Magnetic properties of Nd-Fe-B sintered magnet powders recovered by Yb metal vapor sorption

Fine ground powders of Nd-Fe-B sintered magnet bulks (particle size=46-125 μm in diameter) were coated and alloyed with Yb metal by sorbing them. A significant recovery of the decreased magnetic properties of the ground powders (remanence B_r=-0.95 T, coercivity H_cj= -227 kA·m^-1 and maximum energy product (BH)_max=-48.8 kJ·m^-3) was observed in accordance with increasing temperature up to 800°C. The sorbing temperature and time for Yb metal vapor were optimized and after heating at 800°C for 90 min and annealing subsequently at 610°C for 60 min, the B_r, H_cj and (BH)_max values were increased to be 0.98 T, 712 kA·m^-1 and 173 kJ·m^-3, respectively. From the microstructural characterizations of resulting samples by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalyzer (EPMA), it is found that the sorbed Yb metal uniformly covers the surface and diffuses to the Nd-rich grain boundary of fine ground powders of Nd-Fe-B sintered magnet bulks forming a (Nd, Yb)Fe₂ phase.