[P1] RE-Fe-B Magnets

[P1-17] Novel Aspects in Nd-Fe-B Grain Boundary Engineering: Integrating (Electro)chemistry and Materials Science

*Kristina Zuzek¹, Tomaz Tomse¹, Mihaela Rebernik¹, Fabian Burkhardt¹, Amit Mishra¹, Crt Saksida¹, Laurence Schieren², Carlo Burkhardt², Johann Fischbacher³, Thomas Schrefl³, Sourur Semsari Parapari¹, Saso Sturm (1. Jozef Stefan Institute (Slovenia), 2. Pforzheim University (Germany), 3. Danube University Krems (Austria))

Keywords:

Nd-Fe-B,recycling,processing,grain boundary engineering

Controlling the interfaces between the primary Nd₂Fe₁₄B phase and its surrounding subphases is reported to be crucial for enhancing coercivity¹. Our study focuses on these interfaces—especially between the Nd₂Fe₁₄B phase and the grain-boundary phase—as they play a key role in magnetization reversal. To address this, we introduce a novel approach: single-grain in-situ grain boundary engineering, where magnets are directly created from Nd₂Fe₁₄B matrix grains. The feedstock was thus produced from end-of-life Nd-Fe-B magnets via selective electrochemical and organic acid treatments that have been upgraded to effectively recover Nd₂Fe₁₄B matrix phase grains without any Nd-rich phase and the Nd-oxide phases^2,3. As it is known that trace amounts of copper positively influence the coercivity, we employed an electrochemical approach as a proof-of-concept to deposit Cu on the Nd₂Fe₁₄B matrix grains. We have used Na₂SO₄ for the supporting electrolyte instead of the commonly used H₂SO₄ which enabled us to electrodeposit copper on easily oxidizable substrates like Nd-Fe-B. It was found that potentials, such as −0.5 V, resulted in a negative current throughout the whole deposition experiment, indicating prevailing Cu reduction over the Nd-Fe-B oxidation, representing a critical first step in advancing grain-boundary engineering in the Nd-Fe-B system, introducing new phases that enhance corrosion resistance and promote cost and resource efficiency. Further we succeed in processing of Nd-Fe-B permanent magnets using the matrix Nd₂Fe₁₄B phase grains, with recourse efficient grain boundaries based on Nd-Cu consolidated using spark plasma sintering. The impact of Nd₇₀Cu₃₀ addition ranging from 0 to 30 wt.%, on the microstructure and magnetic properties was investigated. The bulk density and the remanence saturated at 10 wt.% of added Nd₇₀Cu₃₀ , reaching 7.63 g/cm³ and 1.03 T, respectively. Given the strong affinity of Nd to oxygen, part of the Nd from the two-phase Nd₇₀Cu₃₀ alloy interacts with oxygen present at the Nd₂Fe₁₄B grain surfaces, forming Nd-rich oxides, which segregate in triple pockets, as indicated by SEM and TEM. We demonstrate that the removal of oxygen from the grain boundaries significantly enhances the coercivity, which increased from 50 to 825 kA/m for 2.5 and 30 wt.% addition. The positive influence of the texture improvements and the oxygen redistribution on the final coercivity was also proven by micromagnetic calculations. Novel electro and chemical recycling routes opened up new possibilities for reengineering Nd-Fe-B magnets, breaking away from conventional approaches and potentially improving magnet performance. Acknowledgment: ARRS P2-0084, HEU REESILIENCE (101058598), HEU GREENE (101129888).

References
[1] Hono, K. and H. Sepehri-Amin, Strategy for high-coercivity Nd–Fe–B magnets. Scripta Materialia, 2012. 67(6): p. 530-535.
2] X. Xu et al, 2019, Direct recycling of Nd-Fe-B magnets based on the recovery of Nd₂Fe₁₄B grains by acid-free electrochemical etching. ChemSusChem, 2019, 12, 21, 4754-4758
[3] A. Mishra et al. Short-loop recycling of Nd-Fe-B permanent magnets: a sustainable solution for the RE₂Fe₁₄B matrix phase recovery. Materials. Oct. 2023, vol. 16, 1-13