FEGSEM backscattered-electron images of the microstructure od Tb doped Nd-Fe-B magnets showed that in doped magnets a core-shell-type structure is formed with a thin Tb-rich, (Nd,Tb)-Fe-B reaction phase around the primary Nd2Fe14B matrix grains. These intergranular reaction phases had a thickness from a few tens of nanometers up to a few micrometers. The analyses were undertaken in a JEOL JSM-7600F FEGSEM equipped with an Oxford INCA Energy 350 System and an EDS-SDD X-max-20 silicon drift detector. The results of the quantitative low-voltage EDS analyses verified that after the diffusion of Tb an intergranular reaction phase is formed with a composition described generally by (NdxTb(1–x))2Fe14B. We also found that the diffusion process is completed with an equilibrium Tb concentration that corresponds to x = 0.5, i.e., with an equiatomic Nd/Tb ratio of 1/1. A relatively sharp Tb concentration gradient from the shell to the core occurs within a length of ≈ 0.5 um, while the Fe concentration remains unchanged in both the reaction phase and the matrix grains.
COBISS.SI-ID: 26740007
In the study, the so-called grain-boundary diffusion process (GBDP) was introduced. This is a post-sintering process, where the diffusion of Dy or Tb along grain-boundaries and into the outer parts of Nd2Fe14B grains occurs and this contributes to a higher local constant of crystalline anisotropy and consequently higher coercivity. The result is a “core-shell” microstructure where the core is represented by the Dy-free Nd2Fe14B phase and the shell is rich on Dy (NdDyFe14B). When rough calculations were made, to figure out how high is the Dy-concentration after GBDP, it was determined that in a whole magnet there is 0,6 wt. % of Dy and magnetic properties are even better or as good as in the case of magnet produced by the conventional powder metallurgy route that contain 6-10 wt. % of Dy. For this reason, GBDP based on EPD is an extremely attractive process, which leads to a large coercivity-improvement (up to 30 %) with a small drop in remanence. The highest coercivities achieved so far were 20 KOe.
COBISS.SI-ID: 268167424
In the invited talk we reported of exeptional saving in the amount of heavy rare earthe, which is needed for 30 % improvement of coercivity with no consequence on the remanence. As such, these magnets can be used for electric and hybrid vehicles and generators for wind turbines where the working temeprature can be up to 180 deg. C. The amount of Dy used was 10 times lower than in the conventional metalurgical process.
COBISS.SI-ID: 27008295