195 research outputs found
Universal phase transitions of B1 structured stoichiometric transition-metal carbides
The high-pressure phase transitions of B1-structured stoichiometric
transition metal carbides (TMCs, TM=Ti, Zr, Hf, V, Nb, and Ta) were
systematically investigated using ab initio calculations. These carbides
underwent universal phase transitions along two novel phase-transition routes,
namely, B1\rightarrowdistorted TlI (TlI')\rightarrowTlI and/or
B1\rightarrowdistorted TiB (TiB')\rightarrowTiB, when subjected to pressures.
The two routes can coexist possibly because of the tiny enthalpy differences
between the new phases under corresponding pressures. Four new phases result
from atomic slips of the B1-structured parent phases under pressure. After
completely releasing the pressure, taking TiC as a representative of TMCs, only
its new TlI'-type phase is mechanically and dynamically stable, and may be
recovered.Comment: [email protected]
Hydrogen storage systems from waste Mg alloys
The production cost of materials for hydrogen storage is one of the major issues to be addressed in order to consider them suitable for large scale applications. In the last decades several authors reported on the hydrogen sorption properties of Mg and Mg-based systems. In this work magnesium industrial wastes of AZ91 alloy and Mg-10 wt.% Gd alloy are used for the production of hydrogen storage materials. The hydrogen sorption properties of the alloys were investigated by means of volumetric technique, in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and calorimetric methods. The measured reversible hydrogen storage capacity for the alloys AZ91 and Mg-10 wt.% Gd are 4.2 and 5.8 wt.%, respectively. For the Mg-10 wt.% Gd alloy, the hydrogenated product was also successfully used as starting reactant for the synthesis of Mg(NH2)2 and as MgH2 substitute in the Reactive Hydride Composite (RHC) 2LiBH4 + MgH 2. The results of this work demonstrate the concrete possibility to use Mg alloy wastes for hydrogen storage purposes
Phase transition kinetics of superionic H 2 O ice phases revealed by Megahertz X-ray free-electron laser-heating experiments
H2O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H2O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H2O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers
Stellar Populations in the Galactic Center
We discuss the stellar content of the Galactic Center, and in particular,
recent estimates of the star formation rate (SFR). We discuss pros and cons of
the different stellar tracers and focus our attention on the SFR based on the
three classical Cepheids recently discovered in the Galactic Center. We also
discuss stellar populations in field and cluster stars and present some
preliminary results based on near-infrared photometry of a field centered on
the young massive cluster Arches. We also provide a new estimate of the true
distance modulus to the Galactic Center and we found
14.490.02(standard)0.10(systematic) mag (7.91 kpc).
Current estimate agrees quite well with similar photometric and kinematic
distance determinations available in the literature. We also discuss the
metallicity gradient of the thin disk and the sharp change in the slope when
moving across the edge of the inner disk, the Galactic Bar and the Galactic
Center. The difference becomes even more compelling if we take into account
that metal abundances are based on young stellar tracers (classical Cepheids,
Red Supergiants, Luminous Blue Variables). Finally, we briefly outline the
possible mechanisms that might account for current empirical evidence.Comment: To be published in the Astrophysics and Space Science Proceeding
Trends in liver transplantation for primary biliary cirrhosis in the Netherlands 1988-2008
Background: A decrease in the need for liver transplantations (LTX) in Primary Biliary Cirrhosis (PBC), possibly related to treatment with ursodeoxycholic acid (UDCA), has been reported in the USA and UK. The aim of this study was to assess LTX require
X-ray free electron laser heating of water and gold at high static pressure
The study of water at high pressure and temperature is essential for understanding planetary interiors but is hampered by the high reactivity of water at extreme conditions. Here, indirect X-ray laser heating of water in a diamond anvil cell is realized via a gold absorber, showing no evidence of reactivity
HST Paschen-alpha Survey of the Galactic Center: Data Reduction and Products
Our HST/NICMOS Pa survey of the Galactic center (GC) provides a uniform,
panoramic, high-resolution map of stars and ionized diffuse gas in the central
416 arcmin^2 of the Galaxy. This survey was carried out with 144 HST orbits
using two narrow-band filters at 1.87 and 1.90 micron in NICMOS Camera 3. In
this paper, we describe in detail the data reduction and mosaicking procedures
followed, including background level matching and astrometric corrections. We
have detected ~570,000 near-IR sources and are able to quantify photometric
uncertainties of the detections. The source detection limit varies across the
survey field but the typical 50% completion limit is ~17th mag (Vega System) in
the 1.90 micron band. A comparison with the expected stellar magnitude
distribution shows that these sources are primarily Main-Sequence massive stars
(>7M) and evolved lower mass stars at the distance of the GC. In particular,
the observed source magnitude distribution exhibits a prominent peak, which
could represent the Red Clump stars within the GC. The observed magnitude and
color of these RC stars support a steep extinction curve in the near-IR toward
the GC. The flux ratios of our detected sources in the two bands also allow for
an adaptive and statistical estimate of extinction across the field. With the
subtraction of the extinction-corrected continuum, we construct a net Pa
emission map and identify a set of Pa-emitting sources, which should mostly be
evolved massive stars with strong stellar winds. The majority of the identified
Pa point sources are located within the three known massive GC stellar
clusters. However, a significant fraction of our Pa-emitting sources are
located outside the clusters and may represent a new class of `field' massive
stars, many of which may have formed in isolation and/or in small groups. The
maps and source catalogues presented here are available electronically.Comment: Accepted for publication in MNRA
Considerations for management strategy evaluation for small pelagic fishes
Management strategy evaluation (MSE) is the state-of-the-art approach for testing and comparing management strategies in a way that accounts for multiple sources of uncertainty (e.g. monitoring, estimation, and implementation). Management strategy evaluation can help identify management strategies that are robust to uncertainty about the life history of the target species and its relationship to other species in the food web. Small pelagic fish (e.g. anchovy, herring and sardine) fulfil an important ecological role in marine food webs and present challenges to the use of MSE and other simulation-based evaluation approaches. This is due to considerable stochastic variation in their ecology and life history, which leads to substantial observation and process uncertainty. Here, we summarize the current state of MSE for small pelagic fishes worldwide. We leverage expert input from ecologists and modellers to draw attention to sources of process and observation uncertainty for small pelagic species, providing examples from geographical regions where these species are ecologically, economically and culturally important. Temporal variation in recruitment and other life-history rates, spatial structure and movement, and species interactions are key considerations for small pelagic fishes. We discuss tools for building these into the MSE process, with examples from existing fisheries. We argue that model complexity should be informed by management priorities and whether ecosystem information will be used to generate dynamics or to inform reference points. We recommend that our list of considerations be used in the initial phases of the MSE process for small pelagic fishes or to build complexity on existing single-species models.publishedVersio
Novel experimental setup for megahertz X-ray diffraction in a diamond anvil cell at the High Energy Density (HED) instrument of the European X-ray Free-Electron Laser (EuXFEL)
The high-precision X-ray diffraction setup for work with diamond anvil cells (DACs) in interaction chamber 2 (IC2) of the High Energy Density instrument of the European X-ray Free-Electron Laser is described. This includes beamline optics, sample positioning and detector systems located in the multipurpose vacuum chamber. Concepts for pump-probe X-ray diffraction experiments in the DAC are described and their implementation demonstrated during the First User Community Assisted Commissioning experiment. X-ray heating and diffraction of Bi under pressure, obtained using 20 fs X-ray pulses at 17.8 keV and 2.2 MHz repetition, is illustrated through splitting of diffraction peaks, and interpreted employing finite element modeling of the sample chamber in the DAC
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