Temperature Dependence Of Constants Research Articles

Superior High-Temperature Strength in a Supersaturated Refractory High-Entropy Alloy.

Refractory high-entropy alloys (RHEAs) show promising applications at high temperatures. However, achieving high strengths at elevated temperatures above 1173K is still challenging due to heat softening. Using intrinsic material characteristics as the alloy-design principles, a single-phase body-centered-cubic (BCC) CrMoNbV RHEA with high-temperature strengths (beyond 1000MPa at 1273 K) is designed, superior to other reported RHEAs as well as conventional superalloys. The origin of the high-temperature strength is revealed by in situ neutron scattering, transmission-electron microscopy, and first-principles calculations. The CrMoNbV's elevated-temperature strength retention up to 1273 K arises from its large atomic-size and elastic-modulus mismatches, the insensitive temperature dependence of elastic constants, and the dominance of non-screw character dislocations caused by the strong solute pinning, which makes the solid-solution strengthening pronounced. The alloy-design principles and the insights in this study pave the way to design RHEAs with outstanding high-temperature strength.

Degradation kinetic modeling of bioactive compounds and enzyme activity in wheat germ during stabilization

The stability of α-tocopherol (α-toco), total phenolic content (TPC), antioxidant capacity (DPPH and ABTS•+ radical scavenging capacity assays), activity of lipase (LA) and lipoxygenase (LOX) were investigated during stabilization of raw wheat germ with an industrial convection oven at different temperatures (120, 130, 140, 150 and 160 °C) and times (0, 5, 10, 15, and 20 min). Degradation kinetics were best fitted by first-order reaction models for all measured parameters with high R2 and low root mean square (RMSE) values, except ABTSassay which was modeled accurately with the second-order model. The temperature dependence of rate constants for all degradation kinetic parameters were calculated according to the Arrhenius, Eyring-Polanyi, and Ball models. Following the Arrhenius and Eyring models, activation energy (Ea) ranged from 20.69 to 55.04 kJ mol−1, activation enthalpy (ΔH*) ranged from 20.44 to 51.61 kJ mol−1 whereas the activation entropy (ΔS*) varied between −179.313 and −213.447 J mol−1K−1 for the stabilized germ, respectively. It can be concluded that heat treatment at 120 °C for 20 min was the best processing conditions to stabilize the raw germ to preserve its valuable bioactive compounds, antioxidant activity and the models can be useful to design different heat treatment conditions.

High-temperature phonon transport properties of SnSe from machine-learning interatomic potential

As a promising thermoelectric material, tin selenide (SnSe) is of relatively low thermal conductivity. However, the phonon transport mechanisms in SnSe are not fully understood due to the complex phase transition, dynamical instability, and strong anharmonicity. In this work, we perform molecular dynamics simulations with a machine-learning interatomic potential to explore the thermal transport properties of SnSe at different temperatures. The developed interatomic potential is parameterized using the framework of moment tensor potential, exhibiting satisfactory predictions on temperature-dependent lattice constants and phonon dispersion, as well as phase transition temperature. From equilibrium molecular dynamics simulations, we obtained the thermal conductivity tensor from 200 K to 900 K. The origins of temperature-dependent thermal conductivity anisotropy and the roles of four-phonon scatterings are identified. The obtained interatomic potential can be utilized to study the mechanical and thermal properties of SnSe and related nanostructures in a wide range of temperatures.

Phase Transitions in a Perovskite Thin Film Studied by Environmental In Situ Heating Nano-Beam Electron Diffraction.

The rich phase diagram of bulk Pr1-x Cax MnO3 resulting in a high tunability of physical properties gives rise to various studies related to fundamental research as well as prospective applications of the material. Importantly, as a consequence of strong correlation effects, electronic and lattice degrees of freedom are vigorously coupled. Hence, it is debatable whether such bulk phase diagrams can be transferred to inherently strained epitaxial thin films. In this paper, the structural orthorhombic to pseudo-cubic transition for x= 0.1 is studied in ion-beam sputtered thin films and differences to the respective bulk system are pointed out by employing in situ heating nano-beam electron diffraction to follow the temperature dependence of lattice constants. In addition, it is demonstrated that controlling the environment during heating, that is, preventing oxygen loss, is crucial in order to avoid irreversible structural changes, which is expected to be a general problem of compounds containing volatile elements under non-equilibriumconditions.

Pressure and temperature dependence of second-order elastic constants from third-order elastic constants in TMC (TM=Nb, Ti, V, Zr)

In this paper, we present an efficient and effective method to predict the pressure dependence and temperature dependence of second-order elastic constants (SOECs) by introducing third-order elastic constants (TOECs) in the monocarbide ultrahigh temperature ceramics. The method is validated by comparing with experiments and previous calculations in four TMCs (TM = Nb, Ti, V, Zr). Using this method, we investigate the derivatives of SOECs against pressure and temperature as well as the anisotropic properties of polycrystalline modulus. In addition, we fit the SOECs with pressure and temperature under the framework of CALPHAD for practice usage.

Characterization of Ion Association and Solvation in NaPF6 Carbonate Electrolytes

In this work we aimed to determine the ions association and solvation in electrolytes comprised of NaPF6 in binary EC:PC, and ternary 5EC:3PC:2DEC carbonate solvents. We studied their local and bulk transport properties through multi-Nuclear (1H, 19F and 23Na) Magnetic Resonance spin-lattice relaxation time (T1 ) and self-diffusion coefficient (D) measurements. These studies were complemented with ionic conductivity and solution viscosity measurements. Results show several interesting features. Firstly, the Walden Product – which relates the molar conductivity and viscosity through a temperature-dependent constant – showed a dependence on temperature (see Figure 1). This suggested the dissociation of NaPF6 is dependent on temperature and is lower for the less polar electrolytes. Secondly, the sodium ions are preferentially solvated by PC molecules over EC. Thirdly, significant cation-anion association was observed for the less polar electrolytes. Lastly, this association was independent of temperature indicating that the temperature dependence of the Walden Product is primarily due to that of the viscosity. These results can aid in the improvement of ‘beyond lithium ion’ electrolytes for energy storage applications. Figure 1

Strengthening mechanisms in CrMoNbTiW refractory high entropy alloy

We have investigated the strengthening mechanisms of as-cast CrMoNbTiW BCC refractory high entropy alloy (RHEA). The cast RHEA was deformed at a constant strain rate of 10−3 s−1 and a temperature range of 1100–1300 °C. The various factors contributing to the overall strength of the cast alloy are explored with comprehensive experimental evidence. In as-cast RHEA, solid solution strengthening is the dominant mechanism among other factors. A modified Varvenne's solid solution strengthening model is used to predict the yield strength (YS) of the RHEA at high temperatures. The experimentally determined YS exhibits a strong temperature dependence, and the predicted YS values are significantly affected by the temperature-dependent material constants. The absolute value of the predicted YS depends on the shear modulus, whereas the variation of YS with temperature is affected by the Poisson's ratio. Moreover, our study demonstrates that it is feasible to predict the high temperature YS using material constants from literature besides DFT studies.

Intermolecular dynamics and paramagnetic properties of Tm-Na heterometallic complex with macrocyclic ligand by 23Na dynamic NMR

The temperature dependence of lanthanide-induced shifts (LIS) and chemical exchange rate constants are studied. The analysis of chemical exchange rate constants was carried out using the method of analysis of the bandshape taking into account the temperature change in the LIS. The value of Gibbs free energy for the activation of intermolecular dynamics is ΔG≠(298 K) = 55 ± 2 kJ/mol. Method of analysis of the bandshape taking into account the temperature change in the paramagnetic lanthanide-induced shifts used for estimation the activation energy of intermolecular dynamics ΔG≠(298 K) = 55 ± 2 kJ/mol.

Contribution of the carbon-originated hole trap to slow decays of photoluminescence and photoconductivity in homoepitaxial n-type GaN layers

N-type GaN epitaxial layers grown via metal organic vapor-phase epitaxy typically exhibit a yellow luminescence (YL) band owing to carbon-related deep levels in the photoluminescence spectra. The decay of YL after pulse excitation involves a long time constant (∼0.2 ms at room temperature), whereas microwave photoconductivity decay (μ-PCD) curves show the corresponding component of the time constant. To clarify the origin of the long decay time, the temperature-dependent time constants of YL decay and μ-PCD curves are analyzed using a numerical model based on rate equations for trapping and emission through a deep level. The characteristics of the decays are well reproduced by a recombination model using a hole trap H1 at an energy of EV + 0.88 eV because of the acceptor-like state of carbon on a nitrogen site (CN) whose electron capture cross section (σn) is estimated to be 3 × 10−21 cm2. The slow decay in μ-PCD signals indicates that the electrons before being captured to H1 traps are free electrons in the conduction band. These findings indicate that the slow recombination process through CN results in tail currents in the turn-off switching periods of devices.

Thermodynamic properties of fcc metals using reparameterized MEAM potentials

The modified embedded atom method (MEAM) potentials of Jin et al. (Appl Phys A 120:189, 2015) are reparameterized through fitting the available experimental data and ab initio results of several physical properties better for seven face-centered cubic metals, Ag, Al, Au, Cu, Ni, Pd, and Pt. Results for the structure stabilities, point defects, and phonon dispersion properties are in better agreement with the measured values than those from the original MEAM potentials, showing the reliability of the reparameterized potentials. Also, the thermodynamic properties for these metals are studied within the quasi-harmonic approximation. Calculation results for the temperature dependence of lattice constants, thermal expansion coefficients, molar heat capacities at constant volume and/or constant pressure, isothermal and adiabatic bulk moduli, Gruneisen parameters, and Debye temperatures are compared to those from the earlier embedded atom methods and ab initio method calculations as well as the experimental data.

Evaluation of physico-chemical changes and FT-NIR spectra in fresh egg pasta packed in modified atmosphere during storage at different temperatures

Physico-chemical properties (i.e. water activity, colour, water absorption index and hardness) influence stability, quality and consumer acceptability of fresh pasta. The aim of this study was to investigate the effect of storage time on physico-chemical properties of fresh pasta stored at different temperatures (0, 5 and 10 °C). Storage time significantly affected the macroscopic properties (water absorption index and hardness) of fresh pasta and its water status (water activity, freezable water). During storage at all temperatures, water activity and freezable water decreased, confirming a reduction in water mobility in the product. Storage time increased pasta hardness and caused a reduction in the water absorption index. Fresh egg pasta retained its yellow colour, while lipid oxidation indices were only slightly affected. With the increase of storage temperature, the kinetic rate of water activity and gelatinisation degree decreased. In contrast, the kinetic rate of pasta hardness increased with the temperature. The Arrhenius relationship explained the temperature dependence of reaction constants, while FT-NIR analysis showed a structural reorganisation of pasta matrix during storage. Generally, pasta matrix reorganisation involved water, starch and proteins interactions, and storage temperature was an important factor affecting these modifications with specific effects on physico-chemical pasta properties (e.g. hardness).

Quantum Self-Consistent Ab-Initio Lattice Dynamics

The Quantum Self-Consistent Ab-Initio Lattice Dynamics package (QSCAILD) is a python library that computes temperature-dependent effective 2nd and 3rd order interatomic force constants in crystals, including anharmonic effects. QSCAILD’s approach is based on the quantum statistics of a harmonic model. The program requires the forces acting on displaced atoms of a solid as an input, which can be obtained from an external code based on density functional theory, or any other calculator. This article describes QSCAILD’s implementation, clarifies its connections to other methods, and illustrates its use in the case of the SrTiO3 cubic perovskite structure. Program summaryProgram Title: QSCAILDCPC Library link to program files:https://doi.org/10.17632/y4c922fwtf.1Licensing provisions: GNU General Public License version 3.0Programming language: PythonExternal routines/libraries: MPI, NumPy, SciPy, spglib, phonopy, sklearnNature of problem: Calculation of effective interatomic force constants at finite temperatureSolution method: Regression analysis of forces from density functional theory coupled with a harmonic model of the quantum canonical ensemble, performed in an iterative way to achieve self-consistency of the phonon spectrum

Neural network potential energy surface for the low temperature ring polymer molecular dynamics of the H2CO + OH reaction.

A new potential energy surface (PES) and dynamical study of the reactive process of H2CO + OH toward the formation of HCO + H2O and HCOOH + H are presented. In this work, a source of spurious long range interactions in symmetry adapted neural network (NN) schemes is identified, which prevents their direct application for low temperature dynamical studies. For this reason, a partition of the PES into a diabatic matrix plus a NN many-body term has been used, fitted with a novel artificial neural network scheme that prevents spurious asymptotic interactions. Quasi-classical trajectory (QCT) and ring polymer molecular dynamics (RPMD) studies have been carried on this PES to evaluate the rate constant temperature dependence for the different reactive processes, showing good agreement with the available experimental data. Of special interest is the analysis of the previously identified trapping mechanism in the RPMD study, which can be attributed to spurious resonances associated with excitations of the normal modes of the ring polymer.

Electrochemical Behavior of Titanium Complexes in the KCl-KF Melt with Additions of Alkaline Earth Metal Cations

The charge transfer kinetics for the Ti(IV)/Ti(III) redox couple in the KCl-KF(10 wt.%)-K2TiF6 melt was studied. The standard rate constants of charge transfer (k s ) for the redox couple Ti(IV)/Ti(III) in the KCl-KF(10 wt.%)-K2TiF6 melt were calculated. The temperature dependence of the standard rate constants of charge transfer was described by the empirical equation: logk s = (2.01 ± 0.40) – (4178 ± 810)/Т. The activation energy of charge transfer was equal to (80 ± 15) kJ mol–1. Influence of strongly polarizing cations (Mg2+, Ca2+, Sr2+ and Ва2+) on the charge transfer kinetics was studied. It was determined the linear dependence of k s on the ionic potential of alkaline earth metal cations. Values of activation energy of charge transfer for systems with strongly polarizing cations were considerably less than activation energy of the initial system and decrease in case of ionic potential increasing.

Factors Influencing Ultrasonic Wave Propagation in Tourmaline

P-wave propagation velocity in tourmaline is related to its microstructure, chemical composition, temperature, and other factors. The effects of tourmaline density, FeO contents, and temperature on P-wave propagation were studied in this paper. The results show that the temperature affects P-wave velocity due to the temperature dependence of thermal expansion, and elastic and dielectric constants of tourmaline. The propagation velocity of P-wave decreases with increasing porosity or FeO content. The content of FeO in tourmaline can be used as the basis for the determination of different species of tourmaline, which provides an important experimental basis for ultrasonic waves to be used as a nondestructive testing technology to distinguish different types of tourmaline.

Temperature Dependence of Henry's Law Constants of Fenpropidin and Pyrimethanil: Impact on their Atmospheric Partitionnings and Lifetimes

Henry's law constants (HLC) play a key role in the environmental fate of pesticides and their distribution between the different phases, i.e., between air, water and soil. For certain compounds such as fenpropidin and pyrimethanil, HLC values are poorly documented in the literature and in particular their temperature dependence. This work reports the experimental HLC values of two pesticides, namely fenpropidin and pyrimethanil, determined by means of a dynamic equilibrium system coupled to an off-line analysis performed by Gas Chromatography–Photoionisation Detection (GC–PID). The measurements were conducted over the range 278–293 K. In pure water, the experimental average values of HLC at 293 K were: HLC293K(fenpropidin) = (10.0 ± 3.1) × 104 M atm–1, HLC293K(pyrimethanil) = (8.2 ± 0.7) × 104 M atm–1. The obtained data were used to derive the following Arrhenius expressions where the quoted errors represent 2σ: ln HLC (fenpropidin) = (6060 ± 2420)/T – (9.1 ± 8.3); ln HLC (pyrimethanil) = (14570 ± 1800)/T – (38.4 ± 6.2). The environmental implications are then discussed in terms of lifetime or partitionning between the different atmospheric compartments.

Fluorescent Visualization of Nucleolar G-Quadruplex RNA and Dynamics of Cytoplasm and Intranuclear Viscosity

The nucleolus, the locus of ribosome biogenesis, was found to be the predominant intracellular target of a new fluorescent probe, V-P1. In solution, the probe demonstrated both a selectivity to RNA...

Stable and reversible phase change performance of TiO2 coated VO2 nano-columns: Experiments and theoretical analysis

We have fabricated VO2 nano-columns (NCs) on the aluminized quartz glass substrates using glancing angle deposition (GLAD) technique inside the electron beam (e-beam) evaporator chamber. An e-beam deposited TiO2 thin cap-layer has been introduced on the top of the VO2 NCs to protect VO2 phase which is usually unstable and turns to stable higher oxidation state, i.e., V2O5. The X-ray diffraction (XRD) analysis of TiO2 thin film (TF) coated VO2 NCs shows the presence of VO2 phases along with the other oxidation states. The Fourier-Transform-Infrared-Spectroscopy (FTIR) confirms the presence of Ti–O–V, V–O vibrations bands in VO2 NCs/TiO2 TF samples. The VO2 NCs/TiO2 TF sample possesses a sharp drop (~103 order) in resistance at the transition temperature (Tc) ~66 °C. This sample also shows a stable semiconducting to metallic phase (vice versa as well) transition temperature (PTT) hysteresis even after one month which ensures the stable performance in terms of phase transition characteristic. Small-radius polaron model (SRPM) is used to calculate the sheet resistance of the VO2 NCs, where we consider the effect of thermal lattice vibrations on the overlap integral of the crystal atoms. The temperature dependency of constants A and ε have been observed uniquely. The Fermi-Dirac (F-D), Migdal distribution (MD) model have been taken into account for the first time to calculate the conduction band carrier density of VO2 NCs and ratio of the resistance in the semiconducting (RS), metallic phase (RM) is also calculated. The calculated electron concentration (nc) is 3.1 × 1019 cm-3 at the point of PTT (66 °C).

Evaluation of the temperature dependence of dissociation constants for the marine carbon system using pH and certified reference materials

Due to the uptake of anthropogenic carbon dioxide (CO2) from the atmosphere, and the resulting ocean acidification, long term monitoring of the marine carbon cycle is of utmost importance and requires high precision and accuracy across many laboratories over several decades. Despite this, many uncertainties still remain in carbon system measurements and calculations. Calculations of the carbon system are facilitated by publicly available software, but the large number of options for various constants make data and study inter-comparability challenging. Here, the carbon system is evaluated using internal consistency calculations and spectrophotometric pH measurements on two batches of Certified Reference Material over the full oceanic range of temperatures (−1.7–40 °C). The choice of formulation for the bisulfate dissociation constant is insignificant over at least the salinity range of the CRM (S = 33.4–33.8). The choice of formulation of the hydrogen fluoride dissociation constant adds a small amount of uncertainty, but the best option is unclear. The total boron concentration significantly impacts the calculated pH, with the value of Lee et al. (2010) being more internally consistent and, thus, recommended. Of the 8 carbonic acid dissociation constant sets evaluated, nearly all remain internally consistent at near-surface open-ocean salinities (~31–36) within the accuracy of pH measurements. Nevertheless, the values of Mehrbach et al. (1973), as refit by Dickson and Millero (1987), and Lueker et al. (2000) were the most internally consistent and therefore are preferred for current surface ocean studies (S ≈ 31–36, T = −1.7–40 °C, fCO2 < 500 μatm). The pH was more internally consistent at lower temperatures than higher temperatures.

Mode- and Bond-Selected Reaction of H with Local Mode Molecule HDS.

Understanding mode- and bond-selected dynamics of elementary chemical reactions is of central importance in molecular reaction dynamics. The initial state-selected time-dependent wave packet method is employed to study the mode and bond selectivity, isotopic branching ratio, and temperature dependence of rate constants of the two-channel reaction of H with local mode molecule HDS. For the abstraction channel, fundamental excitation of the HS (DS) bond of the reactant HDS significantly enhances the H-abstraction (D-abstraction) reaction, whose efficacy is higher than the same amount of translational energy except at low energies just above the energy threshold. This is in sharp contrast to the prediction of Polanyi rules: translational energy is more efficient than vibrational energy in enhancing a reaction with an early barrier. The recent sudden vector projection model is then applied to rationalize the observed mode specificity, which, however, shows that the translational mode vector has a larger coupling with the reaction coordinate than the stretching vector of the active bond, implying a reversed relative efficacy on promoting the reaction as well. In contrast, the mode and bond specificity for the exchange channel is not as strong as for the abstraction channel due to the regulation of the shallow well along the reaction path.