28ideally higher performance at gas-compression (1) Unveiling the microstructure origin of high coercivity in (Nd, Dy)-Fe-B magnet The low coercivity (Hc), which is numerically limited to around 30 % of the anisotropy field (HA) of the main phase for Nd-Fe-B magnets, is a bottleneck for boosting their performance. The results of my research (Figure 1(a)) showed the coercivity of (Nd0.8Dy0.2)-Fe-B sintered magnets is in fact beyond this theoretical prediction; as large as 40 % of its anisotropy field (HA) of the matrix grains. My fundamental study using multi-scale microstructure characterizations and micromagnetic simulations has revealed that the exceptionally large value of Hc/HA, 0.4, originates from the reduced magnetization of the thin intergranular phase. The dissolution of 4 at.% Dy in the intergranular phase significantly reduces its magnetization due to its antiferromagnetic coupling with Fe, contributing to a large coercivity of 3.32 T. This induces a change in the hardening mechanism from the pinning-type Kondorsky model for Dy-free magnets to the nucleation-dominated Stoner–Wohlfarth model Research Digest technique for the Dy-containing magnets as shown in Figure 1(b). Consequently, a Hc/HA value that is twice larger than that of the Dy-free magnet is realized for the (Nd, Dy)-Fe-B sintered magnet [1]. (2) Giant and reversible MCEs for cryogenic applications.Figure 2 shows giant magnetocaloric effect (MCE) in magnetic entropy change (-∆Sm > 0.2 J cm-3K-1) covering fully the practical application temperature window (20-77 K) required for hydrogen the Er(Ho)Co2-based compounds. The liquefaction using magnetic this study yielded significantly larger entropy changes than those of conventional magnetic refrigerants. For example, the -ΔSm values reported in this work are 55% and 93 % larger than those of HoAl2 and DyAl2 at the same transition temperatures. This implies the use of materials developed in this work for practical applications can boost the efficiency of hydrogen liquefaction [2]. Figure 1: The large coercivity in (Nd, Dy)-Fe-B magnet and its coercivity mechanism. Figure 2 Giant and reversible MCEs for hydrogen liquefaction[1] X. Tang, et al. NPG Asia Materials 15, 50 (2023)[2] X. Tang, et al. Nat. Commun. 13 (2022) 1817.refrigerants developed in 1. Outline of ResearchNd-Fe-B-based magnets dominate the permanent magnet market due to their largest energy density and abundance in constituent elements. However, the low coercivity of Nd-Fe-B magnets limits their elevated temperature applications. Hence, (Nd, Dy)-Fe-B-based permanent magnets are so far the only option for application on the engine of hybrid/electric vehicles and wind turbines. The currently used (Nd, Dy)-Fe-B-based sintered magnets contain a considerably large amount of Dy; ~9 wt. %. The scarcity of Dy in the earth crust has urged to develop Dy-lean/free Nd-Fe-B-based magnets with comparable properties to those of currently used (Nd, Dy)-Fe-B sintered magnets. This requires a better understanding of the coercivity mechanism of large coercivity in (Nd, Dy)-Fe-B sintered magnet.Hydrogen is a zero-carbon-emission energy carrier, which is considered to play a major role in the realization of carbon-neutrality in future society. However, hydrogen gas has a large volume and its liquefaction is necessary for its storage and transportation. Liquefaction of hydrogen is currently realized by the conventional gas-compression approach (Joule-Thompson method), which is costly with low efficiency. To overcome these problems, a novel gas liquefaction approach needs to be developed. Magnetic refrigeration (MR) based on magnetocaloric than effect (MCE) has the conventional cryogenic temperatures. However, the lack of MR materials with large magnetic entropy change in a wide temperature range required for the hydrogen liquefaction is a bottleneck for practical applications of MR cooling systems. I have developed a series of materials that can satisfy the materials needs for hydrogen liquefaction. 2. Research ActivitiesHigh Performance Magnetic Materials for Green TechnologyXin TANG
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