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.
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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.
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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.
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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.
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5) High Pressure effects on strong correlated electronic system (CMR/GMR, Superconductor et. al.)
Strong correlated system is the core of modern condensed matter physics. It has been found that a strong coupling exists among the lattice, spin, and electronic degree of freedom that is manifested by complex phase diagrams. The physical properties of depend strongly on subtle changes in the structure and chemistry of the system induced by changing the ion size, bond distance and bond angles, in which chemical doping, low/high temperature and external magnetic field are traditional ways. Now with the development and application of high pressure technology in this field, many more new materials have been created and novel properties have been found, which have prominently improved human’s life.
As a typical CMR system, high-pressure resistivity and x-ray diffraction measurements were conducted on LaxMnO3, x=0.75, 0.85 and La0.33Ca0.67MnO3 [Z. Chen et. al., J. Magn. Magn. Mater., 322, 3049-3052 (2010)].
Pressure can covert a semiconductor BiTeI into a “topological insulator” (TI), a state of matter in which a material’s interior insulates but its edges or surfaces conduct.[Xi and Chen et. al., Phys. Rev. Lett. 111, 155701(2013)].
6) High pressure synchrotron technology development
Synchrotron-based high pressure is the key way to solve the correlations of structures and properties. Many technologies like XRD, x-ray absorption, total scattering, x-ray image, IR/UV spectroscopy, inelastic scattering have been developed and under development. Synchrotron itself also needs to be upgraded with beam brilliance, beam focusing, broader energy range and precise energy calibration. Accurate X-ray energy calibration is indispensable for X-ray energy-sensitive scattering and diffraction experiments, but there is still a lack of effective methods to precisely calibrate energy over a wide range, especially when normal transmission monitoring is not an option and complicated micro-focusing optics are fixed in place.
We proposed a new method of precise and fast absolute X-ray energy calibration over a wide energy range using an iterative X-ray diffraction based method [Hong, and Chen, Review of Scientific Instruments, 83, 063901-10 (2012)], which can be readily varied with high precision on the order of 10-5-10-6 spatial resolution using gauge blocks.