Microstructural Improvement of NdFeB Magnetic Powders by the Zn Vapor Sorption Treatment

Abstract

The microstructural modification for NdFeB magnetic powders (MQP-B) after Zn vapor sorption has been investigated by transmission electron microscopy (TEM). It is found that a coating layer about 100 nm thick was formed on the surface of powder. Selected area electron diffraction (SAED) pattern and high resolution transmission electron microscopy (HRTEM) analyses show that the coating layer consists of condensed nano-scale crystallites of Fe 11 Zn 40 , NdZn 5 and Zn. After the temperature for Zn vapor sorption is raised to 648 K, a thin layer has been formed on boundaries of Nd 2 Fe 14 B grains locally in the rim region of the powders. The morphology of the grain-boundary thin-layers is very similar to that in HDDR-NdFeB magnet powder after Zn vapor sorption treatment. Since the latter has a good thermal stability of magnetic property, the Zn vapor sorption treatment is expected to improve also the thermal stability of MQP-B magnet.

Corroded microstructure of HDDR-NdFeB magnetic powders

“…The observation on the corroded microstructures of HDDR-NdFeB powders shows that the altered layer covers the surface and the wetting layer is formed along the Nd 2 (Fe,Co) 14 (Figs. 2 and 3).…”

“…Co 11.5 B 6.0 Zr 0.1 (at%), and reported that the powders are made of homogeneous Nd 2 (Fe,Co) 14 B grains about 300 nm in diameter with clean boundaries. This would be that the Nd composition of the HDDR-NdFeB powders is closer to the stoichiometry of Nd 2 (Fe,-Co) 14 B. HDDR-NdFeB powders with such a microstructure are suitable for the production of the bonded magnets with high magnetic properties, because the size of Nd 2 (Fe,Co) 14 B grains near to the critical single magnetic domain size leads to high coercivity. The present alloy contains a higher Co content and a small amount of Ga in order to increase the Curie temperature and the magnetic anisotropy.…”

“…It is not clear at present if amorphous Fe oxide is formed on the way of oxidation process. The corrosion process mentioned above brings the decrease of net volume of Nd 2 (Fe,-Co) 14 B phase and increase of a-(Fe,Co) grains in the powders. This is a main origin of the degradation of the magnetic powders.…”

“…As to the surface coating of HDDR-NdFeB powder, it is desirable to modify the grain boundary structure in addition to form only the surface coating layer because the corrosion attack occurs at the surface and then proceeds to the interior of powders via grain boundaries. It was recently reported that well-designed Zn-coating on NdFeB powders produces Zn compounds along grain boundaries as well as on the surface of the powders [14]. Such modified microstructures are considered to be effective to protect Nd 2 (Fe,-Co) 14 B grains from the corrosion attack.…”

“…From these results of TEM and HRTEM observations mentioned above, it is revealed that the altered layer is of a dense mixture where fine a-Fe grains disperse within very fine h- higher than h-Nd 2 O 3 crystal. The preferentialgrowing of a-Fe crystals would be attributed to Fe segregation in the decomposition due to its high atom fraction in Nd 2 (Fe,Co) 14 …”