The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Keon Wook Kang - One of the best experts on this subject based on the ideXlab platform.
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predicting the dislocation nucleation rate as a function of temperature and stress
Journal of Materials Research, 2011Co-Authors: Keon Wook KangAbstract:Predicting the dislocation nucleation rate as a function of temperature and stress is crucial for understanding the plastic deformation of nanoscale crystalline materials. However, the limited time scale of molecular dynamics simulations makes it very difficult to predict the dislocation nucleation rate at experimentally relevant conditions. We recently develop an approach to predict the dislocation nucleation rate based on the Becker–Doring theory of nucleation and umbrella sampling simulations. The results reveal very large Activation entropies, which originated from the anharmonic effects, that can alter the nucleation rate by many orders of magnitude. Here we discuss the thermodynamics and algorithms underlying these calculations in greater detail. In particular, we prove that the Activation Helmholtz free energy equals the Activation Gibbs free energy in the thermodynamic limit and explain the large difference in the Activation entropies in the constant stress and constant strain ensembles. We also discuss the origin of the large Activation entropies for dislocation nucleation, along with previous theoretical estimates of the Activation Entropy.
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entropic effect on the rate of dislocation nucleation
Proceedings of the National Academy of Sciences of the United States of America, 2011Co-Authors: Keon Wook KangAbstract:Dislocation nucleation is essential to our understanding of plastic deformation, ductility, and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited timescale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the Activation free energy from umbrella sampling. Our data reveal very large Activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The Activation Entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational Entropy. The Activation Entropy at constant stress is significantly larger than that at constant strain, as a result of thermal softening. The large Activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used for rate estimation in solids. Similar behaviors are expected to occur in other nucleation processes in solids.
F Vallianatos - One of the best experts on this subject based on the ideXlab platform.
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a thermodynamic approach of self and hetero diffusion in gaas connecting point defect parameters with bulk properties
RSC Advances, 2016Co-Authors: V Saltas, Alexander Chroneos, F VallianatosAbstract:The self- and hetero-diffusion in GaAs is investigated in terms of the cBΩ thermodynamic model, which connects point defect parameters with the bulk elastic and expansion properties. Point defect thermodynamic properties, such as Activation enthalpy, Activation volume, Activation Gibbs free energy, Activation Entropy and isobaric specific heat of Activation, are calculated as a function of temperature for Ga, H and various n- and p-type dopants (Si, Be, Cr, Fe and Zn) diffused in GaAs. The present calculations are in good agreement with the reported experimental results. The pressure dependence of Ga self-diffusion is also investigated and the diffusivities and Activation volumes are predicted at different temperatures from ambient pressure up to 10 GPa, above which GaAs is transformed into the orthorhombic structure. The Activation volumes of dopants are also estimated at high temperature (1124 K), as a function of pressure.
Alexander Chroneos - One of the best experts on this subject based on the ideXlab platform.
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a thermodynamic approach of self and hetero diffusion in gaas connecting point defect parameters with bulk properties
RSC Advances, 2016Co-Authors: V Saltas, Alexander Chroneos, F VallianatosAbstract:The self- and hetero-diffusion in GaAs is investigated in terms of the cBΩ thermodynamic model, which connects point defect parameters with the bulk elastic and expansion properties. Point defect thermodynamic properties, such as Activation enthalpy, Activation volume, Activation Gibbs free energy, Activation Entropy and isobaric specific heat of Activation, are calculated as a function of temperature for Ga, H and various n- and p-type dopants (Si, Be, Cr, Fe and Zn) diffused in GaAs. The present calculations are in good agreement with the reported experimental results. The pressure dependence of Ga self-diffusion is also investigated and the diffusivities and Activation volumes are predicted at different temperatures from ambient pressure up to 10 GPa, above which GaAs is transformed into the orthorhombic structure. The Activation volumes of dopants are also estimated at high temperature (1124 K), as a function of pressure.
Gideon Schreiber - One of the best experts on this subject based on the ideXlab platform.
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regular articleexperimental assignment of the structure of the transition state for the association of barnase and barstar1
Journal of Molecular Biology, 2001Co-Authors: Christian Frisch, Alan R Fersht, Gideon SchreiberAbstract:Association of a protein complex follows a two step reaction mechanism, with the first step being the formation of an encounter complex which evolves into the final complex. Here we present new experimental data for the association of the bacterial ribonuclease barnase and its polypeptide inhibitor barstar which shed light on the thermodynamics and structure of the transition state and preceding encounter complex of association at diminishing electrostatic attraction. We show that the Activation Entropy at the transition state is close to zero, with the Activation enthalpy being equal to the free energy of binding. This observation was independent of the magnitude of the mutual electrostatic attraction, which were altered by mutagenesis or by addition of salt. The low Activation Entropy implies that the transition state is mostly solvated at all ionic strengths. The structure of the transition state was probed by measuring pairwise interaction energies using double-mutant-cycles. While at low ionic strength all proximal charge-pairs form contacts, at high salt only a subset of these interactions are maintained. More specifically, charge-charge interactions between partially buried residues are lost, while exposed charged residues maintain their ability to form specific interactions even at the highest salt concentration. Uncharged residues do not interact at any ionic strength. The results presented here suggest that the barnase-barstar binding sites are correctly aligned during the transition state even at diminishing electrostatic attraction, although specific short range interactions of uncharged residues are not yet formed. Furthermore, most of the interface desolvation (which contributes to the Entropy of the system) has not yet occurred. This picture seems to be valid at low and high salt. However, at high salt, interactions of the activated complex are limited to a more restricted set of residues which are easier approached during diffusion, prior to final docking. This suggest that the steering region at high salt is more limited, albeit maintaining its specificity.
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experimental assignment of the structure of the transition state for the association of barnase and barstar
Journal of Molecular Biology, 2001Co-Authors: Christian Frisch, Alan R Fersht, Gideon SchreiberAbstract:Abstract Association of a protein complex follows a two step reaction mechanism, with the first step being the formation of an encounter complex which evolves into the final complex. Here we present new experimental data for the association of the bacterial ribonuclease barnase and its polypeptide inhibitor barstar which shed light on the thermodynamics and structure of the transition state and preceding encounter complex of association at diminishing electrostatic attraction. We show that the Activation Entropy at the transition state is close to zero, with the Activation enthalpy being equal to the free energy of binding. This observation was independent of the magnitude of the mutual electrostatic attraction, which were altered by mutagenesis or by addition of salt. The low Activation Entropy implies that the transition state is mostly solvated at all ionic strengths. The structure of the transition state was probed by measuring pairwise interaction energies using double-mutant-cycles. While at low ionic strength all proximal charge-pairs form contacts, at high salt only a subset of these interactions are maintained. More specifically, charge-charge interactions between partially buried residues are lost, while exposed charged residues maintain their ability to form specific interactions even at the highest salt concentration. Uncharged residues do not interact at any ionic strength. The results presented here suggest that the barnase-barstar binding sites are correctly aligned during the transition state even at diminishing electrostatic attraction, although specific short range interactions of uncharged residues are not yet formed. Furthermore, most of the interface desolvation (which contributes to the Entropy of the system) has not yet occurred. This picture seems to be valid at low and high salt. However, at high salt, interactions of the activated complex are limited to a more restricted set of residues which are easier approached during diffusion, prior to final docking. This suggest that the steering region at high salt is more limited, albeit maintaining its specificity.
Chen Sanping - One of the best experts on this subject based on the ideXlab platform.
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Thermochemistry of gadolinium chloride hydrate with diethylammonium diethyldithiocarbamate
The Journal of Chemical Thermodynamics, 2005Co-Authors: Gao Sheng-li, Chen Sanping, Bian Jiang, Jiao Bao-juan, Zhao Feng-qi, Shi Qi-zhenAbstract:Abstract The complex of gadolinium chloride lower hydrate with diethylammonium diethyldithiocarbamate (D-DDC) has been synthesized in absolute alcohol under a N2 atmosphere. Chemical analyses showed that the complex had a general formula Et2NH2[Gd(S2CNEt2)4]. It was characterized by Infra-red (IR) spectroscopy. The enthalpies of solution of gadolinium chloride hydrate and D-DDC in absolute alcohol at T=298.15 K and the enthalpy of formation of Et2NH2[Gd(S2CNEt2)4] as a function of temperature were determined by microcalorimetry. On the basis of experimental results, three thermodynamic parameters, the Activation enthalpy, the Activation Entropy and the Activation free energy, the rate constant and three kinetic parameters, the apparent Activation energy, the pre-exponential constant and the reaction order of liquid phase reaction of formation were obtained.
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coordination behavior of erbium chloride hydrate with diethylammonium diethyldithiocarbamate
Journal of Thermal Analysis and Calorimetry, 2004Co-Authors: F Xuezhong, Chen Sanping, Bian Jiang, R Yixia, Jiao Baojuan, Gao Shengli, Shi QizhenAbstract:The complex of erbium chloride lower hydrate with diethylammonium diethyldithiocarbamate (D-DDC) has been synthesized conveniently in absolute alcohol and dry N2 atmosphere. The title complex was identified as Et2NH2[Er(S2CNEt2)4] by chemical and elemental analyses, the bonding characteristics of which was characterized by IR. The enthalpies of solution of erbium chloride hydrate and D-DDC in absolute alcohol at 298.15 K and the enthalpies change of liquid-phase reaction of formation for Et2NH2[Er(S2CNEt2)4] at different temperatures were determined by microcalorimetry. On the basis of experimental and calculated results, three thermodynamic parameters (the Activation enthalpy, the Activation Entropy and the Activation free energy), the rate constant and three kinetic parameters (the apparent Activation energy, the pre-exponential constant and the reaction order) of liquid phase reaction of formation were obtained. The enthalpy change of the solid-phase title reaction at 298.15 K was calculated by a thermochemical cycle.
