.K.: J. Magn. Magn. Mater. 285, 233–239 (2005) 13. Gheisari, K., Javadpour, S., Shokrollahi, H., Hashemi, B.: J. Magn. Magn. Mater. 320, 1544–1548 (2008) 14. Taghvaei, A.H., Ebrahimi, A., Ghaffari, M., Janghorban, K.: J. Magn. Magn. Mater. 322, 808–813 (2010) 15. Wu, S., Sun, A.Z., Zhai, F.Q., Wang, J., Zhang, Q., Xu, W.H., Logan, P., Volinsky, A.A.: J. Magn. Magn. Mater. 324, 818–822 (2012) 16. Zhang, Z.M., Xu, W., Guo, T., Jiang, Y.Z., Yan, M.: J. Alloy. Compd. 594, 153–157 (2014) 17. Maeda, T., Toyoda, H., Igarashi, N., Hirose, K., Mimura, K., Nishioka, T., Ikegaya, A.: SEI Tech. Rev. 60, 3–9 (2005)