Significant progress of grain boundary diffusion process for cost-effective rare earth permanent magnets: A review

Rare earth permanent magnets are essential in a plethora of electrical machine and renewable energy. Nd-Fe-B magnets account for half of the market share of permanent magnets due to their excellent room temperature magnetic properties. However, the next generation applications require high coercivity at elevated temperatures. For this purpose, heavy rare earths (HRE) have been used to enhance the anisotropy field of the magnets but it not only increases the cost but also reduces the remanence. Grain boundary diffusion process (GBDP), as an emerging technology appeared in this century, offers the best route to improve the coercivity of Nd-Fe-B magnets with low HRE consumption, and has been well studied and rapidly industrialized. In this review, based on the world-wide progress and the results from the authors' groups, a comprehensive critical description is given on the fundamentals and development of GBDP. The GBDPs based on various diffusion sources and processes, and those for different types of magnets are compared and summarized. The coercivity enhancement mechanisms are discussed in detail to understand the structure-properties relationships. The cost reduction and high-efficient use of critical elements are analyzed. The challenges and opportunities are also highlighted for future development of GBDP and cost-effective permanent magnets.