Magnetic and microstructural characteristics of DyF₃/DyH _x dip-coated Nd-Fe-B sintered magnets

We investigated the microstructural and magnetic property changes of DyF₃/DyH_x dip-coated Nd-Fe-B sintered magnets, as a function of the DyF₃/DyH_x mixing ratio. As the fraction of DyH_x in a DyF₃/DyH_x mixed solution was increased (0 → 50 → 100%), there was a coincident gradual increase in coercivity (14.9 → 15.4 → 16.5 kOe). Similarly, the diffusion depth of Dy from the magnet surface also increased with an increasing fraction of DyH_x. In the diffused region, we observed well-developed core-shell microstructures in the DyH_x dip-coated magnet, but this was not seen in the case of the DyF₃ dip-coated magnet. Diffusion of Dy to the main phase was via lattice diffusion predominantly, rather than through grain boundary diffusion in the DyF₃ dip-coated magnet, but the opposite was observed in the DyH_x dip-coated magnet, which is believed to be due to the different influences of F^- and H⁺ ions. Therefore, the use of DyH_x solution is considered more suitable for developing a core-shell microstructure by encouraging grain boundary diffusion, and creating a Dy-saving effect in the dip-coating process.