Kinetic aspects of the solid hydrogenation-disproportionation-desorption-recombination process in Nd13.67Co15.74Al0.77Ga0.27Zr 0.03Fe62.2B7.33 alloys

RODRÍGUEZ TORRES, C.E.; FERNÁNDEZ VAN RAAP, M.B.; SÁNCHEZ, F.H.; MENDOZA ZÉLIS, L.; ACTIS, F.

JOURNAL OF APPLIED PHYSICS

AMER INST PHYSICS

Año: 1998 vol. 84 p. 3786 - 3791

0021-8979

The hydrogenation-disproportionation-desorption-recombination process is known as a novel method for producing permanent magnets. The hydrogen driven reactions involved in this process are carefully studied here using Mössbauer effect, x-ray diffraction and scanning microscopy, in the Nd13.67Co15.74Al0.77Ga0.27Zr 0.03Fe62.2B7.33 alloy. The temperature dependence of the hydrogenation disproportionation in the 119 kPa isochore line was obtained. Hydrogen absorption seems to occur in two steps: absorption from Nd at the grain boundaries followed by absorption from the Nd2Fe14B type phase. In order to determine the recombination kinetics, a typical mass of the alloy was submitted to disproportionation reaction at 800 °C under a hydrogen pressure of 119 kPa, followed by recombination, which was achieved by vacuum desorption at 800 °C and subsequent quenching to room temperature after various time intervals. It was found that the relative fraction of the : Nd2(Fe,Co)14B phase, as obtained from the Mössbauer effect, increases as f = 1 - exp(-ktn) with n = 0.99±0.05. From this growing law it is inferred that recombination occurs without long range diffusion and starts at the grain boundaries with zero nucleation rate. An activation energy of 0.64±0.13 eV/atom was obtained for this process. © 1998 American Institute of Physics.