Since its discovery in 1983, Nd-Fe-B is the most important magnet material on the market. It combines the highest maximum energy product at room temperature with a satisfactory coercivity and temperature coefficient, so it is the choice of material for many applications. Due to this properties, Nd-Fe-B-magnets are of strategic importance in various fields relevant for society today and key technologies for the future, like e-mobility, robotics and renewable energy. However, mining and conditioning of the rare earth (RE) containing ores do not only have considerable negative effects on the environment but are also limited to a few locations worldwide. The reliability of supply is considered highly critical [1,2] – a situation that, with the upcoming transitions in the energy as well as in the mobility sectors, will be even intensified in the future.

Industrial recycling of RE permanent magnets from end-of-life applications is one possibility to mitigate criticality to some extent. Re-feeding scrap magnet material back into the process chain not only lessens the supply risk, but magnets made of recycled material are also ecologically favorable due to the reduction of mined and refined rare earth elements. Our study focusses on functional (magnet-to-magnet) recycling via hydrogen and we demonstrate that recycled magnets are a suitable option for various applications [3,4,5,6,7]. This approach has the advantage of directly re-feeding the intermetallic compound Nd-Fe-B into the production chain. It therefore avoids metallurgical and chemical dissolving processes, which involve significant amounts of energy and chemicals. By using hydrogen, it is also possible to separate the magnetic material from the devices without additional processing steps in a simple manner, making it also a feasible technology to be used in countries with high labor costs [7,8].

Scrap magnets, having a determined rare earth content of 29 wt.-%, a remanence of 1.38 T and a coercivity of 1080 kA/m, served as input material. For applications with higher temperatures (like in traction motors) an increased content of highly critical heavy rare earth (HRE) elements like dysprosium or terbium is necessary. In order to optimize the required HRE content, we analyzed in detail the element distribution and mechanism of coercivity increase at atomic level via 3-dimensional atom probe (3DAP) tomography (Figure 1) for two different recycling approaches: (i) Blending the recycled magnet powder with separately processed HRE hydrides (Nd, Dy, Tb and Ce) or by (ii) application of a grain boundary diffusion process (GBDP) with these elements. We also compared the obtained magnetic and chemical properties and the needed HRE input of both processes. In case of GBDP the same coercivity increase can be gained with the half of HRE content. Finally, we build up a test motor with magnets with an optimized HRE distribution after microstructural engineering and show that these magnets compare with the primary ones.

References:
[1] K.P. Skokov and O. Gutfleisch, Heavy rare earth free, free rare earth and rare earth free magnets - vision and reality, Scripta Materialia View Point Set, 154 (2018) 289-294.
[2] Study on the review of the list of Critical Raw Materials 2017, European Commission.
[3] O. Gutfleisch, I. R. Harris, J. Phys. D: Appl. Phys. 29 (1996) 2255.
[4] O. Gutfleisch, K. Güth, T.G. Woodcock, L. Schultz, Recycling Used Nd-Fe-B Sintered Magnets via a Hydrogen-Based Route to Produce Anisotropic, Resin Bonded Magnets, Advanced Energy Materials 3 (2013) 151-155.
[5] Y. Yang, A. Walton, R. Sheridan, K. Güth, R. Gauß, O. Gutfleisch, M. Buchert, B.-M. Steenari, T. Van Gerven, P.T. Jones, K. Binnemans, REE recovery from end-of-life NdFeB permanent magnet scrap: a critical review, Journal of Sustainable Metallurgy 3 (2017) 122-149.
[6] O. Diehl, M. Schönfeldt, E. Brouwer, A. Dirks, K. Rachut, J. Gassmann, K. Güth, A. Buckow, R. Gauß, R. Stauber, O. Gutfleisch, Towards an alloy recycling of Nd-Fe-B permanent magnets in a circular economy, Journal of Sustainable Metallurgy (2018) 1-13; doi.org/10.1007/s40831-018-0171-7.
[7] M. Zakotnik, I. R. Harris , A. J. Williams , J. Alloys Compd. 450 (2008) 525.
[8] Walton A, Williams A, Rare earth recovery. Mater World 19 (2011) 24–26.

Microstructural engineering through grain boundary modification for improved recycled rare earth permanent magnets

Our 2 powder method* is a novel way to enrich Nd 2 Fe 14 B grains with a shell of high magnetic anisotropy. Thus preventing the nucleation of reversed domains in large sintered blocks without magnet size limitation and at the same time, the minimization of the total heavy rare earth (HRE) content in the large magnet volume. Here, two powders with differences in particle size and chemical composition were blended and sintered together. The powder with the smaller average particle size (AP anisotropy powder) includes HREs like Dy or Tb and therefore, shows a significantly higher magneto crystalline anisotropy than the second powder with larger average particle size (MP main phase powder). Further, the average particle size of the MP is at least 50 100 larger than the particle size of the AP During sintering, the anisotropy powder will turn into the liquid phase first, because of the higher surface energy of smaller particles . This results in a homogeneous distribution of the HREs around the 2 14 1 grains and finally forms a core shell structure .Therefore, by use of the 2 powder method, size independent HRE lean magnets can be produced, which is not the case for the grain boundary diffusion (GBDP) and no additional diffusion treatment after the normal sintering procedure is necessary, making the 2 powder method useful to integrate in the commercial used mass production for high performance didymium based sintered magnets.

Upscaling the 2-Powder Method for the Manufacturing of Heavy Rare Earth-Lean Sintered didymium-based Magnets

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**Session Chair:**
<br>Alex Aubert

Poster Session

poster

It is our great pleasure to invite you to attend the 26th International Workshop on Rare Earth and Future Permanent Magnets and Their Applications (REPM2021) which will be held virtually on June 7-10, 2021. Please note, this is an updated schedule due to COVID-19 travel restrictions which forced us to postpone the conference from summer of 2020. The REPM workshop takes place every two years and is the premier conference on permanent magnets covering a wide range of topics including:

* Rare earth magnets
* Rare earth-free magnets
* Raw materials, mining and recycling
* Advanced synthesis, processing and characterization techniques
* Theoretical modeling of coercivity
* Fundamental properties of advanced magnetic materials
* Thin film magnets
* Applications of permanent magnets

***Workshop***
The workshop will be chaired by Professor George C. Hadjipanayis, University of Delaware, Newark, DE and Dr. Jinfang Liu, President & COO of Electron Energy Corporation, Landisville, PA.
The objectives of the workshop are to bring together engineers and scientists who work on permanent magnets and their applications to present their latest results and exchange ideas on a wide range of relevant topics including resources, raw materials, recycling, processing, fundamental and hard magnetic properties and applications of rare earth and future magnets.

To regsiter for this event, please follow the link below.

Oliver Diehl

2

Presentations

Microstructural engineering through grain boundary modification for improved recycled rare earth permanent magnets

Upscaling the 2-Powder Method for the Manufacturing of Heavy Rare Earth-Lean Sintered didymium-based Magnets

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