Presentation Information
[O1-5]Enabling the production of large HRE lean magnets with homogeneous microstructure - the particle size effect in the 2-powder method and core-shell development in large magnets
*Konrad Opelt1, Mario Schönfeldt1, Chi-Chia Lin1, Jürgen Gassmann1, Imants Dirba2, Abdullatif Durgun2, Matic Jovičević-Klug3, Oliver Gutfleisch2 (1. Fraunhofer IWKS, Fraunhofer Research Institution for Materials Recycling and Resource Strategies, Aschaffenburger Str. 121, 63457 Hanau (Germany), 2. TU Darmstadt, Department of Materials and Geosciences, Functional Materials, Peter-Grünberg-Str. 16, 64287 Darmstadt (Germany), 3. Max-Planck-Institute for Sustainable Materials GmbH, Department of Microstructure Physics and Alloy Design, Max-Planck-Str. 1, 40237 Düsseldorf (Germany))
Keywords:
Nd-Fe-B,life-cycle-assessment,permanent magnets,core-shell structure,sustainable manufacturing
Climate change and the expansion of renewable energies make green technologies like e-mobility and wind energy increasingly important. For the most efficient use, Nd-Fe-B-based magnets are the material of choice in generators of wind turbines and traction motors of electric vehicles. Because of operation temperatures up to 180 °C the use of heavy rare earths (HREs) like Dy or Tb is necessary to increase the temperature stability. Geopolitical reasons and the high price of HREs make them very critical. Therefore, the grain boundary diffusion process (GBDP) is industrially used as a resource-saving approach since it allows engineering a so-called core-shell microstructure within the magnets where the HREs are only located in the outer regions of the magnetic grains. As a result, the coercivity is increased without a significantly decreasing remanence.
The 2-powder method (2PM) is a promising approach to reducing the amount of HREs. In this method, two powders with different particle sizes are blended and sintered. The finer powder shows a higher magnetocrystalline anisotropy because of a higher HRE content. Like in the GBDP, a core-shell structure develops throughout the magnet. However, the advantages of the 2PM are the avoidance of cost-intensive and time-consuming coating and diffusion treatment processes and the production of magnets independent of their size.
In the first part of this study, different HRE-free main phase powders (MP) with D50 values of 3.7 µm and 5.4 µm, and finer HRE-containing anisotropy powders (AP) with D50 values of 2.5 µm, 3.1 µm, 3.6 µm, and 4.0 µm were produced via jet-milling. After blending and sintering, the magnetic properties were measured to investigate the particle size effect, and SEM investigations were performed to analyze the core-shell development. The different particle size ratios and resulting magnet properties are listed in Table 1. For the powder blends with the fine MP (MPf) the coercivity gain of the magnets after applying the 2PM is about 450 kA/m and for the powder blends with the large MP (MPl) it is about 350 kA/m leading to the assumption that the difference of 100 kA/m is related to the particle size effect, meaning that smaller grain sizes of a magnet lead to fewer surface defects at which the magnetization reversal may initiate.
The core-shell structure was observed for all produced magnets, even if the different powders showed similar particle sizes. The observed shell thicknesses seemed not to be affected by the particle size ratio because the shell thicknesses for all magnets are in the same range of 2 – 3 µm and are, therefore, assumed to be only related to the presence of the APs in the powder blends before sintering. So, the mechanism for the core-shell development is considered to be the precipitation of the HREs during the sintering procedure.
In the second part of this study, a huge 340 g magnet with approx. 45 mm in height and 40 mm in diameter was produced with the 2PM to investigate this hypothesis of precipitation as the mechanism for core-shell development on the one hand and to demonstrate the possibility of engineering a core-shell structure in magnets independent of their size (Figure 1). Smaller samples with 5 mm in height and 12 mm in diameter were cut out of the huge 340 g magnet for advanced microstructural characterization using SEM, MOKE, EBSD, TEM, and 3D atom probe tomography.
Finally, a Life Cycle Assessment (LCA) was done to compare the global warming potential of conventional processing of sintered magnets, the GBDP, and the 2PM approach on the pilot plant for magnet manufacturing at Fraunhofer IWKS, Germany. This pilot plant includes all necessary production steps in an industrially relevant batch size starting with up to 50 kg of strip casting, hydrogen decrepitation, jet-milling, alignment and compaction, sintering, cutting, and coating as well as physical vapor deposition for performing the GBDP.
The 2-powder method (2PM) is a promising approach to reducing the amount of HREs. In this method, two powders with different particle sizes are blended and sintered. The finer powder shows a higher magnetocrystalline anisotropy because of a higher HRE content. Like in the GBDP, a core-shell structure develops throughout the magnet. However, the advantages of the 2PM are the avoidance of cost-intensive and time-consuming coating and diffusion treatment processes and the production of magnets independent of their size.
In the first part of this study, different HRE-free main phase powders (MP) with D50 values of 3.7 µm and 5.4 µm, and finer HRE-containing anisotropy powders (AP) with D50 values of 2.5 µm, 3.1 µm, 3.6 µm, and 4.0 µm were produced via jet-milling. After blending and sintering, the magnetic properties were measured to investigate the particle size effect, and SEM investigations were performed to analyze the core-shell development. The different particle size ratios and resulting magnet properties are listed in Table 1. For the powder blends with the fine MP (MPf) the coercivity gain of the magnets after applying the 2PM is about 450 kA/m and for the powder blends with the large MP (MPl) it is about 350 kA/m leading to the assumption that the difference of 100 kA/m is related to the particle size effect, meaning that smaller grain sizes of a magnet lead to fewer surface defects at which the magnetization reversal may initiate.
The core-shell structure was observed for all produced magnets, even if the different powders showed similar particle sizes. The observed shell thicknesses seemed not to be affected by the particle size ratio because the shell thicknesses for all magnets are in the same range of 2 – 3 µm and are, therefore, assumed to be only related to the presence of the APs in the powder blends before sintering. So, the mechanism for the core-shell development is considered to be the precipitation of the HREs during the sintering procedure.
In the second part of this study, a huge 340 g magnet with approx. 45 mm in height and 40 mm in diameter was produced with the 2PM to investigate this hypothesis of precipitation as the mechanism for core-shell development on the one hand and to demonstrate the possibility of engineering a core-shell structure in magnets independent of their size (Figure 1). Smaller samples with 5 mm in height and 12 mm in diameter were cut out of the huge 340 g magnet for advanced microstructural characterization using SEM, MOKE, EBSD, TEM, and 3D atom probe tomography.
Finally, a Life Cycle Assessment (LCA) was done to compare the global warming potential of conventional processing of sintered magnets, the GBDP, and the 2PM approach on the pilot plant for magnet manufacturing at Fraunhofer IWKS, Germany. This pilot plant includes all necessary production steps in an industrially relevant batch size starting with up to 50 kg of strip casting, hydrogen decrepitation, jet-milling, alignment and compaction, sintering, cutting, and coating as well as physical vapor deposition for performing the GBDP.
