Improving the magnetic properties of Nd–Fe–B sintered magnets by the grain boundary diffusion of Pr-LRE-Tb-Cu-Al (LRE = None, La, Nd, and Ce) alloys using stamp coating

Nd–Fe–B sintered magnets exhibit a high maximum energy product and have been utilized in a wide range of applications. Herein, we report that the stamp coating method using powder-type diffusion sources improves the magnetic properties of Nd–Fe–B sintered magnets through the grain boundary diffusion (GBD) process. Compared with the conventional dip-coating method based on slurry-type diffusion sources, stamp coating provided a comparable coercivity improvement with simplified processing. Based on the stamp coating method, diffusion sources in which 80 wt% of heavy rare-earth (HRE) elements were substituted with light rare-earth (LRE) elements and transition metals were investigated. The Pr₆₀Tb₂₀Cu₁₅Al₅-GBD magnet exhibited magnetic properties comparable to those of a pure Tb-GBD magnet, while significantly reducing Tb usage. Moreover, in pursuit of a more cost-effective diffusion source, Pr₄₀LRE₂₀Tb₂₀Cu₁₅Al₅-GBD magnets (LRE = La, Nd, Ce) with the additional partial substitution of expensive Pr were evaluated. The increase in coercivity per Tb usage (kOe/wt.% Tb) of the Pr₆₀Tb₂₀Cu₁₅Al₅- and Pr₄₀LRE₂₀Tb₂₀Cu₁₅Al₅-GBD magnets (∼20 kOe/wt.%) were considerably higher than those of Tb₁₀₀-GBD magnets (6.4 kOe/wt.%). Furthermore, the Tb-reduced GBD magnets showed improved thermal stability compared to that of the Tb-GBD magnet. These findings demonstrate that the cost of the GBD process can be effectively reduced through stamp coating and the substitution of costly HRE and LRE elements, thereby providing a promising strategy for the development of high-performance Nd–Fe–B sintered magnets.