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*.