A ND-FE-O INTERGRANULAR PHASE IN ND-FE-B SINTERED MAGNETS AND ITS EFFECT ON COERCIVITY

The intergranular microstructure and coercivity of Nd richer NdxFe93-x B7 (x = 16-28) sintered magnets have been systematically studied by me ans of scanning electron microscopy, analytical transmission electron microscopy, scanning auger multiprobe, thermomagnetic analysis, magnet ic measurement and annealing experiments. A new stable Nd-Fe-O ferroma gnetic phase with a composition NdFe2Ox (x almost-equal-to 0.3) and a Curie temperature of 145-degrees-C is found in the intergranular regio ns of the magnets, due to the introduction of a considerable amount of oxygen during magnet processing and its segregation in the intergranu lar regions. The phase forms at approximately 650-degrees-C and its vo lume fraction reaches a maximum of approximately 4% in the Nd22Fe71B7 magnets. This new phase has a strong hindrance effect on the propagati on of reversed domain walls between Nd2Fe14B grains, and thus leads to a considerable enhancement of coercivity. Raising the Curie temperatu re of this phase would be a new important approach to improve the high -temperature oercivity of Nd-Fe-B magnets.