Decomposition of DyF3 and its effect on magnetic performance of DyF3-doped Nd-Fe-B-type hot-deformed magnet

Abstract

Decomposition of DyF3 and its effect on the magnetic performance of the hot-pressed compact and die-upset magnet of melt-spun Nd-Fe-B-type material were investigated. DyF3 was thermally decomposed above 660 °C, and this decomposition was linked closely to the coercivity enhancement. When the DyF3 doped flakes were hot-pressed above the decomposition temperature of DyF3, the diffusion of Dy into the flakes was promoted, and leading to profound coercivity enhancement. Coercivity of the hot-pressed magnet was further enhanced by post-hot-press annealing, and coercivity as high as 24.5 kOe was obtained after the optimum annealing. The DyF3 doped hot-deformed magnet exhibited enhanced magnetic performance (iHc = 17.5 kOe, Br = 12.8 kG, (BH)max = 37.6 MGOe) with respect to the un-doped magnet without sacrificing significant remanence. Coercivity was improved by 30%. In magnet in which the decomposition of DyF3 and Dy diffusion were fully accomplished, the region originally occupied by added DyF3 was completely replaced by NdF3.

Significant Progress for Hot-Deformed Nd-Fe-B Magnets: A Review

“…In early studies, Dy fluoride was investigated as a diffusion source to improve the coercivity of HD magnets through the DADP [ 60 , 61 , 62 ]. The prevailing hypothesis is that DyF 3 powders mainly collect at the interface of ribbons and decompose at ~660 °C, followed by Dy diffusion into the interior of powder ribbons during the hot-press and hot-deformation process.…”

“…The prevailing hypothesis is that DyF 3 powders mainly collect at the interface of ribbons and decompose at ~660 °C, followed by Dy diffusion into the interior of powder ribbons during the hot-press and hot-deformation process. However, on account of the high melting point of Dy, the diffusion efficiency of Dy infiltrating into ribbons was limited and most of the Dy aggregated at the ribbon interfaces, resulting in relatively low coercivity levels of 1.6–1.9 T in the 2 wt.% DyF 3 -doped magnets [ 60 , 61 , 62 ]. Recently, Xia et al [ 63 ] found that the addition of a trace amount of nano-Cu facilitated the infiltration of Dy from the ribbon interfaces into the interior, resulting in higher coercivity levels over 2.0 T ( Figure 15 ).…”

Grain boundary diffusion in nanocrystalline Nd-Fe-B permanent magnets with low-melting eutectics

“…The combination of all three approaches promises a high performance and a resource-efficient permanent magnet, but needs a reduction of the annealing temperature close to the melting point of the Nd-rich phase in order to avoid grain growth. To compensate the therefore limited diffusion one can use low-melting eutectics [19e23] instead of pure Dy or DyF 3 [24,25]. Furthermore the low deformation temperatures (700e800 C) and short pressing times (45e450 s) of hot-deformed compared to sintered magnets (1000 C/2-4 h), make it possible to coat the NdFe-B powder with the low-melting eutectics before densification instead of coating the final magnet and still ending up with a distinct Dy shell, which is not the case for sintered magnets [26e29].…”