160 research outputs found
Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications
Based on the arrangement of two-dimensional 'melon', we construct a unit cell
for polymeric carbon nitride (PCN) synthesized via thermal polycondensation,
whose theoretical diffraction powder pattern includes all major features
measured in x-ray diffraction. With the help of this unit cell, we describe
the process-temperature-induced crystallographic changes in PCN that occur
within a temperature interval between 510 and 610 °C. We also discuss further
potential modifications of the unit cell for PCN. It is found that both
triazine- and heptazine-based g-C3N4 can only account for minor phases within
the investigated synthesis products
Clinical microbiological case: a 35-year-old HIV-positive man with intermittent fever and chronic diarrhea
An optical quasimonomer
A comprehensive investigation of the luminescent properties of carbon nitride
polymers, based on tri-s-triazine units, has been conducted. Steady-state
temperature- and excitation-power-dependent as well as time-resolved
measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong
photoluminescence, covering the visible spectrum. The spectral, thermal, and
temporal features of the photoluminescence can be satisfactorily described by
the excitation and radiative recombination of molecular excitons, localized at
single tri-s-triazine units. The discussed model is in accordance with the
recently reported absorption features of carbon nitride polymers. Thus, from
the point of view of optical spectroscopy, the material effectively behaves as
a monomer
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Photophysics of polymeric carbon nitride: An optical quasimonomer
A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-s-triazine units, has been conducted. Steady-state temperature- and excitation-power-dependent as well as time-resolved measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-s-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer
Structure and properties of a novel fulleride Sm6C60
A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory.Comment: 9 pages, 3 figures, submitted to Phys. Rev. B (March 12, 1999
Orbital character of O 2p unoccupied states near the Fermi level in CrO2
The orbital character, orientation, and magnetic polarization of the O 2
unoccupied states near the Fermi level () in CrO was determined using
polarization-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic
circular dichroism (XMCD) from high-quality, single-crystal films. A sharp peak
observed just above is excited only by the electric field vector () normal to the tetragonal -axis, characteristic of a narrow band
( 0.7 eV bandwidth) constituted from O 2 orbitals perpendicular to
(O 2) hybridized with Cr 3 states. By comparison
with band-structure and configuration-interaction (CI) cluster calculations our
results support a model of CrO as a half-metallic ferromagnet with large
exchange-splitting energy ( 3.0 eV) and
substantial correlation effects.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. B Rapid
Com
Transcatheter arterial embolisation in upper gastrointestinal bleeding in a sample of 29 patients in a gastrointestinal referral center in Germany
Sub-surface Oxygen and Surface Oxide Formation at Ag(111): A Density-functional Theory Investigation
To help provide insight into the remarkable catalytic behavior of the
oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface
oxygen, and structures involving both on-surface and sub-surface oxygen, as
well as oxide-like structures at the Ag(111) surface have been studied for a
wide range of coverages and adsorption sites using density-functional theory.
Adsorption on the surface in fcc sites is energetically favorable for low
coverages, while for higher coverage a thin surface-oxide structure is
energetically favorable. This structure has been proposed to correspond to the
experimentally observed (4x4) phase. With increasing O concentrations, thicker
oxide-like structures resembling compressed Ag2O(111) surfaces are
energetically favored. Due to the relatively low thermal stability of these
structures, and the very low sticking probability of O2 at Ag(111), their
formation and observation may require the use of atomic oxygen (or ozone, O3)
and low temperatures. We also investigate diffusion of O into the sub-surface
region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the
adsorption of atomic oxygen and ozone-like species. The present studies,
together with our earlier investigations of on-surface and
surface-substitutional adsorption, provide a comprehensive picture of the
behavior and chemical nature of the interaction of oxygen and Ag(111), as well
as of the initial stages of oxide formation.Comment: 17 pages including 14 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Quantitative ultrasound tissue characterization in shoulder and thigh muscles – a new approach
BACKGROUND: The echogenicity patterns of ultrasound scans contain information of tissue composition in muscles. The aim was: (1) to develop a quantitative ultrasound image analysis to characterize tissue composition in terms of intensity and structure of the ultrasound images, and (2) to use the method for characterization of ultrasound images of the supraspinatus muscle, and the vastus lateralis muscle. METHODS: Computerized texture analyses employing first-order and higher-order grey-scale statistics were developed to objectively characterize ultrasound images of m. supraspinatus and m. vastus lateralis from 9 healthy participants. RESULTS: The mean grey-scale intensity was higher in the vastus lateralis muscle (p < 0.05) than in the supraspinatus muscle (average value of middle measuring site 51.4 compared to 35.0). Furthermore, the number of spatially connected and homogeneous regions (blobs) was higher in the vastus lateralis (p < 0.05) than in the supraspinatus (average for m. vastus lateralis: 0.092 mm(-2 )and for m. supraspinatus: 0.016 mm(-2)). CONCLUSION: The higher intensity and the higher number of blobs in the vastus lateralis muscle indicates that the thigh muscle contained more non-contractile components than the supraspinatus muscle, and that the muscle was coarser. The image analyses supplemented each other and gave a more complete description of the tissue composition in the muscle than the mean grey-scale value alone
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