1) Plastic and Elastic Deformation of Nanomaterials

How nanocrystals deform remains controversial. Various observations and mechanisms have been reported. It has been proposed that below a critical length scale dislocation activity gives way to GB sliding, diffusion, and grain rotation. However, our recent deformation experiments at high pressure on polycrystalline nickel suggest that dislocation activity is still operative in 3 nm crystals [B. Chen et al. Science 338, 1448 (2012)]. The observations of pressure-promoted texturing indicate that under high external pressures, dislocation activity can be extended down to a few-nanometer length scale. Are dislocation-mediated mechanisms operative in nano-ceramics? Why do the elastic moduli of nanocrystals vary with particle size? More explorations are expected.

2) High pressure Phase Transition of Nanocrystals (Size Effects and Mechanisms)

From previous studies, elevated phase transition pressures in smaller crystals have been found for some materials, and reduced phase transition pressure for some others. However, so far the reported studies either include very few particle sizes or the sample size fall in a narrow size range. For more systematic view on this issue, the size dependence of phase transition pressure of nanocrystalline materials sized in a wide range is expected.

It is generally believed that nanocrystals are nearly defect free. The deformation of nanocrystals is hence viewed to occur via homogeneous deformation mechanisms, and their phase transitions as involving a single nucleation site. However, recent studies of ours indicate that defect can be induced in very fine nanocrystals by external stress. The role of defects in high pressure phase transition remains to be explored.

3) High Pressure and High Temperature Rheology of Minerals

Understanding deformation of mineral phases is important for interpreting seismic anisotropy in Earth’s interior. The seismologic observations indicate that the Earth’s interior possess strong anisotropy due to the preferred orientation of the iron crystals and minerals. Rheological properties of minerals play an important role in geodynamics. Because of technical limitation, in-situ high pressure and high temperature rheology of minerals are poorly studied. We are making efforts on this.

4) High Pressure Metallization

In Feynman’s Lectures on Physics, he said “Some materials are electrical ‘conductors’–because their electrons are free to move about: others are ‘insulators’ (non-metals)–because their electrons are held tightly to atoms. We shall consider later how some of these properties come about, but that is a very complicated matter”. Even today it is still unclear why some substances are metals, while others are non-metals.

In recent decades, high pressure metallization has attracted a lot of interest, including the quest for metallic hydrogen, but the exploration on the metallization of nanomaterials and the effects of disorder is rare. We propose such study for better understanding what the metal is.