ICYS Annual Report 2023

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ICYS Annual Report 2023 Nd-Fe-B-based magnets dominate the permanent magnet market due to their high energy density and the relative abundance of their constituent elements. However, their low coercivity limits their use in high-temperature applications [1]. As a result, (Nd, Dy)-Fe-B-based permanent magnets remain the only viable option for use in hybrid/electric vehicle engines and wind turbines. Currently, sintered (Nd, Dy)-Fe-B magnets contain a significant amount of Dy (~9 wt.%), but the scarcity of Dy in the earth's crust has driven efforts to develop Dy-free Nd-Fe-B magnets with comparable properties. Achieving this requires a deeper understanding of the coercivity mechanism and the factors contributing to low coercivity and developing high-performance Dy-free magnets is crucial to addressing material criticality and ensuring sustainable long-term solutions.Hydrogen energy is vital for achieving a carbon-neutral society [2]. However, gaseous hydrogen's large volume and low energy density pose significant challenges, making liquid hydrogen a significantly more efficient fuel source. Traditionally, hydrogen liquefaction has relied on energy-intensive gas-compression systems. In contrast, magnetic refrigeration technology, based on the magnetocaloric effect (MCE), has emerged as a high-efficiency alternative. This requires high-performance magnetocaloric materials that operate in the temperature range of 20-77 K. The Er(Ho)Co2-based compounds show promise due to their giant and reversible MCE [3], suitable for practical applications. However, their MCE in magnetic entropy change degrades below 0.2 J/cm³K as the temperature exceeds 40 K. Therefore, developing materials that exhibit a magnetocaloric effect with a giant magnetic entropy change at above 40 K is essential. The low coercivity (Hc) of Nd-Fe-B magnets, typically limited to ~20% of their anisotropy field (HA), hinders performance improvement. However, my research (Figure 1) shows that (Nd₀.₈Dy₀.₂)-Fe-B sintered magnets surpass this limit, achieving up to 40% of HA [4]. Through multi-scale microstructural characterization and micromagnetic simulations, I found that this high Hc/HA ratio (0.4) results from reduced magnetization in the thin intergranular phase. The dissolution of 4 at.% Dy in this phase lowers its magnetization via antiferromagnetic coupling with Fe, contributing to a coercivity of 3.32 T. Building on these insights, I aim to achieve high coercivity without Dy. As shown in Figure 1, ultrahigh coercivity can be realized by controlling both grain size and the magnetism of the grain boundary phase. This approach opens new possibilities for Dy-free permanent magnets.Additionally, the goal of achieving giant and reversible magnetocaloric effects above 40 K can be realized by controlling in Er(Ho)Co₂-based compounds or by phase transitions Research Digest 1. Outline of Research 2. Research activities Xin TANGFig. 1. High performance Nd-Fe-B magnets for elevated temperature application. Fig. 2. Giant and reversible MCEs for hydrogen liquefaction. References1) J. Li, et al. Acta Mater. 161 (2018) 171-181.2) T. Numazawa, et al. Cryogenics, 62 (2014)185-192.3) X. Tang, et al. Nat. Commun. 13 (2022) 1817.4) X. Tang, et al. NPG Asia Materials 15 (1), 50technology designing novel materials with enhanced magnetocaloric effects (MCE) compared to existing materials. Our preliminary results indicate that the objectives are achievable through a combination of theoretical and experimental approaches.26High Performance Magnetic Materials for Green Technology