Several Issues on the Development of Grain Boundary Diffusion Process for Nd-Fe-B Permanent Magnets
LIU Zhongwu(), HE Jiayi
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
Cite this article:
LIU Zhongwu, HE Jiayi. Several Issues on the Development of Grain Boundary Diffusion Process for Nd-Fe-B Permanent Magnets. Acta Metall Sin, 2021, 57(9): 1155-1170.
Nd-Fe-B based permanent magnets have been widely used in many industries, including renewable energy, information and communication, and intelligent manufacturing. The applications in the electric vehicle drive motors and wind power system generators set high requirements on the elevated temperature performance and coercivity for Nd-Fe-B magnets. Heavy rare earth (HRE) of Tb and Dy have been frequently used to substitute Nd to enhance the anisotropy field of the magnets. However, introducing these HRE elements reduces the remanence of magnets and increases the total price of end-products. The grain boundary diffusion (GBD) process, invented at the beginning of this century, is a significant progress in the field of rare earth permanent magnet manufacturing. The coercivity can be significantly improved by diffusing HRE elements or rare earth alloys into the magnet along the grain boundary. Simultaneously, the reduced heavy rare earth consumption and enhanced performance-cost ratio can also be realized. Although the GBD process has attracted much attention and has been quickly industrialized since its appearance, some key issues still exist on technical and theoretical levels. Based on the latest domestic and overseas developments and the research results from the authors' group, this paper summarizes the urgent problems and feasible solutions for the GBD process. Several issues are described in this report, including the GBD process for thick magnets, utilization of anisotropic behavior of GBD, selection of low-cost diffusion sources, combination of GBD with the existing process, influence of GBD on the service performance, and advancement of GBD related theories. The challenges and opportunities in the future development of the GBD process for Nd-Fe-B based magnets are also highlighted.
Fig.1 Grain structure of sintered Nd-Fe-B magnets (backscattered electron image) (a) and schematic of grain boundary diffusion (GBD) (b)
Fig.2 Schematic of coating method of grain boundary diffusion source in industrial production (a), and the effectiveness and efficiency of different heavy rare earth (HRE) containing diffusion sources for sintered magnets (The diffusion sources include fluoride, oxide, hydride, metal, and alloy of HRE) (b)[16,20-28]
Fig.3 Schematic of improving the diffusion efficiency via doping and alloying (The solid arrows represent the orientation of grains, and the dash arrows represent the GBD of diffusion sources)
Fig.4 Schematics of in situ grain boundary diffusion process (GBDP) of Dy coated Nd-Fe-B powder (a1) and diffusion sources mixed Nd-Fe-B powder (a2), and demagnetization curves for the samples with different amounts of Dy88Mn12 (mass fraction) at 20oC (b) and 100oC (c), respectively[39]
Fig.5 The 2nd quadrant demagnetization curves and magnetic properties of spark plasma sintered (SPSed) magnets after multiplane GBD process (Hcj—intrinsic coercivity, Jr—remanent magnetic polarization, diff.—diffusion)
Fig.6 Schematic of development of the three generations of GBD source (LRE—light rare earth)
Fig.7 Schematic illustration of GBD with HRE (The solid arrows represent the orientation of grains and the dash arrows represent the diffusion of rare earth atoms)
A&D
Oxide
Fluoride
Hydride
Metal/alloy
Diffusion efficiency
Low
Moderate
High
High
Cost advantage
High
Moderate
Low
Low
Level of “green”
Moderate
Low
High
High
Table 1 Advantages and disadvantages (A&D) of different HRE containing diffusion sources
Fig.8 Schematic illustration of GBD with light rare earth (a) and demagnetization curves for N50 sintered magnet and the magnets with Pr-Al-Cu diffusion (b)[52]
Fig.9 Effect of Cu addition on microstructure of sintered magnets (GB—grain boundary)
Fig.10 Schematic illustration of GBD with ZnO (a) and demagnetization curves for sintered magnet and the magnets with ZnO diffusion (b)[61]
Fig.11 Demagnetization curves of Pr7.03Nd21.84Ho2(Fe, M)balB0.95 magnets with and without DyHx diffusion (Curves (a) as-sintered magnet, (b) annealed without diffusion, (c) diffused perpendicular to the c-axis, and (d) diffused along the c-axis)[64]
Fig.12 Micromagnetic simulation model of GBD heterogeneous behavior and grain structure (a), and magnetization distributions of T1 + B1 (b) and L1 + R1 (c) (To study the effect of diffusion direction, the diffusions from the top surface (pole surface), the bottom surface (pole surface), the left side surface, and the right side surface of the magnets are labelled as T, B, L, and R, respectively. To investigate the effect of diffusion depth, numbers 1, 2, and 3 are used to describe that two, three, and four layers of grains are changed to the core-shell structure, respectively; GBP—grain boundary phase)[67]
Fig.13 The combination of GBD and existing process for sintered magnets
Fig.14 Polarization curves of the original magnet and Pr-Al-Cu, La-Al-Cu, and Al-Cu diffused magnets (a) and their microstructures after corrosion (b-e) (The yellow arrows refer to pits)[49]
Fig.15 Stress-strain curves of the original and Pr-Al-Cu diffused magnets (a) and SEM image of the fracture morphology of Pr-Al-Cu diffused magnet (b)[78]
Fig.16 A comparison of the structure-property relationships between sintered and hot-deformed Nd-Fe-B magnets (The black and blue arrows represent the orientations of the 2∶14∶1 phase and reversed domain, respectively, while the red arrows represent the motion of domain wall)
Fig.17 Schematic of preparing bonded magnets with high coercivity and remanence by GBD
Fig.18 Schematic of superferrimagnetic structure with Nd2Fe14B grains antiferromagnetically coupled to the GdFe2 layer (The white and black arrows refer to the orientations of the Nd2Fe14B grains and GdFe2 layers, respectively)
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