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

Phase formation and magnetization reversal process in Nd1+XFe11Ti compounds

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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MATERIALS FOR HUMANITY (MH 21)

MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

poster

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.


An overview on Rare-Earths market and recycling

Structural refinement and magnetic properties of SmCo5-Cu nanocomposites after high pressure torsion

Heat magnetizing method for high-performance neodymium magnets (hot-deformed Nd-Fe-B magnets)

1. Background
Neodymium magnets have high magnetic flux density and coercive force. For this reason, neodymium magnets are used in permanent magnet synchronous motors (PMSMs) such as automobile drive motors and industrial motors that require miniaturization and high output. When magnetizing magnets set on a PMSM, a method of magnetizing after setting the magnet on a rotor is often used. In recent years, motors have come into practical use with a structure in which magnets are embedded in multiple layers inside a rotor. The application of hot-deformed Nd-Fe-B magnets that do not use heavy rare earth elements to automobile drive motors is increasing. With such a change in the industry, the generated magnetic field required for full magnetization tends to increase.

2. Conventional magnetization technology
In recent years, the strength of the applied magnetic field has tended to be as large as 2400 kA / m to 3200 kA / m for full magnetization. With a conventional magnetized yoke, it has become difficult to magnetize.

3.Heat magnetizing method and analysis technology
In general, it is known that when a coercive force is reduced by heating a magnet, the intensity of a magnetic field applied for magnetization can be reduced. However, it has not yet been put to practical use. It was confirmed that by magnetizing a heated magnet (hot-deformed magnets), it was possible to sufficiently magnetize with a magnetic field strength lower than the applied magnetic field at room temperature. In order to find the magnetization conditions necessary for full magnetization, it is necessary to design a process including the magnetic circuit and the magnet temperature during magnetization, and it is essential to use a magnetization analysis method. Using general-purpose magnetic field analysis software, we clarified the importance of the input data such as the magnetization rate and improved the accuracy of the magnetization analysis.

4.Heat magnetizing method experiment for practical use and results.
A rotor and a magnetization yoke were designed and manufactured, and a test was conducted at a practical heating temperature to determine whether magnetizing was possible under conventional magnetizing conditions. When the rotor was heated, it was confirmed that there was a temperature distribution at the position of the rotor, and that the applied magnetic field also had a magnetic field distribution depending on the location. Based on the temperature distribution and the magnetic field distribution, temperature setting and magnetic field strength for uniformly performing full magnetization were determined, and magnetization was performed. Heating the rotor to a practical temperature of 100 ° C confirmed that the required magnetic field strength could be reduced by about 30%.

5. Summary
We predict the magnetic field conditions for full magnetization and the heating temperature of the rotor in advance. It is possible to design a rotor assembling process including a magnetizing process. We report on the method of magnetizing by heating and the method of magnetizing analysis which are clues for practical application.



Phase formation and magnetization reversal process in Nd1+XFe11Ti compounds

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

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

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