In the 21st century, the rapid advancement of technology in fields such as industrial internet, renewable energy, high-speed rail transportation, 5G communications, and intelligent manufacturing has significantly influenced global industrial development. The concept of "new infrastructure construction" introduced in China in 2018 further underscores the importance of these sectors in national economic growth. Among the materials crucial to these advancements, neodymium-iron-boron (Nd-Fe-B) permanent magnets play a pivotal role due to their exceptional magnetic properties, occupying more than 50% of the permanent magnet market. However, traditional Nd-Fe-B magnets face challenges, particularly in applications requiring high temperature stability, such as electric vehicle drive motors and wind turbine generators.
Challenges in Traditional Nd-Fe-B Magnets
One of the primary limitations of commercial sintered Nd-Fe-B magnets, especially those without heavy rare earths (HREs), is their low coercivity (around 1200 kA/m) and Curie temperature (Tc = 312°C). Coercivity, an extrinsic magnetic property, is closely related to the composition and microstructure of the magnet. Increasing coercivity can effectively resist demagnetization at higher temperatures, thereby enhancing the magnet's thermal stability.
To address this issue, several approaches have been explored. One method involves adding cobalt (Co) alloy elements to replace some of the iron (Fe) in the Nd2Fe14B phase, thereby increasing the Curie temperature (Tc) of the magnet. However, excessive Co not only raises material costs but also degrades the hard magnetic properties. Another strategy is to incorporate HRE elements such as dysprosium (Dy) and terbium (Tb) to replace neodymium (Nd) in the 2:14:1 phase, forming a (HRE, Nd)2Fe14B phase with a higher magnetic anisotropy field. Unfortunately, HREs are scarce, with their abundance in the Earth's crust being less than 10% of that of Nd, leading to extremely high prices. Introducing HREs through traditional melting significantly increases the cost of the magnet, with HRE materials accounting for 30% to 50% of the final price of Nd-Fe-B magnets. Furthermore, the antiferromagnetic coupling between HRE atoms and Fe atoms inevitably reduces the remanence and magnetic energy product of the magnet.
Grain Boundary Diffusion (GBD) Technology
In response to the growing demand for high-temperature Nd-Fe-B magnets, the development of GBD technology has emerged as a promising solution. Proposed by Nakamura et al. in 2005, GBD technology utilizes single elements or compounds of HREs as diffusants. Through diffusion heat treatment, HREs penetrate the magnet from the surface along grain boundaries, distributing in the grain boundaries and on the grain surfaces to enhance the coercivity of the Nd-Fe-B magnet. The diffusion process typically occurs at temperatures above the melting point of the grain boundary-rich rare earth phase in the Nd-Fe-B magnet, facilitating rapid diffusion along grain boundaries in the liquid-rich rare earth phase.
The core principle of GBD is that the reversal of magnetization domains first forms on the grain surfaces, making them the weakest link within the magnet. By increasing the anisotropy field on the grain surfaces, the formation of reversal domains can be delayed, thereby enhancing the coercivity of the entire magnet. GBD distributes HREs predominantly in the grain boundaries, minimizing their entry into the grain interiors. This approach not only boosts coercivity but also mitigates the adverse effects of HREs on remanence, achieving excellent overall magnetic properties.
Advantages of GBD Technology with Ce
GBD technology offers several advantages, particularly when cerium (Ce) is incorporated. Ce is a cost-effective alternative to Nd and HREs due to its abundance. By substituting a portion of Nd with Ce, the material cost can be reduced while maintaining or even enhancing magnetic performance. Studies have shown that Ce-containing Nd-Fe-B magnets processed through GBD exhibit significant improvements in coercivity. For instance, Tb-diffusion into sintered (Ce,Nd)-Fe-B magnets prepared by the dual-main-phase method has been found to enhance coercivity by up to 65%, with a relatively minor reduction in remanence and maximum energy product.
Recent Developments and Research Directions
In recent years, GBD technology has rapidly advanced and achieved industrialization. Practical industrial applications of GBD have reduced HRE consumption by more than 50%. However, several technical and theoretical challenges remain. One critical issue is increasing the thickness of magnets that can be effectively treated by GBD. Current industrial practice mainly focuses on magnets with thicknesses less than 4 mm, with few exceeding 8 mm. For applications requiring higher safety margins, such as motors and generators operating above 125°C, thicker magnets are preferred.
To address this challenge, researchers are exploring methods to enhance the diffusion rate of diffusants, aiming to increase the diffusion depth in thicker magnets. One approach involves modifying the composition of the diffusants, such as through alloying or doping, to provide more efficient diffusion channels. For example, the use of Al + TbH2 mixtures as diffusants has been found to significantly improve the coercivity of 6.5 mm thick magnets compared to using TbH2 alone. The introduction of Al promotes the formation of continuous thin-layer grain boundary phases between 2:14:1 grains, enhancing demagnetizing coupling and thus improving coercivity.
Future Perspectives
The future of GBD technology with Ce holds considerable promise for developing high-performance, cost-effective Nd-Fe-B magnets. Research continues to focus on optimizing diffusant compositions, enhancing diffusion efficiencies, and understanding the microstructure-property relationships in GBD magnets. Additionally, there is a need to develop methods for treating thicker magnets effectively, expanding the application range of GBD technology.
In conclusion, the latest developments in GBD technology with Ce represent a significant step forward in addressing the limitations of traditional Nd-Fe-B magnets. By leveraging the unique properties of Ce and optimizing the GBD process, researchers are paving the way for the widespread adoption of high-performance, cost-effective permanent magnets in various industries, driving innovation and sustainability in the 21st century.
