[P1] RE-Fe-B Magnets

[P1-27] Fabrication of fine-grained Nd-Fe-B anisotropic magnets using single-crystalline particles synthesized by reduction-diffusion process

*Keunki Cho¹, Seol-mi Lee¹, Sumin Kim¹, Tae-Hoon Kim¹, Jung-Goo Lee¹ (1. Department of Magnetic Materials, Korea Institute of Materials Science (Korea))

Keywords:

Reduction-diffusion,Permanent magnet,Nd-Fe-B

A recent development of Nd-Fe-B sintered magnets focuses on reducing the size of 2-14-1 grains (D) to enhance the coercivity of magnets following the H_c=1/lnDⁿ [1]. To reduce the grain size of Nd-Fe-B magnets, the size of initial powders should be reduced first; however, the size of initial powders achievable with the conventional ‘top-down’ powder preparation approach (strip casting-hydrogen decrepitation-N₂ jet milling) is limited to ~3 μm [2]. The reduction-diffusion (RD) process has emerged as a promising ‘bottom-up’ approach to obtain ~1 μm sized Nd-Fe-B particles [3]. In the RD process, Nd₂O₃ is reduced to pure Nd by the reductant Ca, then the pure Nd is diffused with B into the Fe particles to form the 2-14-1 particles [4]. Therefore, the size of Fe particles determines the size of 2-14-1 particles, which means that the fine 2-14-1 particles with the size of ~1 μm can be easily obtained by preparing the Fe particles with the size below 1 μm [5]. Despite such advantages, the RD process has not yet been widely applicated for preparing the initial Nd-Fe-B powders because of the difficulty in obtaining low-oxygen particles. The major issue is in the washing step using water that is required to remove the CaO [6]. Using water in the washing step of RD leads to the severe oxidation of the final Nd-Fe-B particles, resulting in deteriorating their sinterability. Thus, in this study, we developed a novel washing process for obtaining low-oxygen Nd-Fe-B particles with a size of ~1 μm by RD process. The Nd₂O₃, B, minor elements (Co, Cu, Al), and Ca powders were mixed with the ~1 μm sized carbonyl iron powders. The mixture was compacted and subjected to the RD process at 900 ℃ under an Ar atmosphere. Then, the pulverized RD product was anti-oxidation coated with the ethylenediamine. A two-step washing process involving NH₄NO₃ and dilute acetic acid was performed, followed by vacuum drying. The RD powders underwent sintering using a Pressless Process (PLP) in a vacuum furnace at 1040–1090 ℃. As shown in Fig. 1, the washing process developed in this work enabled to synthesize the Nd-Fe-B particles with the magnetic properties (4πM_s ~ 15.9 kG and H_cJ ~ 1.5 kOe) comparable with those of the commercial jet milled powders. Thereby, for the first time, we could achieve fully dense Nd-Fe-B sintered magnets with the 4πM_r of 13 kG and H_cJ of 10 kOe using the RD powders as an initial powder, as shown in Fig. 2. In this presentation, the washing process that can effectively remove the CaO while preventing the Nd-Fe-B particles from the oxidation will be explained in detail. Based on the results, we will show a potential of RD process in fabricating fine-grained Nd-Fe-B sintered magnets.


References
[1] K. Hono, and H. Sepehri-Amin. Scr. Mater. 67.6 (2012): 530-535.
[2] Li, W. F., et al. J. Magn. Magn. Mater. 321.8 (2009): 1100-1105.
[3] S. Kim, et al. J. Alloy. Compd. 869 (2021): 159337.
[4] Liu, T. Y., et al. J. appl. Phys. 70.10 (1991): 6612-6614.
[5] R. K. Sidhu, R. K. J. Alloy. Compd. 346.1-2 (2002): 250-254.
[6] V. Galkin, et al. J. Magn. Magn. Mater. 578 (2023): 170832.

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