Direct hydriding of Mg87Al7Ni3Mn3 by reactive mechanical milling in hydrogen atmosphere and influence of particle size on the dehydriding reaction

Nanocrystalline Mg87Al7Ni3Mn3-H was prepared by reactive mechanical milling (RMM) in hydrogen atmosphere. It was found that the amount of the hydride (mainly beta-MgH2) formed during RMM increases in the first 15 h and then reaches an almost constant value of about 34 wt.% (46 vol.%). The thermal stability of the hydride (230degreesC) and the enthalpy of H-desorption (55 kJ/mol H-2) determined are lower than those corresponding to conventional polycrystalline MgH2 as well as to nanocrystalline MgH2 with different transition metal and transition metal oxide catalysts. The first dehydriding reaction of the Mg87Al7Ni3Mn3 hydrides with different particles size was investigated. The finer particle fractions contain essentially a larger amount of MgH2 than the coarser ones, as the thickness of the hydride layer was estimated to be approximately the same for all particles, about 2 mum. This value was found to be close to the critical thickness of the impermeable hydride layer formed under the experimental conditions of RMM applied in the present study. Isothermal kinetics analysis reveals that the first dehydriding reaction after mechanical grinding in hydrogen atmosphere is surface controlled. In the advanced stage of the dehydriding process diffusion controlled hydrogen desorption was detected. An interface controlled growth model was found to describe some of the kinetics curves, as well. Activation energies of dehydriding of 100-115 kJ/mol were obtained for the nanocrystalline Mg97Al7Ni3Mn3-H-x. (C) 2004 Elsevier B.V. All rights reserved.