Coercivity enhancement of waste Nd-Fe-B magnets by Pr70Cu30 grain boundary diffusion process

Xiao Jun-Ru; Liu Zhong-Wu; Lou Hua-Shan; Zhan Hui-Xiong

doi:10.7498/aps.67.20172551

Abstract

Considerable quantities of Nd-Fe-B magnet wastes are produced every year worldwide. Some Nd-Fe-B magnet wastes in the bulk form, produced during manufacturing, have low coercivity and cannot meet the requirements for applications. Finding an effective way to reuse those wastes by improving the coercivity, without powdering or reproducing process, becomes very important for saving energy and raw materials in manufacture. In this work, the grain boundary diffusion process is carried out on waste Nd-Fe-B sintered magnets by using Pr70Cu30 as a diffusion medium. The effects of diffusion temperature, diffusion time, and annealing time on the magnetic properties of the magnets are investigated. It is found that the coercivity increases when the diffusion temperature increases from 500 to 800℃, the diffusion time increases from 1 to 3 h, or the annealing time increases from 1 to 3 h. By comparing the diffused sample with the simply heat treated sample, we find that the coercivity enhancement by grain boundary diffusion process indeed results from the infiltration of Pr and Cu elements. The coercivity of the magnet increases by 51.9%, from 7.88 kOe (1 Oe=79.5775 A/m) to 11.97 kOe, after 4-hour diffusion at 800℃ followed by 3-hour annealing, with a negligible reduction of remanence Br, achieving a 99.8% recovery of coercivity compared with the commercial N35 magnet. It is noted that 500℃ annealing for 3 h after 800℃ diffusion only slightly increases the coercivity by 4.6%, from 11.44 kOe to 11.97 kOe, which indicates that the annealing process after Pr-Cu grain boundary diffusion may be not indispensable. Based on the microstructure analysis, the diffusion of Pr and Cu is confirmed. However, the distributions of Pr and Cu are inhomogeneous within a range of tens of microns near the surface even though the diffusion has spread throughout the magnet. The structure of main phase grains separated by the continuous grain boundary phase is formed after the grain boundary diffusion process while the core-shell structure is not observed, which suggests that the modification of the grain boundary structure is the main reason for the coercivity improvement. Cu element plays an important role in forming continuous grain boundary phase. In addition, the electrochemical corrosion test shows that higher corrosion current is obtained in the diffused magnet than in the original magnet, though the corrosion potential is improved. The reduced corrosion resistance may be related to the increased RE-rich phase content and the formation of continuous grain boundary phase. The present work is of great importance for increasing the production yield of Nd-Fe-B magnets.

Keywords:

Authors and contacts

1.
School of Material Science and Engineering, South China University of Technology, Guangzhou 510640, China;
2.
School of Mechanical and Electrical Engineering, Liuzhou Vocational and Technical College, Liuzhou 545000, China;
3.
Guangdong Yutong New Material Technology Co. Ltd, Chaozhou 515738, China

Corresponding author: Liu Zhong-Wu, zwliu@scut.edu.cn

Funds: Project supported by the National Natural Science Foundation of China (Grant No. 51774146) and the Guangdong Provincial Science and Technology Program, China (Grant No. 2015B010105008).

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[29]	Hirosawa S, Matsuura Y, Yamamoto H, Fujimura S, Sagawa M, Yamauchi H 1986 J. Appl. Phys. 59 873
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[35]	Isotahdon E, Huttunen-Saarivirta E, Kuokkala V T, Paju M 2012 Mater. Chem. Phys. 135 762

Cited By

[1]	Xu T 2004 Chin. Rare Earths 25 31 (in Chinese) [许涛 2004 稀土 25 31]
[2]	Chen Y H, Wang H Y, Pei Y C, Ren J, Wang J J 2015 ACS Sustain. Chem. Eng. 3 3167
[3]	Saito T, Sato H, Motegi T 2006 J. Alloys Compd. 425 145
[4]	Sepehri-Amin H, Ohkubo T, Zakotnik M, Prosperi D, Afiuny P, Tudor C O, Hono K 2017 J. Alloys Compd. 694 175
[5]	Li X T, Yue M, Liu W Q, Li X L, Yi X F, Huang X L, Zhang D T, Chen J W 2015 J. Alloys Compd. 649 656
[6]	Zakotnik M, Tudor C O 2015 Waste Manage. 44 48
[7]	Li C, Liu W Q, Yue M, Liu Y Q, Zhang D T, Zuo T Y 2014 IEEE Trans. Magn. 50 2105403
[8]	Kawasaki T, Itoh M, Ken-Ichi M 2003 Mater. Trans. 44 1682
[9]	Ma T Y, Wang X J, Liu X L, Wu C, Yan M 2015 J. Phys. D:Appl. Phys. 48 215001
[10]	Oono N, Sagawa M, Kasada R, Matsui H, Kimura A 2011 J. Magn. Magn. Mater. 323 297
[11]	Suzuki H, Satsu Y, Komuro M 2009 J. Appl. Phys. 105 07A734
[12]	Guo S, Zhang X F, Ding G F, Chen R J, Lee D, Yan A R 2014 J. Appl. Phys. 115 17A754
[13]	Watanabe N, Itakura M, Kuwano N, Li D, Suzuki S, Ken-Ich M 2007 Mater. Trans. 48 915
[14]	Soderžnik M, Korent M, Soderžnik K Ž, Katter M, stner K, Kobe S 2016 Acta Mater. 115 278
[15]	Tang M H, Bao X Q, Lu K C, Lu S, Li J H, Gao X X 2016 Scripta Mater. 117 60
[16]	Liang L P, Ma T Y, Pei Z, Jin J Y, Mi Y 2014 J. Magn. Magn. Mater. 355 131
[17]	Ji W X, Liu W Q, Yue M, Zhang D T, Zhang J X 2015 Physica B 476 147
[18]	Chen F G, Zhang T Q, Jing W, Zhang L T, Zhou G F 2015 Scripta Mater. 107 38
[19]	Akiya T, Liu J, Sepehri-Amin H, Ohkubo T, Hioki K, Hattori A, Hono K 2014 J. Appl. Phys. 115 17A766
[20]	Tang X, Chen R, Yin W, Wang J Z, Lee D, Yan A R 2013 Appl. Phys. Lett. 102 72409
[21]	Sepehri-Amin H, Liu J, Ohkubo T, Hioki K, Hattori A, Hono K 2013 Scripta Mater. 69 647
[22]	Sepehri-Amin H, Ohkubo T, Nagashima S, Yano M, Shoji T, Kato A, Schrefl T, Hono K 2013 Acta Mater. 61 6622
[23]	Sepehri-Amin H, Ohkubo T, Nishiuchi T, Hirosawa S, Hono K 2010 Scripta Mater. 63 1124
[24]	Kronmller H, Durst K D, Sagawa M 1988 J. Magn. Magn. Mater. 74 291
[25]	Hono K, Sepehri-Amin H 2012 Scripta Mater. 67 530
[26]	Liu S, Kang N, Yu J, Kwon H, Lee J 2016 J. Magn. 21 51
[27]	Li W F, Ohkubo T, Akiya T, Kato H Hono K 2009 J. Mater. Res. 24 413
[28]	Sepehri-Amin H, Liu L H, Ohkubo T, Yano M, Shoji T, Kato A, Schrefl T, Hono K 2015 Acta Mater. 99 297
[29]	Hirosawa S, Matsuura Y, Yamamoto H, Fujimura S, Sagawa M, Yamauchi H 1986 J. Appl. Phys. 59 873
[30]	Haynes W M 2016 CRC Handbook of Chemistry and Physic (96th Ed.) (BOCA Raton:CRC Press) pp5-81-5-83
[31]	Cui X G, Yan M, Ma T Y, Yu L Q 2008 Physica B 403 4182
[32]	Sun C, Liu W Q, Sun H, Yue M, Yi X F, Chen J W 2012 J. Mater. Sci. Technol. 28 927
[33]	He Q J, Li W 2001 Met. Funct. Mater. 8 8 (in Chinese) [贺琦军, 李卫 2001 金属功能材料 8 8]
[34]	Liu W Q, Yue M, Zhang J X, Wang G P, Li T 2007 Rare Metal. Mat. Eng. 36 1066 (in Chinese) [刘卫强, 岳明, 张久兴, 王公平, 李涛 2007 稀有金属材料与工程 36 1066]
[35]	Isotahdon E, Huttunen-Saarivirta E, Kuokkala V T, Paju M 2012 Mater. Chem. Phys. 135 762

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Vol. 67, Issue 6 - 2018-03-20

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