Homogeneous core-shell structure formation in Nd-Fe-B sintered magnets through advanced spark plasma sintering and internal grain boundary diffusion

Abstract

Nd-Fe-B sintered magnets are essential for high-performance applications, including traction motors in electric vehicles (EVs) and robots. However, enhancing coercivity at high temperatures requires the addition of heavy rare earth (HRE) elements, such as Tb and Dy, which present challenges due to their limited availability and high cost. This study addresses these challenges by combining spark plasma sintering (SPS) and internal grain boundary diffusion (i-GBD). The SPSed magnet at 750 °C, 50 MPa for 5 min achieves near-theoretical density with minimal grain growth. A post-sintering heat treatment at 1000 °C significantly enhances coercivity and refines the microstructure. Microstructural analysis reveals that i-GBD enables uniform and deep Tb diffusion, forming homogeneous core-shell structures throughout the magnet. This overcomes the limitations of conventional grain boundary diffusion (c-GBD) in terms of diffusion depth and structural uniformity. In addition, i-GBD ensures consistent coercivity across varying magnet thicknesses, making it suitable for industrial-scale production. This study highlights the effectiveness of i-GBD in reducing HRE usage while maintaining superior magnetic properties. The integration of SPS and i-GBD enables the production of large magnets that can be customized for specific applications through post-manufacturing modifications. This approach holds significant potential for the fabrication of Nd-Fe-B magnets used in EV and robotic traction motors, as well as in large-scale applications such as wind turbines. © 2025 Elsevier B.V., All rights reserved.