Effects of Dy diffusion time on magnetic properties of Nd–Fe–B sintered magnets

In recent years, significant research efforts were undertaken to enhance coercivity ( H cj ) of Nd-Fe-B sintered magnets. One of the methods is Dy diffusion, which is very effective in improving H cj while maintaining the same level of remanence ( B r ). Experiments reported in this work revealed that Dy diffusion differently affected H cj of Nd-Fe-B sintered magnets at three diffusion stages: (1) from 0 to 4 h of Dy diffusion, H cj increased rapidly from 32.3 to 36.5 kOe as Dy content increased rapidly from 0 to 0.27 wt%; (2) from 4 to 24 h, when Dy content changed from 0.27 to 0.56wt%, H cj increased from 36.5 to 38.7 kOe; (3) from 24 to 30 h, H cj and Dy content barely changed. Thus, extending Dy diffusion time beyond 24 h period would not yield any improvements. From 0 to 30 h, B r slightly decreased from 12.40 to 12.22 kGs. During Dy diffusion, intergranular grain boundaries became continuous and homogeneous, and some micro-defects were repaired. As a result, from 0 to 24 h, H cj enhanced significantly while B r barely changed. Thus, the Nd-Fe-B sintered magnets with high properties ( B r = 12.22 kGs, H cj  = 38.9 kOe and ( BH ) max  = 36.87 MGOe) were obtained.