Effects of CE substitution on the microstructures and intrinsic magnetic properties of Nd–Fe–B alloy

Effects of CE substitution on the microstructures and intrinsic magnetic properties of Nd–Fe–B alloy

Journal of Magnetism and Magnetic Materials 393 (2015) 551–554 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials...

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Journal of Magnetism and Magnetic Materials 393 (2015) 551–554

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Effects of CE substitution on the microstructures and intrinsic magnetic properties of Nd–Fe–B alloy Zhi Li a,b, Weiqiang Liu a,b,n, Shanshun Zha a,b, Yuqing Li a,b, Yunqiao Wang a,b, Dongtao Zhang a,b, Ming Yue a,b, Jiuxing Zhang a,b, Xiulian Huang c,d a

College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China The Key Laboratory of Advanced Functional Materials, Ministry of Education, Beijing University of Technology, Beijing 100124, China c Anhui Province Key Laboratories of Rare Earth Permanent Magnet Materials, Anhui 231500, China d Anhui Earth-panda Advance Magnetic Material co., Ltd., Anhui 231500, China b

art ic l e i nf o

a b s t r a c t

Article history: Received 20 January 2015 Received in revised form 1 June 2015 Accepted 15 June 2015 Available online 17 June 2015

(Nd1  xCex)30Fe69B (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) alloys were prepared by inducting melting, and the effect of substitution of Ce for Nd on their microstructure and intrinsic magnetic properties were investigated. With the increase of Ce content, Curie temperature (Tc) decreases from 582.4 to 504.8 K, saturation magnetization (Ms) decreases from 15.88 to 12.71 kGs, and anisotropy field (HA) decreases from 67.4 to 52.7 kOe. However, the reductions of the intrinsic magnetic properties are relatively gentle, and they still have potential to be prepared as permanent magnets. Moreover, further microstructure observations show that Ce is tending to diffuse into the Nd-rich grain boundary phase instead of main phase during the substitute process. Such aggregation behavior is beneficial to fabricate Ce containing magnet with high Ms. & 2015 Elsevier B.V. All rights reserved.

Keywords: Nd–Fe–B alloy Ce substitution Microstructure Intrinsic magnetic properties

1. Introduction Sintered Nd–Fe–B permanent magnetic alloys are widely used in many fields including electric, electronic, instrument, and medical devices because of their excellent magnetic properties [1]. With the expansion of the rare earth industry, non-renewable rare earth resources have become fewer and fewer, which leads to price increase of rare earth metals (Pr, Nd and Dy, etc.), and therefore increase the cost of the sintered Nd–Fe–B magnets as well as restraint the development of Nd–Fe–B industry [2]. At the same time, La and Ce metals separated with Pr and Nd metal are abundant but useless. The reasonable use of La and Ce not only benefits to reduce the cost of the sintered Nd–Fe–B, but also to balance the use of rare earth resources. Whereas the intrinsic magnetic properties of Ce2Fe14B (saturation magnetization 4πMs ¼11.7 kGs and anisotropy field HA ¼26 kOe at 295 K, Curie temperature Tc ¼424 K) are inferior to those of Nd2Fe14B (4πMs ¼16 kGs, HA ¼73 kOe, Tc ¼585 K) [3]. Recently, the substitution of cheaper rare earth elements such as La and Ce to Nd in Nd–Fe–B magnets has been developed by different researchers [4,5]. The R–Fe–B (R, rare earth) sintered magnets prepared with n Corresponding author at: College of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China. E-mail address: [email protected] (W. Liu).

http://dx.doi.org/10.1016/j.jmmm.2015.06.028 0304-8853/& 2015 Elsevier B.V. All rights reserved.

different ratio of alloys of MM–Fe–B (MM, misch-metal) and Nd– Fe–B by dual alloy method were investigated [6]. Up to now, the effect of Ce substitution on the intrinsic magnetic properties and microstructure of Nd–Fe–B alloys has not been studied in detail. In this paper, (Nd1  xCex)30Fe69B (x ¼0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) alloy were prepared by induct melting, and the effect of substitution of Ce for Nd on their microstructure and intrinsic magnetic properties were investigated.

2. Experimental Alloy ingot with nominal composition of (Nd1  xCex)30Fe69B (x ¼0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) was prepared by induction melting, and then was homogenized by annealing in 1353 K under 48 h. Subsequent ball milling was applied to obtain Nd–Fe–B powders with good size distribution. Magnetic alignment was achieved by applying a static magnetic field of 2 T for 4 h. The microstructure and composition of the magnets was analyzed using Scanning Electron Microscope (SEM) (FEI Nova Nano 200) with Energy Dispersive X-ray Detector (EDX). Measurements of magnetization-field curves (M–H) along both magnetically easy and hard direction of the magnets and magnetization–temperature curves (M–T) were carried out in a vibrating sample magnetometer (VSM) (Quantum Design) with a maximum magnetic field of 3 T.

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3. Results and discussion The SEM images of Nd30Fe69B and (Nd0.4Ce0.6)30Fe69B ingots before and after annealing were shown in Fig. 1. As shown in Fig. 1 (a) and (c), the alloys are mainly composed of Nd2Fe14B main phase (gray area) and Nd-rich grain boundary phase (white area) as well as amount of α-Fe phase (dark area) due to the low rare earth content of alloys. As shown in Fig. 1(b) and (d), α-Fe phase could be eliminated in both alloys after annealing. (Nd1  xCex)30Fe69B (x¼ 0.1, 0.2, 0.3, 0.4, and 0.5) alloys show similar results. To study the effect of substitution of Ce for Nd on their intrinsic magnetic properties of (Nd1  xCex)30Fe69B alloys after annealing, the M–H and M–T curves of were measured by VSM. Fig. 2(a) and (b) shows the magnetization curves of Nd30Fe69B and (Nd0.4Ce0.6)30Fe69B alloys along their magnetically easy and hard directions under 3 T magnetic fields, respectively. In the experiment, the magnetization curves in the easy axis and hard axis were measured by the same sample. After measured the magnetization curve in the easy axis, the samples still remained a little residual magnetism, which resulted in the different in the initial stages of the magnetization curve. But it has little effect on the Ms and HA. It is found that under 3 T field, both magnets were magnetized to saturation in their magnetically easy direction, and the of the Nd30Fe69B and saturation magnetization, Ms,

(Nd0.4Ce0.6)30Fe69B alloys are 15.88 to 12.71 kGs, respectively. The reduction of the Ms in (Nd0.4Ce0.6)30Fe69B alloys was mainly due to the magnetic dilution effect from the Ce addition. On the other hand, to obtain the HA of the magnets, the magnetization curves of both magnets along their magnetically easy and hard directions were elongated to a point of intersection, whose x-coordinate value was determined as the HA of the magnet. Therefore, the HA of the Nd30Fe69B and (Nd0.4Ce0.6)30Fe69B alloys were determined as 67.4 to 52.7 kOe, respectively, indicating a pronounced reduction due to the Ce substitution. Furthermore, Ce substitution in the Nd2Fe14B grains was expected to exist as Ce2Fe14B and/or (Nd, Ce)2Fe14B phase because of the weaken HA of the Nd–Fe–B magnet. The intrinsic magnetic properties of the (Nd1  xCex)30Fe69B alloys as a function of Ce content are plotted in Fig. 3. With the increase of Ce content, saturation magnetization (Ms) and anisotropy field (HA) decreases monotonously. As the same time, Fig. 3 also graphically displays the Curie temperature (Tc) as a function of Ce content came from M–T curves. As the increase of Ce content from 0 to 0.6, Tc decreases from 582.4 to 504.8 K, which means that the maximum working temperature of the alloy decreases gradually. However, the reductions of the intrinsic magnetic properties are relatively gentle, and they still have potential to be prepared as permanent magnets. To clarify the distribution of the Ce element in the Nd–Fe–B magnet, the microstructure as well as the concentration

Fig. 1. The SEM image of Nd30Fe69B ingots and (Nd0.4Ce0.6)30Fe69B ingots before and after annealing.

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Fig. 2. The magnetization curves of Nd30Fe69B and (Nd0.4Ce0.6)30Fe69B alloys along their magnetically easy and hard directions under 3 T magnetic field.

Fig. 3. The intrinsic magnetic properties magnetic properties of the (Nd1  xCex)30Fe69B alloys as a function of Ce content (x ¼0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6).

distribution of Ce and Nd elements going through the Nd-rich phase and the Nd2Fe14B matrix phase grains were examined, and the result was shown in Fig. 4. It is found that the Ce content in the Nd-rich phase and the Nd2Fe14B matrix phase grains of (Nd1  xCex)30Fe69B alloys augments gradually, as x increases from 0 to 0.6. However, the Nd content shows the opposite tendency. It should be noted that the improvement of Ce content in the Ndrich phase is far bigger than that in the Nd2Fe14B matrix phase grains, which means that Ce is tending to diffuse into the Nd-rich phase instead of Nd2Fe14B matrix phase grains during the substitute process. In our previous work, the intrinsic magnetic properties and microstructure of (Nd1  xLax)30Fe69B alloys were

Fig. 4. The Ce and Nd content in Nd2Fe14B matrix phase and Nd-rich phase of (Nd1  xCex)30Fe69B alloys as a function of Ce content (x ¼ 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6).

studied and the similar results were obtained. The results verify the theory calculation by Liu [7] that the substitution energies of La in 2:14:1 are positive (0.41 eV/atom) and La tends to be expelled from 2:14:1 phase to Nd-rich grain boundary phase. Base on the same reason, the doping with Ce will lower the structural stability of 2:14:1 phase and tends to distribute in the Nd-rich grain boundary phase. On the contrary, Nd prefers to enter the 2:14:1 phase. Such aggregation behavior is beneficial to fabricate Ce containing magnet with high Ms.

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4. Conclusion The intrinsic magnetic properties of (Nd1  xCex)30Fe69B (x ¼0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) alloys were systematically investigated. With the increase of Ce content, saturation magnetization, anisotropy field and Curie temperature decreases gradually. However, the reductions of the intrinsic magnetic properties are relatively gentle, and they still have potential to be prepared as permanent magnets. Moreover, the aggregation behavior of Ce in the Nd-rich grain boundary phase during the substitute process is beneficial to fabricate Ce containing magnet with high Ms.

Acknowledgment This work was supported by the National High Technology Research and Development Program of China (2012AA063201), National Natural Science Foundation of China (51001002 and 51371002), Jinghua Talents of Beijing University of Technology, Rixin Talents of Beijing University of Technology, 2011 Project, The

Importation and Development of Talents Project of Beijing Municipal Institutions, Technology Foundation for Selected Overseas Chinese Scholar of Beijing.

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