Optimizing terbium content in Tb-Cu-Al grain boundary diffusion to significantly enhance coercivity of Nd-Fe-B sintered magnets

IF 9.6 1区化学 Q1 CHEMISTRY, APPLIED

Journal of Rare Earths Pub Date : 2026-07-01 Epub Date: 2025-09-05 DOI:10.1016/j.jre.2025.09.008

Tao Luo, Jianqiang Lin, Chengzhi Sun, Xin Ming, Lichen Wang, Jungang Cheng, Jian Zhang, Baogen Shen

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Abstract

Developing high-coercivity grain boundary diffusion (GBD) Nd-Fe-B thick magnets (>5 mm) is crucial for advanced motor applications. Multicomponent alloys combining heavy rare earth (HRE) with auxiliary elements (e.g., Cu/Al) are currently the most widely used diffusion sources in GBD, but the optimal composition remains unclear, and diffusion sources with high HRE content (∼70 at%) are mostly adopted, which limits the further improvements in coercivity and HRE utilization efficiency of diffused magnets. This study focuses on the commonly used diffusion elements Tb, Cu, and Al, systematically optimizing the ratio between HRE (Tb) and auxiliary elements (Cu/Al) in Tb-Cu-Al diffusion sources. Through one-step diffusion with a reduced HRE content (50 at%) diffusion source, we achieved a breakthrough in coercivity enhancement (ΔH_cj) for thick magnets while simultaneously obtaining dual high performance in both coercivity and its enhancement. The Tb_xAl_50−x/2Cu_50−x/2 (x = 40 at%, 50 at%, 65 at%) diffusion sources were fabricated by magnetron sputtering. After diffusion treatment, the Tb₅₀Cu₂₅Al₂₅ (50 at% Tb) diffused thick magnet (6 mm) exhibits a remarkable ΔH_cj of 11.49 kOe (from 16.73 to 28.22 kOe), significantly outperforming magnets with higher (65 at%, ΔH_cj = 9.96 kOe) and lower Tb content (40 at%, ΔH_cj = 10.57 kOe) diffusion sources. And the thermal stability of Tb₅₀Cu₂₅Al₂₅ diffused magnets improves markedly, with the coercivity temperature coefficient (β_Hcj) improving from −0.535%/°C in the original magnet to −0.457%/°C. The diffusion mechanism study reveals that the Tb₅₀Cu₂₅Al₂₅ diffusion source suppresses the chemically induced liquid film migration (CILFM) during GBD, weakens the abnormal shell thickening and grain coarsening near the surface of the diffused magnet, leading to the formation of continuous and thin (Nd,Tb)₂Fe₁₄B shell structures. Concurrently, the highly efficient wetting effect of Cu/Al facilitates deeper Tb diffusion and more uniform Tb distribution, optimizing the diffusion kinetics and thereby significantly enhancing coercivity. Additionally, this study proposes the regulatory role of the HRE-to-auxiliary element ratio in HRE GBD kinetics, elucidating the “Tb-Cu/Al ratio balancing effect” at intermediate HRE content diffusion source (50 at%): excessive HRE content (65 at%) in the diffusion source leads to severe Tb accumulation on magnet surface, while insufficient HRE content (40 at%) results in formation of incomplete core–shell structure in the deeper regions. This finding challenges the conventional 70 at% HRE: 30 at% auxiliary element diffusion source paradigm, further enhancing Tb utilization efficiency and providing a new strategy for developing cost-effective, high-coercivity magnets.

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优化Tb-Cu-Al晶界扩散中铽的含量，显著提高Nd-Fe-B烧结磁体的矫顽力

开发高矫顽力晶界扩散（GBD） Nd-Fe-B厚磁体（>5 mm）对于先进的电机应用至关重要。重稀土（HRE）与辅助元素（如Cu/Al）复合的多组分合金是目前GBD中应用最广泛的扩散源，但最佳成分尚不明确，多采用高HRE含量（~ 70 at%）的扩散源，这限制了扩散磁体矫顽力和HRE利用效率的进一步提高。本研究以Tb-Cu-Al扩散源中常用扩散元素Tb、Cu和Al为研究对象，系统优化了Tb-Cu-Al扩散源中HRE （Tb）与辅助元素（Cu/Al）的比例。通过一步扩散，降低HRE含量（50 at%）扩散源，我们实现了厚磁体矫顽力增强的突破（ΔHcj），同时获得了矫顽力和增强的双高性能。采用磁控溅射法制备了TbxAl50−x/2Cu50−x/2 （x = 40 at%, 50 at%, 65 at%）扩散源。扩散处理后，Tb50Cu25Al25 （50 at% Tb）扩散厚磁体（6 mm）的扩散系数ΔHcj为11.49 kOe (16.73 ~ 28.22 kOe)，显著优于高扩散源（65 at%， ΔHcj = 9.96 kOe）和低扩散源（40 at%， ΔHcj = 10.57 kOe）扩散源的磁体。Tb50Cu25Al25扩散磁体的热稳定性得到明显改善，矫顽力温度系数（βHcj）由原磁体的- 0.535%/°C提高到- 0.457%/°C。扩散机理研究表明，Tb50Cu25Al25扩散源抑制了GBD过程中化学诱导的液膜迁移（CILFM），减弱了扩散磁体表面附近异常的壳增厚和晶粒粗化，形成了连续而薄的（Nd,Tb）2Fe14B壳结构。同时，Cu/Al的高效润湿效应使Tb扩散更深，Tb分布更均匀，优化了扩散动力学，从而显著提高了矫顽力。此外，本研究提出了HRE-辅助元素比在HRE- GBD动力学中的调节作用，阐明了中等HRE含量扩散源（50 at%）下的“Tb- cu /Al比例平衡效应”：扩散源中HRE含量过高（65 at%）导致磁体表面严重的Tb积聚，而HRE含量不足（40 at%）导致磁体深层形成不完整的核壳结构。这一发现挑战了传统的70 at% HRE: 30 at%辅助元素扩散源模式，进一步提高了Tb的利用效率，并为开发具有成本效益的高矫顽力磁体提供了新的策略。

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来源期刊

Journal of Rare Earths 化学-应用化学

CiteScore

8.70

自引率

14.30%

发文量

374

审稿时长

1.7 months

期刊介绍： The Journal of Rare Earths reports studies on the 17 rare earth elements. It is a unique English-language learned journal that publishes works on various aspects of basic theory and applied science in the field of rare earths (RE). The journal accepts original high-quality original research papers and review articles with inventive content, and complete experimental data. It represents high academic standards and new progress in the RE field. Due to the advantage of abundant RE resources of China, the research on RE develops very actively, and papers on the latest progress in this field emerge every year. It is not only an important resource in which technicians publish and obtain their latest research results on RE, but also an important way of reflecting the updated progress in RE research field. The Journal of Rare Earths covers all research and application of RE rare earths including spectroscopy, luminescence and phosphors, rare earth catalysis, magnetism and magnetic materials, advanced rare earth materials, RE chemistry & hydrometallurgy, RE metallography & pyrometallurgy, RE new materials, RE solid state physics & solid state chemistry, rare earth applications, RE analysis & test, RE geology & ore dressing, etc.