Sintered Nd-Fe-B magnets with different grain sizes are prepared by adjusting rotational speed of the classifying wheel during the jet milling process. The microstructures of the magnets, the element contents of the magnets, the relationship between powder particle size and grain size and the magnetic properties of the magnets are studied by X-ray diffracton (XRD), scanning electron microscopy (SEM), optical microscopy (OM), electron backscattered diffraction (EBSD), inductively coupled plasma (ICP) and permanent magnet tester. The effect of grain size on thermal stability is characterized by the temperature coefficient of coercivity from room temperature to working temperature and the irreversible magnetic flux loss of the magnet at high temperature. The results show that the particle size of the alloy powder decreases with the increase of the rotational speed of the classifying wheel. When the alloy powder is formed and sintered, it is found that the grain size of the prepared magnet decreases with the decrease of the particle size of the alloy powder. The finer the grain size of the magnet, the clearer the grain boundary. As the grain size of the magnet decreases, the coercivity of the magnet increases, the remanence of the magnet decreases slowly and the thermal stability of the magnet becomes better. For the magnet with a rotational speed of the classifying wheel of 4900 r/min, the average grain size of the magnet is 4.70 mu m, the coercivity (Hcj) H cj ) is 15.47 kOe, and the remanence (Br) B r ) is14.09 kGs. At a temperature of 80 degrees C, the coercivity temperature coefficient of the magnet reaches-0.745 %/degrees C, degrees C, and the irreversible loss of magnetic flux hirr is 16.49 %. In addition, the magnet prepared by the tail material produced a large number of voids and pores due to the high oxygen content. Tail material is not good enough in terms of magnetic property as well as thermal stability.

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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 Nd2Fe14B main phase, resulting in increasing the magnet density and coercivity. As a consequence, we have produced the magnet with Br = 1.43 T, Hcj = 1049 kA/m and (BH)max = 393.2 kJ/m3, which is fully superior to conventional magnet with Br = 1.41 T, Hcj = 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.

The Pr80Al20 (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 Pr80Al20 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. (c) 2023 Elsevier B.V. All rights reserved.

High-quality TbF3 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 TbF3 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 TbF3 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)2Fe14B shell formed at the periphery of Nd2Fe14B 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.

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 RE6(Fe, TM)11Al3 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 RE6(Fe, TM)11Al3 phase preferentially nucleates and grows during the process of cooling; iii) The interfacial tension among Nd-rich liquid phase, main phase and RE6(Fe, TM)11Al3 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 & DEG;C, the coercivity temperature coefficient varies from-0.767 to-0.680 %/& DEG;C and the temperature stability obviously ameliorates about 11.3%.

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% (Pr0.25Nd0.75)80Al20 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 hirr 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 Re6(Fe, TM)11Al3 tetragonal phase, that compensated the coercivity of Ce magnets without critical elements.

To satisfy the application of different environments, grain boundary doping is commonly used in the preparation of sintered magnets to improve the coercivity and the corrosion resistance. In this paper, the alloys were prepared by mixing different ratios of the master alloy (Ce,Pr,Nd)-Fe-B and the sintering aid (Pr,Nd)-Al. The coercivity of sintered (Ce,Pr,Nd)-Fe-B magnet is substantially enhanced by doping 2 wt% of (Pr,Nd)-Al, while the maximum energy product decreases slightly. We systematically investigated the corrosion behavior and microstructure of the sintered magnets in order to determine the mechanism of the degradation. The sintered (Ce,Pr,Nd)-Fe-B magnets with 2 wt% of (Pr,Nd)-Al addition exhibit the decreasing corrosion rate compared with others, due to the distribution of intergranular phases. The electrode potential difference between the main phase and the RE-rich phase is reduced by the addition of Al, improving the potential and stability of RE-rich phase due to the higher electrode potential of Al than that of Nd, Pr or Ce. In addition, the element distribution of the magnets doped by (Pr,Nd)-Al indicates that the Al-rich shell formed at the marginal area of the Ce-rich phase improves its stability. Therefore, intergranular adding ternary (Pr,Nd)-Al alloy powders results in both high coercivity and good corrosion resistance synchronously.