Anisotropic Nd-Fe-B bonded magnets made from HDDR powders

Anisotropic Nd-Fe-B magnet powders can be produced by the hydrogenation-decomposition-desorption-recombination (HDDR) process from Nd-Fe-B-Co-M (M = Ga, Zr, Nb, Hf, and Ta) alloys. The present status of those HDDR powders and the bonded magnets made from them are reviewed with regards to the powder particle size dependence of their magnetic properties, their magnetic thermal stability, and their magnetization behavior. The results of a mechanistic study on the recombination steps are also presented. The magnetic properties of the anisotropic HDDR powder depend relatively little on the powder particle size. Bonded magnets with a density of similar to 6.20 g/cm(3) and a BHmax of 18.5-20.5 MGOe can be produced from anisotropic HDDR powders with particle sizes of below 300 mu m diam. The temperature coefficient of the intrinsic coercive force H-i(c), of the bonded magnet is -0.55%/degrees C in the temperature range from 25 to 100 degrees C. The magnetization force needed for full magnetization of the bonded magnet is about twice the coercive force of the magnet, indicating that the magnetization mechanism is different from that of the rapidly solidified isotropic Nd-Fe-B magnet. In the early stage of the recombination step of the HDD process (1 min desorption), three phases are produced, i,e., alpha-(Fe,Co), spherical NdH2 and rimlike Nd-2(Fe,Co)(14)B surrounding the NdH2 particle, Further desorption makes the Nd-2(Fe,Co)(14)B phase grow. (C) 1996 American Institute of Physics.