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 used in electric vehicles (EVs) and robots. However, enhancing coercivity at high temperatures requires heavy rare earth (HRE) elements, such as Tb and Dy, leading to challenges such as resource scarcity and high costs. 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 1,000 °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 diffusion depth and structural uniformity limitations of conventional GBD. 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 EV and robotic traction motors, as well as in large-scale applications, such as wind turbines.

Keywords: Nd-Fe-B sintered magnets, Heavy rare earth, Spark plasma sintering, Grain boundary diffusion, coercivity