Microstructural observation of SPSed Nd-Fe-B magnet and interal grain boundary diffusion with TbH

Abstract

Nd-Fe-B sintered magnets exhibit limitations due to a decrease in coercivity at high temperatures. To address this issue, the Grain Boundary Diffusion Process (GBDP) is utilized to enhance room-temperature coercivity. This process enhances coercivity by coating heavy rare-earth (HRE) elements such as Dy and Tb to the surface of the magnet, followed by heat treatment to facilitate diffusion along the grain boundaries, forming an HRE-rich shell. However, the GBDP faces problems such as limited diffusion depth, non-uniform diffusion, and grain growth. In this study, high-density Nd-Fe-B sintered magnets were fabricated using Spark Plasma Sintering (SPS). SPS's short sintering time makes grain boundary formation difficult. However, following heat treatment formed grain boundaries, leading to a significant increase in coercivity. Grain growth was also suppressed. TbH was selected as the diffusion source and mixed with Nd-Fe-B powder to fabricate the SPSed magnets. For comparison, TbH was applied to SPSed magnets using the GBDP process, and the content of the diffusion source was established at 1 wt%. Analysis of the coercivity and microstructure of the Mixing magnets and GBDP magnets was performed, and the impact on thicker magnets will be discussed.