The metallization of hydrogen directly would require pressure in excess of 400 gigapascals (GPa), out of the reach of present experimental techniques. The dense group IVa hydrides attract ...considerable attention because hydrogen in these compounds is chemically precompressed and a metallic state is expected to be achievable at experimentally accessible pressures. We report the transformation of insulating molecular silane to a metal at 50 GPa, becoming superconducting at a transition temperature of Tc = 17 kelvin at 96 and 120 GPa. The metallic phase has a hexagonal close-packed structure with a high density of atomic hydrogen, creating a three-dimensional conducting network. These experimental findings support the idea of modeling metallic hydrogen with hydrogen-rich alloy.
The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H−TaS2) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and ...magnetic transport properties. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field Hc1 data, the temperature dependence of the Hc1 can be well described by a single-gap anisotropic s-wave order parameter.
Abstract
A comprehensive analysis of the state of air inside an industrial workshop with electroplating production was carried out. The data of quantitative distribution of suspended particles by ...size fractions (PM
0.3
, PM
0.5
, PM
1
, PM
3
, PM
5
, PM
10
) are presented for 15 main processes of electroplating. Morphometric and chemical composition of the surface of particles were studied. We observed particles of rounded shape, various agglomerates with complex geometric shapes, acute-angular particles, which when inhaled pose a maximum threat to human health. Chemical analysis of these particles showed an absolute predominance of oxides of non-ferrous metals, the percentage of which varied depending on the type of electroplating bath. The content of highly hazardous substances of the 1st (Zn, Pb, and Cd) and the 2nd (Cu, Cr, Ni, Co, and Mo) hazard classes in each sample was recorded.
Carbon dioxide (CO2) ice clouds have been routinely observed in the
middle atmosphere of Mars. However, there are still uncertainties concerning
physical mechanisms that control their altitude, ...geographical, and seasonal
distributions. Using the Max Planck Institute Martian General Circulation
Model (MPI-MGCM), incorporating a state-of-the-art whole atmosphere
subgrid-scale gravity wave parameterization (Yiğit et al., 2008), we
demonstrate that internal gravity waves generated by lower atmospheric
weather processes have a wide-reaching impact on the Martian climate.
Globally, GWs cool the upper atmosphere of Mars by ∼10 % and
facilitate high-altitude CO2 ice cloud formation. CO2 ice
cloud seasonal variations in the mesosphere and the mesopause region
appreciably coincide with the spatio-temporal variations of GW effects,
providing insight into the observed distribution of clouds. Our results
suggest that GW propagation and dissipation constitute a necessary physical
mechanism for CO2 ice cloud formation in the Martian upper atmosphere
during all seasons.
The stress-strain state of the rubber tire of the track roller of the caterpillar mover is determined when considering the problem with an absolutely rigid rim and with an elastic rim. The effect of ...the rim elasticity on the radial stiffness of the track roller, on the distribution of contact pressure, and also on the distribution of the specific strain energy, which is a criterion for the fatigue strength of rubber, is shown. Two cases of the interaction of the track roller with the track are considered. The first case corresponds to the location of the axis of the track roller parallel to the chain link, the second case, when the plane of the track is located at an angle relative to the axis of the track roller. The influence of the angle of inclination of the support surface on the stress-strain state of the external rubber tire of the support roller is shown. The mechanical properties of a rubber tire are described by the elastic Treloar potential, the rim material of the track roller is considered linear elastic material.
The results of a study on the deactivation of an industrial Co–Al
2
O
3
/SiO
2
catalyst under the conditions of highly productive Fischer–Tropsch synthesis of long-chained hydrocarbons at high ...pressure (6 MPa) are presented. It was found that an increase in the synthesis temperature led to a decrease in the rate of catalyst deactivation. The effect of the thermal agglomeration of cobalt particles and the carbonization of catalyst surface on the activity of the catalyst under the test conditions was insignificant. A correlation between the rate of catalyst deactivation and the selectivity for С
19+
hydrocarbons was found; this correlation indicated the blocking of chain growth centers by synthesized waxes as the main reason for the rapid loss of catalyst activity.
ABSTRACT
We examine the long-term (rest-frame time-scales from a few months to ∼20 yr) X-ray variability of a sample of 2344 X-ray bright quasars from the Sloan Digital Sky Survey (SDSS) data release ...14 quasar (DR14Q) catalogue, based on the data of the Spectrum-Roentgen-Gamma (SRG)/eROSITA All-Sky Survey complemented for ∼7 per cent of the sample by archival data from the XMM–Newton Serendipitous Source Catalogue. We characterize variability by a structure function, SF2(Δt). We confirm the previously known anticorrelation of the X-ray variability amplitude with luminosity. We also study the dependence of X-ray variability on black hole mass, MBH, and on an X-ray-based proxy of the Eddington ratio, λX. Less massive black holes prove to be more variable for given Eddington ratio and time-scale. X-ray variability also grows with decreasing Eddington ratio and becomes particularly strong at λX of less than a few per cent. We confirm that the X-ray variability amplitude increases with increasing time-scale. The SF2(Δt) dependence can be satisfactorily described by a power law, with the slope ranging from ∼0 to ∼0.4 for different (MBH, λX) subsamples (except for the subsample with the lowest black hole mass and Eddington ratio, where it is equal to 1.1 ± 0.4).
Using incoherent multiple Andreev reflection effect spectroscopy of planar break-junctions, we have studied the superconducting gap structure of CaKFe
4
As
4
pnictides (the 1144 family), those ...turning to the superconducting state in the stoichiometric composition with critical temperature
K. We have shown a coexistence of three bulk superconducting order parameters
, and directly determined their magnitudes and corresponding characteristic ratios
, and 1.3 at
. We compare the gap structure of CaKFe
4
As
4
and sister pnictides of the 1144 and 122 families studied by us earlier.
The Diels–Alder reaction is a cornerstone of modern organic synthesis. Despite this, it remains essentially inaccessible to biosynthetic approaches. Only a few natural enzymes catalyze even a formal ...4 + 2 cycloaddition, and it remains uncertain if any of them proceed via the Diels–Alder mechanism. In this study, we focus on the 4 + 2 cycloaddition step in the biosynthesis of spinosyn A, a reaction catalyzed by SpnF enzyme, one of the most promising “true Diels–Alderase” candidates. The four currently proposed mechanisms (including the Diels–Alder one) for this reaction in water (as a first-order approximation of the enzymatic reaction) are evaluated by an exhaustive quantum mechanical search for possible transition states (728 were found in total). We find that the line between the recently proposed bis-pericyclic J. Am. Chem. Soc. 2016, 138 (11), 3631 and Diels–Alder routes is blurred, and favorable transition states of both types may coexist. Application of the Curtin–Hammett principle, however, reveals that the bis-pericyclic mechanism accounts for ∼83% of the reaction flow in water, while the classical Diels–Alder mechanism contributes only ∼17%. The current findings provide a route for modeling this reaction inside the SpnF active site and inferring the catalytic architecture of possible Diels–Alderases.
The Atmospheric Chemistry Suite (ACS) package is an element of the Russian contribution to the ESA-Roscosmos ExoMars 2016 Trace Gas Orbiter (TGO) mission. ACS consists of three separate infrared ...spectrometers, sharing common mechanical, electrical, and thermal interfaces. This ensemble of spectrometers has been designed and developed in response to the Trace Gas Orbiter mission objectives that specifically address the requirement of high sensitivity instruments to enable the unambiguous detection of trace gases of potential geophysical or biological interest. For this reason, ACS embarks a set of instruments achieving simultaneously very high accuracy (ppt level), very high resolving power (>10,000) and large spectral coverage (0.7 to 17 μm—the visible to thermal infrared range). The near-infrared (NIR) channel is a versatile spectrometer covering the 0.7–1.6 μm spectral range with a resolving power of ∼20,000. NIR employs the combination of an echelle grating with an AOTF (Acousto-Optical Tunable Filter) as diffraction order selector. This channel will be mainly operated in solar occultation and nadir, and can also perform limb observations. The scientific goals of NIR are the measurements of water vapor, aerosols, and dayside or night side airglows. The mid-infrared (MIR) channel is a cross-dispersion echelle instrument dedicated to solar occultation measurements in the 2.2–4.4 μm range. MIR achieves a resolving power of >50,000. It has been designed to accomplish the most sensitive measurements ever of the trace gases present in the Martian atmosphere. The thermal-infrared channel (TIRVIM) is a 2-inch double pendulum Fourier-transform spectrometer encompassing the spectral range of 1.7–17 μm with apodized resolution varying from 0.2 to 1.3 cm
−1
. TIRVIM is primarily dedicated to profiling temperature from the surface up to ∼60 km and to monitor aerosol abundance in nadir. TIRVIM also has a limb and solar occultation capability. The technical concept of the instrument, its accommodation on the spacecraft, the optical designs as well as some of the calibrations, and the expected performances for its three channels are described.
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