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To improve the coercivity and temperature stability of Nd-Fe-B sintered magnets for high-temperature applications, the eutectic Tb80Fe20 (wt%) alloy powders were intergranular added into the Nd-Fe-B sintered magnets to enhance the coercivity (Hcj) and thermal stability. The microstructure, magnetic properties and thermal stability of the Nd-Fe-B magnets with different Tb80Fe20 contents were studied. The experimental results demonstrate that the coercivity (Hcj) of the sintered Nd-Fe-B magnet is significantly enhanced to 27.78 kOe from 14.12 kOe, and the remanence (Br) decreases not obviously by introducing 4 wt% Tb80Fe20 alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) and remanence (α) of the Nd-Fe-B magnets are increased from −0.5634%/°C to −0.4506%/°C and −0.1276%/°C to −0.1199%/°C at 20–170 °C, respectively. The Curie temperature (TC) of the Nd-Fe-B magnet is slightly enhanced with the increase of Tb80Fe20 content. Moreover, the irreversible flux magnetic loss (hirr) is obviously reduced as Tb80Fe20 addition. Further analysis of the microstructure revealed that a modified microstructure, i.e. clear and continuous RE-rich grain boundary layer, is acquired in the sintered magnets by introducing Tb80Fe20 alloy. The associated mechanisms on improved coercivity and thermal stability are comprehensively researched.

The as-solution-treated Sm2Co17-type magnets exhibiting a single 1:7H phase with different average grain sizes (D) are designed. Anisotropy of bending strength (Rbb) and compressive strength (Rmc) of the magnets was investigated. Moreover, the Rbb increases from 86 to 173 MPa with D decreasing from ∼52 to ∼18 μm for group c//h samples. The Hall-Petch correlation was employed to reveal the effect of grain size on mechanical properties of the magnets, giving deep understand the mechanical anisotropy characteristics. The relatively high Hall-Petch coefficient KRbb (0.79 MPa·m1/2) gives rise to the largest Rbb (173MPa) for group c//h samples. The mechanical anisotropy of the samples is well explained based on crystal structure and grain size features (grain boundaries). Grain refinement is an effective way to enhance the mechanical properties of Sm2Co17-type sintered magnets.

The main difficulty for the recovery of Nd-Fe-B bonded magnet wastes is how to completely remove the epoxy resins. In this study, chemical reaction and physical dissolution were combined to remove the epoxy resins by adding ammonia-water and mixed organic solvents. Ammonia-water can react with the epoxy functional group of epoxy resin to generate polyols. Mixed organic solvents of alcohol, dimethyl formamide (DMF), and tetrahydrofuran (THF) can dissolve the generated polyols and residual epoxy resins. Under the optimum processing conditions, the epoxy resins in the waste magnetic powders are substantially removed. The oxygen and carbon contents in the recycled magnetic powder are reduced from 13500 × 10−6 to 1600 × 10−6 and from 19500 × 10−6 to 2100 × 10−6 with the reduction ratio of 88.1% and 89.2%, respectively. The recycled magnetic powder presents improved magnetic properties with Ms of 1.306 × 10−1 A∙m2/g, Mr of 0.926 × 10−1 A∙m2/g, Hcj of 1.170 T, and (BH)max of 125.732 kJ/m3, which reach 99.8%, 99.4%, 95.9%, and 96.9% of the original magnetic powders, respectively.

The influence of Tb80Fe20/Al co-adding on coercivity and temperature stability of sintered Nd–Fe–B magnets was studied. Compared with the Tb80Fe20 added samples, the coercivity of the Tb80Fe20/Al co-added samples enhances with the increase of Al contents. The excellent magnetic properties are acquired in the 1 wt% Tb80Fe20/0.2 wt% Al co-added magnets. The coercivity (Hcj) is enhanced from 14.33 to 19.08 kOe, and the remanence (Br) decreases slightly by introducing 1 wt% Tb80Fe20/0.2 wt% Al alloy. Meanwhile, the reversible temperature coefficients of coercivity (β) of the Nd–Fe–B magnets are increased from − 0.598 to − 0.579%/°C at 20–140 °C. Moreover, the irreversible flux magnetic loss (hirr) of the Tb80Fe20/Al co-added magnets is obviously reduced with the addition of Al. Further analysis of the microstructure reveals that a modified microstructure, i.e., continuous and clear Nd-rich grain boundary (GB) layers, is acquired in the sintered magnets by introducing Tb80Fe20/Al alloy. In addition, the average grain size decreases to 3.7 from 4.5 μm, which indicates that Tb80Fe20/Al co-adding provides an effective way to prepare for the excellent-performance of sintered Nd–Fe–B magnets.

The grain boundary diffusion (GBD) technology was used to prepare high performance Nd-Fe-B sintered magnets by NdH3 and TbH3 nanoparticle diffusion. The factors affecting the coercivity of GBD magnets include distribution of rare earth rich grain boundary phase and substitution of the heavy rare earth. In order to distinguish the influence of various factors on the coercivity, the microstructure and magnetic domain evolution of the original, reference, Nd-diffused, and Tb-diffused magnets were analyzed. The core-shell structure formed by heavy rare earth substitution is the main factor of coercivity enhancement, and it can transform the magnetic domain reversal mode from easy-nucleation (EN) to difficult-nucleation (DN). With increasing the diffusion depth, the shell of the core-shell structure gradually becomes thinner, DN grains gradually decrease while the EN grains gradually increase, indicating that the magnetic domain reversal mode is directly related to the core-shell structure.

Nd–Fe–B sintered magnets were heat-treated in Dy vapor atmosphere for achieving (Nd,Dy)–Fe–B magnets by grain boundary diffusion (GBD). The magnetic properties, microstructure, and corrosion resistance of (Nd,Dy)–Fe–B magnets were systematically investigated. Results show that the intrinsic coercivity of magnets increases significantly after Dy diffusion. Dy atoms penetrate into the magnets through the grain boundaries, which not only increase the total amount of rare earth and widen the grain boundary but also form (Nd,Dy)2Fe14B phase with higher anisotropic field on the grain surface layer. The intrinsic coercivity $H_{\mathrm {cj}}$ of (Nd,Dy)–Fe–B magnets increases from 16.86 to 21.60 kOe. When the testing time of highly accelerated aging tester is 96 h, the weight loss of pristine Nd–Fe–B magnets is 0.436 mg/cm2, while that of (Nd,Dy)–Fe–B magnets is 1.922 mg/cm2. The results of weight loss test and potentiodynamic polarization curves show that the corrosion resistance of (Nd,Dy)–Fe–B magnets is worse than that of pristine magnets due to more rare earth amount and wider grain boundary.

Nd-Fe-B sintered magnets with different thicknesses of up to 10 mm were fabricated by combining TbH3 nanoparticles as diffusion sources and the grain boundary diffusion (GBD) technique. TbF3 microparticle-diffused magnets were also prepared for comparison. TbH3 nanoparticles diffusion increased the coercivity by 6.9 kOe for the magnets with thicknesses of 10 mm, which is 5.21 kOe larger than that of TbF3 microparticle-diffused magnets. Deeper, thicker and consecutive (Nd,Tb)2Fe14B layers are observed in TbH3 nanoparticle-diffused magnets compared to TbF3 microparticles, indicating the superiority of TbH3 nanoparticles in the GBD process, which can effectively inhibit the nucleation of reversed domains and thus enhance the coercivity.

The effect of heat treatment on the microstructure and thermal stability of sintered Nd-Fe-B magnets was investigated. The results show that the microstructure of the magnets was greatly affected by heat treatment. The intrinsic coercivity (Hcj) was increased and the temperature coefficient of coercivity (β) was decreased by heat treatment. As the annealing temperature changes from 500°C to 770°C, Hcj first increases and reaches a maximum, then starts to decrease, but β remains unchanged from 500°C to 680°C and increases rapidly when annealed over 680°C. A magnet with Hcj= 30.55 kOe, β (from 20°C to 200°C) = -0.4079%°C was obtained after annealed at 620°C for 2 h. The relationship between the annealing temperature and magnetic properties was studied.

The effects of DyHx addition on magnetic properties and microstructure of Nd-Fe-B sintered magnets were investigated. In order to obtain the best magnetic properties, the effects of sintering temperature and annealing time on magnetic properties were also investigated. The results shown that the addition of DyHx can affect the microstructure, increase the intrinsic coercivity (Hcj), and improve the thermal stability of sintered Nd-Fe-B magnets. The remanence (Br) and Hcj showed different variation trends as sintering temperature changes. Within a reasonable range, sintering at higher temperature can get higher Br but lower Hcj. The Hcj was further increased by heat treatment and annealing time was extended.

The effects of hydriding degree on the microstructure and magnetic properties of sintered NdFeB magnets have been studied. The degree of crushing depends on the absorption content of hydrogen and affects the magnetic properties of final magnet. Insertion of hydrogen atoms leads to a significant increase of the unit cell volume. And the crush mechanism depends on the internal stress resulting from differences in the expansion rates of the two phases. The remanence of final magnets increases monotonously while the coercivity decreases sharply with the increasing of hydriding degree, attributed to the strip fracture features and the morphology of particles.