Study of high-coercivity sintered NdFeB magnets

Study of high-coercivity sintered NdFeB magnets

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 308 (2007) 20–23 www.elsevier.com/locate/jmmm Study of high-coercivity sintered NdFeB m...

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ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 308 (2007) 20–23 www.elsevier.com/locate/jmmm

Study of high-coercivity sintered NdFeB magnets G. Baia,b, R.W. Gaoa,, Y. Suna, G.B. Hana, B. Wanga,c a

School of Physics and Microelectronics, Shandong University, Jinan, 250100, PR China Department of Mathematics and Physics, Xi’an Institute of Technology, Xi’an, 710032, PR China c Baotou Rare Earth Research Institute, Batou 014030, PR China

b

Received 10 January 2006; received in revised form 21 April 2006 Available online 24 May 2006

Abstract Magnetic powders for sintered NdFeB magnets have been prepared by using an advanced processing method including strip casting, hydrogen decrepitation, jet milling and rubber isotropic press. The effects of Dy, Ga and Co addition on the microstructure and magnetic properties of sintered magnets have been investigated. By adopting a suitable component ratio and adjusting proper technological parameters, we have prepared high-coercivity sintered NdFeB magnets with hard magnetic properties of jHc ¼ 25.6 kOe, Br ¼ 13.2 kG and (BH)max ¼ 39.9 MGOe. The temperature coefficient of coercivity of the magnets (between 20 and 150 1C) is –0.53%/1C. The magnetic properties at high temperature satisfy the needs of permanent magnet motors. r 2006 Elsevier B.V. All rights reserved. PACS: 75.30.E; 75.30.G; 75.50.T Keywords: Sintered NdFeB magnets; High coercivity; Advanced processing

1. Introduction Because of its high hard magnetic properties, NdFeB magnets have been developed rapidly and are called the new generation of permanent magnets. The theoretical magnetic energy product of NdFeB can reach as high as 64 MGOe [1]. The experimental value of the energy product of NdFeB has reached 56 MGOe [2], but the low coercivity (9.8 kOe) may limit its applications. NdFeB magnets with excellent properties, especially high-coercivity sintered NdFeB magnets, have extensive applications in the field of permanent magnet motors. In recent years, permanent magnet motors have gradually replaced traditional motors because they have high efficiency and run on lower energy. Permanent magnet motors often operate at high temperature and therefore require magnets with high temperature stability. High-temperature-stable magnets can be obtained by enhancing the Curie temperature and coercivity of the magnets. The theoretical value of Corresponding author. Tel.: +86 531 8377035 8329;

fax: +86 531 8377031. E-mail address: [email protected] (R.W. Gao). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.04.029

coercivity of NdFeB magnets is about 70 kOe [3]. Generally, the coercivity of sintered NdFeB magnets reaches about 1/3–1/5 of the theoretic value. Therefore, there is much potential for increasing the value of coercivity of sintered NdFeB magnets. The elements addition is an effective approach to enhancing the coercivity. A coercivity of 22.8 kOe has been obtained by adding 0.5 wt% Ga to Nd16.5Dy16.0Fe53.45Co13.0B1.05 (wt%) alloys [4]. In this paper, we report on the preparation of sintered NdFeB magnets by an advanced processing method and investigate the effects of the elements addition on the properties of the magnets. Previously, it has been shown that Co addition can increase the Curie temperature and temperature stability [5]. Here, we examine the effect of Dy and Ga addition on both the microstructure and hard magnetic properties of sintered NdFeB magnets prepared by advanced processing. 2. Experimental Alloys with nominal composition of Nd31xDyxFebalCo2B1 (x ¼ 1–7 wt%) and Nd27Dy4 FebalCo2GayB1 (y ¼ 0.5, 1.0, 1.5, 2.0 wt%) were prepared by using the advanced

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processing method described as follows. The alloy ingot was prepared in an intermediate frequency induction furnace. Strip flakes were prepared by the strip casting (SC) technique. The melted alloy was ejected onto a spinning copper wheel with speeds ranging from 1.5 to 2.0 m/s. The rapid cooling of the strip flakes suppresses the growth of soft a-Fe branch crystal grains. The thickness of the strip flakes was about 0.25–0.35 mm. Hydrogen decrepitation (HD), which uses the expansion of hydrogen to break up the flakes, was followed by milling in a jet mill (JM). The particles were then accelerated to supersonic speeds by a current of inert gases where they collided with each other so as to obtain the fine powders. Powders of suitable sizes were obtained by controlling the velocity of classification wheels. The green compacts were prepared by pressing powders with 0.05% lubricant under rubber isotropic pressing (RIP) with a pulse magnetic field, which improves the alignment and density of compacts efficiently and decreases the shrink ratio when the compacts are sintered. The compacts were then sintered at 1020–1070 1C and subsequently annealed at 880 and 600 1C, respectively. The microstructural analysis of the sintered magnets was performed using scanning electron microscopy. The magnetic properties of the magnets were measured with a NIM-10000 H hysteresigraph after being fully magnetized by a 5 T magnetic field. 3. Results and discussion 3.1. Microstructure of SC strip The microstructure of the strip flakes is related to the wheel speed V. Based on the result of Liu et al. [6], we chose a wheel speed of V ¼ 2.0 m/s. Fig. 1 shows the microstructure of the flakes melt-spun at V ¼ 2 m/s. The columnar grains of the hard phases (Nd:Fe:B 2:14:1) with thickness of about 3 mm were separated by a uniformly

Fig. 1. SEM backscattered electron micrograph of the SC strip for NdDyFeCoGaB alloy with V ¼ 2 m/s.

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distributed Nd-rich phase with thickness of 0.2–0.5 mm, while no soft magnetic phase (a-Fe) is present. The ideal microstructure of magnets with high coercivity consists of the hard phase (2:14:1) with grain sizes of 3–5 mm uniformly distributed and separated by the Nd-rich phase with no surface defects. In addition, the soft magnetic phase (a-Fe) should not be present. The magnetic powders and compacts made from the strip flakes spinning at V ¼ 2 m/s using HD, JM and RIP satisfy these criteria.

3.2. Effect of Dy addition on the microstructure and magnetic properties Fig. 2 shows the variation of magnetic properties in Nd31xDyxFebal Co2B1 (x ¼ 1–7 wt%) magnets with varying Dy content. It can be seen that the intrinsic coercivity, jHc, obviously increases while both Br and (BH)max decrease with increasing Dy content. This is due to the effects of Dy addition on the anisotropy field of the hard phase and microstructure of magnets. Additive Dy atoms enter the hard (2:14:1) phase, substitute into Nd sites and form Dy2Fe14B [7], which has a larger anisotropy field (15.8 T) than that of Nd2Fe14B (7.6 T) and thus increases the coercivity. Secondly, Dy addition can suppress the separation of a-Fe branch crystals when the melted alloy solidifies, making the grains of the magnets uniform and fine [8]. The surface defects of the grains are decreased, reducing the possibility of nucleation of reversal walls that may deteriorate the coercivity. Compared to the traditional processing for preparing sintered magnets, the coercivity is increased by using the SC processing method. Because the magnetic moment of Dy is reverse to 3d transition metal element (Fe), the saturation magnetic polarization of Dy2Fe14B (0.7 T) is much lower than that of Nd2Fe14B (1.6 T) [9]. Low saturation polarization decreases both Br and (BH)max as shown in Fig. 2. Therefore, if the content of Dy is too high, it is not suitable for use as a

Fig. 2. Magnetic properties in Nd31xDyxFebal Co2B1 (x ¼ 1–7 wt%) magnets with varying Dy content.

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G. Bai et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 20–23

magnet. A content of 3–4 wt% of Dy can obtain optimum values of both jHc and (BH)max. 3.3. Effect of Ga addition on the microstructure and magnetic properties Fig. 3 shows the variation of magnetic properties in Nd27Dy4 FebalCo2GayB1 (y ¼ 0.5, 1.0, 1.5, 2.0 wt%) magnets with varying Ga content. The data of Fig. 3 show that jHc initially increases drastically with increasing y from 0 to 0.5 wt%, and then slowly increases as y further increases. The effect of Ga content on (BH)max is not as clearly observed, but decreases a little when the Ga content is over 1.5 wt%. Two reasons lead to jHc increasing with increasing Ga content. First, nonmagnetic Ga atoms enter the hard (2:14:1) phase to substitute Fe of J2 site preferentially [10], which leads to increasing uniaxial anisotropy and subsequently enhances the coercivity of magnets. Secondly, Ga doping improves the soakage property of liquid phases and lubricates the grain boundary to form Nd-rich intergranular phases containing Ga [4]. This more effectively separates the magnetic grains, weakens the exchange couple demagnetization effect and enhances the coercivity of magnets. Furthermore, Ga addition can reduce the irreversible loss and improve temperature stability [4]. However, the nonmagnetic atoms of Ga may decrease the saturation polarization and remanence. The opposing effect of Ga content on jHc and Br explains the small effect of Ga content on (BH)max as seen in Fig. 3. In addition, Ga is an expensive metal and thus the Ga content should be kept low. In this experiment, the addition of Ga is retained to about 0.5 wt%. 3.4. Effect of composite additions of Dy, Ga and Co on the microstructure and magnetic properties Compared to single addition of Dy, Ga or Co and traditional processing [4], the combined additions of Dy,

Fig. 4. B/J–H demagnetization curve at different temperatures for sintered Nd27Dy4 FebalCo2Ga0.5B1(wt%) magnets.

Ga and Co are especially effective to increase the coercivity and temperature stability of sintered NdFeB magnets prepared by adopting SC processing. With the composite addition of Dy, Ga and Co and adopting the optimum processing parameters discussed above, we prepared Nd27Dy4FebalCo2Ga0.5B1 (wt%) sintered magnets with excellent magnetic properties of jHc ¼ 25.57 kOe, Br ¼ 13.2 KG and (BH)max ¼ 39.9 MGOe. Fig. 4 shows the B/J–H demagnetization curve of the Nd27Dy4FebalCo2Ga0.5B1 magnet at different temperatures. The temperature coefficient of coercivity is defined as b¼

H cjTO  H cjTR  100%. H cjTO ðT O  T R Þ

According to this equation and based on the data shown in Fig. 4, the temperature coefficient of coercivity (20 1C–150 1C) of Nd27Dy4FebalCo2Ga0.5B1 is calculated as 0.530%/1C. The temperature coefficients of coercivity for NEOMAX NdFeB magnets of 46H, 39SH, 48BH and 38VH types are 0.58,0.55, 0.58 and 0.49, respectively. The coercivity temperature coefficient and temperature stability of magnets prepared by adopting the SC processing as described above are essentially equivalent to NEOMAX NdFeB magnets. 4. Conclusions

Fig. 3. Magnetic properties in Nd27Dy4 FebalCo2GayB1 (y ¼ 0.5–2.0 wt%) magnets with varying Ga content.

1. The strip casting (SC), hydrogen decrepitation (HD), jet milling (JM) and rubber isotropic press are the advanced processing to prepare sintered magnets. By properly adjusting processing parameter, optimizing the microstructure of strip flakes, powders, compacts and magnets, sintered magnets with high coercivity and low temperature coefficients can be obtained. 2. Dy addition can increase the anisotropy field, suppress the separation of a-Fe branch crystals, refine the grains and increase the coercivity. However, the addition of Dy decreases the saturation polarization, remanence and

ARTICLE IN PRESS G. Bai et al. / Journal of Magnetism and Magnetic Materials 308 (2007) 20–23

the energy product of the magnet. The optimal content of Dy was found to be 3–4 wt%. 3. The addition of Ga increases uniaxial anisotropy, forms the Nd- rich intergranular phase containing Ga at the grain boundaries, and weakens the exchange couple demagnetization effect. The nonmagnetic Ga also decreases the remanence and energy product. The optimum content of Ga was found to be about 0.5 wt%. 4. The addition of Co can increase the Curie temperature, decrease the temperature coefficient and enhance the temperature stability. The combined additions of Dy, Ga and Co are especially effective to increase the coercivity and temperature stability of sintered NdFeB magnets prepared by SC processing. The magnets of Nd27Dy4 FebalCo2Ga0.5B1(wt%) prepared by adopting the SC, HD and JM processing have optimum magnetic properties of jHc ¼ 25.57 kOe, Br ¼ 13.2KG and (BH)max ¼ 39.9 MGOe. The temperature coefficient of coercivity (20–150 1C) of the magnets is 0.530%/1C, which satisfies the needs of permanent magnet motors.

Acknowledgements This work was supported by the National Natural Science Foundation of China (50371046) and the Research

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Fund for Doctoral Programme of High Education of China (20040422012). Thanks to Dr. T.D. Corrigan for editing the manuscript. References [1] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [2] W. Rodewald, B. Wall, M. Katter, K. Uestuener, IEEE Trans. Magn. 38 (2002) 2955. [3] S.Z. Zhou, Q.F. Dong, Supermagnets: Rare-earth & Iron System Permanent Magnet, Metallurgical Industry Press, China, 1999, p. 230. [4] W.H. Cheng, W. Li, C.J. Li, S.Z. Dong, J. Magn. Magn. Mater. 234 (2001) 274. [5] A.S. Kim, F.E. Camp, T. Lizzi, J. Appl. Phys. 79 (1996) 4840. [6] H.Q. Liu, B. Wang, G.B. Han, R.W. Gao, J. Magn. Magn. Mater. 285 (2005) 23. [7] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa, J. Appl. Phys. 59 (1986) 873. [8] S.Z. Zhou, Q.F. Dong, Supermagnets: Rare-earth & Iron System Permanent Magnet, Metallurgical Industry Press, China, 1999, p. 272. [9] L.Q. Yu, Y.H. Wen, M. Yan, J. Magn. Magn. Mater. 283 (2004) 353. [10] R. Gro¨ssinger, S. Heib, G. Hilscher, H.R. Kirchmayr, R. Krewenka, G. Wiesingeret al, J. Magn. Magn. Mater. 80 (1989) 61.