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Tomoo Katsura - One of the best experts on this subject based on the ideXlab platform.
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electrical conductivity of enstatite as a function of water content implications for the electrical structure in the upper mantle
Earth and Planetary Science Letters, 2012Co-Authors: Tomoo Katsura, Takashi Yoshino, Shuangming Shan, Takuya Matsuzaki, Baohua ZhangAbstract:Abstract The electrical conductivity of Ca-free aluminous enstatite with various water contents has been determined at a pressure of 3 GPa in a Kawai-type multi-anvil apparatus. Impedance spectroscopy was performed for both hydrogen-doped and -undoped samples in a frequency range from 0.1 Hz to 1 MHz to examine the effect of water on conductivity. Two conduction mechanisms were identified for hydrogen-undoped samples at temperature of 1000–1723 K and for hydrogen-doped samples at relatively lower temperature range of 500–900 K to minimize dehydration of samples. For the hydrogen-undoped samples, the Activation Enthalpy is around 1.9 eV at the higher temperatures range (>1300 K) suggesting that the dominant charge transfer mechanism is Fe 2+ −Fe 3+ hopping (small polaron) conduction. For the hydrogen-doped samples measured below 900 K, the Activation Enthalpy decreases from 1.11 to 0.70 eV, and the conductivity values systematically increase with increasing water content, suggesting that proton conduction is the dominant conduction mechanism. Taking hopping conduction and water content dependence of Activation Enthalpy for proton conduction into account, all electrical conductivity data were fitted to the formula σ = σ 0 h exp( −H h / kT )+ σ 0 p C w exp[−( H p 0 − αC w 1/3 )/ kT ], where σ 0 is pre-exponential factor, C w is the water content in weight percent, H is the Activation Enthalpy, H p 0 is the Activation Enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolution temperature and subscripts h and p represent hopping and proton conductions, respectively. Using the present results, a laboratory-based conductivity-depth profile in the Earth's upper mantle has been constructed as a function of water content. Comparison of our model with the currently available geophysical observations beneath the Eastern Pacific Rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine.
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the effect of water on the electrical conductivity of olivine aggregates and its implications for the electrical structure of the upper mantle
Earth and Planetary Science Letters, 2009Co-Authors: Takashi Yoshino, Takuya Matsuzaki, Anton Shatskiy, Tomoo KatsuraAbstract:The electrical conductivity of San Carlos olivine aggregate of various water content was measured at a pressure of 10 GPa in a Kawai-type multi-anvil apparatus. Conductivity measurements were performed on two sets of samples to determine the effect on conductivity of water in olivine: 1) a hydrogen-doped sample and 2) a hydrogen-undoped sample. To minimize water escape from the hydrogen-doped samples, the conductivity measurement was carried out below 1000 K. Three conduction mechanisms were identified from the Arrhenian behavior of the undoped samples, which include a small amount of water. A change in the Activation Enthalpy indicated that the dominant conduction mechanism changed from proton conduction to small polaron conduction with increasing temperature. At temperatures above 1700 K, the Activation Enthalpy exceeds 2 eV suggesting that the dominant mechanism of charge transport would be ionic conduction. The conductivity increased with increasing water content. The Activation Enthalpy for proton conduction tends to decrease slightly with increasing water content. The Activation Enthalpy determined for each run had similar values (~ 0.9 eV). Taking the water concentration dependence of Activation Enthalpy into account for proton conduction, all data were fitted to the electrical conductivity formula σ=σ0Iexp[−EI/kT]+σ0Hexp[−EH/kT]+σ0PCWexp[−(E0−αCW1/3)/kT], where σ0 represents a pre-exponential term, CW is the water content in weight percent, E is the Activation Enthalpy, E0 is the Activation Enthalpy for proton conduction at very low water concentration, α is the geometrical factor, k is the Boltzmann constant, T is absolute temperature and subscripts I, H and P denote ionic, hopping (small polaron) and proton conductions, respectively. The conductivity jump at the 410 km discontinuity (olivine–wadsleyite transition) is much smaller than that previously predicted. Since the contribution of proton conduction to the bulk electrical conductivity decreases with increasing temperature the high conductivity anomaly at the top of the asthenosphere cannot be explained by olivine hydration.
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effect of iron content on electrical conductivity of ringwoodite with implications for electrical structure in the transition zone
Physics of the Earth and Planetary Interiors, 2009Co-Authors: Takashi Yoshino, Tomoo KatsuraAbstract:Abstract Electrical conductivity of ringwoodite with various iron contents [Fe/(Fe + Mg) = 0.09, 0.2 and 0.3] was measured at pressure (20 GPa) and temperature (up to 1900 K) conditions of the lower part of the mantle transition zone in a Kawai-type multi-anvil apparatus. The conductivity increased with increasing total iron content. All electrical conductivity data were fitted to the formula of electrical conductivity σ = σ0 XFe exp(−H/kT), where σ0 is the pre-exponential term, XFe is the mole fraction of iron content in the Mg site, H is the Activation Enthalpy, k is the Boltzmann constant and T is absolute temperature. The Activation Enthalpy becomes higher at a certain temperature. At high temperatures, the Activation Enthalpy decreased from 1.44 to 0.92 eV with increasing total Fe content. At low temperatures less than 1000 K, the Activation Enthalpy also decreases from 1.15 to 0.74 eV with total Fe content. Dependence of the Activation Enthalpy on Fe content suggests that the dominant mechanism of charge transport is Fe2+–Fe3+ hopping (small polaron). Recent electrical conductivity-depth profiles of the transition zone beneath the Pacific Ocean obtained from the electromagnetic induction study shows that the conductivity values between 520 and 660 km depths may be explained by ringwoodite with Fe/(Fe + Mg) = 0.10. On the other hand, assuming a normal geotherm, conductivity values beneath the continent or stable craton are considerably lower than those of ringwoodite with Fe/(Fe + Mg) = 0.10. Taking into consideration results from the global seismic tomographic studies, relatively low conductivity in these regions can be explained by the existence of a cooler region compared with the surrounding mantle, rather than the presence of iron-poor ringwoodite, or a combination of both.
Jack F Douglas - One of the best experts on this subject based on the ideXlab platform.
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mass dependence of the Activation Enthalpy and entropy of unentangled linear alkane chains
Journal of Chemical Physics, 2015Co-Authors: Cheol Jeong, Jack F DouglasAbstract:The mass scaling of the self-diffusion coefficient D of polymers in the liquid state, D ∼ Mβ, is one of the most basic characteristics of these complex fluids. Although traditional theories such as the Rouse and reptation models of unentangled and entangled polymer melts, respectively, predict that β is constant, this exponent for alkanes has been estimated experimentally to vary from −1.8 to −2.7 upon cooling. Significantly, β changes with temperature T under conditions where the chains are not entangled and at temperatures far above the glass transition temperature Tg where dynamic heterogeneity does not complicate the description of the liquid dynamics. Based on atomistic molecular dynamics simulations on unentangled linear alkanes in the melt, we find that the variation of β with T can be directly attributed to the dependence of the Enthalpy ΔHa and entropy ΔSa of Activation on the number of alkane backbone carbon atoms, n. In addition, we find a sharp change in the melt dynamics near a “critical” cha...
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mass dependence of the Activation Enthalpy and entropy of unentangled linear alkane chains
Journal of Chemical Physics, 2015Co-Authors: Cheol Jeong, Jack F DouglasAbstract:The mass scaling of the self-diffusion coefficient D of polymers in the liquid state, D ∼ M(β), is one of the most basic characteristics of these complex fluids. Although traditional theories such as the Rouse and reptation models of unentangled and entangled polymer melts, respectively, predict that β is constant, this exponent for alkanes has been estimated experimentally to vary from -1.8 to -2.7 upon cooling. Significantly, β changes with temperature T under conditions where the chains are not entangled and at temperatures far above the glass transition temperature Tg where dynamic heterogeneity does not complicate the description of the liquid dynamics. Based on atomistic molecular dynamics simulations on unentangled linear alkanes in the melt, we find that the variation of β with T can be directly attributed to the dependence of the Enthalpy ΔHa and entropy ΔSa of Activation on the number of alkane backbone carbon atoms, n. In addition, we find a sharp change in the melt dynamics near a "critical" chain length, n ≈ 17. A close examination of this phenomenon indicates that a "buckling transition" from rod-like to coiled chain configurations occurs at this characteristic chain length and distinct entropy-Enthalpy compensation relations, ΔSa ∝ ΔHa, hold on either side of this polymer conformational transition. We conclude that the Activation free energy parameters exert a significant influence on the dynamics of polymer melts that is not anticipated by either the Rouse and reptation models. In addition to changes of ΔHa and ΔSa with M, we expect changes in these free energy parameters to be crucial for understanding the dynamics of polymer blends, nanocomposites, and confined polymers because of changes of the fluid free energy by interfacial interactions and geometrical confinement.
David Rodney - One of the best experts on this subject based on the ideXlab platform.
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first principles prediction of kink pair Activation Enthalpy on screw dislocations in bcc transition metals v nb ta mo w and fe
Physical Review B, 2015Co-Authors: Lucile Dezerald, Lisa Ventelon, Laurent Proville, F Willaime, David RodneyAbstract:The atomistic study of kink pairs on screw dislocations in body-centered cubic (bcc) metals is challenging because interatomic potentials in bcc metals still lack accuracy and kink pairs require too many atoms to be modeled by first principles. Here, we circumvent this difficulty using a one-dimensional line tension model whose parameters, namely the line tension and Peierls barrier, are reachable to density functional theory calculations. The model parameterized in V, Nb, Ta, Mo, W, and Fe, is used to study the kink-pair Activation Enthalpy and spatial extension. Interestingly, we find that the atomistic line tension is more than twice the usual elastic estimates. The calculations also show interesting group tendencies with the line tension and kink-pair width larger in group V than in group VI elements. Finally, the present kink-pair Activation energies are shown to compare qualitatively with experimental data and potential origins of quantitative discrepancies are discussed.
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Activation Enthalpy for kink-pair nucleation on dislocations: Comparison between static and dynamic atomic-scale simulations
Physical Review B, 2007Co-Authors: David RodneyAbstract:We show in the case of a high-Peierls-stress Lomer dislocation in an aluminum crystal that the dependence of the kink-pair Activation Enthalpy on the stress obtained from static nudged elastic band method calculations agrees with that extracted from dynamical, constant strain-rate simulations. In order to perform the dynamical simulations, we first propose flexible boundary conditions to replace the rigid conditions that are usually applied. This removes the spurious forces on the dislocation that arise because of the mismatch between the elastic strain imposed by the rigid conditions and the plastic strain associated with the dislocation motion. Second, we present a statistical analysis to rigorously extract Enthalpy-stress relations from dynamical simulations. We find that the Activation Enthalpy becomes zero for a stress (which we call the Peierls stress for kink nucleation) smaller than that required to move athermally a rigid straight dislocation (called here the Peierls stress for rigid motion). This effect may explain the discrepancy often reported in the literature between the Peierls stress predicted by atomistic calculations, determined on short two dimensional dislocations, i.e., the Peierls stress for rigid motion, and the Peierls stress extracted from experiments, which corresponds to that when kink pairs form on three dimensional dislocations without the help of thermal fluctuations, i.e., the Peierls stress for kink nucleation.
Kenneth A Johnson - One of the best experts on this subject based on the ideXlab platform.
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a new general method for simultaneous fitting of temperature and concentration dependence of reaction rates yields kinetic and thermodynamic parameters for hiv reverse transcriptase specificity
Journal of Biological Chemistry, 2017Co-Authors: An Li, Jessica L Ziehr, Kenneth A JohnsonAbstract:: Recent studies have demonstrated the dominant role of induced fit in enzyme specificity of HIV reverse transcriptase and many other enzymes. However, relevant thermodynamic parameters are lacking, and equilibrium thermodynamic methods are of no avail because the key parameters can only be determined by kinetic measurement. By modifying KinTek Explorer software, we present a new general method for globally fitting data collected over a range of substrate concentrations and temperatures and apply it to HIV reverse transcriptase. Fluorescence stopped-flow methods were used to record the kinetics of enzyme conformational changes that monitor nucleotide binding and incorporation. The nucleotide concentration dependence was measured at temperatures ranging from 5 to 37 °C, and the raw data were fit globally to derive a single set of rate constants at 37 °C and a set of Activation Enthalpy terms to account for the kinetics at all other temperatures. This comprehensive analysis afforded thermodynamic parameters for nucleotide binding (Kd , ΔG, ΔH, and ΔS at 37 °C) and kinetic parameters for enzyme conformational changes and chemistry (rate constants and Activation Enthalpy). Comparisons between wild-type enzyme and a mutant resistant to nucleoside analogs used to treat HIV infections reveal that the ground state binding is weaker and the Activation Enthalpy for the conformational change step is significantly larger for the mutant. Further studies to explore the structural underpinnings of the observed thermodynamics and kinetics of the conformational change step may help to design better analogs to treat HIV infections and other diseases. Our new method is generally applicable to enzyme and chemical kinetics.
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a new general method for simultaneous fitting of temperature and concentration dependence of reaction rates yields kinetic and thermodynamic parameters for hiv reverse transcriptase specificity
Journal of Biological Chemistry, 2017Co-Authors: Jessica L Ziehr, Kenneth A JohnsonAbstract:Recent studies have demonstrated the dominant role of induced fit in enzyme specificity of HIV reverse transcriptase and many other enzymes. However, relevant thermodynamic parameters are lacking, and equilibrium thermodynamic methods are of no avail because the key parameters can only be determined by kinetic measurement. By modifying KinTek Explorer software, we present a new general method for globally fitting data collected over a range of substrate concentrations and temperatures and apply it to HIV reverse transcriptase. Fluorescence stopped-flow methods were used to record the kinetics of enzyme conformational changes that monitor nucleotide binding and incorporation. The nucleotide concentration dependence was measured at temperatures ranging from 5 to 37 °C, and the raw data were fit globally to derive a single set of rate constants at 37 °C and a set of Activation Enthalpy terms to account for the kinetics at all other temperatures. This comprehensive analysis afforded thermodynamic parameters for nucleotide binding (Kd, ΔG, ΔH, and ΔS at 37 °C) and kinetic parameters for enzyme conformational changes and chemistry (rate constants and Activation Enthalpy). Comparisons between wild-type enzyme and a mutant resistant to nucleoside analogs used to treat HIV infections reveal that the ground state binding is weaker and the Activation Enthalpy for the conformational change step is significantly larger for the mutant. Further studies to explore the structural underpinnings of the observed thermodynamics and kinetics of the conformational change step may help to design better analogs to treat HIV infections and other diseases. Our new method is generally applicable to enzyme and chemical kinetics.
Alexander Stoppa - One of the best experts on this subject based on the ideXlab platform.
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Molar Conductivities and Association Constants of 1-Butyl-3-methylimidazolium Chloride and 1-Butyl-3-methylimidazolium Tetrafluoroborate in Methanol and DMSO
Journal of Chemical & Engineering Data, 2010Co-Authors: Marija Bešter-rogač, Johannes Hunger, Alexander StoppaAbstract:Molar conductivities, Λ, of dilute solutions of the ionic liquids 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium tetrafluoroborate in methanol and DMSO were determined as a function of temperature (methanol, T = (273.15 to 313.15) K; DMSO, T = (293.15 to 318.15) K). The data were analyzed with Barthel’s low-concentration chemical model (lcCM) model to obtain the limiting molar conductivities, Λ∞(T), and association constants, KA°(T), of these electrolytes. From Λ∞(T), the Eyring Activation Enthalpy of charge transport and (where possible) the ion conductivity of the cation, λ∞([bmim]+), were determined.
