Crystallographic and Material Characteristics

Under increasing hydrogen pressure, substantial changes occnr in the Mg-H system. Bastide et al. [5] investigated the behavior of MgH phase nnder high pressures up to 80 kbar and found that at ambient temperature (20°C) and 80 kbar of 

Varin et ai., Nanomaterials for Solid State Hydrogen Storage, 

Phase Composition at.% H Space group Strukturbericht designation Prototype 

The estimated value of -67 nm (no lattice strains) [6] is at the border of nanocrystallinity if one defines it as the grain size smaller than 100 nm [7]. Our result correlates very well with the grain size of 78 nm reported very recently by Kojima et al. [8] for their commercial MgH. Both resnlts suggest that certain commercial varieties of MgH could be subjected to either ball milling or other type of postdeformation in the proprietary mannfactnring process, which results in the final nearly nanosize grains (crystallites). 

The second type of MgH powder was purchased from ABCR GmbH Co. KG, sold under the trade name MG-5026. Its average purity claimed by the supplier is 98% (remaining Mg). For simplicity, it will be referred to hereafter as the ABCR powder. Volumetric desorption tests were very reproducible, giving the average value of desorbed hydrogen equal to 7.5 wt.% which is nearly identical to the theoretical purity-corrected capacity of 7.51 at 98% purity. This testifies to 

and 81.5% only at 4.8 MPa [16], In absorption process, the reaction of magnesium with hydrogen is a nucleation and growth mechanism where the nucle-ation rate is pressure dependent. They estimated the enthalpy and corresponding entropy of MgH2 formation as -70.0 kJ/mol and -126 J/mol K, respectively. 

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