United States

Sm-Fe-N is one of prospective candidates for new generation permanent magnet materials due to its extremely high anisotropy field as well as a quite high Curie temperature. A serious drawback of this material is that sintering at temperatures above 620C results in thermal decomposition, making impossible to produce bulk specimens by conventional powder metallurgical processes. Previously, our research group employed pressure current sintering and succeeded in producing isotropic bulk Sm-Fe-N magnets. However, it was also revealed that the coercivity of the original powder was significantly degraded by sintering. In the later study we proposed a hypothesis that the surface oxide might be reduced to form alpha-Fe during sintering. Our most recent study actually proved this hypothesis. To overcome this problem, our group recently developed a low-oxygen powder metallurgy technique wherein all the powder metallurgical processes are carried out inside glove boxes. This technique enabled us to produce extremely high performance Sm-Fe-N fine powder, making it possible to produce anisotropic bulk magnet without coercivity degradation due to the reduction of oxide. 
Nevertheless, another newly arisen problem is the maximum energy product of the sintered anisotropic magnet is not as great as that of the aligned original powder although the high coercivity is maintained. It is quite possible that the particles are damaged during the pulverization process. In this study, the crystallinity and the microstructure of the Sm-Fe-N powder produced by our low oxygen powder metallurgical process.
Commercially available Sm-Fe-N powder was subjected to pulverization by jet-milling with nitrogen gas inside a glove box. The obtained fine powder was gently oxidized at a pass box and then taken out to outside glove box. X-ray diffraction (XRD) was performed to check the crystallinity of the powder. The fine powder was then brought inside a vacuum chamber of focused ion beam – scanning electron microscope (FIB-SEM) apparatus to pick up microsample for transmission electron microscopy (TEM) observation. The microsample was picked up so that the microstructure of the whole particle can be visualized. The TEM observation was carried out using FEI Talos operated at 200 kV. The magnetic properties were measured by using physical property measurement system equipped with vibrating sample magneto meter (PPMS-VSM, Quantum Design/ Dynacool 7).
The particle diameters were significantly reduced down to submicron region, and the coercivity was significantly increased consequently. The maximum energy product of the aligned powder exhibited 43 MGOe in a good case. However, careful analyses of the XRD patterns of those as-milled powders revealed that the full width half maxima (FWHM) significantly increased after jet-milling. Also, the TEM observation revealed that there are polycrystalline regions at the surfaces of pulverized particles. It is likely that these polycrystalline regions are the critical factor of the deterioration of the magnet properties after. During the talk, a possible strategy to improve the performance of the sintered magnets will be discussed.


REPM 2021: The 26th International Workshop on Rare-Earth and Future Permanent Magnets and their Applications

Roles of local texture in crystallinity and magnetic properties of highly coercive Sm-Fe-N fine powder by jet-milling

dislocation

nanocrystalline

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INTERMAG 2021

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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.

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Phase formation and magnetization reversal process in Nd1+XFe11Ti compounds

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

Nd(Fe,Mo)12 nitrogenation reaction kinetics versus temperature and nitrogen pressure

The criticality of rare-earth elements and its impact on the permanent magnet market have initiated a search for alternative material systems, either to fill the magnetic performance gap between ferrites and Nd-Fe-B magnets or to even replace the existing benchmark systems. Ce-based systems are attractive due to the abundance of cerium, however the formation of secondary phases leads to challenges for Ce-based compounds.
In this work we investigated the phase formation properties of CeFe11Ti system together with the microstructural and magnetic properties. Additionally we carried out introduction of interstitial nitrogen and hydrogen atoms to the parent system. The structural and magnetic characterizations of the interstitially doped systems are also investigated by means of experiments and theoretical calculations. A significant reduction is observed for the anisotropy field values of nitrogenated CeFe11TiNx composition and the reason for this reduction is associated to the Kondo screening of the Ce-4f magnetic moment in the ThMn12-type system. The calculations show the enhancement of the screening of the 4f moment for the nitrogenated samples [1]. 
Temperature dependent structural analysis of the CeFe11Ti composition indicate an Invar-type anomaly around the Curie temperature where a M2 behavior is observed as expected from the magnetostriction theory. Microstructural analysis results indicate the formation of twinned grains which are also observed for other ThMn12-type structures with a twin angle of 58°±2° [2]. 
Isothermal magnetization measurements of CeFe11Ti compound under hydrostatic pressure were carried out and a slight decrease in the anisotropy field values is observed. This reduction of the anisotropy is associated to the expected reduction of the Curie temperature under 0.45 GPa hydrostatic pressure.
This work was supported by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) RE-MAP project with the project number 316912154 and MagHEM commissioned by NEDO, Japan.
[1] A. Galler, S. Ener, F. Maccari, I. Dirba, K. P. Skokov, O. Gutfleisch, S. Biermann, L.V. Pourovskii, njp Quantum Materials, 6 (2021) 2
[2] F. Maccari, S. Ener, D. koch, I. Dirba, K. P. Skokov, E. Bruder, L. Schäfer and O. Gutfleisch, Journal of Alloys and Compounds, 867 (2021) 158805


Correlating changes of the unit cell and microstructure with the magnetic properties of CeFe11TiX compounds

Additive manufacturing of Nd-Fe-B permanent magnets by means of Laser powder bed fusions (LPBF) offers the possibility to overcome geometrical restrictions of the traditional powder metallurgical fabrication route for sintered magnets. LPBF is a layer-by-layer additive manufacturing technique, which utilizes lasers to selectively melt metallic powders, which then fuse during solidification. The final product of this manufacturing technique is a dense metallic part. The unique manufacturing process, which includes local melting of a powder material, rapid cooling, directional heat transfer and several re-heating and re-melting, creates special microstructural features and can improve magnetic properties of the final magnet significantly. For example, the coercivity of a LPBF-processed magnet can be increased by more than 30% - from 700 kA/m to more than 900 kA/m - compared to an Nd-lean powder, which was originally optimized for polymer bonded magnets. However, the right choice of the processing parameters is of critical importance for the successful application of LPBF to permanent magnets. The interaction of the major influencing variables, such as laser powder or scan velocity, is difficult to treat in physically based models. An alternative approach for the creation of a suited process model is a statistically based Design of Experiment, which can be used to find an optimal parameter set in a given experimental space. In our current work we use the DoE approach for the comparison of different LPBF machines and for prediction of optimal magnetic properties within a predefined manufacturing parameter range.

Laser Powder Bed Fusion of Nd-Fe-B Permanent Magnets: Solving the Optimization Problem using Design of Experiments and Statistics

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

Production of Nd-Fe-B based powders for Laser-Powder-Bed-Fusion – the 3DREMAG Project
Stefan Riegg1, Joni Reijonen2, Olivier Tosoni3, Juergen Gassmann4, Lukas Schaefer1 and Oliver Gutfleisch1
1) Technical University of Darmstadt, Functional Materials, DE-64287 Darmstadt, Germany 
2) Technical Research Centre of Finland Ltd. (VTT), FI-02044 Espoo, Finland
3) Commissariat à l'énergie atomique et aux énergies alternatives – CEA-Liten, FR-38000 Grenoble, France
4) Fraunhofer IWKS, Magnetic Materials, DE-63457 Hanau, Germany
Keywords: Nd-Fe-B, Permanent Magnets, Additive Manufacturing, Powder Production
During the past years additive manufacturing (AM) became a part of the repertoire of standard industrial process routes. The method integrates effectively the increasing digitalization of production to the materials development. This is well established for the standard-metal-materials available for construction parts starting to create ways of mechanical functionalization by adapting the process parameters locally or by novel design ideas. [1] We want to address a different kind of functionality by defining additional functionality of the printed parts as an additional physical property such as electrical conductivity or a specific magnetic stray field. AM allows near-net-shape manufacturing of complex magnet shapes, avoiding significant machining waste during conventional manufacture of the final shape from sintered magnet blocks. However, the development of the AM process requires a qualified starting material for powder bed fusion, binder jetting or material extrusion. Since the supply of metal powders for AM is limited in general, and there is no Nd-Fe-B based powder designed for use in AM, the development of a Nd-Fe-B-based starting material for commercial production is the goal of the EIT Raw Materials funded project 3DREMAG [2]. 
Commercial powders designed for injection molding are already used for different AM techniques in combination with different polymers and binders [3 and references therein]. As a different example, self-produced Mn-Al powders were used in electron beam melting to produce hard magnetic and rare-earth element free fully dense bodies [4]. The printing process was followed by a heat treatment to achieve 90% of the hard magnetic tau-phase in the samples.
It is very challenging to obtain (a) fully dense Nd-Fe-B magnets by AM and (b) the specific microstructure of uniform micrometer sized Nd2Fe14B grains separated by a rare-earth rich grain boundary phase that has to be established to achieve high remanence and coercivity. To obtain sufficient amounts of powder materials that is qualified to be processed by powder bed fusion,the production route is outlined starting from ground strip cast flakes followed by a plasma spheroidization step. Extremely spherical particle morphology and proper particle size distribution to assure good flowability and packing density for the powder are desired. Special considerations are given to the process challenges such as oxidation, particle shape and size distribution, density of printed parts as well as magnetic anisotropy. Newest results will be discussed to encourage powder production for AM magnets with specific shapes for sensor or actuator applications.
[1] Matthias Gebauer, Markus Oettel, Bernhard Müller, Fraunhofer IWU: http://publica.fraunhofer.de/eprints/urn_nbn_de_0011-n-5592221.pdf
[2] https://3DREMAG.eu
[3] Christian Huber, Gerald Mitteralmskogler, Michael Goertler, Iulian Teliban, Martin Groenefeld, Dieter Suess, Additive manufactured isotropic NdFeB magnets by stereolithography, fused filament fabrication, and selective laser sintering, Materials 13 (2020) 1916
[4] I. Radulov, V. Popov Jr, A. Koptyug, F. Maccari, A. Kovalevsky, S. Essel, J. Gassmann, K. Skokov, M. Bamberger, Production of net-shape Mn-Al permanent magnets by electron beam melting, Additive Manufacturing 30 (2019) 100787


Production of Nd-Fe-B based powders for Laser-Powder-Bed-Fusion - the 3DREMAG Project

We demonstrate a 20% Ce-substituted Nd-Fe-B hot-deformed magnet with a large coercivity (µ0Hc) of 1.69 T and aremanent magnetization of 1.32 T, which are comparable to those for Nd-Fe-B sintered magnets. The temperature coefficient of coercivity (β) of the Ce-substituted magnet was -0.593%/°C in the range of 27–127C, which is superior to that of a Nd-Fe-B sintered magnet (β = -0.669%/°C) with a comparable coercivity at room temperature. Furthermore, the coercivity and its thermal stability can be increased to µ0Hc =1.83 T and β = -0.51 %/°C by applying the Nd80Cu20 eutectic grain boundary diffusion process to an optimized (Nd0.75Ce0.25)-Fe-B hot-deformed sample. A coercivity of 0.74 T was achieved at 150°C in the Nd-Cu diffusion processed hot-deformed magnets, which is much higher than that of sintered magnets. Microstructure investigations and micromagnetic simulations indicate that the modification of grain boundary phase and formation of a Nd-rich shell near the grain boundaries of the developed (Nd,Ce)-Fe-B hot-deformed magnet is the cause of the improved coercivity at room temperature. Thus, in this work, we realize a microstructure-optimized low-cost (Nd0.8Ce0.2)-Fe-B hot-deformed magnet that can outperform Nd-Fe-B sintered magnets.

High performance Ce-substituted hot-deformed magnets with 20% Nd reduction

Several models for explaining of the effect of grain size on the coercivity of sintered magnets are exposed and discussed. Two situations are discussed: i) grain size bellow the single domain size ii) grain size slightly above the single domain size. For the region (i), the Stoner-Wohlfarth model is the basis for the discussion, and it can sucessfully model the hysteresis of high coercivity magnets, total or partially. For the region (ii), a model based on energy balance predicts that the coercivity decreases with the inverse of the square root of the grain size. A nucleation model is able to predict the angular dependence of the coercivity. The models are general and can be applied to any magnetic material, with or without rare-earths.


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Phase formation and magnetization reversal process in Nd1+XFe11Ti compounds

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