Terms Of Temperature Dependence Research Articles

High-temperature thermodynamics of silver: Semi-empirical approach* *Project supported by the Major Research Project, UGC, New Delhi, India (Grant No. 42-771/2013 (SR)).

We report high-temperature thermodynamics for fcc silver by combining ab initio phonon dynamics to empirical quadratic temperature-dependent term for anharmonic part of Helmholtz free energy. The electronic free energy is added through an interpolation scheme, which connects ambient condition free electron gas model to Thomas–Fermi results. The present study shows good agreement with experimental and reported findings for several thermal properties, and the discrepancy observed in some caloric properties is addressed. The decreases in the product of volume thermal expansion coefficient and isothermal bulk modulus and in the constant volume anharmonic lattice specific heat at high temperature are the clear evidences of proper account of anharmonicity. The present study also reveals that T 2–dependent anharmonic free energy is sufficient for correct evaluation of thermal pressure and conventional Grüneisen parameter. We observe that the intrinsic phonon anharmonicity starts dominating above characteristic temperature, which is attributed to higher order anharmonicity and can be related to higher order potential derivatives. We conclude that the uncorrelated and largeamplitude lattice vibrations at high temperature raise dominating intrinsic thermal stress mechanism, which surpasses the phonon-anharmonism and requires future consideration.

Controlled-release solid dispersions of Eudragit® FS 100 and poorly soluble spironolactone prepared by electrospinning and melt extrusion

Eudragit® FS (EudFS), a novel anionic metachrylate terpolymer with pH dependent solubility was processed using electrospinning (ES) for the first time and melt extrusion (EX) for controlled delivery of a poorly water-soluble model drug, spironolactone (SPIR). Optimization of ES showed good spinnability of EudFS. Nanofibrous products could be obtained using fairly polar solvents (acetone, dichloromethane-ethanol (1:1), dimethylformamide). Drug-loaded nanofibers were prepared using novel alternating current electrospinning at multiple times higher throughput than direct current electrospinning. Melt extrusion proved to be also a feasible technique to prepare EudFS-based solid dispersions above 100°C. The viscoelasticity of EudFS melts was characterized in terms of temperature, frequency and heating time dependence. Solid state analyses revealed amorphous SPIR content in the electrospun fibers, however, signs of residual crystallinity could be detected in the extruded sample with 20% SPIR according to XRPD and EDS mapping. In vitro dissolution of the EudFS-based solid dispersions showed superior control of drug release compared to the physical mixture of crystalline SPIR and EudFS. At gastric pH where the polymer is insoluble the drug was more restrained in the ground extrudates than in the electrospun fibers. This state was followed by a quick burst as the pH rose to 7.4. EudFS, well-processed by electrospinning or melt extrusion, proved to be a promising matrix for oral drug delivery applications especially targeting the colon.

Depth perception: the need to report ocean biogeochemical rates as functions of temperature, not depth.

For over 50 years, ocean scientists have oddly represented ocean oxygen consumption rates as a function of depth but not temperature in most biogeochemical models. This unique tradition or tactic inhibits useful discussion of climate change impacts, where specific and fundamental temperature-dependent terms are required. Tracer-based determinations of oxygen consumption rates in the deep sea are nearly universally reported as a function of depth in spite of their well-known microbial basis. In recent work, we have shown that a carefully determined profile of oxygen consumption rates in the Sargasso Sea can be well represented by a classical Arrhenius function with an activation energy of 86.5 kJ mol-1, leading to a Q10 of 3.63. This indicates that for 2°C warming, we will have a 29% increase in ocean oxygen consumption rates, and for 3°C warming, a 47% increase, potentially leading to large-scale ocean hypoxia should a sufficient amount of organic matter be available to microbes. Here, we show that the same principles apply to a worldwide collation of tracer-based oxygen consumption rate data and that some 95% of ocean oxygen consumption is driven by temperature, not depth, and thus will have a strong climate dependence. The Arrhenius/Eyring equations are no simple panacea and they require a non-equilibrium steady state to exist. Where transient events are in progress, this stricture is not obeyed and we show one such possible example.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.

Nonlinear response and avalanche behavior in metallic glasses

The response to different stress amplitudes at temperatures below the glass transition temperature is analyzed by mechanical oscillatory excitation of Pd40Ni40P20 metallic glass samples in single cantilever bending geometry. While low amplitude oscillatory excitations are commonly used in mechanical spectroscopy to probe the relaxation spectrum, in this work the response to comparably high amplitudes is investigated. The strain response of the material is well below the critical yield stress even for highest stress amplitudes, implying the expectation of a linear relation between stress and strain according to Hooke’s Law. However, a deviation from the linear behavior is evident, which is analyzed in terms of temperature dependence and influence of the applied stress amplitude by two different approaches of evaluation. The nonlinear approach is based on a nonlinear expansion of the stress-strain-relation, assuming an intrinsic nonlinear character of the shear or elastic modulus. The degree of nonlinearity is extracted by a period-by-period Fourier-analysis and connected to nonlinear coefficients, describing the intensity of nonlinearity at the fundamental and higher harmonic frequencies. The characteristic timescale to adapt to a significant change in stress amplitude in terms of a recovery timescale to a steady state value is connected to the structural relaxation time of the material, suggesting a connection between the observed nonlinearity and primary relaxation processes. The second approach of evaluation is termed the incremental analysis and relates the observed response behavior to avalanches, which occur due to the activation and correlation of local microstructural rearrangements. These rearrangements are connected with shear transformation zones and correspond to localized plastic events, which are superimposed on the linear response behavior of the material.

Temperature-dependent crystal-plasticity model for magnesium: A bottom-up approach

A crystal-plasticity model is developed to account for temperature-dependent mechanical behaviour of magnesium in this paper. The constitutive description of plastic deformation accounts for crystalline slip and twining as well as their interactions. The temperature dependence is incorporated into the constitutive equations for both slip and twin modes based on experimental observations. A bottom-up computational modelling framework is proposed to validate the developed constitutive model. First, the crystal-plasticity model is calibrated with experimental results for plane compression at micro-scale. At meso-scale, a three-dimensional representative element volume was adopted to represent the microstructure of polycrystalline magnesium. In the combination with the proposed constitutive theory, the effects of temperature on mechanical response and evolution of twins and texture in polycrystalline magnesium were predicted. Comprehensive experimental validations at meso-scale were performed to consolidate further the developed crystal-plasticity model incorporating temperature dependence in terms of stress-strain curves, the Hall-Petch relationship and texture evolution. This work provides a useful modelling tool for understanding temperature-dependent behaviour of magnesium, which could be used to improve the formability of this family of materials.

Ab Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2.

In single-layer WSe2, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K± valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a "valleytronic" device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter-free description of the valley dynamics. By using an atomistic, ab initio approach, we fully disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time-dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron-phonon-mediated processes that induce spin-flip intervalley transitions.

Incoherent superconductivity well above in high- cuprates—harmonizing the spectroscopic and thermodynamic data

Cuprate superconductors have long been known to exhibit an energy gap that persists high above the superconducting transition temperature (). Debate has continued now for decades as to whether it is a precursor superconducting gap or a pseudogap arising from some competing correlation. Failure to resolve this has arguably delayed explaining the origins of superconductivity in these highly complex materials. Here we effectively settle the question by calculating a variety of thermodynamic and spectroscopic properties, exploring the effect of a temperature-dependent pair-breaking term in the self-energy in the presence of pairing interactions that persist well above . We start by fitting the detailed temperature-dependence of the electronic specific heat and immediately can explain its hitherto puzzling field dependence. Taking this same combination of pairing temperature and pair-breaking scattering we are then able to simultaneously describe in detail the unusual temperature and field dependence of the superfluid density, tunneling, Raman and optical spectra, which otherwise defy explanation in terms a superconducting gap that closes conventionally at . These findings demonstrate that the gap above in the overdoped regime likely originates from incoherent superconducting correlations, and is distinct from the competing-order ‘pseudogap’ that appears at lower doping.

How have recent temperature changes affected the efficiency of ocean biological carbon export?

AbstractThe ocean's large, microbially mediated reservoirs of carbon are intimately connected with atmospheric CO2 and climate, yet quantifying the feedbacks between them remains an unresolved challenge. Through an idealized mechanistic model, we consider the impact of documented climate change during the past few decades on the efficiency of biological carbon export out of the surface ocean. This model is grounded in universal metabolic phenomena, describing export efficiency's temperature dependence in terms of the differential temperature sensitivity of phototrophic and heterotrophic metabolism. Temperature changes are suggested to have caused a statistically significant decrease in export efficiency of 1.5% ± 0.4% over the past 33 yr. Larger changes are suggested in the midlatitudes and Arctic. This interpretation is robust across multiple sea surface temperature and net primary production data products. The same metabolic mechanism may have resulted in much larger changes e.g., in response to the large temperature shifts between glacial and interglacial time periods.

On the Prediction of α-Martensite Temperatures in Medium Manganese Steels

A new composition-based method for calculating the α-martensite start temperature in medium manganese steel is presented and uses a regular solution model to accurately calculate the chemical driving force for α-martensite formation, $$ \Delta G_{\text{Chem}}^{\gamma \to \alpha } $$ . In addition, a compositional relationship for the strain energy contribution during martensitic transformation was developed using measured Young’s moduli (E) reported in literature and measured values for steels produced during this investigation. An empirical relationship was developed to calculate Young’s modulus using alloy composition and was used where dilatometry literature did not report Young’s moduli. A comparison of the $$ \Delta G_{\text{Chem}}^{\gamma \to \alpha } $$ normalized by dividing by the product of Young’s modulus, unconstrained lattice misfit squared (δ 2), and molar volume (Ω) with respect to the measured α-martensite start temperatures, $$ M_{\text{S}}^{\alpha } $$ , produced a single linear relationship for 42 alloys exhibiting either lath or plate martensite. A temperature-dependent strain energy term was then formulated as $$ \Delta G_{\text{str}}^{\gamma \to \alpha } \left( {{\text{J}}/{\text{mol}}} \right) = E\varOmega \delta^{2} (14.8 - 0.013T) $$ , which opposed the chemical driving force for α-martensite formation. $$ M_{\text{S}}^{\alpha } $$ was determined at a temperature where $$ \Delta G_{\text{Chem}}^{\gamma \to \alpha } + \Delta G_{\text{str}}^{\gamma \to \alpha } = 0 $$ . The proposed $$ M_{\text{S}}^{\alpha } $$ model shows an extended temperature range of prediction from 170 K to 820 K (−103 °C to 547 °C). The model is then shown to corroborate alloy chemistries that exhibit two-stage athermal martensitic transformations and two-stage TRIP behavior in three previously reported medium manganese steels. In addition, the model can be used to predict the retained γ-austenite in twelve alloys, containing e-martensite, using the difference between the calculated $$ M_{\text{S}}^{\varepsilon } $$ and $$ M_{\text{S}}^{\alpha } $$ .

Scaling of size distributions of C60 and C70 fullerene surface islands

We present experimental data and a theoretical analysis for the size distributions of C60 and C70 surface islands deposited onto In-modified Si(111)√3×√3-Au surface under different conditions. We show that both fullerene islands feature an analytic Vicsek-Family scaling shape where the scaled size distributions are given by a power law times an incomplete beta-function with the required normalization. The power exponent in this distribution corresponds to the fractal shape of two-dimensional islands, confirmed by the experimentally observed morphologies. Quite interestingly, we do not see any significant difference between C60 and C70 fullerenes in terms of either scaling parameters or temperature dependence of the diffusion constants. In particular, we deduce the activation energy for surface diffusion of ED=140±10meV for both types of fullerenes.

Anisotropic temperature-dependent thermal conductivity by an Al2O3 interlayer in Al2O3/ZnO superlattice films

The thermal conductivity of superlattice films is generally anisotropic and should be studied separately in the in-plane and cross-plane directions of the films. However, previous works have mostly focused on the cross-plane thermal conductivity because the electrons and phonons in the cross-plane direction of superlattice films may result in much stronger interface scattering than that in the in-plane direction. Nevertheless, it is highly desirable to perform systematic studies on the effect of interface formation in semiconducting superlattice films on both in-plane and cross-plane thermal conductivities. In this study, we determine both the in-plane and cross-plane thermal conductivities of Al2O3 (AO)/ZnO superlattice films grown by atomic layer deposition (ALD) on SiO2/Si substrates in the temperature range of 50–300 K by the four-point-probe 3-ω method. Our experimental results indicate that the formation of an atomic AO layer (0.82 nm) significantly contributes to the decrease of the cross-plane thermal conductivity of the AO/ZnO superlattice films compared with that of AO/ZnO thin films. The cross-plane thermal conductivity (0.26–0.63 W m−1 K−1 of the AO/ZnO superlattice films (with an AO layer of ∼0.82 nm thickness) is approximately ∼150%–370% less than the in-plane thermal conductivity (0.96–1.19 W m−1 K−1) of the corresponding film, implying significant anisotropy. This indicates that the suppression of the cross-plane thermal conductivity is mainly attributed to the superlattice, rather than the nanograin columnar structure in the films. In addition, we theoretically analyzed strong anisotropic behavior of the in-plane and cross-plane thermal conductivities of the AO/ZnO superlattice films in terms of temperature dependence.

A Precision SiGe Reference Circuit Utilizing Si and SiGe Bandgap Voltage Differences

Silicon–germanium (SiGe) BiCMOS technology platform provides designers with a unique opportunity to have access to both Si–Si and SiGe–Si p-n junctions. By taking advantage of the coexistence of these two p-n junctions, this paper presents a new temperature compensation technique for SiGe reference circuits. The source of the appearance of curvature in the thermal characteristics of reference circuits is due to the fact that the temperature-dependent nonlinearities in the base–emitter junction are not completely canceled across the temperature range of interest. Here, it is shown that under specific biasing conditions, the two Si–Si and SiGe–Si p-n junction voltages exhibit similar nonlinear temperature dependences. As such, a two-step temperature compensation technique is proposed: first, via a weighted subtraction of two currents, one proportional to the base–emitter junction of Si BJT and the other proportional to the base–emitter junction of SiGe heterojunction bipolar transistor, major nonlinear temperature-dependent terms are canceled; second, via the addition of a proportional to the absolute temperature current, the remaining linear temperature-dependent term is canceled. As a result of this two-step temperature compensation technique, an output voltage proportional to the difference of the bandgap voltages of Si and SiGe can be obtained. The proposed compensation technique is utilized to implement a precision reference circuit in IBM’s SiGe BiCMOS 8HP technology. Measurement results verify the stability of the circuit against temperature variations. As the generated output voltage of the circuit is proportional to the difference of Si and SiGe bandgap voltages, the proposed circuit can be utilized to experimentally evaluate the Ge-induced bandgap offset in a given SiGe technology platform.

Local chemical potential, local hardness, and dual descriptors in temperature dependent chemical reactivity theory.

In this work we establish a new temperature dependent procedure within the grand canonical ensemble, to avoid the Dirac delta function exhibited by some of the second order chemical reactivity descriptors based on density functional theory, at a temperature of 0 K. Through the definition of a local chemical potential designed to integrate to the global temperature dependent electronic chemical potential, the local chemical hardness is expressed in terms of the derivative of this local chemical potential with respect to the average number of electrons. For the three-ground-states ensemble model, this local hardness contains a term that is equal to the one intuitively proposed by Meneses, Tiznado, Contreras and Fuentealba, which integrates to the global hardness given by the difference in the first ionization potential, I, and the electron affinity, A, at any temperature. However, in the present approach one finds an additional temperature-dependent term that introduces changes at the local level and integrates to zero. Additionally, a τ-hard dual descriptor and a τ-soft dual descriptor given in terms of the product of the global hardness and the global softness multiplied by the dual descriptor, respectively, are derived. Since all these reactivity indices are given by expressions composed of terms that correspond to products of the global properties multiplied by the electrophilic or nucleophilic Fukui functions, they may be useful for studying and comparing equivalent sites in different chemical environments.

Bimaterial infinite plane with a cavity on the interface subjected to different temperatures

Stress analysis is applied to two bonded materials of infinite extent with a single notch at their interface. The two materials, represented in normal view as two joined half planes with a notch, are subjected to different temperatures. As illustration, a bimaterial infinite plane with an elliptical hole at the interface is analyzed. To achieve a closed-form solution, a rational mapping function and a complex variable method are used. By changing the mapping function, other geometries for the notch can be analyzed. The cavity at the interface constitutes a small defect because its extent is small compared with the infinite plane. A mathematical difficulty arises with the presence of the points at infinity of the half planes. The coefficients of the homogeneous part of the stress function are expressible in terms of Dundurs’ parameters, but the temperature-dependent loading term is not. Stress distributions without and with debonding for two pairings of bimaterial are shown. The stress intensity of debonding is defined, and values are investigated for various debonding lengths for the elliptical holes. Also the debonding extension is investigated. As a special case, the stress intensity factors for T-shaped cracks are also investigated.

Nuclear level density of even-even nuclei with temperature-dependent pairing energy

The influence of using a temperature-dependent pairing term on the back-shifted Fermi gas (BSFG) model of nuclear level density of some even-even nuclei has been investigated. We have chosen an approach to determine the adjustable parameters from theoretical calculations, directly. The exact Ginzburg-Landau (EGL) theory was used to determine the temperature-dependent pairing energy as back-shifted parameter of the BSFG model. The level density parameter of the BSFG model has been determined through the Thomas-Fermi approximation. The level densities of 96Mo, 106,112Cd, 106,108Pd, 164Dy, 232Th, 238U and heat capacities of 96Mo and 164Dy nuclei were calculated. Good agreement between theory and experiment was observed.

Carrier Localization Effects in InGaN/GaN Multiple-Quantum-Wells LED Nanowires: Luminescence Quantum Efficiency Improvement and "Negative" Thermal Activation Energy.

Two-dimensional InGaN/GaN multiple-quantum-wells (MQW) LED structure was nanotextured into quasi-one-dimensional nanowires (NWs) with different average diameters with a combination approach of Ni nanoislands as mask + dry etching. Such nanotexturing bring out several appealing effects including deeper localization of carriers and significant improvement in quantum efficiency (e.g., from 4.76% of the planar MQW structure to 12.5% of the 160 nm MQW NWs) of light emission in the whole interested temperature range from 4 K to 300 K. With the aid of localized-state ensemble (LSE) luminescence model, the photoluminescence spectra of the samples are quantitatively interpreted in the entire temperature range. In terms of distinctive temperature dependence of photoluminescence from these samples, a concept of “negative” thermal activation energy is tentatively proposed for the MQW NWs samples. These findings could lead to a deeper insight into the physical nature of localization and luminescence mechanism of excitons in InGaN/GaN nanowires.

Numerical simulation of dual-phase-lag bioheat transfer model during thermal therapy

This paper theoretically investigates the thermal behavior in a living biological tissue under various coordinate systems and different non-Fourier boundary conditions with the dual-phase-lag bioheat transfer model during thermal therapy. The properties of Legendre wavelets together with the finite difference scheme are used to find an approximate analytical solution of the present problem. It has been observed that surrounding healthy tissues are less affected in second and third kind of boundary condition when applied along with spherical symmetric coordinate system. Also greater temperature rise and fast achievement of peak hyperthermia temperature is achieved when second and third kind of boundary conditions are used in combination with Cartesian coordinate system. It is observed that due to the presence of blood perfusion and temperature dependent metabolic heat generation term, the dual-phase-lag bioheat transfer model reduces to Pennes bioheat transfer model only when τq=τT=0s, not for arbitrary τq=τT. Further, in case of dual-phase-lag bioheat transfer model wave-like or diffusion-like behavior will dominate depends whether the ratio τq/τT > 1 or τq/τT < 1. Effect of temperature dependent metabolic heat generation rate, thermal conductivity and blood perfusion rate on dimensionless temperature are discussed in details. The whole analysis is presented in dimensionless form.

Back shifted Fermi gas model with temperature dependent pairing energy: Thermal properties of 98Mo

The effect of using a temperature dependent pairing term in back-shifted Fermi-gas (BSFG) formula of nuclear level density has been studied. We have used the mean order parameter formula of modified Ginzburg–Landau (MGL) theory as a simple possible choice for temperature dependency of the pairing term. The level density and heat capacity of [Formula: see text]Mo have been calculated with this formalism and compared with the experimental data. We observed good agreement between the heat capacity of this model and the experimental data.

Average electronic energy is the central quantity in conceptual chemical reactivity theory

We show that, at both zero and nonzero temperature, the average electronic energy is the central quantity for the study of chemical reactivity using density functional theory. Specifically, we show that regardless of the temperature and the ensemble framework one chooses, the average electronic energy exhibits the functional dependence with respect to external potential and electron number of the zero-temperature case, but with the presence of an additional temperature-dependent term that vanishes in the zero-temperature limit. This reconciles the results from the finite-temperature chemical reactivity theory with those of the traditional zero-temperature approach, and resolves apparent discrepancies between the canonical and grand canonical ensembles. However, the grand canonical ensemble provides a more convenient mathematical framework for studying chemical reactivity because it regards chemical species as open quantum (sub)systems. The response functions of the average electronic energy reduce to the zero-temperature reactivity descriptors in the appropriate limit, helping to justify the use of idealized zero-temperature approximations for describing experimental results obtained at nonzero temperature. Extending this approach to the entropy, the responses of the entropy are shown to give a different set of interesting chemical reactivity response functions. The expressions for the heat capacity of an electronic system are especially elucidative.

Study of heat conduction through a self-heated composite cylinder by Laplace transfer functions

A study of heat conduction through a composite cylinder showing large differences in scale is presented. The system consists of a thin electrically conductive sensing layer, subjected to Joule self-heating and placed in thermal contact with an inner core and an outer concentric cylindrical sheath. The formalism of Laplace transfer functions is used to relate the operating temperature of the sensing layer to the unknown outermost temperature, to the Joule self-heating and to the thermal contact resistances. Analytical expressions for the overheating temperatures are deduced from these transfer functions. Digital filters are derived from the Laplace transfer functions by using Padé approximants and a bilinear transformation. The real-time direct and inverse heat conduction problems are solved without resorting to Laplace inversion, but rather by using the recursion relations associated with the Padé digital filters. This method allows one to easily solve the problems of heat conduction through a self-heated composite cylinder, even in the nonlinear case of a temperature dependent source term. This approach could be applied to optimize and accurately describe the operation of commonly used wire-wound resistance temperature detectors.