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• Sintered Ce magnets with various Pr-Nd-Al alloy additions is investigated. • The coercivity of Ce magnet changes from 12.7 to 14.93 kOe. • Sandwich-structure grain boundary phase is helpful to improve the thermal stability of Ce magnet. • Formation mechanism of sandwich-structure grain boundary phase is revealed. In this study, the formation mechanism of sandwich-structure grain boundary phase in sintered Ce magnets with various Pr-Nd-Al alloy additions is investigated in detail. The sandwich-structure grain boundary between the main phases is composed of RE 6 (Fe, TM) 11 Al 3 phase at the center and symmetrical Nd-rich phase at both sides. Based on the analysis of phase diagrams, the formation mechanism is revealed from the following aspects: i) The amorphous formation ability of Nd-rich liquid phase in Ce magnet during sintering can be enhanced by introducing Pr-Nd-Al alloy; ii) The RE 6 (Fe, TM) 11 Al 3 phase preferentially nucleates and grows during the process of cooling; iii) The interfacial tension among Nd-rich liquid phase, main phase and RE 6 (Fe, TM) 11 Al 3 phase contributes to the formation of the sandwich-structure grain boundary. With the addition of Pr-Nd-Al alloy, the activity of Al entering the main phase is inhibited, resulting in almost no reduction of remanence and Curie temperature. The structural stability and decoupling effect of the sandwich-structure grain boundary phase are more conducive to the improvement of the coercivity and thermal stability of the Ce magnets. The coercivity of Ce magnet changes from 12.7 to 14.93 kOe (the increase extent is 17.6%). In the temperature range 20–100 ℃, the coercivity temperature coefficient varies from -0.767 to -0.680 %/℃ and the temperature stability obviously ameliorates about 11.3%. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel sandwich-like structure design of the grain boundaries was proposed. • The coercivity temperature coefficient and the irreversible loss of magnetic flux for Ce magnets added with Pr-Nd-Al alloy are enhanced. • The thermal stability analyses from various perspectives provide sufficient evidence for the proposed design concept. • The self-compensation effect of thermal stability is experimentally verified in Pr-Nd-Al Ce magnet. A novel design concept was proposed for optimizing the grain boundaries from the theoretical aspect, which could play the self-compensation effect on the thermal stability of the coercivity. The Pr-Nd-Al Ce magnet without critical elements was investigated that experimentally verified the self-compensation effect of grain boundaries structure on the thermal stability of the coercivity. As a result, the Pr-Nd-Al Ce magnet with 3 wt% (Pr 0.25 Nd 0.75) 80 Al 20 as the additive inside the grain boundaries had better thermal stability than that of Nd-Fe-B magnet. The coercivity temperature coefficient β of Pr-Nd-Al Ce magnet was improved by 5.7%, and the irreversible loss of magnetic flux h irr was enhanced by 58% at the temperature of 100 ℃ compared with the Nd-Fe-B magnet. The improvement of thermal stability was attributed to the special sandwich-like structure consisting of two amorphous nonmagnetic phases sandwiching a layer of Re 6 (Fe, TM) 11 Al 3 tetragonal phase, that compensated the coercivity of Ce magnets without critical elements. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nd-Fe-B magnets with excellent properties were obtained through novel powder rounding modification. • Good flowability of the rounded powder enhanced the alignment degree of the magnet. • The thin and continuous RE-rich phase promoted the improvement of density and coercivity. • Synchronous enhancement mechanism in coercivity and remanence of the magnets was revealed. Two types of sintered Nd-Fe-B magnets were fabricated by the process of powder metallurgy technology through two different powder preparation processes i.e., conventional powder preparation process and novel powder rounding modification. The microstructure, composition and properties of the two type powders have been studied including the particle size distributions and flowabilities. Furthermore, magnetic properties, microstructure and alignment degree of the magnets prepared by the corresponding powder also have been investigated. The novel powder rounding modification rounds the sharp edges and corners of the particles compared to the conventional powder preparation process, thereby reducing friction between the particles and improving flowability of the powder. As a result, the alignment degree of the sintered magnet is enhanced, which contribute to the improvement of the remanence and the energy product density. The powder that experienced rounding process changed the distribution of RE (rare earth)-rich particles, causing tiny RE-rich particles to adhere around the main phase particles, facilitating the compact of bulk during liquid-phase sintering process and achieving uniform distribution of RE-rich phases around the Nd 2 Fe 14 B main phase, resulting in increasing the magnet density and coercivity. As a consequence, we have produced the magnet with B r = 1.43 T, H cj = 1049 kA/m and (BH) max = 393.2 kJ/m3, which is fully superior to conventional magnet with B r = 1.41 T, H cj = 969 kA/m and (BH) max = 383.7 kJ/m3 by adopting powder rounding process. This indicates that the novel powder rounding modification is an efficient approach for synchronous improvement of magnetic performance of sintered Nd-Fe-B without any heavy rare earth elements. [ABSTRACT FROM AUTHOR]

The Pr 80 Al 20 (at%) alloy was diffused into the dual-main-phase Ce magnets (Ce/TRE = 40 wt.%, TRE: total rare earth) with different diffusion time by grain boundary diffusion process (GBDP). The magnetic property and the microstructure of a series of magnets with different diffusion time were investigated. The coercivity gradually increased as the diffusion time increased, while the remanence remained basically constant. The improvement of coercivity originated from the synergistic contribution of modified grain boundaries with decoupling effect and Ce-rich main phase containing Pr with improved magnetocrystalline anisotropy. The percentage of the irreversible portion and the nucleation field of the reverse domain calculated from recoil loops curves further confirmed that the diffusion of Pr 80 Al 20 alloy enhanced the coercivity. The enhanced anisotropy of the Ce-rich grains compensates for the decay of the remanence carried by the diffusion-induced increase in the nonmagnetic grain boundary phase, which ultimately results in the remanence remaining essentially constant. The above phenomena further broaden the room-temperature applications of dual-main-phase magnets with high Ce content without heavy rare-earth grain boundary diffusion. • Dual-main-phase magnets with high Ce content were diffused with light rare earth Pr-Al alloy. • Pr element mapping contrast clearly indicated sufficient diffusion inside the magnet. • Small amounts of Pr entered into the Ce-rich main phase grains to improve magnetocrystalline anisotropy. • Synchronous enhancement mechanism in coercivity and maximum energy product of the magnets was revealed. [ABSTRACT FROM AUTHOR]

High-quality TbF 3 coating used for the grain boundary diffusion process (GBDP) of sintered Nd-Fe-B was prepared by the suspension plasma spraying (SPS) technique. The obtained TbF 3 coating with SPS process showed a dense and continuous structure with the uniform distribution as well as a good adhesion with the Nd-Fe-B substrate. Compared with the uncoated Nd-Fe-B magnet without heavy rare earth, the coercivity of TbF 3 coated Nd-Fe-B magnet enhanced from 1160 kA/m to 1824 kA/m after the GBDP heat treatment. The microstructure and composition analysis indicated that the (Nd, Tb) 2 Fe 14 B shell formed at the periphery of Nd 2 Fe 14 B grains throughout the whole sample magnet. The shell with high magnetocrystalline anisotropy effectively suppressed the nucleation of reverse domain and repaired the defects on the grain surface of the sintered Nd-Fe-B magnets, thus enhancing the coercivity. As a consequence, the SPS technology has advantage of high depositing rate, flexible production process, good boundary adhesion and homogeneous-dense structure, which is expected to become a promising candidate way of depositing diffusion coating for the GBDP of sintered Nd-Fe-B in industrial production. • High-quality TbF 3 coatings are obtained on the Nd-Fe-B surface by SPS technique. • Core-shell structures are formed in the 5.5 mm thick GBDP Nd-Fe-B magnets. • The coercivity of Nd-Fe-B magnets enhanced 57.2 % by the SPS- GBDP technique. • SPS technology is a promising way of coating deposition for diffusion of Nd-Fe-B. [ABSTRACT FROM AUTHOR]