High Pressure Quenched Glasses: unique structures and properties.

Zr-based metallic glasses are prepared by quenching supercooled liquid under pressure. These glasses are stable in ambient conditions after decompression. The High Pressure Quenched glasses have a distinct structure and properties. The pair distribution function shows redistribution of the Zr-Zr interatomic distances and their shift towards smaller values. These glasses exhibit higher density, hardness, elastic modulus, and yield stress. Upon heating at ambient pressure, they show volume expansion and distinct relaxation behavior, reaching an equilibrated state above the glass transition. These experimental results are consistent with an idea of pressure-induced low to high density liquid transition in the supercooled melt.

Effect of interaction of external surfaces on the symmetry and lattice distortion of CdSe nanocrystals by molecular dynamics simulations.

The effect of interaction of low-index atomic planes, (100), (110), and (111) terminating CdSe platelet nanocrystals is examined using molecular dynamics (MD) simulations. Asymmetry of the environment of atoms at the end surface layers leads to anisotropic deformation of the cubic lattice and to a relative shift of Cd and Se sub-lattices. Interference of distortions of the crystal lattice originating at the terminal surfaces leads to changes of symmetry of the CdSe lattice in the whole sample volume. In the models, 2-3 nm thick, for all types of surfaces under examination, the initial cubic lattice symmetry gets lost in the whole sample volume. Graphical abstractᅟ.

Pressure Induced Liquid-to-Liquid Transition in Zr-based Supercooled Melts and Pressure Quenched Glasses.

Through high-energy x-ray diffraction and atomic pair density function analysis we find that Zr-based metallic alloy, heated to the supercooled liquid state under hydrostatic pressure and then quenched to room temperature, exhibits a distinct glassy structure. The PDF indicates that the Zr-Zr distances in this glass are significantly reduced compared to those quenched without pressure. Annealing at the glass transition temperature at ambient pressure reverses structural changes and the initial glassy state is recovered. This result suggests that pressure causes a liquid-to-liquid phase transition in this metallic alloy supercooled melt. Such a pressure induced transition is known for covalent liquids, but has not been observed for metallic liquids. The High Pressure Quenched glasses are stable in ambient conditions after decompression.

Stacking faults in SiC nanowires.

SiC nanowires were obtained by a reaction between vapor silicon and multiwall carbon nanotubes, CNT, in vacuum at 1200 degrees C. Raman and IR spectrometry, X-ray diffraction and high resolution transmission electron microscopy, HRTEM, were used to characterize properties of SiC nanowires. Morphology and chemical composition of the nanowires was similar for all samples, but concentration of structural defects varied and depended on the origin of CNT. Stacking faults were characterized by HRTEM and Raman spectroscopy, and both techniques provided complementary results. Raman microscopy allowed studying structural defects inside individual nanowires. A thin layer of amorphous silicon carbide was detected on the surface of nanowires.

TiC nanocrystal formation from carburization of laser-grown Ti/O/C nanopowders for nanostructured ceramics.

Refractory carbide ceramics (TiC and ZrC) raise interest as promising materials for high-temperature applications such as structural materials for the future generation of nuclear reactors. In this context, nanostructured ceramics are expected to exhibit improved thermomechanical properties as well as better behavior under irradiation when compared to conventional materials. It is therefore necessary to synthesize carbide nanocrystals of such materials to elaborate the ceramics. We report here the formation study of TiC nanocrystals through the direct carburization of Ti/O/C nanopowders grown by laser pyrolysis. A spray of titanium tetraisopropoxide was laser pyrolyzed with ethylene as the sensitizer, leading to Ti/O/C nanopowders with various C contents controlled by the synthesis conditions. Annealing treatments performed on these nanopowders under an inert atmosphere without any C addition enabled the formation of TiC grains through the carburization of the oxide phase by free C incorporated during the synthesis. The powders were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The final TiC grain size was about 80 nm, and the grains were monocrystalline. The influence of the free C content on the grain growth during the annealing step, together with its effects on the densification of the ceramics after sintering by high-pressure flash sintering, was examined. A 93% densification was finally achieved.