Hydrogen absorption and desorption characteristics of high coercivity NdDyFeCoNbCuB sintered magnet. I. Low temperature hydrogen decrepitation treatments

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 vicinity of 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.