Microstructural and compositional characterization of terbium-doped Nd-Fe-B sintered magnets

摘要

Anisotropic sintered magnets based on the Nd_2Fe_(14)B phase doped with Tb were prepared using a grain-boundary diffusion process (GBDP) in order to enhance their coercivity. A FEG-SEM microstructural analysis revealed that these GBDP magnets had a core-shell structure, where thin, Tb-rich, (NdTb)_2Fe_(14)B shells are formed on the original matrix Nd_2Fe_(14)B grains after diffusion of the Tb. This shell thickness varies from a few tens of nanometres in the middle of the magnet up to a few micrometers near the edge. The exact chemical composition of these shells was determined using EDS and WDS electron-probe microanalyses, which were modified and optimized for submicrometer scale analyses. When analyzing the common Nd-Lα, Tb-Lα and Fe-Kα lines a mutual multiple overlap in the EDS spectra is present and, as a result, an accurate quantitative analysis was only feasible when using WDS. Using this technique we were able to achieve a lateral analytical resolution of 0.4 μm. A further improvement in resolution, down to 0.15 μm, was realized with a dedicated set-up using low-voltage EDS, analyzing the "atypical" low-energy Nd-Mα, Tb-Mα and Fe-Lα lines. Quantitative analyses confirmed that the reaction phase (Nd_xTb_(1-x))_2Fe_(14)B is formed after the diffusion of Tb with the equilibrium concentration of Tb being equal to x=0.5, i.e., with the atomic ratio of Nd/Tb equal to 1/1. We also found that a relatively sharp Tb concentration gradient from the shell to the core occurs within a length of =0.5 nm, while the Fe concentration remains unchanged. In terms of magnetic properties, the Tb-doping significantly increased coercivity by =30% while the remanence remained at the same value as in the undoped Nd-Fe-B.