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

Abstract Fine powders by grinding Nd–Fe–B sintered magnet bulks (particle size=46–125 μm in diameter) were coated and alloyed with Zn, Mg or Yb metal by sorbing them, for which the boiling points were low. Among these volatile metals, Yb showed a good ability to recover the decreased magnetic property of as-ground Nd–Fe–B powders, viz. ∼0.95 T for remanence ( B r ), ∼227 kA/m for coercivity ( H cJ ) and ∼48.8 kJ/m 3 for maximum energy product (( BH ) max ), by sorbing (coating and alloying) its vapor at 550–850°C in every pyrex or quartz glass tube sealed under an evacuated condition (∼10 −6 Torr). The sorbing temperature and time for Yb metal vapor on the magnetic properties of Nd 2 Fe 14 B were optimized and, after heating at 800°C for 90 min and annealing subsequently at 610°C for 60 min, the poor B r , H cJ and ( BH ) max values were increased to be 0.98 T, 712 kA/m and 173 kJ/m 3 , respectively. It was found on the basis of the microstructural characterizations that the sorbed Yb metal uniformly covered the surface and penetrated to the grain boundary to form a (Nd,Yb)Fe 2 phase for the fine powders of Nd–Fe–B sintered magnet bulks. For the sorption of Zn or Mg metal vapor, no effective improvement of the magnetic properties was observed for the ground Nd–Fe–B powders: The Zn metal vapor sorption resulted in the formation of α -Fe lowering considerably the magnetic properties. For the sample powders sorbed with Mg metal vapor, since Mg mainly reacted with the boundary phase of Nd–Fe–B sintered magnet powders to form a Mg–Nd phase, the coercivity was still more decreased although the B r value was not changed.