Grain Boundary Diffusion Sources and Their Coating Methods for Nd-Fe-B Permanent Magnets

Abstract

Nd-Fe-B magnets containing no heavy rare earth (HRE) elements exhibit insufficient coercivity to withstand the demagnetization field at elevated temperatures. The grain boundary diffusion (GBD) process provides the best route to fabricate high-coercive Nd-Fe-B magnets with low consumption of expensive HRE resources. Here we give a special review on the grain boundary diffusion sources and their coating methods. Up to now, various types of grain boundary sources have been developed, starting from the earliest Tb or Dy metal. The HRE-M eutectic alloys were firstly proposed for reducing the cost of the diffusion source. After that, the diffusion sources based on light rare earth and even non rare earth elements have also been proposed, leading to new understanding of GBD. Now, the diffusion sources including inorganic compounds, metals, and alloys have been employed in the industry. At the same time, to coat the diffusion source on the magnets before diffusion treatment, various methods have been developed. Different from the previous review articles for GBD, this review gives an introduction of typical types of diffusion sources and their fabrication approaches. The effects of diffusion source on the microstructure and magnetic properties are summarized briefly. In particular, the principles and applicability of different coating approaches were discussed in detail. It is believed that this review can provide a technical guidance for the industry for designing the diffusion process and products meeting specific requirements.

Advances in Metal-Containing Magnetic Materials and Magnetic Technologies

“…Two papers on permanent magnets are presented in this issue. He et al [1] reviewed sources of grain boundary diffusion (GBD) and their coating methods for Nd-Fe-B permanent magnets. The GBD process, which is one of the most exciting technologies for rare earth permanent magnets that has emerged in this century, provides the best route for fabricating highly coercive Nd-Fe-B magnets with low levels of consumption of expensive HRE resources.…”

Grain refinement and coercivity enhancement of sintered Nd–Fe–B alloys by doping eutectic alloy (Nd0.75Pr0.25)70Cu30.

“…The coercivity is being improved by modifying the microstructure, particularly the intergranular zones, and diffusing the rare earth-rich alloy into the grain boundaries of the parent compound [9][10][11][12]. Grain boundary sources, which have been reported to dramatically improve the coercivity, include heavy RE (HRE) compounds, light RE (LRE) compounds and RE-free compounds.…”

“…On the other hand, the grain boundary source is normally coated in thick block magnets and heat treated followed by a simple post-annealing process [9]. In this work, NdFeB powders were mixed with 0 to 5 wt.…”