Providing and using green energy is the main approach to mitigate the climate change, this required transformation is also a material transition [1]. In this context, two applications are becoming more and more relevant: Wind turbines for producing green electricity and electrical (hybrid) cars for reducing carbon dioxide emissions if they are using renewable energy. Installations of wind turbines as well as registrations of electrical (hybrid) cars show a large increase all over the world, leading to a quickly rising demand of Nd-Fe-B based high performance permanent magnets [2]. These kind of rare-earth (RE) based permanent magnets are often not substitutable because of their outstanding performance. In particular the heavy rare earth elements (HRE) like Dy and Tb are classified as highly critical in terms of supply, but are needed to ensure the excellent magnetic performance of Nd-Fe-B magnets at elevated temperatures (120 – 200 °C), typical for electrical machines. Therein, the coercivity decreases up to one half of the coercivity at room temperature when HREs are omitted. HREs increase the magneto crystalline anisotropy but also couple antiferromagnetic with the Fe sublattice, leading to a decrease in remanence and the energy product [1,2].
Using our patented 2-powder method [3], we can produce high performance Nd-Fe-B sintered magnets with low contents of critical elements even for large magnet blocks. We have investigated the 2-powder method (2PM) in an industrial relevant production scale of 20 kg by using the powder metallurgy route including strip casting, hydrogen decrepitation, target jet milling, compacting in a magnetic field and sintering. As starting material a HRE containing (10 wt.% Dy) and a HRE-free strip-cast batch were produced. These batches were target jet milled into a coarse HRE free main phase powder (D50 = 5.5 µm) and a fine HRE containing anisotropy powder (D50 = 2.1 µm). Subsequently the powders were blended in different ratios to get a final HRE content of 0, 1, 2 and 3 wt.% in the sintered magnets. In the magnet with 3 wt.% Dy an increase of about 750 kA/m (Figure 1) was measured without a significant loose in remanence. Forming of a core-shell structure was observed via secondary electron microscopy in the whole magnet volume (Figure 2). Finally, we compare the 2PM with the grain boundary diffusion process (GBDP) by producing magnets with 10 mm thickness, underlining the advantage of the 2PM.