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

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• 作者：Zoran Samardž ; ija^a ; ^b ; ^{zoran.samardzija@ijs.si} ; Paul McGuiness^a ; ^b ; ^{paul.mcguiness@ijs.si} ; Marko Soderž ; nik^a ; ^{marko.soderznik@ijs.si} ; Spomenka Kobe^a ; ^{spomenka.kobe@ijs.si} ; Masato Sagawa^c ; ^{sagawa@intermetallics.co.jp}
• 关键词：Nd&ndash ; Fe&ndash ; B magnets ; Terbium ; Microstructure ; Microanalysis ; WDS ; EDS
• 刊名：Materials Characterization
• 出版年：2012
• 期刊代码：57_10445803
• 类别：ms
• 出版时间：May, 2012
• 卷：67
• 期：Complete
• 页码：27-33
• 文件大小：1318 K

摘要

Anisotropic sintered magnets based on the Nd₂Fe₁₄B phase doped with Tb were prepared using a grain-boundary diffusion process (GBDP) in order to enhance their coercivity. A FEGSEM microstructural analysis revealed that these GBDP magnets had a core-shell structure, where thin, Tb-rich, (NdTb)₂Fe₁₄B shells are formed on the original matrix Nd₂Fe₁₄B 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-L伪, Tb-L伪 and Fe-K伪 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 渭m. A further improvement in resolution, down to 0.15 渭m, was realized with a dedicated set-up using low-voltage EDS, analyzing the 鈥渁typical鈥?low-energy Nd-M伪, Tb-M伪 and Fe-L伪 lines. Quantitative analyses confirmed that the reaction phase (Nd_xTb_{1 鈭?#xA0;x})₂Fe₁₄B is formed after the diffusion of Tb with the equilibrium concentration of Tb being equal to x 鈮?#xA0;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 鈮?#xA0;0.5 渭m, while the Fe concentration remains unchanged. In terms of magnetic properties, the Tb-doping significantly increased coercivity by 鈮?#xA0;30%while the remanence remained at the same value as in the undoped Nd-Fe-B.