Hydriding/dehydriding of Mg87Ni3Al3Mm7 (Mm = La, Ce-rich mischmetal) alloy produced by mechanical milling

Nanocrystalline Mg87Ni3Al3Mm7 hydrides were produced by reactive mechanical milling (RMM) under hydrogen atmosphere. Milling first under argon, and then under hydrogen atmosphere was also carried out in order to study the effect of the gas atmosphere on the microstructure of the products of milling as well as on their hydriding properties. In both cases, the main product of the milling process turned out to be MgH2 of the type β-MgH2. When the milling was carried out entirely under hydrogen atmosphere the amount of MgH2 (∼34 wt.%) was slightly larger than that resulted from milling under argon followed by milling under hydrogen atmosphere. X-ray diffraction studies revealed that the main product of the milling process is a nanocrystalline material with an average crystallite size of about 10-15 nm. The presence of a highly disordered (amorphous) phase was revealed as well. The first dehydriding reaction of the alloys (after RMM) was studied by thermogravimetry (TG) and differential scanning calorimetry (DSC). These studies allowed to determine the temperature and enthalpy of desorption (Tdes, ΔHdes) as well as the amount of hydrogen released during heating. The Tdes as well as the enthalpy of dehydriding of the alloys obtained by milling in different atmospheres were found not to differ significantly. The as-milled nanocrystalline materials were annealed to relieve the strain resulted from the milling and improve the material's crystallinity. Heat treatment at 350 °C for 1 h reduced the amount of the disordered phase in the sample milled only under hydrogen atmosphere. PCT analysis reveals an equilibrium pressure of about 2.5 atm for the alloy milled under Ar and hydrogen and about 2 atm for the alloy milled only under hydrogen. Both equilibrium pressures are higher than that found with pure Mg, indicating some thermodynamic destabilization of the hydride as a result of the alloying. Hydrogen-absorption kinetics was also studied at isothermal conditions. The hydrogen-absorption process in both nanocrystalline materials was found to be very fast most likely due to the alloying and fine particle size. The rate of H-absorption in the sample milled under argon and hydrogen was found to be higher then that in the sample obtained by milling only under hydrogen atmosphere.