- Email: [email protected]
Origin of the coercivity difference in sintered Nd-Fe-B magnets by grain boundary diffusion process using TbH3 nanoparticles and TbF3 microparticles
Intermetallics 110 (2019) 106464
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Origin of the coercivity difference in sintered Nd-Fe-B magnets by grain boundary diffusion process using TbH3 nanoparticles and TbF3 microparticles
T
Zhao Zhoua,b, Weiqiang Liua, Dan Wua, Ming Yuea,∗, Qingmei Lua, Dongtao Zhanga, Hongguo Zhanga, Jinliang Zhaob, Youhao Liuc, Shanshun Zhac, Xiaofei Yic, Gongping Wang4 a
College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology, Beijing, 100124, China College of Applied Sciences, Beijing University of Technology, Beijing, 100124, China c State Key Laboratory of Rare Earth Permanent Magnetic Materials, Hefei, 231500, China 4 Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Electronics and Communication, Jiangxi Normal University, Nanchang, 330022, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nd-Fe-B sintered magnets Grain boundary diffusion TbH3 nanoparticles Thickness Magnetic domain TbF3 microparticle
Nd-Fe-B sintered magnets with different thicknesses of up to 10 mm were fabricated by combining TbH3 nanoparticles as diffusion sources and the grain boundary diffusion (GBD) technique. TbF3 microparticle-diffused magnets were also prepared for comparison. TbH3 nanoparticles diffusion increased the coercivity by 6.9 kOe for the magnets with thicknesses of 10 mm, which is 5.21 kOe larger than that of TbF3 microparticle-diffused magnets. Deeper, thicker and consecutive (Nd,Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets compared to TbF3 microparticles, indicating the superiority of TbH3 nanoparticles in the GBD process, which can effectively inhibit the nucleation of reversed domains and thus enhance the coercivity.
1. Introduction Due to their outstanding magnetic properties, Nd-Fe-B sintered magnets are widely used in low-carbon economy industries, such as permanent magnet motors for electric vehicles, hybrid vehicles and wind power generation [1]. However, the application of Nd-Fe-B sintered magnets in permanent magnet motors is limited by their poor thermal stability due to their high operating temperatures more than 150 °C [2,3]. In order to improve the intrinsic coercivity and thermal stability of Nd-Fe-B sintered magnets, partial substitutions for Nd by heavy rare earth (HRE) elements such as Dy or Tb were investigated [4–6], forming (Nd, Dy)2Fe14B or (Nd, Tb)2Fe14B with high magnetocrystalline anisotropy fields. However, the remanence and energy products decreased with the addition of Dy or Tb elements because of the antiferromagnetic coupling between Fe and Dy or Tb atoms. More importantly, the addition of HRE elements contributes to the increase of production costs [7]. To overcome these problems, grain-boundary diffusion (GDB) technology was introduced into the post-treatment of Nd-Fe-B sintered magnets and good results were achieved [8]. During the GBD process, the surface of Nd-Fe-B sintered magnets is coated with HRE elements in the form of oxides, fluorides, pure metals or alloys
∗
through electrophoretic deposition, sputtering or by vapor deposition, and then the heat treatments is carried out to make the HRE elements to diffuse into the magnets through grain boundaries [9–16]. Liu et al. [13] used TbH3 suspensions to form a coating on the surfaces of the magnets with the thickness of 4 mm. After annealing, the coercivity of the magnets increased by 8.92 kOe. Ion sputtering and three-dimensional sputtering were applied by Watanabe et al. [14] and Li et al. [15] to carry out grain boundary diffusion for Nd-Fe-B sintered magnets with a thickness of 2.8 mm, and the coercivity of the magnets increased by 12.87 kOe and 10.75 kOe, for DyF3 and TbF3, respectively. Soderznik et al. [16] employed electrodeposition to deposit submicron TbF3 onto the surfaces of the magnets with a thickness of 3.5 mm. The coercivity of the magnets increased from 9.88 kOe to 19.2 kOe. So far, the studies have been focused on magnets with thicknesses less than 6 mm due to the limitation of the diffusion distance for the heavy rare earth additions. This poses a major challenge in manufacturing thicker magnets. Hydride nanoparticles have the advantage of enhanced chemical activities due to their small particle sizes, which should have a beneficial effect on the performance of the thicker magnets. In previous work [17], we found that TbH3 nanoparticle-diffused magnets showed better magnetic properties than TbF3
Corresponding author. E-mail address: [email protected] (M. Yue).
https://doi.org/10.1016/j.intermet.2019.04.007 Received 25 January 2019; Received in revised form 31 March 2019; Accepted 7 April 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
microscope (MOKE, BH-786IP-PK). Fig. 2 shows the demagnetization curves (a) and coercivity enhancement as a function of thickness (b) for Nd-Fe-B sintered magnets diffused by TbH3 nanoparticles and TbF3 microparticles. It is obvious that the thin magnets diffused by TbH3 nanoparticles and TbF3 microparticles have remarkable coercivity enhancements but with a slight reduction in remanence compared to the original magnets. For the magnets with a thickness of 1 mm, TbH3 nanoparticles and TbF3 microparticles, as diffusion sources, show coercivity enhancement of 12.48 kOe and 9.26 kOe, respectively, with a coercivity enhancement difference of 3.22 kOe. Notably, the coercivity enhancement decreases with the increase in the thickness of the magnet. When the thickness of the magnets reaches 10 mm, the coercivity enhancement for the TbF3 microparticles, as diffusion sources, is only 1.69 kOe, which is far below the acceptable value for practical applications. In contrast, for the same thickness of 10 mm, the coercivity enhancement for TbH3 nanoparticles as diffusion sources, has reached a remarkable value of 6.9 kOe, which is 5.21 kOe larger than that of TbF3 microparticle-diffused magnets. On the basis of these findings, it can be concluded that TbH3 nanoparticles combined with GBD technology could break the thickness limitations, of up to 10 mm, in preparing thick Nd-Fe-B sintered magnets with high coercivity. To achieve a better understanding for the difference of TbH3 nanoparticles and TbF3 microparticles in GBD magnets, the concentration gradient of Tb from the surface to inside in cross section of TbH3 nanoparticles and TbF3 microparticles diffused magnets with different thicknesses were performed by EPMA as shown in Fig. 3(a) and (b). It shows that the Tb concentration, in all GBD magnets, decreases gradually from the surface to the inside. As we know, due to the concentration gradient, the diffusion source diffuses into the magnet through the Nd-rich phase, and undergoes a substitution reaction with the main phase. During the diffusion process, Tb prefers to enter into the Nd2Fe14B matrix phase rather than into the Nd-rich phase, and substitutes for Nd to form (Nd, Tb)2Fe14B phase, which is in agreement with a recent first-principles study of Liu et al. [18]. Comparing the difference between TbH3 nanoparticles and TbF3 microparticle-diffused magnets, it is clear that higher Tb concentrations and deeper diffusion distances are achieved simultaneously in TbH3 nanoparticle-diffused magnets compared to those in TbF3 microparticle-diffused magnets. Given that all magnets have the same weight to gain ratio of TbH3 nanoparticles and TbF3 microparticles, we can conclude that the thicker magnets contain more TbF3 microparticles or TbH3 nanoparticles. Therefore, the concentration and diffusion depth of Tb in the diffused magnets are significantly increased with increasing thickness of the magnets. It is worthwhile to mention that the difference of Tb distribution between TbF3 microparticles and TbH3 nanoparticles diffused magnets becomes conspicuous with increasing thickness of the
Fig. 1. SEM images of TbH3 nanoparticles (a) and TbF3 microparticles (b).
microparticle-diffused magnets. In this paper, we explored the performance of TbH3 nanroparticles in the GBD process for magnets with different thicknesses, and compared then with TbF3 microparticles. The microstructures of the diffused magnets with both types of particles, including the magnetic domains at different diffusion depths were systematically analyzed. Commercial Nd-Fe-B sintered magnets with the grade of N52 (EarthPanda Advance Magnetic Material Co. Ltd. China) were used as the original materials. The magnets were cut into squares with dimensions of 10 mm × 10 mm × X mm (with the thickness, X, having values of 1, 2, 4, 6, 10, along the c-axis). The surfaces of the magnets were polished, cleaned with alcohol, and immediately dried by hot air. TbH3 nanoparticles, prepared by an evaporation-condensation device, and the TbF3 microparticles were mixed with ethyl alcohol in a weight ratio of 50:100, respectively. Fig. 1 shows the SEM images of TbH3 nanoparticles and TbF3 microparticles. The particle size of TbH3 nanoparticles are 100–600 nm in diameter, while that of TbF3 microparticles are 1–4 μm in diameter. The treated magnets were sprayed, in a glove box, by TbH3 nanoparticles or TbF3 microparticle suspensions until the weight gain of the diffusion powders reached 0.7 wt% of the original magnets. Heat treatments were done in vacuum at 925 °C for 8 h and subsequently annealed at 500 °C for 3 h. Magnetic properties were measured using a B-H tracer (NIM-500C) in a 3 T pulsed field. The microstructures of the magnets were observed using a scanning electron microscope (SEM, Nova Nano200) with energy dispersive X-ray spectroscopy (EDX) and electron probe micro-analyzer (EPMA, JEOL JXA800) with a wavelength-dispersive X-ray detector (WDX). Magnetic domains structures were analyzed by a Magneto-optical Kerr
Fig. 2. Demagnetization curves (a) and coercivity enhancement as a function of thickness (b) for Nd-Fe-B sintered magnets diffused by TbH3 nanoparticles and TbF3 microparticles. 2
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
Fig. 3. Concentration gradient of Tb from surface to inside in cross section of TbH3 nanoparticles (a), TbF3 microparticles (b) diffused magnets with 1 mm, 6 mm, 10 mm thicknesses (along the c-axis); Tb content in Nd-rich phases at different depths from surface to inside of TbH3 nanoparticles and TbF3 microparticles diffused magnet with thickness of 10 mm (c); High-magnification BSE image and concentration distribution mapping of Tb, Nd of TbH3 nanoparticles diffused magnets at 250 μm beneath the surface (d); High-magnification BSE image and concentration distribution mapping of F, Tb and Nd of TbF3 microparticle-diffused magnets at 100 μm beneath the surface (e). The color scale for the mass % of the elements is given at the bottom of the figures.
Fig. 3(d). Moreover, the hydride nanoparticles have the advantage of a strong chemical activity due to their small particle sizes; they can easily diffuse into the inside of the magnets. Therefore, it is important to highlight that thicker and consecutive (Nd, Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets 250 μm beneath the surface compared to TbF3 microparticle-diffused magnets at 100 μm beneath the surface, indicating the superiority of TbH3 nanoparticles in the GBD process. Magnets, after the GBD process with TbH3 nanoparticles and TbF3 microparticles, exhibit different microstructures, leading to a significant difference between their coercivities. To further investigate the effects of different microstructures on the coercivity, the magnetic domains of the two types of magnets, at different depths, were observed by MOKE. Fig. 4(a1-c3) shows the backscattered electron (BSE) images, MOKE images at the thermally demagnetized state (TDS) and in the remanence state at the surface region (10–30 μm depth), 200 μm depth and center (∼3500 μm depth) for the TbH3 nanoparticle-diffused magnets. At the surface region, most Tb elements distribute in the matrix of the grains, leading to a higher magnetocrystalline anisotropy field (HA). Therefore, the reversed domains can hardly nucleate, and no domain structure can be observed in the remanence state. On the other hand, the saturation magnetization (Ms) of the matrix grains is reduced because of the enrichment of (Nd, Tb)2Fe14B, leading to a lager domain width dD at TDS due to dD ∼1/Js. [21], where Js is the saturation polarization. At 200 μm depth, Tb is distributed at the outer regions of the matrix grains, leading to a higher HA in these regions. MOKE image in the remanence state shows about 5% reversed domains, indicating that the core-shell structure can effectively suppress the nucleation of
magnets, which is consistent with the observation from magnetic properties. Tb content in Nd-rich phase at different depths from the surface to center of the diffused magnets with thickness of 10 mm was analyzed by EDX, and the results were shown in Fig. 3(c). It can be seen that Tb content of TbF3 microparticles diffused magnets decreases gradually from surface to center, with less than 1 wt % at 2500 μm and almost 0 at 3500 μm. However, it is worth pointing out that TbH3 nanoparticle-diffused magnets still have 7.5 wt % at 2500 μm and 1.7 wt % Tb at the central region of 4500 μm, which can explain the large coercivity enhancement of magnets with a thickness of 10 mm. On the basis of our findings, it can be concluded that the diffusion of TbH3 nanoparticles is more effective and suited for thick magnets. In order to find out the reason why the diffusion effect of TbF3 microparticles is not obvious, we compared the high-magnification EPMA micrograph of TbH3 nanoparticle diffused magnets at 250 μm beneath the surface and TbF3 microparticle-diffused magnets at 100 μm beneath the surface. As shown in Fig. 3(e), the areas marked in black are composed with F and Nd elements in Nd-rich phases of TbF3 microparticles diffused magnets, indicating the formation of Nd-F phase. The existence of Nd-F phase in Nd-rich phases is consistent with the observation from Kim et al. [19] and Xu et al. [20]. The gray Nd-F phase is mainly distributed near the surface layers of the magnets, blocking the diffusion channel and hindering the diffusion of Tb. Moreover, F and Tb elements do not co-existence, indicating a good agreement that F element plays a role in impeding the diffusion of Tb. Unlike TbF3 microparticles, TbH3 nanoparticles can diffuse into the magnets through the Nd-rich phases without obvious obstruction, and the hydrogen will be released during the heat treatments, as shown in 3
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
Fig. 4. The BSE images at the surface region (10–30 μm depth) (a1, d1), 200 μm depth (a2, d2), center (∼3500 μm depth) (a3, d3); MOKE images in thermal demagnetization state (TDS) at the surface region (10–30 μm depth) (b1, e1), 200 μm depth (b2, e2), center (∼3500 μm depth) (b3, e3); MOKE images in the remanence state at surface region (10–30 μm depth) (c1, f1), 200 μm depth (c2, f2), center (∼3500 μm depth) (c3, f3) of TbH3 nanoparticles (a1-c3) and TbF3 microparticles (d1-f3) diffused magnets.
(10–30 μm depth), 200 μm depth, and center (∼3500 μm depth) of TbF3 microparticle-diffused magnets. Similar to Fig. 4(a1-c3), at the surface region, most of the Tb is distributed in the matrix grains, leading to a higher magnetocrystalline anisotropy field (HA) and hindering of the nucleation of the reversed domains. On the other hand, the saturation magnetization (Ms) of the matrix grains are reduced, leading to a larger domain width dD in TDS. As the Tb content in TbF3 microparticle-diffused magnets is less than that of TbH3 nanoparticle-
reversed domains, thereby increasing the coercivity. On the other hand, the domain width at 200 μm depth is smaller than that at the surface region, as shown in Fig. 4(b2), as Tb enriches only in the shell rather than in the core of the matrix grains. At the center of the magnet, only a small amount of Tb exists and core-shell structures cannot be observed, leading to 40% reversed domains due to the lower HA of the matrix grains. Fig. 4(d1-f3) show the BSE images, MOKE images in thermal demagnetization state (TDS) and remanence state at the surface region 4
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
thick Nd-Fe-B sintered magnets with high coercivities.
diffused magnets, the domain width of TbF3 microparticle-diffused magnets is smaller. At 200 μm depth, the core-shell structure is hardly observed on the BSE images, and about 25% reversed domains are observed, which is larger than that for TbH3 nanoparticle-diffused magnets. However, as Fig. 3(a) and (b) show, there is still a small amount of Tb at this depth. It is speculated that there may be a thin "shell" layer on the grains, which is too difficult to be observed by SEM. Therefore, the proportion of reverse domains is smaller than at the center. At the center of the magnet, no Tb element exists, see Fig. 3(c), and certainly no core-shell structure is observed, leading to about 50% reversed domains. Tb element forms a core-shell structure in the diffused magnets, which can effectively inhibit the nucleation of reverse domains, and thus improve the coercivity of the magnets. Combined with the results of EPMA and MOKE, it can be inferred that the diffusion distance of TbH3 nanoparticles is much larger than that of TbF3 microparticles, and thus, more core-shell structures are formed in the magnet, improving the nucleation field and increasing the coercivity.
Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51371002, 51001002, 51331003, 51561011), National Key Research and Development Program of China (2016YFB0700902), the International S&T Cooperation Program of China (No. 2015DFG52020), Foundation of Beijing Municipal Education Commission, China (KM201610005025). We thank Prof. Zaven Altounian for helpful discussion.. References [1] Y. Matsuura, J. Magn. Magn. Mater. 303 (2006) 344–347. [2] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, J. Appl. Phys. 55 (1984) 2078–2082. [3] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083–2087. [4] X. Yang, S. Guo, G.F. Ding, X.J. Cao, J.L. Zeng, J. Song, D. Lee, A. Yan, J. Magn. Magn. Mater. 443 (2017) 179–183. [5] G. Yan, P.J. McGuiness, J.P.G. Farr, I.R. Harris, J. Alloy. Comp. 491 (2010) L20–L24. [6] S.Q. Hu, K. Peng, H. Chen, J. Magn. Magn. Mater. 426 (2017) 340–346. [7] K. Hono, H. Sepehri-Amin, Scripta Mater. 67 (2012) 530–535. [8] K.T. Park, K. Hiraga, M. Sagawa, Proc 16th Workshop Rare-Earth Magnets and Their Applications, (2000), pp. 257–264. [9] H. Sepehri-Amin, D. Prabhu, M. Hayashi, T. Ohkubo, K. Hioki, A. Hattori, Scripta Mater. 68 (2013) 167–170. [10] H. Suzuki, Y. Satsu, M. Komuro, J. Appl. Phys. 105 (2009) 07A734-1-07A734-3. [11] M. Soderžnik, K.Z. Rožman, S. Kobe, K. Spomenka, P.M. Guiness, Intermetallics 23 (2012) 158–162. [12] M. Soderznik, K.Z. Rozman, S. Kobe, P. McGuiness, Intermetallics 23 (2012) 158–162. [13] W.Q. Liu, C. Chang, M. Yue, J.S. Yang, D.T. Zhang, J.X. Zhang, Y.Q. Liu, Rare Met. 36 (9) (2017) 718–722. [14] N. Watanabe, M. Itakura, N. Kuwano, D.S. Li, S. Suzuki, K.I. Machida, Mater. Trans. 48 (2007) 915–918. [15] D. Li, S. Suzuki, T. Kawasaki, K-i. Machida, J. Appl. Phys. 47 (2008) 7876–7878. [16] M. Soderznik, M. Korent, K.Z. Soderznik, M. Katter, K. Üstüner, S. Kobe, Acta Mater. 115 (2016) 278–284. [17] D. Wu, M. Yue, W.Q. Liu, J.W. Chen, X.F. Yi, Mater. Res. Lett. 6 (4) (2018) 255–260. [18] X.B. Liu, Z. Altounian, J. Appl. Phys. 111 (2012) 07A701-1-07A701-3. [19] T.H. Kim, S.R. Lee, H.J. Kim, M.W. Lee, T.S. Jang, J. Appl. Phys. 115 (2014) 17A763-1-17A763-3. [20] F. Xu, L.T. Zhang, X.P. Dong, Q.Z. Liu, M. Komuro, Scripta Mater. 64 (2011) 1137–1140. [21] K. Kobayashi, K. Urushibata, T. Matsushita, S. Sakamoto, S. Suzuki, J. Alloy. Comp. 615 (2014) 569–575.
2. Conclusion In summary, thick Nd-Fe-B sintered magnets up to 10 mm with significant coercivity enhancements were prepared by grain boundary diffusion with TbH3 nanoparticles. The enhancement of coercivity of TbF3 microparticle-diffused magnets is only 1.69 kOe, which is far below that required for practical applications. In contrast, TbH3 nanoparticles, as diffusion sources, could enhance the coercivity to 6.9 kOe. The diffusion distance of TbH3 nanoparticles is much longer than that of TbF3 microparticles. It must also be mentioned that hydrogen would be released while fluorine remains in the magnets impeding the diffusion of Tb during the GBD process. Meanwhile, thicker and consecutive (Nd, Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets compared to TbF3 microparticle-diffused magnets, indicating the efficacy of TbH3 nanoparticles in the GBD process. MOKE images in thermal demagnetization and remanence states reveal that TbH3 nanoparticle-diffused magnets demonstrate larger domain widths and less reversed domains compared to TbF3 microparticle-diffused magnets, indicating that TbH3 nanoparticle diffusion could form more core-shell structures and suppress the nucleation of reversed domains, thereby increasing the coercivity for thicker magnets. Therefore, it can be concluded that TbH3 nanoparticles combining with GBD technology can overcome the thickness limitation of up to 10 mm for preparing
5
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Origin of the coercivity difference in sintered Nd-Fe-B magnets by grain boundary diffusion process using TbH3 nanoparticles and TbF3 microparticles
T
Zhao Zhoua,b, Weiqiang Liua, Dan Wua, Ming Yuea,∗, Qingmei Lua, Dongtao Zhanga, Hongguo Zhanga, Jinliang Zhaob, Youhao Liuc, Shanshun Zhac, Xiaofei Yic, Gongping Wang4 a
College of Materials Science and Engineering, Key Laboratory of Advanced Functional Materials, Ministry of Education of China, Beijing University of Technology, Beijing, 100124, China College of Applied Sciences, Beijing University of Technology, Beijing, 100124, China c State Key Laboratory of Rare Earth Permanent Magnetic Materials, Hefei, 231500, China 4 Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Electronics and Communication, Jiangxi Normal University, Nanchang, 330022, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nd-Fe-B sintered magnets Grain boundary diffusion TbH3 nanoparticles Thickness Magnetic domain TbF3 microparticle
Nd-Fe-B sintered magnets with different thicknesses of up to 10 mm were fabricated by combining TbH3 nanoparticles as diffusion sources and the grain boundary diffusion (GBD) technique. TbF3 microparticle-diffused magnets were also prepared for comparison. TbH3 nanoparticles diffusion increased the coercivity by 6.9 kOe for the magnets with thicknesses of 10 mm, which is 5.21 kOe larger than that of TbF3 microparticle-diffused magnets. Deeper, thicker and consecutive (Nd,Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets compared to TbF3 microparticles, indicating the superiority of TbH3 nanoparticles in the GBD process, which can effectively inhibit the nucleation of reversed domains and thus enhance the coercivity.
1. Introduction Due to their outstanding magnetic properties, Nd-Fe-B sintered magnets are widely used in low-carbon economy industries, such as permanent magnet motors for electric vehicles, hybrid vehicles and wind power generation [1]. However, the application of Nd-Fe-B sintered magnets in permanent magnet motors is limited by their poor thermal stability due to their high operating temperatures more than 150 °C [2,3]. In order to improve the intrinsic coercivity and thermal stability of Nd-Fe-B sintered magnets, partial substitutions for Nd by heavy rare earth (HRE) elements such as Dy or Tb were investigated [4–6], forming (Nd, Dy)2Fe14B or (Nd, Tb)2Fe14B with high magnetocrystalline anisotropy fields. However, the remanence and energy products decreased with the addition of Dy or Tb elements because of the antiferromagnetic coupling between Fe and Dy or Tb atoms. More importantly, the addition of HRE elements contributes to the increase of production costs [7]. To overcome these problems, grain-boundary diffusion (GDB) technology was introduced into the post-treatment of Nd-Fe-B sintered magnets and good results were achieved [8]. During the GBD process, the surface of Nd-Fe-B sintered magnets is coated with HRE elements in the form of oxides, fluorides, pure metals or alloys
∗
through electrophoretic deposition, sputtering or by vapor deposition, and then the heat treatments is carried out to make the HRE elements to diffuse into the magnets through grain boundaries [9–16]. Liu et al. [13] used TbH3 suspensions to form a coating on the surfaces of the magnets with the thickness of 4 mm. After annealing, the coercivity of the magnets increased by 8.92 kOe. Ion sputtering and three-dimensional sputtering were applied by Watanabe et al. [14] and Li et al. [15] to carry out grain boundary diffusion for Nd-Fe-B sintered magnets with a thickness of 2.8 mm, and the coercivity of the magnets increased by 12.87 kOe and 10.75 kOe, for DyF3 and TbF3, respectively. Soderznik et al. [16] employed electrodeposition to deposit submicron TbF3 onto the surfaces of the magnets with a thickness of 3.5 mm. The coercivity of the magnets increased from 9.88 kOe to 19.2 kOe. So far, the studies have been focused on magnets with thicknesses less than 6 mm due to the limitation of the diffusion distance for the heavy rare earth additions. This poses a major challenge in manufacturing thicker magnets. Hydride nanoparticles have the advantage of enhanced chemical activities due to their small particle sizes, which should have a beneficial effect on the performance of the thicker magnets. In previous work [17], we found that TbH3 nanoparticle-diffused magnets showed better magnetic properties than TbF3
Corresponding author. E-mail address: [email protected] (M. Yue).
https://doi.org/10.1016/j.intermet.2019.04.007 Received 25 January 2019; Received in revised form 31 March 2019; Accepted 7 April 2019 0966-9795/ © 2019 Elsevier Ltd. All rights reserved.
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
microscope (MOKE, BH-786IP-PK). Fig. 2 shows the demagnetization curves (a) and coercivity enhancement as a function of thickness (b) for Nd-Fe-B sintered magnets diffused by TbH3 nanoparticles and TbF3 microparticles. It is obvious that the thin magnets diffused by TbH3 nanoparticles and TbF3 microparticles have remarkable coercivity enhancements but with a slight reduction in remanence compared to the original magnets. For the magnets with a thickness of 1 mm, TbH3 nanoparticles and TbF3 microparticles, as diffusion sources, show coercivity enhancement of 12.48 kOe and 9.26 kOe, respectively, with a coercivity enhancement difference of 3.22 kOe. Notably, the coercivity enhancement decreases with the increase in the thickness of the magnet. When the thickness of the magnets reaches 10 mm, the coercivity enhancement for the TbF3 microparticles, as diffusion sources, is only 1.69 kOe, which is far below the acceptable value for practical applications. In contrast, for the same thickness of 10 mm, the coercivity enhancement for TbH3 nanoparticles as diffusion sources, has reached a remarkable value of 6.9 kOe, which is 5.21 kOe larger than that of TbF3 microparticle-diffused magnets. On the basis of these findings, it can be concluded that TbH3 nanoparticles combined with GBD technology could break the thickness limitations, of up to 10 mm, in preparing thick Nd-Fe-B sintered magnets with high coercivity. To achieve a better understanding for the difference of TbH3 nanoparticles and TbF3 microparticles in GBD magnets, the concentration gradient of Tb from the surface to inside in cross section of TbH3 nanoparticles and TbF3 microparticles diffused magnets with different thicknesses were performed by EPMA as shown in Fig. 3(a) and (b). It shows that the Tb concentration, in all GBD magnets, decreases gradually from the surface to the inside. As we know, due to the concentration gradient, the diffusion source diffuses into the magnet through the Nd-rich phase, and undergoes a substitution reaction with the main phase. During the diffusion process, Tb prefers to enter into the Nd2Fe14B matrix phase rather than into the Nd-rich phase, and substitutes for Nd to form (Nd, Tb)2Fe14B phase, which is in agreement with a recent first-principles study of Liu et al. [18]. Comparing the difference between TbH3 nanoparticles and TbF3 microparticle-diffused magnets, it is clear that higher Tb concentrations and deeper diffusion distances are achieved simultaneously in TbH3 nanoparticle-diffused magnets compared to those in TbF3 microparticle-diffused magnets. Given that all magnets have the same weight to gain ratio of TbH3 nanoparticles and TbF3 microparticles, we can conclude that the thicker magnets contain more TbF3 microparticles or TbH3 nanoparticles. Therefore, the concentration and diffusion depth of Tb in the diffused magnets are significantly increased with increasing thickness of the magnets. It is worthwhile to mention that the difference of Tb distribution between TbF3 microparticles and TbH3 nanoparticles diffused magnets becomes conspicuous with increasing thickness of the
Fig. 1. SEM images of TbH3 nanoparticles (a) and TbF3 microparticles (b).
microparticle-diffused magnets. In this paper, we explored the performance of TbH3 nanroparticles in the GBD process for magnets with different thicknesses, and compared then with TbF3 microparticles. The microstructures of the diffused magnets with both types of particles, including the magnetic domains at different diffusion depths were systematically analyzed. Commercial Nd-Fe-B sintered magnets with the grade of N52 (EarthPanda Advance Magnetic Material Co. Ltd. China) were used as the original materials. The magnets were cut into squares with dimensions of 10 mm × 10 mm × X mm (with the thickness, X, having values of 1, 2, 4, 6, 10, along the c-axis). The surfaces of the magnets were polished, cleaned with alcohol, and immediately dried by hot air. TbH3 nanoparticles, prepared by an evaporation-condensation device, and the TbF3 microparticles were mixed with ethyl alcohol in a weight ratio of 50:100, respectively. Fig. 1 shows the SEM images of TbH3 nanoparticles and TbF3 microparticles. The particle size of TbH3 nanoparticles are 100–600 nm in diameter, while that of TbF3 microparticles are 1–4 μm in diameter. The treated magnets were sprayed, in a glove box, by TbH3 nanoparticles or TbF3 microparticle suspensions until the weight gain of the diffusion powders reached 0.7 wt% of the original magnets. Heat treatments were done in vacuum at 925 °C for 8 h and subsequently annealed at 500 °C for 3 h. Magnetic properties were measured using a B-H tracer (NIM-500C) in a 3 T pulsed field. The microstructures of the magnets were observed using a scanning electron microscope (SEM, Nova Nano200) with energy dispersive X-ray spectroscopy (EDX) and electron probe micro-analyzer (EPMA, JEOL JXA800) with a wavelength-dispersive X-ray detector (WDX). Magnetic domains structures were analyzed by a Magneto-optical Kerr
Fig. 2. Demagnetization curves (a) and coercivity enhancement as a function of thickness (b) for Nd-Fe-B sintered magnets diffused by TbH3 nanoparticles and TbF3 microparticles. 2
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
Fig. 3. Concentration gradient of Tb from surface to inside in cross section of TbH3 nanoparticles (a), TbF3 microparticles (b) diffused magnets with 1 mm, 6 mm, 10 mm thicknesses (along the c-axis); Tb content in Nd-rich phases at different depths from surface to inside of TbH3 nanoparticles and TbF3 microparticles diffused magnet with thickness of 10 mm (c); High-magnification BSE image and concentration distribution mapping of Tb, Nd of TbH3 nanoparticles diffused magnets at 250 μm beneath the surface (d); High-magnification BSE image and concentration distribution mapping of F, Tb and Nd of TbF3 microparticle-diffused magnets at 100 μm beneath the surface (e). The color scale for the mass % of the elements is given at the bottom of the figures.
Fig. 3(d). Moreover, the hydride nanoparticles have the advantage of a strong chemical activity due to their small particle sizes; they can easily diffuse into the inside of the magnets. Therefore, it is important to highlight that thicker and consecutive (Nd, Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets 250 μm beneath the surface compared to TbF3 microparticle-diffused magnets at 100 μm beneath the surface, indicating the superiority of TbH3 nanoparticles in the GBD process. Magnets, after the GBD process with TbH3 nanoparticles and TbF3 microparticles, exhibit different microstructures, leading to a significant difference between their coercivities. To further investigate the effects of different microstructures on the coercivity, the magnetic domains of the two types of magnets, at different depths, were observed by MOKE. Fig. 4(a1-c3) shows the backscattered electron (BSE) images, MOKE images at the thermally demagnetized state (TDS) and in the remanence state at the surface region (10–30 μm depth), 200 μm depth and center (∼3500 μm depth) for the TbH3 nanoparticle-diffused magnets. At the surface region, most Tb elements distribute in the matrix of the grains, leading to a higher magnetocrystalline anisotropy field (HA). Therefore, the reversed domains can hardly nucleate, and no domain structure can be observed in the remanence state. On the other hand, the saturation magnetization (Ms) of the matrix grains is reduced because of the enrichment of (Nd, Tb)2Fe14B, leading to a lager domain width dD at TDS due to dD ∼1/Js. [21], where Js is the saturation polarization. At 200 μm depth, Tb is distributed at the outer regions of the matrix grains, leading to a higher HA in these regions. MOKE image in the remanence state shows about 5% reversed domains, indicating that the core-shell structure can effectively suppress the nucleation of
magnets, which is consistent with the observation from magnetic properties. Tb content in Nd-rich phase at different depths from the surface to center of the diffused magnets with thickness of 10 mm was analyzed by EDX, and the results were shown in Fig. 3(c). It can be seen that Tb content of TbF3 microparticles diffused magnets decreases gradually from surface to center, with less than 1 wt % at 2500 μm and almost 0 at 3500 μm. However, it is worth pointing out that TbH3 nanoparticle-diffused magnets still have 7.5 wt % at 2500 μm and 1.7 wt % Tb at the central region of 4500 μm, which can explain the large coercivity enhancement of magnets with a thickness of 10 mm. On the basis of our findings, it can be concluded that the diffusion of TbH3 nanoparticles is more effective and suited for thick magnets. In order to find out the reason why the diffusion effect of TbF3 microparticles is not obvious, we compared the high-magnification EPMA micrograph of TbH3 nanoparticle diffused magnets at 250 μm beneath the surface and TbF3 microparticle-diffused magnets at 100 μm beneath the surface. As shown in Fig. 3(e), the areas marked in black are composed with F and Nd elements in Nd-rich phases of TbF3 microparticles diffused magnets, indicating the formation of Nd-F phase. The existence of Nd-F phase in Nd-rich phases is consistent with the observation from Kim et al. [19] and Xu et al. [20]. The gray Nd-F phase is mainly distributed near the surface layers of the magnets, blocking the diffusion channel and hindering the diffusion of Tb. Moreover, F and Tb elements do not co-existence, indicating a good agreement that F element plays a role in impeding the diffusion of Tb. Unlike TbF3 microparticles, TbH3 nanoparticles can diffuse into the magnets through the Nd-rich phases without obvious obstruction, and the hydrogen will be released during the heat treatments, as shown in 3
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
Fig. 4. The BSE images at the surface region (10–30 μm depth) (a1, d1), 200 μm depth (a2, d2), center (∼3500 μm depth) (a3, d3); MOKE images in thermal demagnetization state (TDS) at the surface region (10–30 μm depth) (b1, e1), 200 μm depth (b2, e2), center (∼3500 μm depth) (b3, e3); MOKE images in the remanence state at surface region (10–30 μm depth) (c1, f1), 200 μm depth (c2, f2), center (∼3500 μm depth) (c3, f3) of TbH3 nanoparticles (a1-c3) and TbF3 microparticles (d1-f3) diffused magnets.
(10–30 μm depth), 200 μm depth, and center (∼3500 μm depth) of TbF3 microparticle-diffused magnets. Similar to Fig. 4(a1-c3), at the surface region, most of the Tb is distributed in the matrix grains, leading to a higher magnetocrystalline anisotropy field (HA) and hindering of the nucleation of the reversed domains. On the other hand, the saturation magnetization (Ms) of the matrix grains are reduced, leading to a larger domain width dD in TDS. As the Tb content in TbF3 microparticle-diffused magnets is less than that of TbH3 nanoparticle-
reversed domains, thereby increasing the coercivity. On the other hand, the domain width at 200 μm depth is smaller than that at the surface region, as shown in Fig. 4(b2), as Tb enriches only in the shell rather than in the core of the matrix grains. At the center of the magnet, only a small amount of Tb exists and core-shell structures cannot be observed, leading to 40% reversed domains due to the lower HA of the matrix grains. Fig. 4(d1-f3) show the BSE images, MOKE images in thermal demagnetization state (TDS) and remanence state at the surface region 4
Intermetallics 110 (2019) 106464
Z. Zhou, et al.
thick Nd-Fe-B sintered magnets with high coercivities.
diffused magnets, the domain width of TbF3 microparticle-diffused magnets is smaller. At 200 μm depth, the core-shell structure is hardly observed on the BSE images, and about 25% reversed domains are observed, which is larger than that for TbH3 nanoparticle-diffused magnets. However, as Fig. 3(a) and (b) show, there is still a small amount of Tb at this depth. It is speculated that there may be a thin "shell" layer on the grains, which is too difficult to be observed by SEM. Therefore, the proportion of reverse domains is smaller than at the center. At the center of the magnet, no Tb element exists, see Fig. 3(c), and certainly no core-shell structure is observed, leading to about 50% reversed domains. Tb element forms a core-shell structure in the diffused magnets, which can effectively inhibit the nucleation of reverse domains, and thus improve the coercivity of the magnets. Combined with the results of EPMA and MOKE, it can be inferred that the diffusion distance of TbH3 nanoparticles is much larger than that of TbF3 microparticles, and thus, more core-shell structures are formed in the magnet, improving the nucleation field and increasing the coercivity.
Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 51371002, 51001002, 51331003, 51561011), National Key Research and Development Program of China (2016YFB0700902), the International S&T Cooperation Program of China (No. 2015DFG52020), Foundation of Beijing Municipal Education Commission, China (KM201610005025). We thank Prof. Zaven Altounian for helpful discussion.. References [1] Y. Matsuura, J. Magn. Magn. Mater. 303 (2006) 344–347. [2] J.J. Croat, J.F. Herbst, R.W. Lee, F.E. Pinkerton, J. Appl. Phys. 55 (1984) 2078–2082. [3] M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083–2087. [4] X. Yang, S. Guo, G.F. Ding, X.J. Cao, J.L. Zeng, J. Song, D. Lee, A. Yan, J. Magn. Magn. Mater. 443 (2017) 179–183. [5] G. Yan, P.J. McGuiness, J.P.G. Farr, I.R. Harris, J. Alloy. Comp. 491 (2010) L20–L24. [6] S.Q. Hu, K. Peng, H. Chen, J. Magn. Magn. Mater. 426 (2017) 340–346. [7] K. Hono, H. Sepehri-Amin, Scripta Mater. 67 (2012) 530–535. [8] K.T. Park, K. Hiraga, M. Sagawa, Proc 16th Workshop Rare-Earth Magnets and Their Applications, (2000), pp. 257–264. [9] H. Sepehri-Amin, D. Prabhu, M. Hayashi, T. Ohkubo, K. Hioki, A. Hattori, Scripta Mater. 68 (2013) 167–170. [10] H. Suzuki, Y. Satsu, M. Komuro, J. Appl. Phys. 105 (2009) 07A734-1-07A734-3. [11] M. Soderžnik, K.Z. Rožman, S. Kobe, K. Spomenka, P.M. Guiness, Intermetallics 23 (2012) 158–162. [12] M. Soderznik, K.Z. Rozman, S. Kobe, P. McGuiness, Intermetallics 23 (2012) 158–162. [13] W.Q. Liu, C. Chang, M. Yue, J.S. Yang, D.T. Zhang, J.X. Zhang, Y.Q. Liu, Rare Met. 36 (9) (2017) 718–722. [14] N. Watanabe, M. Itakura, N. Kuwano, D.S. Li, S. Suzuki, K.I. Machida, Mater. Trans. 48 (2007) 915–918. [15] D. Li, S. Suzuki, T. Kawasaki, K-i. Machida, J. Appl. Phys. 47 (2008) 7876–7878. [16] M. Soderznik, M. Korent, K.Z. Soderznik, M. Katter, K. Üstüner, S. Kobe, Acta Mater. 115 (2016) 278–284. [17] D. Wu, M. Yue, W.Q. Liu, J.W. Chen, X.F. Yi, Mater. Res. Lett. 6 (4) (2018) 255–260. [18] X.B. Liu, Z. Altounian, J. Appl. Phys. 111 (2012) 07A701-1-07A701-3. [19] T.H. Kim, S.R. Lee, H.J. Kim, M.W. Lee, T.S. Jang, J. Appl. Phys. 115 (2014) 17A763-1-17A763-3. [20] F. Xu, L.T. Zhang, X.P. Dong, Q.Z. Liu, M. Komuro, Scripta Mater. 64 (2011) 1137–1140. [21] K. Kobayashi, K. Urushibata, T. Matsushita, S. Sakamoto, S. Suzuki, J. Alloy. Comp. 615 (2014) 569–575.
2. Conclusion In summary, thick Nd-Fe-B sintered magnets up to 10 mm with significant coercivity enhancements were prepared by grain boundary diffusion with TbH3 nanoparticles. The enhancement of coercivity of TbF3 microparticle-diffused magnets is only 1.69 kOe, which is far below that required for practical applications. In contrast, TbH3 nanoparticles, as diffusion sources, could enhance the coercivity to 6.9 kOe. The diffusion distance of TbH3 nanoparticles is much longer than that of TbF3 microparticles. It must also be mentioned that hydrogen would be released while fluorine remains in the magnets impeding the diffusion of Tb during the GBD process. Meanwhile, thicker and consecutive (Nd, Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets compared to TbF3 microparticle-diffused magnets, indicating the efficacy of TbH3 nanoparticles in the GBD process. MOKE images in thermal demagnetization and remanence states reveal that TbH3 nanoparticle-diffused magnets demonstrate larger domain widths and less reversed domains compared to TbF3 microparticle-diffused magnets, indicating that TbH3 nanoparticle diffusion could form more core-shell structures and suppress the nucleation of reversed domains, thereby increasing the coercivity for thicker magnets. Therefore, it can be concluded that TbH3 nanoparticles combining with GBD technology can overcome the thickness limitation of up to 10 mm for preparing
5