5,166 research outputs found
The high-pressure behavior of CaMoO4
We report a high-pressure study of tetragonal scheelite-type CaMoO4 up to 29
GPa. In order to characterize its high-pressure behavior, we have combined
Raman and optical-absorption measurements with density-functional theory
calculations. We have found evidence of a pressure-induced phase transition
near 15 GPa. Experiments and calculations agree in assigning the high-pressure
phase to a monoclinic fergusonite-type structure. The reported results are
consistent with previous powder x-ray-diffraction experiments, but are in
contradiction with the conclusions obtained from earlier Raman measurements,
which support the existence of more than one phase transition in the pressure
range covered by our studies. The observed scheelite-fergusonite transition
induces significant changes in the electronic band gap and phonon spectrum of
CaMoO4. We have determined the pressure evolution of the band gap for the low-
and high-pressure phases as well as the frequencies and pressure dependences of
the Raman-active and infrared-active modes. In addition, based upon
calculations of the phonon dispersion of the scheelite phase, carried out at a
pressure higher than the transition pressure, we propose a possible mechanism
for the reported phase transition. Furthermore, from the calculations we
determined the pressure dependence of the unit-cell parameters and atomic
positions of the different phases and their room-temperature equations of
state. These results are compared with previous experiments showing a very good
agreement. Finally, information on bond compressibility is reported and
correlated with the macroscopic compressibility of CaMoO4. The reported results
are of interest for the many technological applications of this oxide.Comment: 36 pages, 10 figures, 8 table
Temperature distribution profiles inside biomass under dielectric breakdown conditions
Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.Under the effect of a sufficiently strong electric field, all
materials suffer from a form of breakdown, which involves the
flow of current through them. Although wood is sometimes
utilized as an electrical insulator, it is also subject to breakdown
when exposed to high electric fields. In general, dielectric
breakdown is considered a negative effect for electrically
insulating materials since it implies the loss of insulating
properties of the material. However, the high temperatures
generated inside the material can be used as an efficient way to
induce the thermo-chemical decomposition of biomass with the
purpose of sustainable energy generation. A mathematical
model of the dynamics of temperature and electric field inside a
small piece of biomass is developed to study temperature
distribution and thermal instability growth under thermal
dielectric breakdown conditions. A two-dimensional model is
implemented for different electric field strengths with biomass
dielectric properties obtained from the literature. Temperature,
current and electric potential distributions have been analyzed
and reported for several cases. The temperature development
over time has also been analyzed and reported. The results
show that higher voltages lead to almost instantaneous thermal
breakdown. Similar results are obtained for AC voltage when
the frequency is decreased. These conditions are desired for
efficient gasification of biomass.dc201
The electronic structure of zircon-type orthovanadates: Effects of high-pressure and cation substitution
The electronic structure of four ternary-metal oxides containing isolated
vanadate ions is studied. Zircon-type YVO4, YbVO4, LuVO4, and NdVO4 are
investigated by high-pressure optical-absorption measurements up to 20 GPa.
First-principles calculations based on density-functional theory were also
performed to analyze the electronic band structure as a function of pressure.
The electronic structure near the Fermi level originates largely from molecular
orbitals of the vanadate ion, but cation substitution influence these
electronic states. The studied ortovanadates, with the exception of NdVO4,
undergo a zircon-scheelite structural phase transition that causes a collapse
of the band-gap energy. The pressure coefficient dEg/dP show positive values
for the zircon phase and negative values for the scheelite phase. NdVO4
undergoes a zircon-monazite-scheelite structural sequence with two associated
band-gap collapses.Comment: 35 pages, 11 figures, 2 Tables, 52 reference
A combined high-pressure experimental and theoretical study of the electronic band-structure of scheelite-type AWO4 (A = Ca, Sr, Ba, Pb) compounds
The optical-absorption edge of single crystals of CaWO4, SrWO4, BaWO4, and
PbWO4 has been measured under high pressure up to ~20 GPa at room temperature.
From the measurements we have obtained the evolution of the band-gap energy
with pressure. We found a low-pressure range (up to 7-10 GPa) where
alkaline-earth tungstates present a very small Eg pressure dependence (-2.1 <
dEg/dP < 8.9 meV/GPa). In contrast, in the same pressure range, PbWO4 has a
pressure coefficient of -62 meV/GPa. The high-pressure range is characterized
in the four compounds by an abrupt decrease of Eg followed by changes in
dEg/dP. The band-gap collapse is larger than 1.2 eV in BaWO4. We also
calculated the electronic-band structures and their pressure evolution.
Calculations allow us to interpret experiments considering the different
electronic configuration of divalent metals. Changes in the pressure evolution
of Eg are correlated with the occurrence of pressure-induced phase transitions.
The band structures for the low- and high-pressure phases are also reported. No
metallization of any of the compounds is detected in experiments nor is
predicted by calculations.Comment: 26 pages, 1 table, 6 figure
The structure of a polyketide synthase bimodule core
Polyketide synthases (PKSs) are predominantly microbial biosynthetic enzymes. They assemble highly potent bioactive natural products from simple carboxylic acid precursors. The most versatile families of PKSs are organized as assembly lines of functional modules. Each module performs one round of precursor extension and optional modification, followed by directed transfer of the intermediate to the next module. While enzymatic domains and even modules of PKSs are well understood, the higher-order modular architecture of PKS assembly lines remains elusive. Here, we visualize a PKS bimodule core using cryo-electron microscopy and resolve a two-dimensional meshwork of the bimodule core formed by homotypic interactions between modules. The sheet-like organization provides the framework for efficient substrate transfer and for sequestration of trans-acting enzymes required for polyketide production
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