Compressibility of periodically twinned silicon carbide nanowires is studied using in situ high pressure X-ray diffraction. Twinned SiC nanowires displayed a bulk modulus of 316 GPa, similar to 20-40% higher than previously reported values for SiC of other morphologies. This finding provides direct evidence of a significant effect of twinned structures on the elastic properties of SiC on the nano scale and supports previous molecular dynamics simulations of twin boundary/stacking fault-induced strengthening. Both experiments and simulations indicate that nanoscale twinning is an effective pathway by which to tailor the mechanical properties of nanostructures. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We report on a comprehensive study of thermodynamic and mechanical properties as well as a bond-deformation mechanism on ultra-incompressible Re(2)N and Re(3)N. The introduction of nitrogen into the rhenium lattice leads to thermodynamic instability in Re(2)N at ambient conditions and enhanced incompressibility and strength for both rhenium nitrides. Rhenium nitrides, however, show substantially lower ideal shear strength than hard ReB(2) and superhard c-BN, suggesting that they cannot be intrinsically superhard. An intriguing soft "ionic bond mediated plastic deformation" mechanism is revealed to underline the physical origin of their unusual mechanical strength. These results suggest a need to reformulate the design concept of intrinsically superhard transition-metal nitrides, borides, and carbides.

We investigated the processing conditions of diamond/tungsten carbide (WC) composites using in situ synchrotron x-ray diffraction (XRD) and reactive sintering techniques at high pressure and high temperatures. The as-synthesized composites were characterized by synchrotron XRD, scanning electron microscopy, high-resolution transmission electron microscopy, and indentation hardness measurements. Through tuning of the reaction temperature and time, we produced fully reacted, well-sintered, and nanostructured diamond composites with Vickers hardness of about 55 GPa and the grain size of WC binding matrix smaller than 50 nm. A specific set of orientation relationships between WC and tungsten is identified to gain microstructural insight into the reaction mechanism between diamond and tungsten. (C) 2011 American Institute of Physics. [doi:10.1063/1.3570645]

The effect of pressure on the crystalline structure and superconducting transition temperature (T-c) of the 111-type Na1-xFeAs system using in situ high-pressure synchrotron X-ray powder diffraction and diamond anvil cell techniques is studied. A pressure-induced tetragonal to tetragonal isostructural phase transition was found. The systematic evolution of the FeAs4 tetrahedron as a function of pressure based on Rietveld refinements on the powder X-ray diffraction patterns was obtained. The nonmonotonic T-c(P) behavior of Na1-xFeAs is found to correlate with the anomalies of the distance between the anion (As) and the iron layer as well as the bond angle of As-Fe-As for the two tetragonal phases. This behavior provides the key structural information in understanding the origin of the pressure dependence of T-c for 111-type iron pnictide superconductors. A pressure-induced structural phase transition is also observed at 20 GPa.

Using density functional theory, we show that the long-believed transition-metal tetraborides (TB4) of tungsten and molybdenum are in fact triborides (TB3). This finding is supported by thermodynamic, mechanical, and phonon instabilities of TB4, and it challenges the previously proposed origin of superhardness of these compounds and the predictability of the generally used hardness model. Theoretical calculations for the newly identified stable TB3 structure correctly reproduce their structural and mechanical properties, as well as the experimental x-ray diffraction pattern. However, the relatively low shear moduli and strengths suggest that TB3 cannot be intrinsically stronger than c-BN. The origin of the lattice instability of TB3 under large shear strain that occurs at the atomic level during plastic deformation can be attributed to valence charge depletion between boron and metal atoms, which enables easy sliding of boron layers between the metal ones.

Among transition metal nitrides, tungsten nitrides possess unique and/or superior chemical, mechanical, and thermal properties. Preparation of these nitrides, however, is challenging because the incorporation of nitrogen into tungsten lattice is thermodynamically unfavorable at atmospheric pressure. To date, most materials in the W-N system are in the form of thin films produced by nonequilibrium processes and are often poorly crystallized, which severely limits their use in diverse technological applications. Here we report synthesis of tungsten nitrides through new approaches involving solid-state ion exchange and nitrogen degassing under pressure. We unveil a number of novel nitrides including hexagonal and rhombohedral W2N3. The final products are phase-pure and well-crystallized in bulk forms. For hexagonal W2N3, hexagonal WN, and cubic W3N4, they exhibit elastic properties rivaling or even exceeding cubic-BN. All four nitrides are prepared at a moderate pressure of 5 GPa, the lowest among high-pressure synthesis of transition metal nitrides, making it practically feasible for massive and industrial-scale production.

Hydrostatic pressure, as an alternative of chemical pressure to tune the crystal structure and physical properties, is a significant technique for novel function material design and fundamental research. In this article, we report the phase stability and visible light response of the organolead bromide perovskite, CH3NH3PbBr3 (MAPbBr(3)), under hydrostatic pressure up to 34 GPa at room temperature: Two phase transformations below 2 GPa (from Pm (3) over barm to Im (3) over bar, then to Pnma) and a reversible amorphization starting from about 2 GPa were observed, which could be attributed to the tilting of PbBr6 octahedra and destroying of long-range ordering of MA cations, respectively. The visible light response of MAPbBr3 to pressure was studied by in situ photoluminescence, electric resistance, photocurrent measurements and first-principle simulations. The anomalous band gap evolution during compression with red-shift followed by blue-shift is explained by the competition between compression effect and pressure-induced amorphization. Along with the amorphization process accomplished around 25 GPa, the resistance increased by 5 orders of magnitude while the system still maintains its semiconductor characteristics and considerable response to the visible light irradiation. Our results not only show that hydrostatic pressure may provide an applicable tool for the organohalide perovskites based photovoltaic device functioning as switcher or controller, but also shed light on the exploration of more amorphous organometal composites as potential light absorber.

Recent (sub)millimeter polarization observations of protoplanetary disks reveal toroidally aligned, effectively prolate dust grains large enough (at least ∼ 100 μm) to efficiently scatter millimeter light. The alignment mechanism for these grains remains unclear. We explore the possibility that gas drag aligns grains through gas-dust relative motion when the grain’s center of mass is offset from its geometric center, analogous to a badminton birdie’s alignment in flight. A simple grain model of two non-identical spheres illustrates how a grain undergoes damped oscillations from flow-induced restoring torques which align its geometric center in the flow direction relative to its center of mass. Assuming specular reflection and subsonic flow, we derive an analytical equation of motion for spheroids where the center of mass can be shifted away from the spheroid’s geometric center. We show that a prolate or an oblate grain can be aligned with the long axis parallel to the gas flow when the center of mass is shifted along that axis. Both scenarios can explain the required effectively prolate grains inferred from observations. Application to a simple disk model shows that the alignment timescales are shorter than or comparable to the orbital time. The grain alignment direction in a disk depends on the disk (sub-)structure and grain Stokes number (St) with azimuthal alignment for large St grains in sub-Kepler ian smooth gas disks and for small St grains near the gas pressure extrema, such as rings and gaps.

The suggestion by Zaritsky & Lin (ZL) that a vertical extension of the red clump feature (the VRC) in color-magnitude diagrams (CMDs) of the Large Magellanic Cloud is consistent with a significant population of foreground stars to the LMC that could account for the observed microlensing optical depth has been challenged by various investigators. We respond by (1) examining each of the challenges presented, to determine whether any or all of those arguments invalidate the claims made by ZL, and (2) presenting new photometric and spectroscopic data obtained in an attempt to resolve this issue. We systematically discuss why the objections raised so far do not unequivocally refute ZL's claim. We conclude that although the CMD data do not mandate the existence of a foreground population, they are entirely consistent with a foreground population associated with the LMC that contributes significantly (similar to 50%) to the observed microlensing optical depth. From our new data, we conclude that less than or similar to 40% of the VRC stars are young, massive red clump stars, because (1) synthetic CMDs created using the star formation history derived independently from Hubble Space Telescope data suggest that fewer than 50% of the VRC stars are young, massive red clump stars, (2) the angular distribution of the VRC stars is more uniform than that of the young (<1 Gyr) main-sequence stars, and (3) the velocity dispersion of the VRC stars in the region of the LMC examined by ZL, 18.4 +/- 2.8 km s(-1) (95% confidence limits), is inconsistent with the expectation for a young disk population. Each of these arguments is predicated on assumptions, and the conclusions are uncertain. Therefore, an exact determination of the contribution to the microlensing optical depth by the various hypothesized foreground populations, and the subsequent conclusions regarding the existence of halo MACHOs, requires a detailed knowledge of many complex astrophysical issues, such as the initial mass function, star formation history, and post-main-sequence stellar evolution.

The Transiting Exoplanet Survey Satellite (TESS) will search for planets transiting bright and nearby stars. TESS has been selected by NASA for launch in 2017 as an Astrophysics Explorer mission. The spacecraft will be placed into a highly elliptical 13.7-day orbit around the Earth. During its two-year mission, TESS will employ four wide-field optical CCD cameras to monitor at least 200,000 main-sequence dwarf stars with I-C less than or similar to 13 for temporary drops in brightness caused by planetary transits. Each star will be observed for an interval ranging from one month to one year, depending mainly on the star's ecliptic latitude. The longest observing intervals will be for stars near the ecliptic poles, which are the optimal locations for follow-up observations with the James Webb Space Telescope. Brightness measurements of preselected target stars will be recorded every 2 min, and full frame images will be recorded every 30 min. TESS stars will be 10-100 times brighter than those surveyed by the pioneering Kepler mission. This will make TESS planets easier to characterize with follow-up observations. TESS is expected to find more than a thousand planets smaller than Neptune, including dozens that are comparable in size to the Earth. Public data releases will occur every four months, inviting immediate community-wide efforts to study the new planets. The TESS legacy will be a catalog of the nearest and brightest stars hosting transiting planets, which will endure as highly favorable targets for detailed investigations.