Discovering a new MgH₂ metastable phase

Area of Science:

• Materials Science
• Nanotechnology
• Hydrogen Storage

Background:

• Magnesium hydride (MgH2) is a promising material for hydrogen storage.
• Existing MgH2 phases (beta and gamma) have limitations in storage capacity and kinetics.
• Mechanical deformation is explored to create novel MgH2 phases.

Purpose of the Study:

• To investigate the formation of new MgH2 phases through severe plastic deformation.
• To characterize the properties of any newly formed phases, focusing on hydrogen storage.
• To explore phase transformations in MgH2 under mechanical stress.

Main Methods:

• Mechanical alloying of MgH2 powders.
• Cold rolling to induce severe plastic deformation and micro-lathe formation.
• High-energy ball milling for extended periods (50 and 200 hours) to induce defects and phase transformations.
• X-ray diffraction to analyze crystal structure and phase identification.
• Hydrogen storage capacity and desorption kinetics measurements.

Main Results:

• Cold rolling produced micro-lathes of gamma- and beta-MgH2 phases.
• Long-term ball milling (50 hours) destabilized existing phases, forming a new face-centered cubic (fcc-MgH2) phase.
• The fcc-MgH2 phase has a lattice parameter of 0.4436 nm.
• This novel phase demonstrated a high hydrogen storage capacity (6.6 wt%) and rapid desorption kinetics (7 min at 275 °C).
• Cyclic phase transformations between beta, gamma, and fcc-MgH2 were observed with extended ball milling (200 hours).

Conclusions:

• A new metastable fcc-MgH2 phase was successfully synthesized via mechanical deformation and ball milling.
• The fcc-MgH2 phase offers significant improvements in hydrogen storage capacity and kinetics.
• This work demonstrates the potential of mechanical processing for designing advanced hydrogen storage materials.