HYDROGEN ABSORPTION DESORPTION AND HDDR STUDIES ON ND16FE76B8 AND ND11.8FE82.3B5.9

The disproportionation and recombination reactions in which Nd2Fe14B decomposes into alpha Fe, Nd hydride and Fe2B in hydrogen and then reforms under vacuum, have been investigated in Nd16Fe76B8 and Nd11.8Fe82.3B5.9 (Nd2Fe14B) alloys by TPA, DTA and Faraday balance measurements. The hydrogen content of a range of Nd-Fe-B alloys was found to increase in value with increasing Nd content, and desorption measurements of Nd16Fe76B8 hydride showed that hydrogen desorbs from the Nd-rich grain boundary at 250 degrees C from NdHapproximate to 2.7 to NdH2 and at 660 degrees C from Nd dihydride to Nd metal. Optical microscopy of partially disproportionated Nd16Fe76B8 showed that the disproportionation of the Nd2Fe14B phase begins at the Nd-rich grain boundary, which suggests that the Nd-rich phases at the grain boundary act as hydrogen diffusion paths during the disproportionation reaction. This was supported by TPA measurements, which showed that the temperature range of the disproportionation reaction decreases with increasing proportion of Nd-rich phase. DTA measurements showed that the disproportionation temperature in Nd16Fe76B8 decreases with increasing hydrogen pressure. It was found that, in disproportionated Nd-Fe-B, the hydrogen desorption kinetics for NdHapproximate to 2.7 and the dihydride were different for Nd in the Nd-rich grain boundary and Nd in the disproportionated matrix.