Abstract
The grain boundary diffusion (GBD) process has been widely applied to effectively improve the coercivity of sintered Nd-Fe-B magnets. However, the abnormal grain growth and shallow diffusion depth were not conducive to coercivity enhancement. In this work, 0.2wt.% of Tb3O4 powder addition during the sintering process was applied to inhibit the abnormal grain growth and increase the continuous GB phase, and thus a superior coercivity of 26.22kOe was achieved after Tb GBD by comparing with that of 24.15kOe for the GBD magnet without Tb3O4 powder addition. Correlated with the electron probe microanalyzer (EPMA) and transmission electron microscopy (TEM) characterizations, a deeper diffusion depth, a thicker Tb-rich shell, and finer Nd2Fe14B phase grains make synergetic contributions to this coercivity enhancement. Furthermore, the Nd6Fe13Ga phase is formed surrounding the Tb3O4 intergranular phase, which is conductive to reduce magnetic coupling. The relationships among the coercivity, grain size, and core-shell structure are further confirmed by micromagnetic simulation. This work provides a dependable approach to further coercivity enhancement by grain boundary regulation.
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