纳米晶双主相(Nd,Ce)-Fe-B磁体Ce的价态调控及对磁性的影响机制

The development of dual-main-phase (Nd,Ce)-Fe-B magnets can relieve the pressure of the overused critical rare earth elements such as Pr, Nd, and also make full use of the high abundant rare earth element Ce. The magnetic properties of Ce-based magnets are strongly affected by the valence state of Ce element. But the impact mechanism of Ce valence state on the magnetic properties is still unclear yet. In this project, the nanocrystalline (Nd,Ce)-Fe-B magnet with good thermal stabilities will be applied to regulate the Ce valence state to improve the magnetic properties. The anisotropic nanocrystalline dual-main-phase magnets consisting of Nd-Ce-Fe-B and Ce-Fe-B main phases will be fabricated by dual alloy technique. The distribution of Ce element will be investigated to clarify the migration behaviors and diffusion mechanism of Nd and Ce elements during hot press and hot deformation processes. The valence state of Ce will be regulated by changing the local environment of Ce atom. The variation of valence state weight percentage of Ce element will be characterized by the X-ray absorption near edge fine structure analysis of Ce atom. The relationship among the composition, Ce valence state, and magnetic properties will be established. The magnetic hardening mechanism affected by Ce valence state will also be further studied by magnetic domain structure observation and micromagnetic simulation. This project may shed light on the research and development of Ce-containing permanent magnets.

双主相(Nd,Ce)-Fe-B磁体的开发将缓解Nd、Pr等关键稀土元素的过度消耗，实现高丰度稀土元素Ce的平衡和高值利用。Ce元素的价态与磁体磁性能密切相关，但其影响机制目前尚不清楚。本项目拟以具有优异温度稳定性的纳米晶磁体为研究对象，利用Ce价态的调控，达到提高磁性能的目的。项目采用双合金工艺，制备出含Nd-Ce-Fe-B和Ce-Fe-B两个硬磁主相的各向异性纳米晶磁体。研究热压与热变形过程中Nd、Ce元素的迁移行为和扩散机制，阐明Ce在主相和晶界相中的分布规律；通过成分设计，改变Ce原子局域环境，实现对Ce价态的调控；利用Ce原子X射线吸收近边结构的表征，明确Ce元素价态变化规律，理解Ce价态对磁性能的影响机制并建立磁体成分-Ce价态-磁性能之间的内在关系；结合磁畴结构分析与微磁学模拟计算，进一步揭示Ce价态对磁体磁硬化机制的影响。本项目的成功实施将为Ce磁体的研究与开发提供理论依据。

以铈为主的高丰度稀土元素在稀土永磁中仍未获得大量应用，造成稀土资源的不平衡利用。从原材料成本和国家战略安全角度考虑，高性价比的高丰度稀土永磁的研究与开发势在必行。本项目优化了(Nd,Ce)-Fe-B合金成分，获得了具有双主相结构的纳米晶磁体，研究了制备过程中Nd、Ce元素的迁移行为和磁体磁硬化机制，基于同步辐射X射线吸收近边精细结构方法揭示了Ce元素价态变化规律。.相比初始合金，成分优化后Ce-Fe-B基快淬合金的磁性能明显提高，剩磁增强效应显著，回复曲线完全闭合，平均回复磁导率减小。同步辐射结果表明，Ce3+价态比重由0.345增加至0.483，而Ce4+价态比重由0.655降低至0.517。采用双合金方法，通过热压热变形技术制备了具有双主相结构的纳米晶(Nd,Ce)-Fe-B基磁体，有效改善了磁体的硬磁性能和热稳定性。磁体在300-400K温度范围的矫顽力温度系数由-0.56%/K优化至-0.46%/K。热变形磁体细晶区主相晶粒呈片状，尺寸均匀，而粗晶区团聚的微米级CeFe2晶粒阻碍了主相晶粒的变形，恶化了磁体织构。Nd元素取代Ce2Fe14B相中部分Ce元素，导致热压磁体和热变形磁体中均同时存在Nd-Ce-Fe-B和Ce-Fe-B两个硬磁主相，且磁体第二个相转变温度随着低熔点稀土合金添加量的增加和热压热变形过程的进行而有所提高。低熔点稀土合金添加后，热变形磁体中Ce3+和Ce4+的价态比重较为接近，表明Ce原子的局域化学环境并无明显变化。此外，系统对比了放电等离子烧结和常规热压烧结制得热变形磁体磁性能与显微组织的差异，明确了两种制备技术对纳米晶磁体磁性能的影响机制。.本项目对纳米晶双主相(Nd,Ce)-Fe-B基磁体的结构和性能调控方面做了深入系统的研究，对高丰度稀土永磁材料的进一步研究与开发具有较重要的学术价值和实际意义。