Hoda Emami

Optical Properties of Nanocrystalline Monoclinic Y2O3 Stabilized by Grain Size and Plastic Strain Effects via High-Pressure Torsion

Yttrium oxide (yttria) with monoclinic structure exhibits unique optical properties; however, the monoclinic phase is thermodynamically stable only at pressures higher than ∼16 GPa. In this study, the effect of grain size and plastic strain on the stability of monoclinic phase is investigated by a high-pressure torsion (HPT) method. A cubic-to-monoclinic phase transition occurs at 6 GPa, which is ∼10 GPa below the theoretical transition pressure. Microstructure analysis shows that monoclinic phase forms in nanograins smaller than ∼22 nm and its fraction increases with plastic strain, while larger grains have a cubic structure. The band gap decreases and the photoluminescence features change from electric dipole to mainly magnetic dipole without significant decrease in the photoluminescence intensity after formation of the monoclinic phase. It is also suggested that monoclinic phase formation is due to the enhancement of effective internal pressure in nanograins.

Solid-state reactions and hydrogen storage in magnesium mixed with various elements by high-pressure torsion: experiments and first-principles calculations

Magnesium hydride is widely known as an interesting candidate for solid-state hydrogen storage. However it is too stable and does not desorb hydrogen at ambient conditions. Although MgH2 suffers from slow kinetics, its hydrogenation kinetics can be significantly improved by addition of catalysts and/or decreasing the grain size. Reducing the thermodynamic stability of MgH2 is now the main challenging task. In this study, 21 different elements were added to magnesium in atomic scale by using the High-Pressure Torsion (HPT) technique and different kinds of nanostructured intermetallics and new metastable or amorphous phases were synthesized after HPT (Mg17Al12, MgZn, MgAg, Mg2In, Mg2Sn, etc.) or after post-HPT heat treatment (MgB2, Mg2Si, Mg2Ni, Mg2Cu, MgCo, Mg2Ge, Mg2Pd, etc.). In most of the compounds, the desorption temperature decreases by addition of elements, even though that the ternary hydrides are formed only in limited systems such as Mg–Ni and Mg–Co. Appreciable correlations were achieved between the theoretical binding energies obtained by first-principles calculations and the experimental dehydrogenation temperatures. These correlations can explain the effect of different elements on the hydrogenation properties of the Mg-based binary systems and the formation of ternary hydrides.