Hydrogen absorption and desorption characteristics of high coercivity NdDyFeCoNbCuB sintered magnets. I. Low temperature hydrogen decrepitation treatements.
Résumé
Hydrogen absorption/adsorption properties of high coercivity NdDyFeCoNbCuB sintered magnets were determined. Hydrogenation kinetics were analyzed using both differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Hydrogenation of the Nd-rich intergranular phase results in a rather broad and large peak at ∼100° +/-50°C, then the tetragonal main phase (φ phase) reacts readily close to 195°C. The disproportionation process of the whole magnet initiates at T∼500°C, then accelerates in the vicinityof 600°C and finally ends at T∼780°C. Furthermore, the first hydrogenation reaction, which is associated with the hydrogen diffusion in the bulk via the intergranular Nd-rich phase, was seen to proceed quite differently depending on the heating rate, or the applied plateau temperature. While hydrogen absorption at 50°C is rather slow, it results in higher hydrogen uptake than at 150°C, though there it happens much faster. Three modes of hydrogenation of sintered magnets are discussed in terms of practical operability. Using the optimized hydrogen decrepitation/desorption annealing route leads to a demonstration that the anisotropic NdDyFeCoNbCuB powders obtained by the HD/D technique have recovered most of the magnetic performance initially displayed in the bulk magnets.