Presentation Information
[P1-6]Simultaneous improvement in coercivity and remanence of (Nd, Pr)-ultra-saving Ce-substituted RE-Fe-B sintered magnets by grain boundary diffusion process using low-melting Nd-Cu-Al-Ga alloy
*Sujin Lee1, Sumin Kim1, Tae-Hoon Kim1, Kyoung-Hoon Bae2, Dong-Hwan Kim2, Jung-Goo Lee1 (1. Korea Institute of Materials Science (Korea), 2. Star Group Ind. CO., Ltd. (Korea))
Keywords:
Ce-based magnets,Grain boundary diffusion process,Coercivity,High Nd/Pr-saving,Remanence,High-performance
The Nd-Fe-B-based magnets are widely employed in the various industries, including wind power generation, intelligent robotics, and electric/hybrid vehicles, due to their superior hard magnetic properties derived from high coercivity and remanence of Nd2Fe14B (2-14-1) grains1). However, the increasing demand for the Nd-Fe-B magnets has led to a substantial depletion of critical rare earth elements (RE), particularly Nd and Pr, resulting in concerns on their cost and supply stability2). Therefore, significant research efforts have been directed toward developing high-performance permanent magnets with reduced Nd and Pr3). Among potential substitutes, Ce have emerged as a promising candidate for partially replacing Nd/Pr in the magnets due to its abundant availability and cost-effectiveness4). However, the inferior intrinsic properties of Ce2Fe14B compared to Nd2Fe14B present a significant challenge in achieving high-performance (Nd,Pr)-saving magnets. In particular, excessive Ce substitution (>40 % of the total RE) severely degrades the hard magnetic properties5), which creates a significant limit for increasing Nd/Pr-saving rate of the magnets. In this work, we demonstrate a simultaneous enhancement of coercivity and remanence in ~57 % (Nd, Pr)-saving magnets via the grain boundary diffusion process (GBDP) using a low-melting Nd-Cu-Al-Ga (NCAG) alloy as the diffusion source.
The Nd/Pr-saving base magnets with the composition of (Nd, Pr)13.0Ce14.3Ho2.7B0.9Co1.0Cu0.2Al1.7Ga1.4Febal. (wt.%) were prepared for GBDP. A small amount of Ho, which can increase the anisotropy field of 2-14-1, was co-doped with Ce. Therefore, the Nd/Pr-saving level [(Ce+Ho)/(Nd+Pr+Ce+Ho)] of the magnets prepared for this work was ~57 %. To resolve the deterioration of magnetic properties induced by Ce in 2-14-1 grains, the Nd-based alloy (NCAG) was selected as the diffusion source satisfying two criteria: (1) RE in the GBD source should have a negative substitution energy for Ce in the 2-14-1 lattice to extract the Ce from 2-14-1 to Nd-rich GBP, and (2) the RE in the GBD source should be able to increase both the anisotropy field and saturation magnetization of 2-14-1 in Ce-substituted magnets. The coating amount of NCAG source for the base magnet was 8 wt.%. The coated magnets were heat-treated at 970℃ for 15 h for GBD, followed by post-diffusion annealing at 550℃ for 2h. Subsequently, we examined the magnetic and microstructural analyses for the GBD-treated and post-diffusion annealed magnets.
As shown in the second-quadrant demagnetization curves in Fig.1(a), the coercivity of the as-sintered (Nd, Pr)-saving magnets substantially increased from 0.90 T to 1.53 T by the NCAG-GBDP, and it further increased to 1.65 T after the post diffusion annealing. Additionally, the remanence of the as-sintered magnets also improved to 1.07 T after NCAG-GBDP, exhibiting the gain of 0.03 T. The NCAG-GBDP simultaneously improved the coercivity and the remanence of (Nd, Pr)-saving magnets. Figs. 1(b) and (c) show the back scattered electron (BSE) images and electron probe micro-analyzer (EPMA) maps for Ce and Nd acquired from center region (~1650 μm depth) of the GBD-treated and post-diffusion annealed magnets, respectively. For the GBD-treated magnet, the substitution of Nd with Ce in the outer region of 2-14-1 grains was observed even at the magnet center, indicating that the Nd-rich shell, which can suppress the reverse domain nucleation6), is formed in the entire region of (Nd, Pr)-saving magnets. Note that post-diffusion annealing facilitated further extraction of Ce from the shell, allowing more Nd to diffuse from the Nd-rich GBP into the shell. These results provide a clear evidence for the enhancement of both coercivity and remanence of high Ce-substituted magnets via NCAG-GBDP. In this presentation, we will discuss the detailed microstructural changes induced by the NCAG-GBDP, specifically focused on the compositional changes of shell before and after the GBDP. Based on these results, we will suggest the key factors in the GBDP for improving the hard magnetic properties of high-Ce-content magnets to a level of commercial (Nd, Pr)-Fe-B sintered magnets.
References
1) M. Sagawa et al., J. Appl. Phys. 57 (1985) 4094.
2) K. P. Skokov et al., Scr. Mater. 154 (2018) 289.
3) Z. C. Lin et al., Acta Mater. 200 (2020) 502.
4) O. Gutfleisch et al., Adv. Mater. 23 (2011) 821.
5) X. Song et al., Magn. Magn. Mater. 606 (2024) 172359.
6) F. Chen et al., J. Alloy. Comp. 819 (2020) 152965.
The Nd/Pr-saving base magnets with the composition of (Nd, Pr)13.0Ce14.3Ho2.7B0.9Co1.0Cu0.2Al1.7Ga1.4Febal. (wt.%) were prepared for GBDP. A small amount of Ho, which can increase the anisotropy field of 2-14-1, was co-doped with Ce. Therefore, the Nd/Pr-saving level [(Ce+Ho)/(Nd+Pr+Ce+Ho)] of the magnets prepared for this work was ~57 %. To resolve the deterioration of magnetic properties induced by Ce in 2-14-1 grains, the Nd-based alloy (NCAG) was selected as the diffusion source satisfying two criteria: (1) RE in the GBD source should have a negative substitution energy for Ce in the 2-14-1 lattice to extract the Ce from 2-14-1 to Nd-rich GBP, and (2) the RE in the GBD source should be able to increase both the anisotropy field and saturation magnetization of 2-14-1 in Ce-substituted magnets. The coating amount of NCAG source for the base magnet was 8 wt.%. The coated magnets were heat-treated at 970℃ for 15 h for GBD, followed by post-diffusion annealing at 550℃ for 2h. Subsequently, we examined the magnetic and microstructural analyses for the GBD-treated and post-diffusion annealed magnets.
As shown in the second-quadrant demagnetization curves in Fig.1(a), the coercivity of the as-sintered (Nd, Pr)-saving magnets substantially increased from 0.90 T to 1.53 T by the NCAG-GBDP, and it further increased to 1.65 T after the post diffusion annealing. Additionally, the remanence of the as-sintered magnets also improved to 1.07 T after NCAG-GBDP, exhibiting the gain of 0.03 T. The NCAG-GBDP simultaneously improved the coercivity and the remanence of (Nd, Pr)-saving magnets. Figs. 1(b) and (c) show the back scattered electron (BSE) images and electron probe micro-analyzer (EPMA) maps for Ce and Nd acquired from center region (~1650 μm depth) of the GBD-treated and post-diffusion annealed magnets, respectively. For the GBD-treated magnet, the substitution of Nd with Ce in the outer region of 2-14-1 grains was observed even at the magnet center, indicating that the Nd-rich shell, which can suppress the reverse domain nucleation6), is formed in the entire region of (Nd, Pr)-saving magnets. Note that post-diffusion annealing facilitated further extraction of Ce from the shell, allowing more Nd to diffuse from the Nd-rich GBP into the shell. These results provide a clear evidence for the enhancement of both coercivity and remanence of high Ce-substituted magnets via NCAG-GBDP. In this presentation, we will discuss the detailed microstructural changes induced by the NCAG-GBDP, specifically focused on the compositional changes of shell before and after the GBDP. Based on these results, we will suggest the key factors in the GBDP for improving the hard magnetic properties of high-Ce-content magnets to a level of commercial (Nd, Pr)-Fe-B sintered magnets.
References
1) M. Sagawa et al., J. Appl. Phys. 57 (1985) 4094.
2) K. P. Skokov et al., Scr. Mater. 154 (2018) 289.
3) Z. C. Lin et al., Acta Mater. 200 (2020) 502.
4) O. Gutfleisch et al., Adv. Mater. 23 (2011) 821.
5) X. Song et al., Magn. Magn. Mater. 606 (2024) 172359.
6) F. Chen et al., J. Alloy. Comp. 819 (2020) 152965.
