Dependence of grain size on grain boundary diffusion mechanism of Nd-Fe-B sintered magnets

The methods to improve the grain boundary diffusion efficiency of Nd-Fe-B magnets are mostly focused on the type of diffusion source and the preparation method, while the characteristics of the diffusion matrix are less studied. In this paper, the effect of grain size in diffusion matrix on the grain boundary diffusion mechanism of Tb element is investigated intensively. The results show that with only 0.75 wt. % TbF3 diffusion, the intrinsic coercivity increases by 8.83 kOe for the magnet with finer average grain size, while the increase for coarse-grain magnet is only about 6.99 kOe. The huge increase in the intrinsic coercivity is attributed to the increase in the defect density inside the magnet due to the grain refinement. More defects lead to a reduction in diffusion activation energy and thus an increase in diffusion efficiency. The grain refinement also leads to higher grain orientation. This facilitates a smooth and uniform diffusion of the heavy rare earth elements into deeper regions to form a highly uniform shell structure. In this work, starting from the characteristics of diffusion matrix factors, it is found that the grain refinement is beneficial for the diffusion of heavy rare earth elements, which provides a novel idea for improving the grain boundary diffusion efficiency and magnetic properties.