Search results for "THERMODYNAMICS"
showing 10 items of 2774 documents
Revised Atomistic Models of the Crystal Structure of C–S–H with high C/S Ratio
2016
Abstract The atomic structure of calcium-silicate-hydrate (C1.67–S–H x ) has been studied. Atomistic C–S–H models suggested in our previous study have been revised in order to perform a direct comparison of energetic stability of the different structures. An extensive set of periodic structures of C–S–H with variation of water content was created, and then optimized using molecular dynamics with reactive force field ReaxFF and quantum chemical semiempirical method PM6. All models show organization of water molecules inside the structure of C–S–H. The new geometries of C–S–H, reported in this paper, show lower relative energy with respect to the geometries from the original definition of C–S…
Thermodynamic stability of stoichiometric LaFeO 3 and BiFeO 3 : a hybrid DFT study
2017
BiFeO3 perovskite attracts great attention due to its multiferroic properties and potential use as a parent material for Bi1−xSrxFeO3−δ and Bi1−xSrxFe1−yCoyO3−δ solid solutions in intermediate temperature cathodes of oxide fuel cells. Another iron-based LaFeO3 perovskite is the end member for well-known solid solutions (La1−xSrxFe1−yCoyO3−δ) used for oxide fuel cells and other electrochemical devices. In this study an ab initio hybrid functional approach was used for the study of the thermodynamic stability of both LaFeO3 and BiFeO3 with respect to decompositions to binary oxides and to elements, as a function of temperature and oxygen pressure. The localized (LCAO) basis sets describing th…
BacSr1−cTiO3 perovskite solid solutions: Thermodynamics from ab initio electronic structure calculations
2005
We suggest theoretical prediction for Ba"cSr"1"-"cTiO"3 perovskite solid solutions (BST) combining ab initio DFT/B3PW calculations and alloy thermodynamics. This approach is based on calculations of a series of ordered super-structures in Ba-Sr simple cubic sublattice immersed in the rest TiO"3 matrix. Although these structures are unstable with respect to the decomposition, the results of total energy calculations allow us to extract the necessary energy parameters and to calculate the phase diagram for the solid solutions (alloys). A novel approach applied to the BST system enables to predict that at T>400 K Ba and Sr atom distribution is random. But below this temperature at small c Ba a…
Periodic behaviour in heterogeneous chemical reactions
1992
Abstract The authors present an analytical and numerical analysis for a solid-gas oxidation process represented by a set of coupled reaction rates equations. The equations describe the time evolution of four elementary process that govern the overall heterogeneous kinetics. The description formation of a new oxide unit considers: (1) an internal interface (oxide-metal) reaction by which an activated complex is formed; (2) the dissolution of the complex produce a chemical element σ; (3) the diffusion of σ through the oxide layer; and (4) an external interface (oxide-gas) reaction. The results reported here delinate the parameter region where chemical oscillations are present.
Evidence for a surface self-cleaning sputtering mechanism in fast atom bombardment mass spectrometry
1985
Evidence for a surface self-cleaning sputtering mechanism in fast atom bombardment mass spectrometry involving significant sputtering from the bulk of the glycerol matrix has been obtained from (a) the time dependence of sample ion abundances and chemical noise for bioorganic compounds, (b) determinations of the sputtering volume in glycerol solution, and (c) studies of in situ chemical and biochemical reactions. The relevance of these results for optimal sample/matrix preparation in analytical applications is pointed out.
Simple variable-volume injector for flow-injection systems
1990
Abstract A simple and versatile variable-volume injector is described for the introduction of samples in flow-injection systems. The device delivers precisely and reproducibly (usually r.s.d.
Predicting mass fluxes in the pervaporation process using Maxwell-Stefan diffusion coefficients
2018
Abstract In the past decades, it has been proven that pervaporation is an effective and energy-efficient membrane process for the separation of liquids that are difficult to separate in classical processes. The demand for new process applications has increased the need for mass transfer simulation methods, considering the interactions between the system components and the influence of process parameters on the permeation fluxes and at the same time requiring as few experiments as possible. The aim of the study was to find out whether the calculation of mass fluxes of multicomponent fluids based on the system of generalized Maxwell-Stefan equations (GMSE), using Maxwell-Stefan (M-S) diffusio…
Hydrodynamics of a co-current three-phase solid-bed reactor for foaming systems
2001
The objective of the present study was to evaluate the parameters, which characterize the pulsing flow of the gas and liquid through a bed, namely the frequency of pulsation, the velocity of the pulses and the pulse structure, for foaming systems. The paper presents the results of experiments aimed at determining the effect of the foaming power and the surface tension of liquid phase on the values of the measured parameters.
Substituent effect on the redox potential of substituted (aryl)(2-nitrobenzo[ b ]thiophen-3-yl)amines
2001
Abstract The electronic effect of some meta- or para-substituents on the reduction of the title compounds has been investigated. The reversible reduction potential values of these compounds have been evaluated by cyclic voltammetry at a mercury electrode in 0.1 M tetraethylammoniumtetrafluoroborate, dimethylsulfoxide solutions. The substituent effect depends on both its nature and its position. The reduction potential values of the derivatives studied have been correlated with the Hammett substituent constants.
Molecular association and dynamics in n-pentanol and 2-methyl-2-butanol
1985
Static dielectric constant, viscosity, density, ultrasonic and dielectric relaxation measurements are reported for two pentanol isomers (n-pentanol and 2-methyl-2-butanol) in a wide range of temperatures. The results show a different extent of H-bonds self-association in relation to the molecular geometry of the alcohol. The activation energies obtained from dielectric and ultrasonic studies are compared with those obtained from viscous flow. Such a comparison, together with the analysis of all the dielectric and ultrasonic relaxation data, show that in n-pentanol a correlation between viscosity and ultrasonic relaxation exists, while dielectric relaxation is not correlated with the other t…