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The synthesis of size-controlled Sm2Fe17magnetic particles is a prerequisite for the fabrication of high-performance Sm2Fe17N3permanent magnetic materials. Here, Sm2Fe17was synthesized using a cost-effective reduction-diffusion method. The calcium chloride molten salt was introduced to control the particle size and achieve a single phase of Sm2Fe17. The effects of reduction-diffusion reaction temperature and the amount of added calcium chloride on the phase constitution and microstructure of the final product of reduction-diffusion were systematically investigated. Adding an appropriate amount of calcium chloride can effectively inhibit the overgrowth and sintering of the reduced particles. By employing the strategy of adding 20 wt% of calcium chlorides into the green compacts, we were able to successfully synthesize uniform Sm2Fe17particles that are well-dispersed, with an average size of 2.2 μm. Furthermore, by combining the optimal reduction-diffusion conditions and the nitriding process, the hard magnetic Sm2Fe17N3material was successfully obtained. This study could be useful for the development of high-performance Sm2Fe17N3magnetic materials utilizing reduction-diffusion technology.

The evolution of the microstructure, microchemistry and magnetic properties of the Sm(CobalFe0.28CuyZrx)7.6magnets with different Zr and Cu contents was investigated. It is found that the coercivity of the Sm(Co,Fe,Cu,Zr)zmagnets is sensitive to Zr content. The deficiency of Zr content causes heterogeneity of Cu and Fe distributions, while an excessive Zr content leads to the formation of a SmCoZr impurity phase. The cellular structure and distribution of Cu concentration gradient between the cell boundary phase and cell phase are destroyed by inappropriate Zr content, which results in a reduction of coercivity. The Cu concentration difference between the cell boundary phase and cell phase increases with increasing Cu content. The coercivity of the Sm(CobalFe0.28CuyZr0.02)7.6magnets increases from 10.4 to 25.4 kOe for y = 0.05 and y = 0.07. However, the excess of Cu element destroys the cell boundary phase and enlarges the cell size, resulting in a significant decrease of squareness and energy density. The optimum performance (remanence of 11.4 kG, coercivity of 25.4 kOe, maximum magnetic energy product of 30.4 MGOe) was obtained for the Sm(Co0.63Fe0.28Cu0.07Zr0.02)7.6magnet.

Environmental friendly recycling process for Nd-Fe-B sintered magnet sludges generated in the manufacturing process, which contain large amount of rare earth, including Nd, Pr and Dy, is badly needed so far. In present study, we have developed an effective route to obtain recycled sintered magnets from Nd-Fe-B sintered magnet sludges by calcium reduction-diffusion (RD) process. Compared to conventional recycling process, our research is focused on recovering most of the useful elements, including Nd, Pr, Dy, Co, and Fe together instead of just rare earth elements. To improve the recycling efficiency and reduce pollution, the co-precipitating parameters were simulated and calculated using MATLAB software. Most of useful elements were recovered by a co-precipitation method, and the obtained composite powders were then directly fabricated as recycled Nd-Fe-B powders by a calcium reduction-diffusion (RD) method. The recovery rates are 98%, 99%, 99%, 93%, and 99%, for Nd, Pr, Dy, Co, and Fe, respectively. The amount of useful elements contained in the recovered composite powders is greater than 99.71 wt%. The process of RD for synthesizing Nd2Fe14B and subsequently removing CaO was thoroughly investigated. Furthermore, the recycled Nd-Fe-B magnet exhibits a remanence of 1.1 T, a coercivity of 1053 kA/m, and an energy product of 235.6 kJ/m3, respectively, indicating that recycled Nd-Fe-B sintered magnet was successfully recovered from the severely contaminated sludges via an effective recycling route.

The hot-deformed (HD) Nd-Fe-B magnets show heterogeneous microstructure composed of coarse and fine grain regions. It is significant to fully understand the influence of this complex microstructure on the magnetization reversal process which can give the guidance for the enhancement of the magnetic properties. In this paper, the heterogeneous microstructure of the (HD) Nd-Fe-B magnets were characterized from the morphology, size, macro-texture and micro-structure. In addition, the magnetization reversal process of the HD Nd-Fe-B magnets was systematically analyzed by magnetic measurement, in-situdomain evolution observation and micromagnetic simulation. The results indicate that the HD Nd-Fe-B magnets mainly consist of fine grain regions (FGRs) and coarse grain regions (CGRs). The FGRs show plate-like grains with fine grain size and strong c-axis texture, while the CGRs show equiaxial grains with large grain size and weak c-axis texture. In particular, it is worth noting that the texture in homogeneity exists not only between FGRs and CGRs, but also inside both the FGRs and CGRs. The dominant coercivity mechanism of the HD Nd-Fe-B magnets is domain wall pinning. Also, the experimental analysis shows that the reverse domain is formed and expanded in the CGRs at low reverse applied field, while the reverse domain occurs in the FGRs at higher reverse applied field. The micromagnetic simulation results also confirm the above magnetization reversal process. In addition, micromagnetic simulation results also show that the orientation of the grains also affects the pinning strength, besides the grain size.

Microstructure and magnetic properties were studied for the commercial Sm(CoFeCuZr)zmagnets before and after post annealing treatment. The results show that the phases composition and orientation of the magnet do not change after post annealing treatment, but the substantial redistribution of Cu element within multiscale (the microscale crystal grain and the nanoscale cellular structure) had been observed simultaneously. In detail, as the Cu redistribution, the thickness of the Cu-rich Sm(Co,Cu)5cell boundary becomes thinner, and the Cu concentration in the boundary increases sharply. The pinning field of domain walls and corresponding coercivity increases remarkably with slight remanence and maximum energy product loss, and the overall magnetic performance of (BH)max(MGOe)+Hcj(kOe) increases by 54.3% as a result. Moreover, the thermal stability of the magnet improves as well. On the other hand, Cu-lean phenomenon was observed along the grain boundary region, triggering to magnetic domain reversal process and slightly undermining the squareness of the demagnetization curve of the magnet.

The structural and magnetic properties were studied for recycling Nd–Fe–B powders from Nd–Fe–B sintered magnets sludge via reduction diffusion (RD) with calcium hydride (CaH2) particles. For comparison, traditional reducing agent calcium granules were applied to prepare recycled Nd–Fe–B powders. Finer particle size and better size distribution as well as lower impurity content are achieved by using CaH2instead of Ca. In detail, the average particle size of the recycled Nd–Fe–B powder is reduced from 4.66 to 3.43 μm, and the bimodal distribution disappears. Moreover, the residual calcium content and oxygen content are reduced to about 0.080 wt% and 0.32 wt%. As a consequence, the room-temperature magnetization of the CaH2-recycled Nd–Fe–B powder is increased to 146.30 emu/g, 6.8% and 33%, respectively, higher than that of Ca-reduced powder and the initial sludge. Further analysis indicates that CaH2is able to reduce the sludge at lower temperature to fabricate well-dispersed, uniform recycled powder with high magnetization arising from a combination factors of its low melting point, low thermodynamic behavior, and the release of hydrogen during the reaction.