Coercivity of Nd-Fe-B Sintered Magnets Produced by the Grain Boundary Diffusion Process with Various Rare-Earth Compounds

Abstract

In order to clarify the origin of coercivity enhancement of Nd-Fe-B sintered magnets by the Grain Boundary Diffusion Process (GBDP), microstructural observations were performed and the coercivities of magnets processed with various rare-earth oxides were investigated. Microstructural analysis using FE-EPMA revealed that Dy supplied from the magnet surface diffused into whole magnets with a thickness of 2 mm. At the center of a magnet, the Dy distribution width around a grain boundary is almost comparable to the width of the Nd-rich grain boundary phase. When rare-earth (R) oxides whose R provide higher anisotropy in R2Fe14B than that of Nd2Fe14B, are used for GBDP, the magnets showed higher coercivity than that of the control, and vice versa. Although the diffusion depth of Dy into the Nd2Fe14B grains was unclear at the center of the magnet, the relationship between the kinds of rare-earth elements used for GBDP and the resultant coercivities strongly suggests that the rare-earth elements supplied from the surface of the magnets and absorbed along the grain boundary phase may diffuse into very thin portions of the Nd2Fe14B grain surface.Key words: Nd-Fe-B sintered magnet, grain boundary diffusion, coercivity, rare-earth fluoride, rare-earth oxide 種々の希土類化合物を用いて粒界拡散処理した Nd-Fe-B 系焼結磁石の保磁力 7 Table 1 Contents of absorbed R elements and coercivities (HcJ) of magnets processed with R-O powders. Some intrinsic properties of R2Fe14B 11) are also listed: HA, the anisotropy field at room temperature; K1, the anisotropy constant at room temperature; and Js, the magnetic polarization at room temperature. R Content

Current Status of Research and Development toward Permanent Magnets Free from Critical Elements

“…This mechanism is similar to that proposed previously by Oono et al for formation of a core/shell structure in sintered Nd-Fe-B magnets after grain-boundary diffusion of Dy-Ni-Al alloys 11) . These investigations suggest that the enhancement of coercivity is a result of enhanced magnetic hardness in the HRE-enriched shell as was proposed earlier by Nakamura et al basing on an observation that the change in coercivity (ΔHcJ) caused by the diffusion process using various rare earth elements was roughly proportional to the difference in anisotropy fields of R2Fe14B compounds (Δ(2K1/JS)) where R stands for the rare earth elements used in the experiment 12) . Retrieving data from Ref.…”

Coercivity enhancement of Dy-coated Nd-Fe-B flakes by crystallization

“…The sample was composed of fine grains and the map of the Tb-M line indicates that Tb is concentrated in the vicinity of grain boundaries, which agrees with previous observations of sintered magnets. [3][4][5][6] Detailed results for the TEM observations have been published elsewhere. 11 As Tb and Dy have similar diffusion behaviors, 5 the Dy in the present Dy-coated flakes is expected to be concentrated in the vicinity of grain boundaries after crystallization.…”

“…The coercivity of a sintered Nd-Fe-B-based magnet has been enhanced by diffusing a heavy rare-earth metal from its surface. [3][4][5][6] Sintering has also been used to diffuse a heavy rare-earth element to Nd 2 Fe 14 B grain surfaces. 7,8 In both of these methods, the heavy rare-earth element is diffused by annealing at high temperatures of over 1000 K. A low-temperature process is required for isotropic magnets that are fabricated by crystallizing an amorphous powder because hightemperature annealing causes Nd 2 Fe 14 B grains to grow, which degrades the magnetic properties of isotropic magnet powders.…”

Study of sintered Nd-Fe-B magnet with high performance of Hcj (kOe) + (BH)max (MGOe) > 75

“…It has been observed that the modification of coercivity on Nd-Fe-B magnet by the GBD process is sensitive to rare-earth oxides with different rare earths. 22 The Tb oxide is the best to increase the H cj due to Tb 2 Fe 14 B with the highest uniaxial anisotropy, while the Sm oxide drastically decreases the coercivity due to Sm 2 Fe 14 B with strong planar anisotropy. This indicates that although the magnetocrystalline anisotropy field H a keeps unchanged on the entire Nd 2 Fe 14 B-structure main phase during the GBD process, but the H a of the surface layers of the main phase grains is influenced.…”

“…Dy or Tb atoms diffuse along the grain boundary phase during post-sinter heat treatment at temperature higher than melting point of rare-earth rich (RE-rich) phase but lower than sintering temperature. [20][21][22] H cj can be raised up to 4 ∼ 5 kOe without obvious reduction of B r . Moreover, the composition of Dy or Tb is much lower than those in conventional alloying process, which may reduce magnet cost dramatically.…”