Presentation Information
[O16-2]Accelerated development of thermally stable Nd-Fe-B magnets with light rare-earth elements and their synergetic effects to temperature stability with Co
Xinyi Ji1, *Lanting Zhang1, Hong Zhu1, Zhaozhe Zhong2, Jian Liu2, Zhanji Dong3, Zhongjie Peng3, Kaihong Ding3 (1. Shanghai Jiao Tong University (China), 2. Toyota Motor Technical Research and Service (Shanghai) Co., Ltd (China), 3. Yantai Dongxing Magnetic Materials Inc (China))
Keywords:
Nd-Fe-B magnet,light rare-earth elements,Bayesian optimization,temperature stability,site occupation
Developing Nd-Fe-B-based permanent magnets with exceptional high-temperature stability is critical for extending their use in traction motors operating at temperatures of ~150℃. Traditionally, the temperature stability of the Nd-Fe-B magnet can be improved by elements with high Tc such as Co or heavy rare-earth elements (HRE) such as Dy and Tb, etc, which inevitably increases the cost. Doping with cost-effective light rare-earth elements (LRE) such as La, Ce and Y and leveraging the temperature stability with Co and Ni are highly desired to enhance the competitiveness of magnets in the market. Unlike the conventional trial-and-error way, we applied the ‘adaptive’ learning framework (based on Bayesian Optimization) in the composition optimization of Nd-Fe-B sintered magnets towards improved performance-cost ratio. By systematic substitution in the RE-TM-B pseudo-ternary system where RE=NdPr, La, Ce, Y and TM=Fe, Co, Ni, we designed 24 compositions based on [(Nd0.8Pr0.2)(1-x)(La/Ce/Y)x]13.86[Fe(1-y)(Co/Ni)y]77.10M3.30B5.74 (M=Al, Cu, Ga, Ti, at.%) as the starting point of iteration. These magnets were composed of >97% of the RE2TM14B matrix phase and minor phases such as the fcc-NdOx, Ia-3-NdOx, NdFe4B4, and RETM2 Laves phase. 9 novel compositions were recommended within 3 iterations, which were then experimentally prepared, and their magnetic properties measured. It is found that the La-Co, Ce-Co, and La-Co-Ni combinations outperform the other combinations. The best two candidates identified in the last iteration showed 18.4% and 13.1% improvement in performance-cost ratio to benchmark Nd-Fe-B, respectively. An unexpected synergetic effect by co-doping with LRE and Co to improve the temperature stability of the remanence of the magnet is discovered in the present work. It is shown that the remanence at 150℃ and its temperature coefficient α (20-150℃) are 1.099 T and -0.087%/℃ after La-Co co-doping, outperforming the 1.093 T and -0.134%/℃ of the undoped magnet. Regarding remanence and its α, doping with the La-Co pair shows a favourable effect over the Ce-Co pair. Compared with doping with Co alone, the α (20-150℃) of the La0.25Co0.25 and Ce0.25Co0.25 co-doped magnets are almost the same as that of the Co0.25 one even though their Tc are 87°C and 136°C lower. Statistical results from the atomic resolution STEM-EDS indicate that La preferentially occupies the 4g-site of the 2:14:1 lattice in the La0.25Co0.25 magnet, while Ce, the 4f-site in the Ce0.25Co0.25 magnet. Mössbauer spectroscopy reveals that Co preferentially occupies the 16k1, 16k2 and 8j2 sites in the Co0.25 and La0.25Co0.25 magnets, while the 16k1, 16k2 and 8j1 sites in the Ce0.25Co0.25 magnet. Density functional theory calculations based on the preferential site occupations of Ce, La and Co in the 2:14:1 lattice determined by atomic resolution STEM-EDS and Mössbauer spectrum were performed and found that there are spikes and spin splitting at ~10 eV above the Fermi level by La-Co co-doping, which can explain the improved temperature stability. Compared with the benchmark data, the temperature coefficients of remanence of the La-Co co-doped magnets outperform many commercial and reported ones, which is inspiring to develop thermally stable and cost-effective magnets by LRE without HRE.