Thermodynamic Equilibrium Concentrations Research Articles

Plasma-based dry reforming of CH4: Plasma effects vs. thermal conversion

In this work we evaluate the chemical kinetics of dry reforming of methane in warm plasmas (1000–4000 K) using modelling with a newly developed chemistry set, for a broad range of parameters (temperature, power density and CO2/CH4 ratio). We compare the model against thermodynamic equilibrium concentrations, serving as validation of the thermal chemical kinetics. Our model reveals that plasma-specific reactions (i.e., electron impact collisions) accelerate the kinetics compared to thermal conversion, rather than altering the overall kinetics pathways and intermediate products, for gas temperatures below 2000 K. For higher temperatures, the kinetics are dominated by heavy species collisions and are strictly thermal, with negligible influence of the electrons and ions on the overall kinetics. When studying the effects of different gas mixtures on the kinetics, we identify important intermediate species, side reactions and side products. The use of excess CO2 leads to H2O formation, at the expense of H2 formation, and the CO2 conversion itself is limited, only approaching full conversion near 4000 K. In contrast, full conversion of both reactants is only kinetically limited for mixtures with excess CH4, which also gives rise to the formation of C2H2, alongside syngas. Within the given parameter space, our model predicts the 30/70 ratio of CO2/CH4 to be the most optimal for syngas formation with a H2/CO ratio of 2.

High Concentration Intrinsic Defects in MnSb2Te4.

MnSb2Te4 has a similar structure to an emerging material, MnBi2Te4. According to earlier theoretical studies, the formation energy of Mn antisite defects in MnSb2Te4 is negative, suggesting its inherent instability. This is clearly in contrast to the successful synthesis of experimental samples of MnSb2Te4. Here, the growth environment of MnSb2Te4 and the intrinsic defects are correspondingly investigated. We find that the Mn antisite defect is the most stable defect in the system, and a Mn-rich growth environment favors its formation. The thermodynamic equilibrium concentrations of the Mn antisite defects could be as high as 15% under Mn-poor conditions and 31% under Mn-rich conditions. It is also found that Mn antisite defects prefer a uniform distribution. In addition, the Mn antisite defects can modulate the interlayer magnetic coupling in MnSb2Te4, leading to a transition from the ideal antiferromagnetic ground state to a ferromagnetic state. The ferromagnetic coupling effect can be further enhanced by controlling the defect concentration.

Microstructure and defect engineering of graphite anodes by pulsed laser annealing for enhanced performance of lithium-ion batteries

Nanosecond pulsed laser annealing significantly improves cyclability and current carrying capacity of lithium-ion batteries (LIBs). This improvement is achieved by engineering of microstructure and defect contents present in graphite in a controlled way by using pulsed laser annealing (PLA) to increase the number density of Li+ ion trapping sites. The PLA treatment causes the following changes: (1) creates surface steps and grooves between the grains to improve Li+ ion charging and intercalation rates; (2) removes inactive polyvinylidene difluoride (PVDF) binder from the top of graphite grains and between the grains which otherwise tends to block the Li+ migration; and (3) produces carbon vacancies in (0001) planes which can provide Li+ charging sites. From X-ray diffraction data, we find upshift in diffraction peak or reduction in planar spacing, from which vacancy concentration was estimated to be about 1.0%, which is higher than the thermodynamic equilibrium concentration of vacancies. The laser treatment creates single and multiple C vacancies which provide sites for Li+ ions, and it also produces steps and grooves for Li+ ions to enter the intercalating sites. It is envisaged that the formation of these sites enhances Li+ ion absorption during charge and discharge cycles. The current capacity increases from an average 360 mAh/g to 430 mAh/g, and C–V shows significant reduction in SEI layer formation after the laser treatment. If the vacancy concentration is too high and charge-discharge cycles are long, then trapping of electrons by Li+ may occur, which can lead to Li0 formation and Li plating causing reduction in current capacity.

Method of utilizaion of the forbidden chlorinated pesticides through the recycling of the raw materials that include titanium

The article is dedicated to the actual problem of the recycling of the forbidden Chlorinated pesticides. The capacity of the usage of the forbidden Chlorinated pesticides as the chlorinating agents in the process of ilmenite concentrate recycling on the example of hexachlorobenzene is described. The thermodynamic parameters and equilibrium concentrations of obtaining carbon tetrachloride from hexachlorobenzene are defined. The process of chlorinating of the ilmenite concentrate is researched, the change of equilibrium to the concentration starting reagents and the reactions’ products with the carbon tetrachloride from Mining and Metallurgical Complex of Vilnogirsk with the carbon tetrachloride is calculated. It is also showed that with the help of metabolic reactions of carbon tetrachloride and ilmenite it is possible to avoid the difficult redox transformations, which take place in the traditional technology of titanium obtaining.

High shear spheroidal topological fluid flow induced coating of polystyrene beads with C60 spicules.

Spheroidal spicular like topological fluid flow in an angled vortex fluidic device (VFD) housing a 20 mm diameter tube with a hemispherical base rotating at 4k rpm and tilted at 45° is effective in reducing the thermodynamic equilibrium concentration of fullerene C60 in toluene, with the formation of spicules of the material under continuous flow processing. Under the same operational conditions in the presence of polystyrene beads 2 to 6 μm in diameter, spicules of C60ca. 150 nm in length grow on their surfaces. This establishes that the spheroidal topological fluid flow in the VFD prevails while enveloping spheroidal like particles of such size.

A quantitative phase-field model of gas bubble evolution in UO2

Due to the large formation energy of vacancies and noble gas atoms at interstitial and/or substitutional sites in nuclear fuel (UO2), the thermodynamic equilibrium concentrations of these species are very low in the nuclear fuel matrix even at very high temperature, which imposes difficulties upon the quantitative study of bubble evolution via the phase-field method. In this study, a quantitative phase-field model is proposed to deal with this problem. The free energy density of the system is derived according to the principles of thermodynamics, with consideration of the elastic interaction and internal pressure of each gas bubble, and with the use of material parameters from experiments. The model enables one to study the kinetics of gas bubble growth with very dilute concentrations of vacancy and gas atoms in the matrix. With this model, the growth of a single bubble and multiple bubbles were simulated under different concentrations of vacancy and gas atoms and at different temperatures. The effect of elastic interaction energy and the generation rate of vacancies and gas atoms on bubble growth are analyzed.

Seasonal Variability of the Ion Composition, Phase State, and Mass Concentration of Aerosol in the Rural and Urban Atmosphere over Belgium (2001–2003)

Field data on the ion composition and mass concentration of aerosol in the rural (Wingene) and urban (Antwerp) regions of Belgium and the results of their thermodynamic analysis are presented. Ammonium nitrate and ammonium sulfate are found to be the major water-soluble components of aerosol particles. The seasonal variability of mass concentration, phase state, and ion composition of aerosol particles is largely determined by variations in temperature and relative humidity. It is shown that the content of ammonium nitrate and ammonium sulfate in PM2.5 is close to their thermodynamic equilibrium concentrations.

Xe gas bubbles evolution in UO<sub>2</sub> fuels–A phase field simulation

Owing to the large formation energy of vacancies and inert gas atoms (Xe and Kr) in nuclear fuel (UO2), the thermodynamic equilibrium concentrations of these species are extremely low in the UO2 matrix, which makes it extremely difficult to conduct quantitative study of gas bubbles evolution by phase-field method (PFM). In this study, a more physics based quantitative phase-field model has been proposed. The free energy density of the system was derived according to the principles of thermodynamics and KKS model, the UO2 tri-vacancy, Xe gas atoms and gas bubbles were considered in the system. This model enables one to study the gas bubble growth with extremely low concentrations of vacancy and Xe gas atom in the UO2 matrix. The influence of temperature, vacancy and Xe gas atom generation rates on single and multi-gas bubbles evolution were studied. At high temperature and with high generation rates of vacancies and Xe gas atoms, the gas bubbles had higher growth rate. In addition, the effect of temperature gradient on gas bubble migration was also studied by adding a temperature gradient term in the Cahn-Hillard equations. The gas bubble preferred to migrate to high temperature area. Their shape changed from initial circular shape to a prolate shape along the direction of temperature gradient, which is consistent with the experimental results. The simulation results confirmed the formation of center cavity in the nuclear fuel pellet. The simulation results are consistent with the classical rate theory and experimental observations.

Facile synthesis of monodispersed copper oxalate flaky particles in the presence of EDTA

Monodispersed copper oxalate particles with flaky morphology were prepared via a simple one-pot synthesis method. Scanning electron microscope (SEM), X-ray diffraction (XRD), and fourier transform infrared (FTIR) spectra were used to characterize particle morphology, size, phase composition, and functional groups. It was found that the presence of ethylenediaminetetraacetic acid (EDTA) and the solution pH value had strong influence on the morphological and size evolution of the precipitated particles. On the basis of controlled release of copper ions from a Cu2+–EDTA complex and Weimarn’s law, a strategy for the controlled synthesis of monodispersed copper oxalate particles was designed by referring to the basic mode of the Stober method. The inherent nature of crystallization to form the flaky solid in the early stage of precipitation as well as the driving force of the long-lasting low supersaturation in the growth stage was proposed to explain the size and morphological evolution of the copper oxalate precipitates. Thermodynamic equilibrium concentrations of copper(II) species in the Cu(II)–EDTA–oxalate–H2O solution system were calculated to help explain the possible formation mechanism of copper oxalate precipitates.

Stratified plastic flow in metals

Experimentally determined possibility to control the localization of plastic flow (shear bands) and the mechanical properties of metals (viscosity) during a plastic deformation process that is relevantly changed, validates questioning the nature of the shear bands, the mechanisms behind their existence and their morphological form.In this paper, indications for a hypothesis, that there are possibilities and certain conditions in which a stratified plastic flow similar to a rigid (dislocation less) shear strain in metallic crystals can appear, are presented. The analysis conducted for the paper was based on experiments indicating the possible dominant role of point defects in structural processes (diffusion) and their influence on the mechanical properties of crystals (viscosity). The dependency between the diffusion coefficient and the point defects concentration and their migration enthalpy as well as the physical dependence of the viscosity coefficient on the diffusion coefficient might explain the low viscosity of the metal with high (in relation to thermodynamic equilibrium) concentration of self-interstitial atoms. It was proved that a high supersaturation of metal with such defects can be obtained either through a mechanically activated reverse dislocation movement or through changing the dominant slip system. The defects, generated as a result of collapse of dislocation dipoles, and their spatial distribution, change the properties of the plane in which they were generated.In other words, a planar zone along the active slip plane supersaturated by self-interstitial atoms with very low migration energy levels (which means their bonds within the lattice are weak) is treated as a layer (stratum), where the inter-planar bond in a crystal is weakened. A high saturation of vacancies in a given layer, which lowers the number of bonds within the first coordination sphere, gives similar results. High local saturation of metal with point defects results in the lattice's lower resistance to shear strain, and therefore, lower viscosity.

Thermodynamic Investigation on SO<sub>3</sub> Distribution in Oxy-Fuel Combustion

Thermodynamic equilibrium is proposed to analyze the distribution of SO3 in different flue gas circulation modes to study the migration law of sulfur for oxy-fuel combustion. Results indicate that the concentration of SO2 increases dramatically when flue gas circulation is adopted with the decline of flue gas volume and the increase of oxygen concentration in oxy-fuel combustion, which results the increase of SO3 concentration and formation rate. At 1200°C, thermodynamic equilibrium concentration of SO2 and SO3 are 5.3 times and 6.1 times respectively compared with conventional combustion. In Oxy1 circulation mode, SO3 is the most sensitive to excess air coefficient and sulfur content, followed by Oxy2 circulation mode, while the conventional combustion has the lowest sensitivity. Therefore, SO3 is greatly influenced by different flue gas circulation modes during oxy-fuel combustion.

Equilibrium Analysis on Sulfur Material in Oxyfuel Combustion

To study migration and transformation of sulfur species in oxyfuel combustion, the study attempts to analyze distribution of sulfur compounds with thermodynamic equilibrium. Results show that sulfur-containing gases predominantly are SO2 and SO3, the maximum thermodynamic equilibrium concentration of those in oxyfuel combustion respectively increase by 3.4 and 4.5 times compared with the conventional combustion. Furthermore, SO2 gas formation rate decreases while SO3 increases under oxyfuel combustion. Sulfur-containing gases are generally more sensitive to temperature and excess air coefficient. The amount of sulfur compounds significantly increases in oxyfuel combustion.

Modelling the Influence of the Oxygen Concentration in the Gas Mixture (N2/O2) at Thermodynamic Equilibrium

Knowledge of the chemical composition of plasma is required for calculations and modeling in thermal plasmas. Indeed, from the knowledge of this composition can be calculated thermodynamic properties, transport coefficients and the radiative properties of a plasma environment. In this work, we propose to study the influence of thermodynamic equilibrium concentration of oxygen in a gas mixture N2/O2. Particularly we study the evolution of the species density in the plasma created according to the temperature and pressure variables for O2 mixtures. A thermodynamic equilibrium, when you want to take into account a large number of species, two main methods are usually employed, one is based on the law of mass action and the other on minimization of the Gibbs. We decided in our study to the law of mass action and the method of Newton-Raphson. The results show that when the plasma is in thermodynamic equilibrium densities of the different species present in plasma that are functions of temperature and pressure. They are not independent because they are bound by certain laws of thermodynamic equilibrium

The effect of finite microscopic liquid solute diffusion on macrosegregation formation

We study the effect of solidification kinetics, driven by local limited diffusion in the liquid, on macrosegregation. If the diffusion in the liquid surrounding a growing grain is slow, the local average liquid concentration is lower than the thermodynamic equilibrium concentration at the interface. The redistribution of solute by the flow of intergranular liquid on the macroscopic scale is affected by the modified microsegregation in the liquid. We study this phenomenon using a two-phase model based on the volume-averaging method, describing macroscopic transport coupled to a microscopic grain growth model. The growth kinetics is resolved by accounting for finite diffusion in the liquid and solid phases, assuming an equiaxed globular morphology. To accurately model the diffusion field around the grain, we propose an improved approximation for the solutal boundary layer thickness accounting for the growth conditions and liquid convection. The effect of growth kinetics on macrosegregation is then investigated in the case of solidification of a binary alloy in a small cavity where the solid phase is fixed and fluid flow is driven by natural convection. We show that it is important to accurately model the diffusion field around the grain to capture correctly the effect of growth kinetics on the weakening of macrosegregation.

Thermodynamic stability of petroleum C15 polyalkylated bicyclo[4.4.0]decanes at 500 and 600 K

The thermodynamic equilibrium concentrations of petroleum C15 polymethylated bicyclo[4.4.0]decanes (sesquiterpanes) at 500 and 600 K have been experimentally obtained for the first time via the aluminosilicate-mediated equilibrium isomerization of the saturated 200–300°C fraction of crude oils of different genotypes. It has been found that sesquiterpane isomers with an angular carbon atom and methyl and ethyl substituents in the molecule are the least thermodynamically stable in the equilibrium mixture and their relative concentration increases with increasing temperature from 500 to 600 K; the most stable isomer is 2,3,3,7,7-pentamethylbicyclo[4.4.0]decane.

First-principles study of the incorporation and diffusion of helium in cubic zirconia

The incorporation and the diffusion of helium (He) with and without the intrinsic vacancy defects in the cubic ZrO 2 are investigated through the first-principles total-energy calculations, in which the projector-augmented-wave (PAW) method with the generalized gradient approximation (GGA) is adopted. Ab initio calculations are carried out to identify the dominant defects and understand their electronic properties as well. Both the intrinsic oxygen interstitials and the oxygen vacancies in the cubic ZrO 2 show the negative- U behavior. The incorporation energy of He impurity indicates that it is preferable to occupy the Zr vacancy at first, whereas the solution energy suggests that He would be accommodated in the interstitial site at the thermodynamic equilibrium concentration. By calculating the He migration energies corresponding to both the interstitial diffusion and the vacancy assisted diffusion, we suggest that it is most likely to diffuse by hopping through a single vacancy. Remarkably, our calculated vacancy-assisted diffusion energy barrier of He is consistent well with the experimental measurement.

Steam reforming of glycerol: The experimental activity of La1−xCexNiO3 catalyst in comparison to the thermodynamic reaction equilibrium

The steam reforming of glycerol (1,2,3-propantriol) was investigated with non-substituted and partially Ce substituted La1−xCexNiO3 mixed oxides where x=0, 0.1, 0.3 or 0.7, and the activities were compared with Pt metal catalysts. The catalysts were characterised by temperature-programmed reduction (TPR), X-ray powder diffraction (XRD), BET surface area and carbon content. The Ni was easily reduced in the La0.3Ce0.7NiO3 structure. The experimental results were compared with the thermodynamic equilibrium concentrations, which were calculated for the system with non-stochiometric method. The La0.3Ce0.7NiO3 catalyst was highly active in the glycerol steam reforming with conversions approaching to the equilibrium at temperatures between 500 and 700°C. The formation of carbonaceous deposits on the La0.3Ce0.7NiO3 was smallest among all the investigated La1−xCexNiO3 catalysts. Unchanged catalyst surface area (BET) during operation and low carbon deposition after reaction confirm the efficient operation and high stability of the non-noble, inexpensive catalyst of La0.3Ce0.7NiO3.

Secondary Ion Mass Spectrometry of Vapor−Liquid−Solid Grown, Au-Catalyzed, Si Wires

Knowledge of the catalyst concentration within vapor-liquid-solid (VLS) grown semiconductor wires is needed in order to assess potential limits to electrical and optical device performance imposed by the VLS growth mechanism. We report herein the use of secondary ion mass spectrometry to characterize the Au catalyst concentration within individual, VLS-grown, Si wires. For Si wires grown by chemical vapor deposition from SiCl 4 at 1000 degrees C, an upper limit on the bulk Au concentration was observed to be 1.7 x 10(16) atoms/cm(3), similar to the thermodynamic equilibrium concentration at the growth temperature. However, a higher concentration of Au was observed on the sidewalls of the wires.

Thermodynamics and Kinetics of Intramolecular Water Assisted Proton Transfer in Na+-1-Methylcytosine Water Complexes

High-level ab initio predictions of the tautomerization equilibrium and rate constants of water-assisted proton transfer of 1-methyl-cytosine (MeC) to its MeC* imino tautomeric form in the presence of up to two water molecules (W) and the Na(+) cation were carried out. The calculated energy values were used to obtain the thermodynamic parameters and equilibrium concentration of MeC, its rare tautomer, and their complexes with up to two water molecules and the Na (+) cation. The rate constants for the tautomerization were obtained by using the instanton approach (a quasiclassical method based on the least-action principle). Hydration of MeC by one water molecule leads to an increase of the concentration of the MeC* tautomer in the equilibrium mixture and a decrease of the barrier of the MeC* formation (to 15.6 kcal/mol). If the Na(+) cation is present, the tautomeric form is much less favored, and the tautomerization barrier increases to 25.2 kcal/mol. It was found that MeC monohydrate has both the highest equilibrium (2.9 x 10(-2)) and rate (7.9 x 10(5) s(-1)) constants of tautomerization in comparison to the MeC*NaW and MeC*Na2W complexes containing the Na(+) cation. Moreover, this study also allows one to estimate the concentration of MeC present in the cell during DNA synthesis as the unwanted tautomer, which in forming a mismatched base pair can cause spontaneous point mutations. Kinetic simulations have demonstrated that the low values of equilibrium (10(-14)-10(-13)) and rate constants (10(-17)-10(-16) s(-1)) of tautomerization make contribution of the MeC*Na(+)W and MeC*Na(+)2W complexes to the point mutations in DNA unlikely. In contrast to these Na(+) complexes, MeC*W can reach an equilibrium concentration of 2.9 x 10(-2) within 10(-7) s; thus, it can increase the probability of the point mutations.

Mechanisms of photoluminescence sensor response of porous silicon for organic species in gas and liquid phases

Photoluminescence response of porous silicon in presence of single organic analytes scales within the linear dynamic range with concentration of detected species. We present systematic study of changes in porous silicon photoluminescence intensity in the presence of precisely controlled amounts of linear aliphatic alcohols (from methanol to n-hexanol) in gas and liquid phases. From the concentration dependence of photoluminescence quenching we determined sensitivity of porous silicon sensor response. The sensor response sensitivity revealed nearly monotonous change within the homological set of n-alcohols in both gas and liquid phases. However, while in gas phase the sensitivity of sensor response rose with the length of alcohol chain, in liquid phase we observed the opposite behavior. The mechanism of sensor response in gas and liquid phases is explained by photoluminescence dielectric quenching. The strength of photoluminescence quenching is directly determined by dielectric constant and concentration of analyte in liquid phase whereas in gas phase it primarily depends on effective concentration of analyte inside porous silicon matrix. The thermodynamic equilibrium concentration of analyte inside porous silicon matrix is controlled by capillary condensation effect. A very good correlation between gas phase concentration and room temperature saturated vapor pressure of studied analytes and porous silicon photoluminescence quenching response was obtained.