Understanding the coercivity enhancement mechanism of grain boundary diffused Nd-Fe-B magnets by comparing with commercial equivalent coercivity magnets

Abstract

The grain boundary diffusion (GBD) technology effectively utilizes heavy rare earth (HRE) elements to improve the coercivity of sintered Nd-Fe-B magnet. Significantly, clarifying its coercivity improvement mechanism is essential for further optimizing the process and performance. Through comparative analysis with commercial conventional magnets (CSH) processing equivalent coercivity, this study investigated the specific mechanism by which the GBD method enhances the coercivity of HRE elements in an efficient manner. To achieve the same coercivity (∼21.4 kOe), GBD magnets contained only 0.25 wt% Tb, whereas CSH magnets had 1.17 wt% Dy and 1.18 wt% Tb. The microstructure analysis revealed that the Tb element in the GBD magnet was concentrated in the outer layer of the grain, constituting 17.37 wt% of the total rare earth content. While, in CSH magnets, the HRE element in the main phase grains accounted for only 9.58 wt% of the total rare earth elements, including Dy (3.83 wt%) and Tb (5.75 wt%). The characteristic core-shell structural grains in GBD magnet also endowed it with an uneven magnetic domain structure and a more concentrated “abrupt” magnetization reversal. Based on the characterization results the nucleation positions of the two types of magnets were analyzed by fitting Hci/Ms and Ha/Ms. The results can serve as a guide for enhancing coercivity in grain boundary diffusion sintered Nd-Fe-B magnets, aiding in process and property optimization, and advancing the understanding of nucleation type mechanisms.