Interphase Energies and Nonequilibrium Growth of γ-precipitates in Al-Ag: A DFT Study

2011 ◽  
Vol 1297 ◽  
Author(s):  
D. J. Watts ◽  
D. D. Johnson ◽  
Daniel Finkenstadt
Keyword(s):  
Density Functional ◽  
Growth Models ◽  
Calculated Phase Diagram ◽  
Explicit Calculation ◽  
Experimental Conditions ◽  
Wulff Construction ◽  
Aspect Ratios ◽  
Precipitate Growth ◽  
Approximate Unit ◽  
Nonequilibrium Growth

AbstractDensity-functional theory (DFT) calculations of interphase boundary energies provide useful input for many precipitate growth models in alloy systems [1]. One example is Al-Ag, where a rich variety of precipitate types exist, and the sizes and shapes are determined roughly by a Wulff construction, namely, minimizing surface free energies with respect to geometry. This is only a first approximation, however, as kinetic-considerations and crystallography do not allow for a uniform, isotropic growth. Consequently, a nonequilibrium growth model is developed for γ-plates [2], which attempts to connect semi-coherent (ledge) and incoherent (edge) interface growth rates in a way that incorporates shape and interface energies. Through this connection, we make a DFT model with approximate unit cells that mirror experimental conditions, which gives accurate predictions for precipitate aspect ratios and time-development of nonequilibrium shapes. Starting from an explicit calculation of Suzuki segregation of solute to stacking-faults, we find a mechanism for nucleation of nanoscale γ-plates on quenched defects, identify a bulk structure from a calculated phase diagram that gives the relevant HCP equilibrium precipitate structure occurring at 50 at.% Ag and calculate critical nucleation parameters for γ-precipitate formation. Applications to island-coarsening and lath morphology are also discussed.

A Reinvestigation of the Deceptively Simple Reaction of Toluene with ●OH, and the Fate of the Benzyl Radical. I. a Combined Thermodynamic and Kinetic Study on the Competition Between ●OH Addition and Hydrogen Abstraction Reactions.

2019 ◽  
Author(s):  
Zoi Salta ◽  
Agnie M. Kosmas ◽  
Marc E. Segovia ◽  
Martina Kieninger ◽  
Oscar Ventura ◽  
...  
Keyword(s):  
Density Functional ◽  
Room Temperature ◽  
Hydrogen Abstraction ◽  
Reaction Rates ◽  
Composite Model ◽  
Alternative Mechanism ◽  
Radical Intermediate ◽  
Experimental Conditions ◽  
Methyl Group ◽  
Benzyl Radical

This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both experimentally observed H-abstraction from the methyl group and possible additions to the phenyl ring were investigated. Reaction enthalpies and heights of the barriers suggest that H-abstraction is more favorable than ●OH addition to the ring. The calculated reaction rates at room temperature and the radical-intermediate product fractions support this view. This is somehow contradictory with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that begs for an explanation. In this first part of our work we give the evidences that support the preference of hydrogen abstraction over ●OH addition and suggest an alternative mechanism which shows that cresols can actually arise also from the former reaction and not only from the latter.

A Reinvestigation of the Deceptively Simple Reaction of Toluene with ●OH, and the Fate of the Benzyl Radical. I. a Combined Thermodynamic and Kinetic Study on the Competition Between ●OH Addition and Hydrogen Abstraction Reactions.

2019 ◽  
Author(s):  
Zoi Salta ◽  
Agnie M. Kosmas ◽  
Marc E. Segovia ◽  
Martina Kieninger ◽  
Oscar Ventura ◽  
...  
Keyword(s):  
Density Functional ◽  
Room Temperature ◽  
Hydrogen Abstraction ◽  
Reaction Rates ◽  
Composite Model ◽  
Alternative Mechanism ◽  
Radical Intermediate ◽  
Experimental Conditions ◽  
Methyl Group ◽  
Benzyl Radical

This work reports density functional and composite model chemistry calculations performed on the reactions of toluene with the hydroxyl radical. Both experimentally observed H-abstraction from the methyl group and possible additions to the phenyl ring were investigated. Reaction enthalpies and heights of the barriers suggest that H-abstraction is more favorable than ●OH addition to the ring. The calculated reaction rates at room temperature and the radical-intermediate product fractions support this view. This is somehow contradictory with the fact that, under most experimental conditions, cresols are observed in a larger concentration than benzaldehyde. Since the accepted mechanism for benzaldehyde formation involves H-abstraction, a contradiction arises that begs for an explanation. In this first part of our work we give the evidences that support the preference of hydrogen abstraction over ●OH addition and suggest an alternative mechanism which shows that cresols can actually arise also from the former reaction and not only from the latter.

A Hierarchical Approach to Purify and Functionalize Pristine Carbon Nanotubes

2012 ◽  
Vol 510-511 ◽  
pp. 118-123
Author(s):  
M. Mansoor
Keyword(s):  
Carbon Nanotubes ◽  
Vapor Deposition ◽  
Chemical Vapor ◽  
Wet Oxidation ◽  
Hierarchical Approach ◽  
Experimental Conditions ◽  
Aspect Ratios ◽  
Carbon Impurities ◽  
Metallic Catalyst ◽  
Surfactant Assisted

A step-by-step, hierarchical approach is explored in the present work to purify and functionalize carbon nanotubes synthesized by chemical vapor deposition. Attempts are made to purify and functionalize CNTs without extinguishing their aspect ratios. The carbon impurities are removed by thermal oxidation, whilst the unprotected metallic catalyst particles are eliminated by wet oxidation, subsequently; CNT bundles are de-roped by surfactant assisted sonication. Finally, protected metallic catalyst particles are removed and functional groups (hydroxyl and carboxyl) are attached by acid treatment and wet oxidation, respectively. The derivate CNTs are characterized using zeta potential measurements, TGA, XRD, FTIR and SEM. The characterization showed that in optimum experimental conditions the catalytic particles are removed upto 80%, the carbon impurities are eliminated upto 95% and chemical functionalities of hydroxyl and carboxyl is occurred with noticeable de-roping of the CNT bundles.

scholarly journals First-Principles Mechanistic Insights into the Hydrogen Evolution Reaction on Ni2P Electrocatalyst in Alkaline Medium

Catalysts ◽  
2020 ◽  
Vol 10 (3) ◽  
pp. 307
Author(s):  
Russell W. Cross ◽  
Nelson Y. Dzade
Keyword(s):  
Hydrogen Evolution ◽  
Hydrogen Evolution Reaction ◽  
Density Functional ◽  
Hydrogen Adsorption ◽  
Density Functional Theory Calculations ◽  
Transition Metal Doping ◽  
Experimental Conditions ◽  
Functional Theory ◽  
Alkaline Conditions ◽  
Modification Of The Surface

Nickel phosphide (Ni2P) is a promising material for the electrocatalytic generation of hydrogen from water. Here, we present a chemical picture of the fundamental mechanism of Volmer–Tafel steps in hydrogen evolution reaction (HER) activity under alkaline conditions at the (0001) and (10 1 ¯ 0) surfaces of Ni2P using dispersion-corrected density functional theory calculations. Two terminations of each surface (Ni3P2- and Ni3P-terminated (0001); and Ni2P- and NiP-terminated (10 1 ¯ 0)), which have been shown to coexist in Ni2P samples depending on the experimental conditions, were studied. Water adsorption on the different terminations of the Ni2P (0001) and (10 1 ¯ 0) surfaces is shown to be exothermic (binding energy in the range of 0.33−0.68 eV) and characterized by negligible charge transfer to/from the catalyst surface (0.01−0.04 e−). High activation energy barriers (0.86−1.53 eV) were predicted for the dissociation of water on each termination of the Ni2P (0001) and (10 1 ¯ 0) surfaces, indicating sluggish kinetics for the initial Volmer step in the hydrogen evolution reaction over a Ni2P catalyst. Based on the predicted Gibbs free energy of hydrogen adsorption (ΔGH*) at different surface sites, we found that the presence of Ni3-hollow sites on the (0001) surface and bridge Ni-Ni sites on the (10 1 ¯ 0) surface bind the H atom too strongly. To achieve facile kinetics for both the Volmer and Heyrovsky–Tafel steps, modification of the surface structure and tuning of the electronic properties through transition metal doping is recommended as an important strategy.

Theoretical Investigation of Fluorinated and Hydrocenated Diamond Filks

1992 ◽  
Vol 270 ◽  
Author(s):  
Mark R. Pederson ◽  
Warren E. Pickett
Keyword(s):  
Density Functional ◽  
Film Growth ◽  
Growth Models ◽  
Diamond Films ◽  
Diamond Surface ◽  
Free Standing ◽  
Level Shifts ◽  
Reconstructed Surface ◽  
Experimental Values ◽  
Diamond Film Growth

ABSTRACTTo investigate some of the fundamental differences between halogen and hydrogen assisted diamond film growth we have performed several calculations related to the <100> diamond surface. The models used in these investigations include ten-layer periodic slabs of free standing fluorinated diamond films as well as isolated clusters [C21F6H20]. For purposes of comparison, we have also performed calculations on models of the hydrogenated <100> surface. The calculations are performed within the density-functional framework using LCAO and LAPW computational methods. We have considered two geometries of a monofluoride surface. The first surface, best described as an ideal l×l surface with a monolayer of ionically bonded fluorines, exhibits a metallic density of states in contrast to a 2×l reconstructed surface with chemically bonded fluorines that is found to be insulating. We compare theoretical carbon core level shifts with experimental values and discuss growth models based on these surface calculations.

Kinetic super-roughening and anomalous dynamic scaling in nonequilibrium growth models

1994 ◽  
Vol 49 (1) ◽  
pp. 122-125 ◽  
Author(s):  
S. Das Sarma ◽  
S. V. Ghaisas ◽  
J. M. Kim
Keyword(s):  
Growth Models ◽  
Dynamic Scaling ◽  
Nonequilibrium Growth

Stability, Electronic Properties, and Structural Isomerism in Small Copper Clusters

2012 ◽  
Vol 1479 ◽  
pp. 15-20
Author(s):  
Juan M. Montejano-Carrizales ◽  
Faustino Aguilera-Granja ◽  
Ricardo A. Guirado-López
Keyword(s):  
Electronic Properties ◽  
Density Functional ◽  
Three Dimensional ◽  
Density Functional Theory Calculations ◽  
Energy Barriers ◽  
Elastic Band ◽  
Experimental Conditions ◽  
Atomic Displacements ◽  
Structural Isomerism ◽  
The Stability

ABSTRACTWe present extensive pseudopotential density functional theory calculations dedicated to analyze the stability, electronic properties, and structural isomerism in Cu6 clusters. We consider structures of different symmetries and charge states. Our total energy calculations reveal a strong competition between two- and three-dimensional atomic arrays, the later being mostly energetically preferred for the anionic structures. The bond lengths and electronic spectra strongly depend on the local atomic environment, a result that is expected to strongly influence the catalytic activity of our clusters. Using the nudged elastic band method we analyze the interconversion processes between different Cu6 isomers. Complex atomic relaxations are obtained when we study the transition between different cluster structures; however relatively small energy barriers of approximately 0.3 eV accompany the atomic displacements. Interestingly, we obtain that by considering positively charged Cu6+ systems we reduce further the energy barriers opposing the interconversion process. The previous results could imply that, under a range of experimental conditions, it should be possible to observe different Cu6cluster structures in varying proportions.

scholarly journals Fast anisotropic Mg and H diffusion in wet forsterite

2021 ◽  
Author(s):  
Joshua Muir ◽  
Feiwu Zhang ◽  
Andrew Walker
Keyword(s):  
Density Functional ◽  
Diffusion Rate ◽  
Water Concentration ◽  
Experimental Conditions ◽  
Functional Theory ◽  
Strong Function ◽  
Effect Of Water ◽  
Large Numbers ◽  
Relationship Of ◽  
The Relationship

Mg diffusion, which is important for properties of forsterite such as conductivity and deformation, is a strong function of water content. The mechanism behind this effect, however, has not been fully elucidated. In this study we use Density Functional Theory to predict the diffusivity of 〖(2H)〗_Mg^X and we find that they are around 1000 times slower than H-free Mg vacancies V_Mg^''. In most wet conditions the concentration of 〖(2H)〗_Mg^X is much higher than that of V_Mg^'' and thus the primary effect of water on increasing the Mg-diffusion rate in forsterite is by producing large numbers of H-bearing Mg vacancies. A water induced increase in diffusion rate is predicted to be accompanied by a large increase in diffusional anisotropy primarily in the [001] direction. Using a previously developed model of H distribution in forsterite we predict that the effect of water on Mg diffusion is strongly dependent upon environmental conditions such as pressure or temperature. An exponent (r) describing the relationship of water concentration to Mg diffusion is found to vary between 0.5-1.6 across common experimental conditions with pressure decreasing this exponent and temperature increasing it. With 100 wt. ppm water Mg diffusion rates are predicted to increase by over 2 orders of magnitude at high temperature and low pressure (2000 K, 0 GPa) and by over 3.5 orders of magnitude at low temperature and high pressure (1000 K, 10 GPa) while the anisotropy of diffusion is predicted to increase by ~2/over 5.5 orders of magnitude respectively. A conversion from “dry” to “wet” rheological laws is predicted to occur at <~1 ppm. These results suggest that Mg diffusion in wet forsterite could vary considerably throughout mantle conditions in ways that cannot be captured with a simple one component equation. Finally we considered the effects of the diffusion of H-bearing Mg vacancies on conductivity in forsterite and olivine. We combined our diffusivity results with experimentally determined results for phonon conductivity but this predicted significaly lower conductivities than have been observed experimentally in olivine, particularly at low temperatures (~1000 K). This suggests that the effect of water on olivine conductivity is not primarily due to bulk 〖(2H)〗_Mg^X diffusion and operates via a different unknown mechanism.

scholarly journals Characteristics and Mechanism of Vinyl Ether Cationic Polymerization in Aqueous Media Initiated by Alcohol/B(C6F5)3/Et2O

Polymers ◽  
2019 ◽  
Vol 11 (3) ◽  
pp. 500
Author(s):  
Jinghan Zhang ◽  
Yibo Wu ◽  
Kaixuan Chen ◽  
Min Zhang ◽  
Liangfa Gong ◽  
...  
Keyword(s):  
Vinyl Ether ◽  
Cationic Polymerization ◽  
Density Functional ◽  
Aqueous Media ◽  
Vinyl Ethers ◽  
Experimental Conditions ◽  
Functional Theory ◽  
Air Atmosphere ◽  
Significant Difference ◽  
First Time

Aqueous cationic polymerizations of vinyl ethers (isobutyl vinyl ether (IBVE), 2-chloroethyl vinyl ether (CEVE), and n-butyl vinyl ether (n-BVE)) were performed for the first time by a CumOH/B(C6F5)3/Et2O initiating system in an air atmosphere. The polymerization proceeded in a reproducible manner through the careful design of experimental conditions (adding initiator, co-solvents, and surfactant or decreasing the reaction temperature), and the polymerization characteristics were systematically tested and compared in the suspension and emulsion. The significant difference with traditional cationic polymerization is that the polymerization rate in aqueous media using B(C6F5)3/Et2O as a co-initiator decreases when the temperature is lowered. The polymerization sites are located on the monomer/water surface. Density functional theory (DFT) was applied to investigate the competition between H2O and alcohol combined with B(C6F5)3 for providing a theoretical basis. The effectiveness of the proposed mechanism for the aqueous cationic polymerization of vinyl ethers using CumOH/B(C6F5)3/Et2O was confirmed.

Performance of Response Surface and Davey Model for Prediction of Staphylococcus aureus Growth Parameters under Different Experimental Conditions

2004 ◽  
Vol 67 (6) ◽  
pp. 1138-1145 ◽  
Author(s):  
G. ZURERA-COSANO ◽  
A. M. CASTILLEJO-RODRÍGUEZ ◽  
R. M. GARCÍA-GIMENO ◽  
F. RINCÓN-LEÓN
Keyword(s):  
Staphylococcus Aureus ◽  
Growth Rate ◽  
Response Surface ◽  
Growth Parameters ◽  
Growth Models ◽  
Lag Time ◽  
Anaerobic Conditions ◽  
Experimental Conditions ◽  
Aerobic And Anaerobic ◽  
Aerobic And Anaerobic Conditions

The combined effect of different temperatures (7 to 19°C), pH levels (4.5 to 8.5), sodium chloride levels (0 to 8%), and sodium nitrite levels (0 to 200 ppm) on the predicted growth rate and lag time of Staphylococcus aureus under aerobic and anaerobic conditions was studied. The two predictive models developed, response surface (RS) and the Davey model, provided reliable estimates of the two kinetic parameters studied. The RS provided better predictions of maximum specific growth rate, with bias factors of 1.06 and 1.31 and accuracy factors of 1.17 and 1.37, respectively, in aerobic and anaerobic conditions. The Davey model performed more accurately for lag time, with a bias factor of 1.12 and an accuracy factor of 1.49, for both aerobic and anaerobic conditions. Predictive growth models are a valuable tool, enabling swift determination of Staphylococcus aureus growth rate and lag time. These data are essential for ensuring staphylococcus-relatedquality and safety of food products.

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