Hydrogenation-disproportionation processes in Nd14.01Hf0.08Fe78.91B6.99 alloys

The kinetics of the reactions undergone by the system Nd14.01Hf0.08Fe78.91B6.99 submitted to thermal treatments under H2 atmosphere were studied in detail. Constant number of moles, constant pressure (P≈119 kPa), isothermal or high heating rate experiments were performed at temperatures between 240 and 980°C on samples of ∼200 mg. The state after each treatment was characterized by Mössbauer effect spectroscopy and X-ray diffraction. Three temperature regions were identified. In the first one (T≤580°C) two processes were observed: the formation of Nd-hydride at grain boundaries (2.6 H/Nd) and a solid solution Nd2Fe14BHx in the matrix phase. Activation energy of 36.20±0.02 kJ mol−1 for absorption process was determined. In the second temperature region (680°C≤T≤800°C) two processes of H2 absorption were found: (1) φ-Nd2Fe14B hydrogenation and formation of grain border Nd-hydride, and (2) disproportionation regime. For T≥800°C disproportionation rates are slower for increasing temperatures, the φ-phase incorporates less than 0.6 H/Nd and a strong dependence with the experimental conditions was found in this temperature range. Between 800 and 875°C the disproportionation reaction is fast but saturates, leaving only a small non-disproportionated fraction. This may constitute the best temperature interval for the hydrogenation-disproportionation processes.