Coercivity and Thermal Stability Enhancement for Spark-Plasma-Sintered Nanocrystalline NdFeB Magnets With Dy2O3 and Zn Additions

NdFeB bulk magnets were synthesized by spark plasma sintering (SPS) using the precursors of melt spun nanocrystalline Nd_10.15Pr_1.86Fe_80.41Al_1.67B_5.91 ribbons mixed with Zn and/or Dy₂O₃ powders. The addition of 0.6 wt.% Zn or 2 wt.% Dy₂O₃ is effective in improving the magnetic properties of the magnets, and their coercivities are about 11% or 15%, respectively, higher than that of the additive free magnet. Both additives can suppress the grain growth of Nd₂Fe₁₄B phase. Dy₂O₃ is not beneficial to the densification during SPS, but Dy is considered to partly diffuse into the grains near particle boundaries and to form (Nd, Dy)₂Fe₁₄B phase. NdZn and NdZn₅ phases were observed in the SPSed magnets with Zn additions more than 1 wt.%, which leads to coarse grains and porosities between the particles. The magnet with combined additions of 2 wt.% Dy₂O₃ and 0.6 wt.% Zn showed good thermal stability with small temperature coefficients and optimal magnetic properties with high coercivity and maximum energy product.