Coercivity enhancement of Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Nd-Al-Cu alloys.

摘要

Summary form only given. Nd-Fe-B based sintered magnets have a wide range of applications in energy efficient technologies such as traction motors of hybrid electric vehicles and wind generators [1]. In general, Nd-Fe-B based sintered magnets have a large amount of rare earth elements, such as Nd, Pr and Dy. From an economic point of view, new type of magnets incorporating light rare earth elements such as La and Ce into Nd-Fe-B based sintered magnets has stimulated considerable research efforts due to the analogous structure recently [2]. However, magnets incorporated with Ce exhibit much deteriorated magnetic properties due to the inferior intrinsic magnetic properties of Ce ₂ Fe ₁₄B, which saturation magnetization, ambient anisotropic field and Curie temperature were reported to be 1.17 T, 2.6 T and 152 °C, respectively [3]. Many efforts had been paid to improve the performance. One wellknown method to increase the coercivity of sintered magnets is to refine the grain boundary (GB) structure and chemistry through grain boundary diffusion (GBD) [4], [5]. Previous investigations have demonstrated that the coercivity of Ce-based magnets is enhanced by diffusion process with minor amounts of Nd ₇₀ Cu ₃₀ and Nd₈₀Ag₂₀ [6]. In this work, we investigated the microstructure of Nd-Ce-Fe-B sintered magnets and the eutectic grain boundary diffusion process using Nd-Al-Cu in order to comprehend the origin of the coercivity enhancement by the Nd-Al-Cu diffusion process. Experimental As Nd-Ce-Fe-B sintered magnets with a nominal composition of (Nd, CO3,sFebBi (wt. %) were fabricated by the conventional powder metallurgy method. The magnets were cut into a cube shape with an edge of 10 mm and a height of 4 mm. Nd 8 oAl l0 Cu 10 alloy ribbons as a diffusion source were prepared by the melt spinning technique using the high vacuum quenching system. In the diffusion process, the Nd-Ce-Fe-B sintered magnets were covered by the Nd 8o Al i0 Cu m alloy ribbons. And the subsequent diffusion process was carried out at 800°C for lh and subsequent 580°C for 5 h in a vacuum furnace. 3 Results and discussion Fig. 1 shows the demagnetization curves at 300 K for Nd-Ce-Fe-B magnets before and after diffusion treatment. After diffusion process, the coercivity is substantially enhanced from -0.86 T to -1.27 T, while the remanent magnetization decreased slightly from -1.03 T to -1.02 T. The changes in the magnetic properties were usually related with the microstructure evolution. Fig. 2 shows back -scattered electron(BSE) images and mapping images of the diffusion -processed sample with the c -axis in the plane for magnets. The selected analysis peaks for the elemental maps are Nd, Ce, Fe, Cu, and Al, respectively. As shown in Fig. 2a, the dark gray region indicates the Nd 2 Fe i4 B matrix phases (matrix phase), and the brightly imaged regions correspond to the so-called "Nd-rich phases". It is obviously shown that the Nd-rich layers become continuous, distinct and smooth after the diffusion process. These Nd-rich phases are believed to be non -ferromagnetic based on the low Fe concentration, which would be beneficial to the exchange decoupling between Nd 2 Fe i4 B grains [7]. The Nd-rich shell in the Nd 2 Fe i4 B phase was selectively formed at certain Nd 2 Fe 14 B/Nd-rich phase interfaces, as indicated by arrows in the Fig. 2a. SEM-EDS mapping images of the diffusion -processed sample are shown in Figs. 2b -2f. It is found that most of the intergranular phases are depleted of Fe and enriched with Nd and Ce, which are similar to those of commercial sintered magnets. It is surprising to fi nd that Cu and Al are also enriched in these Nd-rich phases in the diffusion -processed magnet. It claimed that certain Cu addition promotes the forrnation of the C-Nd 2 0 3 phase during annealing, which was attributed to the high coercivity [8]. On the other hand, although the distribution of Al element is inhomogeneous, it indicated that the eutectic decomposition in the intergranular phase into Al -lean regions and the modified wettability of boundary phase through Al is also advantageous for coercivity. This is probably the reason for the enhancement of coercivity of the diffusion -processed magnet. 4 Conclusions In summary, the phase constitution and magnetic properties were investigated in Nd-Ce-Fe-B sintered magnets by the grain boundary diffusion process using Nd-Al-Cu alloys. The coercivity of Nd-CeFe-B sintered magnet was substantially increased from -0.86 T to -1.27 T while smaller degradation of remanence. The detailed microstructure characterization has shown the formation of Nd-rich core/shell microstructure and the distribution of intergranular phases. The coercivity enhancement in this work can be attributed to be the microstructure evolution during the Nd-Al-Cu eutectic grain boundary diffusion process. Finally, these investigations could serve as a reference for the grain boundary diffusion on the Ce-based magnets with moderate performances.