2,634 research outputs found
Apparatus for purging systems handling toxic, corrosive, noxious and other fluids Patent
Fluid transferring system design for purging toxic, corrosive, or noxious fluids and fumes from materials handling equipment for cleansing and accident preventio
Influence of a magnetic field on the viscosity of a dilute gas consisting of linear molecules.
The viscomagnetic effect for two linear molecules, N2 and CO2, has been calculated in the dilute-gas limit directly from the most accurate ab initio intermolecular potential energy surfaces presently available. The calculations were performed by means of the classical trajectory method in the temperature range from 70 K to 3000 K for N2 and 100 K to 2000 K for CO2, and agreement with the available experimental data is exceptionally good. Above room temperature, where no experimental data are available, the calculations provide the first quantitative information on the magnitude and the behavior of the viscomagnetic effect for these gases. In the presence of a magnetic field, the viscosities of nitrogen and carbon dioxide decrease by at most 0.3% and 0.7%, respectively. The results demonstrate that the viscomagnetic effect is dominated by the contribution of the jj¯ polarization at all temperatures, which shows that the alignment of the rotational axes of the molecules in the presence of a magnetic field is primarily responsible for the viscomagnetic effect
Questioning the existence of a unique ground state structure for Si clusters
Density functional and quantum Monte Carlo calculations challenge the
existence of a unique ground state structure for certain Si clusters. For Si
clusters with more than a dozen atoms the lowest ten isomers are close in
energy and for some clusters entropic effects can change the energetic ordering
of the configurations. Isotope pure configurations with rotational symmetry and
symmetric configurations containing one additional isotope are disfavored by
these effects. Comparisons with experiment are thus difficult since a mixture
of configurations is to be expected at thermal equilibrium
Optimization of laser cutting processes using design of experiments
AbstractWe report on the optimization of laser cutting of thin Al2O3 ceramic layers using a design of experiment (DOE) approach. DOE allows to separate the most important influencing factors on the targeted cutting process, to clarify their interaction, to reduce the overall amount of parameter sets that need to be examined and to identify the optimized parameter regions, respectively. Using both, a continuous wave 500 W fiber laser and a 200W CO2 laser, we have optimized and compared the cutting of 250μm thin Al2O3 ceramic substrate layers applying a commercial DOE software. Our results demonstrate the potential of DOE to optimize laser material processes
Cross second virial coefficients and dilute gas transport properties of the (CH4 + CO2), (CH4 + H2S), and (H2S + CO2) systems from accurate intermolecular potential energy surfaces
The cross second virial coefficient and the dilute gas shear viscosity, thermal conductivity, and binary diffusion coefficient have been calculated for (CH4 + CO2), (CH4 + H2S), and (H2S + CO2) mixtures in the temperature range from (150 to 1200) K. The cross second virial coefficient was obtained using the Mayer-sampling Monte Carlo approach, while the transport properties were evaluated by means of the classical trajectory method. State-of-the-art intermolecular potential energy surfaces for the like and unlike species interactions were employed in the calculations. All potential energy surfaces are based on highly accurate quantum-chemical ab initio calculations, with the potentials for the unlike interactions reported in this work and those for the like interactions taken from our previous studies of the pure gases. The computed transport property values are in good agreement with the few available experimental data, which are limited to (CH4 + CO2) mixtures close to room temperature. The lack of reliable data makes the values of the thermophysical properties calculated in this work currently the most accurate estimates for low-density (CH4 + CO2), (CH4 + H2S), and (H2S + CO2) mixtures. Tables of recommended values for all investigated thermophysical properties as a function of temperature and composition are provided
Electronic band structure, Fermi surface, and elastic properties of new 4.2K superconductor SrPtAs from first-principles calculations
The hexagonal phase SrPtAs (s.g. P6/mmm; #194) with a honeycomb lattice
structure very recently was declared as a new low-temperature (TC ~ 4.2K)
superconductor. Here by means of first-principles calculations the optimized
structural parameters, electronic bands, Fermi surface, total and partial
densities of states, inter-atomic bonding picture, independent elastic
constants, bulk and shear moduli for SrPtAs were obtained for the first time
and analyzed in comparison with the related layered superconductor SrPt2As2.Comment: 8 pages, 4 figure
Dilute gas viscosity of n-alkanes represented by rigid Lennard-Jones chains
The shear viscosity in the dilute gas limit has been calculated by means of the classical trajectory method for a gas consisting of chain-like molecules. The molecules were modelled as rigid chains made up of spherical segments that interact through a combination of site-site Lennard-Jones 12-6 potentials. Results are reported for chains consisting of 2, 3, 4, 6, 8, 12 and 16 segments in the reduced temperature range of 0.3 – 50 for site-site separations of 0.25 , 0.333 , 0.40 , 0.60 and 0.80 , where is the Lennard-Jones length scaling parameter. The results were used to determine the shear viscosity of n-alkanes in the zerodensity limit by representing an n-alkane molecule as a rigid linear chain consisting of c − 1 spherical segments, where c is the number of carbon atoms. We show that for a given n-alkane molecule, the scaling parameters ε and σ are not unique and not transferable from one molecule to another. The commonly used site-site Lennard-Jones 12-6 potential in combination with a rigid-chain molecular representation can only accurately mimic the viscosity if the scaling parameters are fitted. If the scaling parameters are estimated from the scaling parameters of other n-alkanes, the predicted viscosity values have an unacceptably high uncertainty
Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach
We develop a first-principles scheme to calculate adiabatic and non-adiabatic
phonon frequencies in the full Brillouin zone. The method relies on the
variational properties of a force-constants functional with respect to the
first-order perturbation of the electronic charge density and on the
localization of the deformation potential in the Wannier function basis. This
allows for calculation of phonon dispersion curves free from convergence issues
related to Brillouin zone sampling. In addition our approach justify the use of
the static screened potential in the calculation of the phonon linewidth due to
decay in electron-hole pairs. We apply the method to the calculation of the
phonon dispersion and electron-phonon coupling in MgB and CaC. In both
compounds we demonstrate the occurrence of several Kohn anomalies, absent in
previous calculations, that are manifest only after careful electron and phonon
momentum integration. In MgB, the presence of Kohn anomalies on the
E branches improves the agreement with measured phonon spectra and
affects the position of the main peak in the Eliashberg function. In CaC we
show that the non-adiabatic effects on in-plane carbon vibrations are not
localized at zone center but are sizable throughout the full Brillouin zone.
Our method opens new perspectives in large-scale first-principles calculations
of dynamical properties and electron-phonon interaction.Comment: 18 pages, 8 figure
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