简介概要

Coercivity,microstructure,and thermal stability of sintered Nd-Fe-B magnets by grain boundary diffusion with TbH₃ nanoparticles

来源期刊：Rare Metals2017年第9期

论文作者：Wei-Qiang Liu Cheng Chang Ming Yue Jing-Shan Yang Dong-Tao Zhang Jiu-Xing Zhang Yan-Qin Liu

文章页码：718 - 722

摘要：Grain boundary diffusion technique with TbH₃ nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient（α） and the coercivity temperature coefficient（β） of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd₂Fe₁₄B grains and is expected to exist as（Nd,Tb）₂Fe₁₄B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH₃ nanoparticles exhibit enhanced coercivity.

稀有金属(英文版) 2017,36(09),718-722

Coercivity,microstructure,and thermal stability of sintered Nd-Fe-B magnets by grain boundary diffusion with TbH₃ nanoparticles

Wei-Qiang Liu Cheng Chang Ming Yue Jing-Shan Yang Dong-Tao Zhang Jiu-Xing Zhang Yan-Qin Liu

College of Materials Science and Engineering, Beijing University of Technology

收稿日期：8 August 2013

基金：financially supported by the National Natural Science Foundation of China (Nos.51001002 and 51371002);the National High Technology Research and Development Program of China (No.2012AA063201);the Key Program of Science and Technology Development Project of Beijing Municipal Education Commission (No.KZ201110005007);Jinghua Talents of Beijing University of Technology;Rixin Talents of Beijing University of Technology;the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions;

Coercivity,microstructure,and thermal stability of sintered Nd-Fe-B magnets by grain boundary diffusion with TbH₃ nanoparticles

Wei-Qiang Liu Cheng Chang Ming Yue Jing-Shan Yang Dong-Tao Zhang Jiu-Xing Zhang Yan-Qin Liu

College of Materials Science and Engineering, Beijing University of Technology

Abstract：

Grain boundary diffusion technique with TbH₃ nanoparticles was applied to fabricate Tb-less sintered NdFe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet are improved by 112% and reduced by 26% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient(α) and the coercivity temperature coefficient(β) of the magnets are improved after diffusion treatment. Microstructure shows that Tb element enriches in the surface region of Nd₂Fe₁₄B grains and is expected to exist as(Nd,Tb)₂Fe₁₄B phase. Thus, the magneto-crystalline anisotropy field of the magnet improves remarkably. As a result, the sintered Nd-FeB magnets by grain boundary diffusion with TbH₃ nanoparticles exhibit enhanced coercivity.

Keyword：

Grain boundary diffusion; TbH₃ nanoparticles; Coercivity; Thermal stability;

Author： Wei-Qiang Liu,e-mail:liuweiqiang77@hotmail.com; Ming Yue,e-mail: yueming@bjut.edu.cn;

Received： 8 August 2013

1 Introduction

Sintered Nd-Fe-B permanent magnets are widely used due to their outstanding magnetic performance [ 1, 2] .Up to now,however,their undesirable thermal stability is still a major concern in some practical applications such as high temperature motor.Substitution of Nd by Tb or Dy in the magnet was reported to effectively improve the intrinsic coercivity and consequent thermal stability by increasing the magneto-crystalline anisotropy field (HA)of the NdFe-B magnet [ 3, 4, 5, 6, 7, 8, 9] .Unfortunately,this method simultaneously results in considerable degradation of the remanence of the magnets and increase of the cost due to more Tb or Dy contents.Recently,techniques for enhancement of the coercivity of sintered Nd-Fe-B magnets by diffusing a continuous layer of Tb or Dy onto the surface of the Nd₂Fe₁₄B matrix grains without obvious reduction of the remanence were developed by different researchers [ 10, 11, 12, 13, 14] .Previous investigations focus on micro-scale Dy₂O₃,DyF₃,and TbF₃ powders diffusion.However,little is known concerning effects of nanoparticles diffusion on the magnetic property of the Nd-Fe-B permanent magnets.Here the effect on the magnetic property of sintered NdFe-B magnets by grain boundary diffusion with TbH₃nanoparticles was reported.

2 Experimental

Blocks of Nd-Fe-B sintered magnets were cut into thin slices with the dimension of 1 mm×3 mm×4 mm.Then the magnets were etched with a nitric acid solution.TbH₃ nanoparticles were prepared by inert gas condensation method,and then were mixed with ethyl alcohol in a ratio of 50:50 in weight.Figure 1 shows the TEM image of TbH₃ nanoparticles.Most of TbH₃ nanoparticles are50-100 nm in diameter.The magnets were immersed in the mixture with applying ultra-sonic for 2 min,and dried immediately by a hot air.Powder-coated magnets were heat-treated at 600-950℃for 3-9 h under Ar atmosphere.Magnetic properties were measured using a vibrating sample magnetometer after applying a pulsed field of 3 T.The microstructures of the magnets were analyzed using electron probe microanalyzer (EMPA) and energy dispersive X-ray spectroscopy (EDX).

Fig.1 TEM image of TbH₃ nanoparticles

Fig.2 Influence of heat treatment temperature on magnetic proper-ties of magnets after grain boundary diffusion process with TbH₃nanoparticles

3 Results and discussion

Figure 2 shows the influence of heat treatment temperature on the magnetic properties of the magnets after grain boundary diffusion process with TbH₃ nanoparticles.It shows that increasing the heat treatment temperature results in gradual augment in the coercivity of the magnets,while the remanence of the magnets drops simultaneously.Considering the factors of the coercivity and remanence,the heat treatment temperature is optimized at 900℃.The coercivity of the magnets increases by 710 kA·m^-1(66%),while the remanence of the magnets reduces by0.09 T (6.6%) compared with the original magnets.The influence of heat treatment time on the magnetic properties of the magnets after grain boundary diffusion process with TbH₃ nanoparticles is shown in Fig.3.The results indicate that increasing the heat treatment time leads to a decrease in the remanence.Different from this case,the coercivity of the magnets remarkably increases first,up to peaks at 7 h.The coercivity of the magnets increases 1,198 kA·m^-1(112%),while the remanence of the magnets reduces0.35 T (26%) compared with the original magnets.

Fig.3 Influence of heat treatment time on magnetic properties of magnet after grain boundary diffusion process with TbH₃nanoparticles

To clarify the distribution of the Tb element in the NdFe-B magnet,the concentration distribution of Nd and Tb elements in different depths from the surface of the magnets was examined,and the results are shown in Figs.4 and5.The Tb content of the magnets from the surface to the internal was analyzed by EDX quantitative as shown in Fig.6.It is found that Tb diffuses into the inside of the magnet through the grain boundary phase,and forms Tbrich shells around the grain boundary of the NdFeB grains by substituting Nd.The Tb content is about 14.3 wt%at the surface of the magnet.Tb element enriches in the main phase grain,resulting in significantly reducing of the magnet remanence.As the diffusion depth increases,Tb content gradually reduces from the surface to the internal.At the 200μm depth position,Tb content is about10.52 wt%and most Tb element enriches in the surface region of Nd₂Fe₁₄B grains and is expected to exist as(Nd,Tb)₂Fe₁₄B phase.This result confirms that Tb elements diffuse into the magnet internal along the grain boundary and preferentially distribute in the surface region of the main phase.At the 500μm depth position,Tb content is still 10.16 wt%,indicating that Tb nanoparticles diffusion depth is beyond 500μm.

Fig.4 EPMA profiles of a Tb and b Nd taken from a portion of a magnet after grain boundary diffusion process with TbH₃ nanoparticles at depth from 0 to 250μm

Fig.5 EPMA profiles of a Tb and b Nd taken from a portion of a magnet after grain boundary diffusion process with TbH₃ nanoparticles at a depth of 200μm from surface

Fig.6 Tb content changing in magnet after grain boundary diffusion process with TbH₃ nanoparticles

Remanence and coercivity plotted against a thickness of removed surface of the grain boundary diffusion processed magnets with TbH₃ nanoparticles are shown in Fig.7.It shows that increasing the thickness of removed surface results in gradual augment in the remanence of the magnets.It is considered that the degradation of the remanence of the magnets is due to a large number of Tb substituting Nd in the surface of the magnets and forming(Nd,Tb)₂Fe₁₄B phase bearing lower M_s compared with Nd₂Fe₁₄B phase.As the diffusion depth increases,the amount of Tb element diffusion into the Nd₂Fe₁₄B phase gradually decreases,which results in the increase of remanence.On the other hand,the improvement of coercivity gradually reduces as the depth increases.Even though the surface layer of more than 500μm is removed,the improvement of the coercivity is also 1,067 kA·m^-1(99%).At the internal position,the Tb element enriches in the surface region of Nd₂Fe₁₄B grains and is expected to exist as (Nd,Tb)₂Fe₁₄B phase.As Kronmüller [ 15] pointed out,the crystal anisotropy of the magnetic grains surface region,which is always lower than that of inside the grains,

Fig.7 Remanence and coercivity plotted against thickness of removed surface of grain boundary diffusion processed magnets with TbH₃ nanoparticles

Fig.8 Demagnetization curves of original magnet and magnet after grain boundary diffusion process with TbH₃ nanoparticles at different temperatures

may lead to an obvious reduction of the magnetic reversal nucleation field (H_N) and subsequent coercivity of the magnet.In this case,the surface regions of the Nd₂Fe₁₄B grains are enriched with (Nd,Tb)₂Fe₁₄B phase with strong crystal anisotropy.The H_N and the coercivity of the NdFe-B sintered magnet diffused by Tb nanoparticles are therefore enhanced noticeably.It can be considered that the improvement of coercivity depends on the formation of(Nd,Tb)₂Fe₁₄B phase in not only Nd₂Fe₁₄B grains but also the surface region of the Nd₂Fe₁₄B grains,and the latter plays an important role.

Demagnetization curves of the magnets before and after grain boundary diffusion process with TbH₃ nanoparticles at different temperatures were measured.As shown in Fig.8,both magnets exhibit lower magnetic properties at200℃than those at 27℃.The magnets after grain boundary diffusion process with TbH₃ nanoparticles possess higher coercivity than that of original magnet at 27 and200℃.Reversible temperature coefficients were calculated in Table 1.The results show that the remanence temperature coefficient (α) and the coercivity temperature coefficient (β) of magnets after grain boundary diffusion process are reduced from-0.24 to—0.18%·℃^-1 and from-0.53 to—0.42%·℃^-1 between 27 and 200℃,respectively.It is concluded that the grain boundary diffusion magnets exhibit better temperature stability than original magnets.

4 Conclusion

In summary,high coercivity magnets were fabricated by applying grain boundary diffusion technology with TbH₃nanoparticles.The coercivity increases by more than1,198 kA·m^-1 (112%) compared with original magnets.Meanwhile,both the remanence temperature coefficient (α)and the coercivity temperature coefficient (β) of the NdFe-B magnet are improved after diffusion treatment.The diffusion Tb element is found to preferentially enrich as(Nd,Tb)₂Fe₁₄B phase in the surface region of the Nd₂Fe₁₄B matrix grains.The special distributed (Nd,Tb)₂Fe₁₄B phase is expected to improve the coercivity of the Nd-Fe-B magnet by enhancing both H_A and H_N of the magnet during demagnetization process.

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Table 1 Magnetic properties and temperature coefficient of original magnet

Acknowledgments This study was financially supported by the National Natural Science Foundation of China (Nos.51001002 and51371002),the National High Technology Research and Development Program of China (No.2012AA063201),the Key Program of Science and Technology Development Project of Beijing Municipal Education Commission (No.KZ201110005007),Jinghua Talents of Beijing University of Technology,Rixin Talents of Beijing University of Technology,the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions.