Effect of Al* generated in situ in hydriding on the dehydriding properties of Mg-Al alloys prepared by hydriding combustion synthesis and mechanical milling

In this work, the effects of Al* generated in situ in hydriding on the dehydriding of Mg-Al alloys prepared by Hydriding Combustion Synthesis (HCS) and Mechanical Milling (MM) were studied. Thermal analysis showed that the peak dehydriding temperature of MgH₂ was reduced by 25 °C (or 11 °C) when 20 at% Al was added in HCS (or MM) process, even though, the same dehydriding reaction of 17MgH₂ + 12Al*/Al → Al₁₂Mg₁₇ + 17H₂ was observed. Isothermal dehydriding at 300 °C for 3 h demonstrated that it took 81 min for HCS+MM-MgH₂ to desorb 50% of hydrogen, while only 24 min was required for HCS+MM-Mg₈₀Al*₂₀. The apparent activation energy for dehydriding of MgH₂ was reduced from 144.3 ± 7.19 kJ/mol for HCS+MM-MgH₂ to 118.8 ± 6.73 kJ/mol for HCS+MM-Mg₉₀Al*₁₀. The scanning electron microscopy/energy dispersive spectroscopy and X-ray photoelectron spectroscopy analyses revealed that the Al* in situ generated from the hydriding of Al₁₂Mg₁₇ in HCS would significantly decrease dehydriding temperature and improve dehydriding kinetics of the MgH₂-Al composites because of a uniform distribution, the shortening reaction diffusion length, providing unoxidized surfaces to react with MgH₂ and the high chemical activity of Al*.