KINETIC ASPECTS OF THE SOLID NATION-DISPROPORTIONATION-DESORPTION-RECOMBINATION PROCESS IN ND13.67CO15.74AL0.77GA0.27ZR0.03FE62.2B7.33 ALLOYS

The nation-disproportionation-desorption-recombination process is know n as a novel method for producing permanent magnets. The hydrogen driv en reactions involved in this process are carefully studied here using Mossbauer effect, x-ray diffraction and scanning microscopy, in the N d13.67Co15.74Al0.77Ga0.27Zr0.03Fe62.2B7.33 alloy. The temperature depe ndence of the hydrogenation disproportionation in the 119 kPa isochore line was obtained. Hydrogen absorption seems to occur in two steps: a bsorption from Nd at the grain boundaries followed by absorption from the Nd2F14B type phase. In order to determine the recombination kineti cs, a typical mass of the alloy was submitted to disproportionation re action at 800 degrees C under a hydrogen pressure of 119 kPa, followed by recombination, which was achieved by vacuum desorption at 800 degr ees C and subsequent quenching to room temperature after various time intervals. It was found that the relative fraction of the Nd-2(Fe,Co)( 14)B phase, as obtained from the Mossbauer effect, increases as f = 1 - exp(-kt(n)) with n = 0.99+/-0.05. From this growing law it is inferr ed that recombination occurs without long range diffusion and starts a t the grain boundaries with zero nucleation rate. An activation energy of 0.64+/-0.13 eV/atom was obtained for this process. (C) 1998 Americ an Institute of Physics. [S0021-8979(98)03617-2]