594 research outputs found
First principles based atomistic modeling of phase stability in PMN-xPT
We have performed molecular dynamics simulations using a shell model
potential developed by fitting first principles results to describe the
behavior of the relaxor-ferroelectric (1-x)PbMg1/3Nb2/3O3-xPbTiO3 (PMN-xPT) as
function of concentration and temperature, using site occupancies within the
random site model. In our simulations, PMN is cubic at all temperatures and
behaves as a polar glass. As a small amount of Ti is added, a weak polar state
develops, but structural disorder dominates, and the symmetry is rhombohedral.
As more Ti is added the ground state is clearly polar and the system is
ferroelectric, but with easy rotation of the polarization direction. In the
high Ti content region, the solid solution adopts ferroelectric behavior
similar to PT, with tetragonal symmetry. The ground state sequence with
increasing Ti content is R-MB-O-MC-T. The high temperature phase is cubic at
all compositions. Our simulations give the slope of the morphotropic phase
boundaries, crucial for high temperature applications. We find that the phase
diagram PMN-xPT can be understood within the random site model.Comment: 27 pages, 9 figure
Carbon dioxide adsorption and interaction with formation fluids of Jordanian unconventional reservoirs
Shales are mostly unexploited energy resources. However, the extraction and production of their hydrocarbons require innovative methods. Applications involving carbon dioxide in shales could combine its potential use in oil recovery with its storage in view of its impact on global climate. The success of these approaches highly depends on various mechanisms taking place in the rock pores simultaneously. In this work, properties governing these mechanisms are presented at technically relevant conditions. The pendant and sessile drop methods are utilized to measure interfacial tension and wettability, respectively. The gravimetric method is used to quantify CO2 adsorption capacity of shale and gas adsorption kinetics is evaluated to determine diffusion coefficients. It is found that interfacial properties are strongly affected by the operating pressure. The oil-CO2 interfacial tension shows a decrease from approx. 21 mN/m at 0.1 MPa to around 3 mN/m at 20 MPa. A similar trend is observed in brine-CO2 systems. The diffusion coefficient is observed to slightly increase with pressure at supercritical conditions. Finally, the contact angle is found to be directly related to the gas adsorption at the rock surface: Up to 3.8 wt% of CO2 is adsorbed on the shale surface at 20 MPa and 60 °C where a maximum in contact angle is also found. To the best of the author’s knowledge, the affinity of calcite-rich surfaces toward CO2 adsorption is linked experimentally to the wetting
behavior for the first time. The results are discussed in terms of CO2 storage scenarios occurring optimally at 20 MPa
Fluoroscopy-guided procedures in cardiology: is patient exposure being reduced over time?
The number of fluoroscopy-guided procedures in cardiology is increasing over time and it is appropriate to wonder whether technological progress or change of techniques is influencing patient exposure. The aim of this study is to examine whether patient dose has been decreasing over the years. Patient dose data of more than 7700 procedures were collected from two cardiology centres. A steady increase in the patient dose over the years was observed in both the centres for the two cardiological procedures included in this study. Significant increase in dose was also observed after the installation of a flat-panel detector. The increasing use of radial access may lead to an increase in the patient exposure. The monitoring of dose data over time showed a considerable increase in the patient exposure over time. Actions have to be taken towards dose reduction in both the centre
Multimodal mate choice: exploring the effects of sight, sound, and scent on partner choice in a speed-date paradigm
Action Contro
Coexistence of the Critical Slowing Down and Glassy Freezing in Relaxor Ferroelectrics
We have developed a dynamical model for the dielectric response in relaxor
ferroelectrics which explicitly takes into account the coexistence of the
critical slowing down and glassy freezing. The application of the model to the
experiment in PMN allowed for the reconstruction of the nonequilibrium spin
glass state order parameter and its comparison with the results of recent NMR
experiment (Blinc et al., Phys. Rev. Lett. 83, No. 2 (1999)). It is shown that
the degree of the local freezing is rather small even at temperatures where the
field-cooled permittivity exceeds the frequency dependent permittivity by an
order of magnitude. This observation indicates the significant role of the
critical slowing down (accompanying the glass freezing) in the system dynamics.
Also the theory predicts an important interrelationship between the frequency
dependent permittivity and the zero-field-cooled permittivity, which proved to
be consistent with the experiment in PMN (A. Levstik et. al., Phys. Rev. B 57,
11204 (1998))
An audit of diagnostic reference levels in interventional cardiology and radiology: are there differences between academic and non-academic centres?
A wide variation in patient exposure has been observed in interventional radiology and cardiology. The purpose of this study was to investigate the patient dose from fluoroscopy-guided procedures performed in non-academic centres when compared with academic centres. Four procedures (coronary angiography, percutaneous coronary intervention, angiography of the lower limbs and percutaneous transluminal angioplasty of the lower limbs) were evaluated. Data on the dose-area product, fluoroscopy time and number of images for 1000 procedures were obtained from 23 non-academic centres and compared with data from 5 academic centres. No differences were found for cardiology procedures performed in non-academic centres versus academic ones. However, significantly lower doses were delivered to patients for procedures of the lower limbs when they were performed in non-academic centres. This may be due to more complex procedures performed in the academic centres. Comparison between the centres showed a great variation in the patient dose for these lower limb procedure
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
First-principles theory of ferroelectric phase transitions for perovskites: The case of BaTiO3
We carry out a completely first-principles study of the ferroelectric phase
transitions in BaTiO. Our approach takes advantage of two features of these
transitions: the structural changes are small, and only low-energy distortions
are important. Based on these observations, we make systematically improvable
approximations which enable the parameterization of the complicated energy
surface. The parameters are determined from first-principles total-energy
calculations using ultra-soft pseudopotentials and a preconditioned
conjugate-gradient scheme. The resulting effective Hamiltonian is then solved
by Monte Carlo simulation. The calculated phase sequence, transition
temperatures, latent heats, and spontaneous polarizations are all in good
agreement with experiment. We find the transitions to be intermediate between
order-disorder and displacive character. We find all three phase transitions to
be of first order. The roles of different interactions are discussed.Comment: 33 pages latex file, 9 figure
Next-generation muscle-directed gene therapy by in silico vector design
There is an urgent need to develop the next-generation vectors for gene therapy of muscle disorders, given the relatively modest advances in clinical trials. These vectors should express substantially higher levels of the therapeutic transgene, enabling the use of lower and safer vector doses. In the current study, we identify potent muscle-specific transcriptional cisregulatory modules (CRMs), containing clusters of transcription factor binding sites, using a genome-wide data-mining strategy. These novel muscle-specific CRMs result in a substantial increase in muscle-specific gene transcription (up to 400-fold) when delivered using adeno-associated viral vectors in mice. Significantly higher and sustained human micro-dystrophin and follistatin expression levels are attained than when conventional promoters are used. This results in robust phenotypic correction in dystrophic mice, without triggering apoptosis or evoking an immune response. This multidisciplinary approach has potentially broad implications for augmenting the efficacy and safety of muscle-directed gene therapy
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