Size-dependent mechanical properties of Mg nanoparticles used for hydrogen storage

Magnesium (Mg) hydride is a promising hydrogen storage material, yet its application has been limited by the slow hydrogen sorption kinetics. Recently, Mg nanoparticles have shown significant improvement of hydrogen storage properties in terms of dimensional stability upon cycling with the trend that the smaller the particle, the better the sorption kinetics. Since the volume change during sorption generates stress, leading to plastic deformation, the fundamentals of the mechanical deformation of the Mg particles are a significant issue. By using in situ transmission electron microscope compression tests and atomistic simulations on Mg nanoparticles, it was observed that deformation in the larger particles was dominated by the nucleation of ⟨a⟩-type dislocations from stress concentrations at the contact surface, while the smaller particles deformed more homogeneously with greater distribution of multiple types of dislocation sources. Importantly, this improvement of plastic deformation with decrease in size is orientation-independent. First-principles calculations suggest that this improved plasticity can be explained by the nearly-isotropic ideal shear strength for Mg, which becomes more important in smaller nanoparticles. As a result, the smaller Mg nanoparticles demonstrated better plastic stability to accommodate volume change upon hydrogen storage cycling.