Improvement of coercivity of sintered NdFeB permanent magnets by grain boundary diffusion process

• Main Authors: Chien-Fu Chen, 陳建甫
• Other Authors: Wen-Cheng Chang
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/14906630609097951891

碩士 === 國立中正大學 === 物理學系暨研究所 === 101 === In this study, grain-boundary-diffusion (GBD) process via the usage of DyF3 powder on the surface of magnets was applied to the sintered NdFeB magnets. The effects of surface morphology, oxygen concentration, thickness and diffusion conditions on coercivity (iHc) of sintered NdFeB magnets have been studied. The results show that pre-polishment of the magnets is effectively for reducing the surface roughness of magnets, leading to the enhancement of iHc. In addition, the magnets with lower oxygen concentration are helpful for Dy diffusion and improve iHc, yet easier to decrease Br of the magnets. Meanwhile, the diffusion of Dy in thinner magnets may easily reach the deeper part and improve the magnetocrystalline anisotropy field of inner grains, iHc of the magnets is easily enhanced, accordingly. In this series samples, the optimal magnetic properties of iHc increment (∆iHc) =4.5 kOe and iHc + (BH)max=67.6 under the diffusion conditions of 900 oC, 4 hrs were obtained. Furthermore, Dy diffusion treatment was applied to the as-sintered magnets without post annealing. It was found that ∆iHc are larger than those magnets treated with two steps annealing. The largest ∆iHc can be up to 6.2 kOe in magnets without two steps annealing. Finally, we changed Dy source from DyF3 powder to Dy film and study the thickness effect of Dy film on ∆iHc of the sintered magnets. The results showed that the ∆iHc was enhanced effectively with increasing the thickness of Dy film. Interestingly, extremely high ∆iHc up to 7.9 kOe can be obtained by introducing 1.75 wt% Dy only to the magnet through this novel GBD method. The optimal magnetic properties of Br = 13.9 kG, iHc=22.9 kOe, (BH)max=48.2 MGOe and iHc + (BH)max=71.1 were obtained in 2-mm thick magnet, as well as Br = 13.8 kG, iHc=22.0 kOe, (BH)max=48.1 MGOe and iHc + (BH)max=70.1 in 3.2-mm thick magnet.