Thermodynamic Consideration on Multi-Step Hydrogenation of Mg<SUB>17</SUB>Al<SUB>12</SUB> Assisted by Phase Separation

Abstract

The multi-step hydrogenation and dehydrogenation on the pressure-composition-temperature relations for the Mg 17 Al 12 -H system was thermodynamically discussed on the based upon the enthalpy for Mg-Al alloy recombination during corresponding reaction with hydrogen. Pressure-composition-temperature diagrams were measured Mg 17 Al 12 -H system by means of the volumetric technique in the temperature range of 598-648 K. The isotherms show two step hydrogenation and dehydrogenation indicating a disproportionate formation of MgH 2 via Mg 2 Al 3 intermetallic phase. The decomposition of Mg from the intermetallic phase causes thermodynamic destabilisation for the formation of Mg hydrides. The molar enthalpies of hydride formation for the investigated systems are: ÁH f ¼ À38:5 AE 0:9 kJ (mol H ) À1 for the Mg-H system;À1 for the first-step hydrogenation in Mg 17 Al 12 ; ÁH f ¼ À31 AE 1 kJ (mol H ) À1 for the second-step hydrogenation in

Thermoelectric Properties of Mg2Si Produced by New Chemical Route and SPS

“…This behavior has been discovered on MgAl alloys [26,27] and was initially investigated for the purpose of hydrogen storage. In the presence of Si, the initial H 2 absorption by Mg is significantly lower than that of the pure Mg-H system, so the number of cycles needed for the formation of Mg 2 Si phase from Mg and Si phases is higher than for other Mg alloys, such as MgAl.…”

“…Hydrogen sorption [22][23][24][25][26][27] on Mg-alloys has been studied not only for the purpose of removing Oxygen from intermetallics, but also extensively over the last two decades for the purpose of finding an easy-to-use Hydrogen storage alloy in order to promote the usage of Hydrogen as alternative fuel for vehicles or energy storage. Besides FeTi [28], Pd [29,30], rare earth elements [25,31], Mg-Fe [32] and MgAl-alloys [26,27] have been considered.…”

“…Besides FeTi [28], Pd [29,30], rare earth elements [25,31], Mg-Fe [32] and MgAl-alloys [26,27] have been considered. On the other hand, Hydrogen causes severe embrittlement at grain boundaries in Al-alloys [33].…”

Application of Electron Microscopy to Investigation of Corrosion of Mg-Al Alloys in Various Electrolyte Solutions

“…In the case of AZ91 alloy, atomic hydrogen reacts with magnesium inside the Mg 17 Al 12 particles. This leads to the creation of magnesium hydride (MgH 2 ) as follows [6]:…”

“…Reaction ( 4) occurs in several stages. The Mg 2 Al 3 phase forms during the intermediate stage, but it reacts with hydrogen, thereby producing magnesium hydride and aluminium [6]. The magnesium hydride is incoherent with the Mg 17 Al 12 phase, and this phase would be brittle fractured when the inner stress produced by hydrogen pressure, and expansion stress due to the formation of magnesium hydride, is higher than the fracture strength.…”