Normal Temperature Dependence Research Articles

First-principles study on thermal and electrical transport properties of NbGe2 and NbSi2: the role of electron-phonon coupling

Abstract We study coupled electron and phonon transport in NbX2 with X=Ge, Si, in which experimental evidence of strong electron-phonon coupling and hydrodynamic transport has been reported. Based on first-principles calculations at the density functional theory level, we report their thermal and electrical transport properties. We find that phonon-electron scattering strongly affects their phonon thermal conductivity (κ ph) and leads to weak temperature dependence of κ ph, instead of normal inverse temperature dependence when anharmonic three phonon scattering dominates. Moreover, κ ph contributes to quarter of the total thermal conductivity, which differs from typical metals where total thermal conductivity is predominantly from electron part. Different from previous numerical study, our results of electrical resistivity agree well with experimental measurement. The anisotropic property of the transport coeffcients is attributed to the electron and phonon dispersion relation. Moreover, we find negligible effect of electron-phonon drag on the transport property, contrary to expectation from a strongly coupled electron-phonon fluid.

Abnormal temperature-dependent transient electroluminescence spikes induced by the leakage of hole carriers in organic light-emitting diodes

The amplitude of the emission spike at the transient electroluminescence (TEL) falling edge is an important benchmark for evaluating the quantities of trapped charges existed in organic light-emitting diodes and often shows a normal temperature dependence which increases with the decreasing temperature. Surprisingly, an unreported abnormal temperature-dependent TEL spike was observed in this work. A series of experimental results relevant to the electroluminescence spectrum and TEL measurements demonstrate that this abnormal temperature-dependent behavior is induced by the leakage of hole carriers from the emission layer (EML) to an electron transport layer (ETL). After the voltage pulse is turned off, these holes already leaked into the ETL drift back toward the EML, subsequently engaging in radiative recombination with trapped electrons on guest molecules to generate a spike at the TEL falling edge. However, the drift process is hindered by the reduced carrier mobility of the ETL material with the decrease in temperature. As a result, the spike intensity weakens as the temperature decreases, which contradicts the conclusions reported in previous literatures. Therefore, this study not only leads to the reconsideration for the judgment criteria of the number of trapped charges but also provides valuable insight into the TEL research field of organic optoelectronic devices.

Crystal structure and thermodynamic properties of the new compound Sc4Sb2.52 in the Sc-Sb system

The new compound Sc4Sb2.52 was first discovered, which was synthesized by the arc-melting under high purity argon. The composition of the new compound was examined by energy dispersive X-ray spectroscopy (EDS). Its crystal structure was first determined using X-ray powder diffraction data by Rietveld method. In order to further verify its structure, it was also observed by transmission electron microscope (TEM). The compound crystallized in a cubic Th3P4-type structure with the space group I4¯3d (No. 220) and the lattice parameter is a = 8.65454(9) Å. Its atomic positions are Sc at 16c (0.0736(3), 0.0736(3), 0.0736(3)), and Sb at 12a (0.375, 0, 0.25). The heat capacity (Cp) of the new compound Sc4Sb2.52 was reported for the first time, which were measured using differential scanning calorimetry (DSC) in a temperature range from 105 K to 450 K. The curve of Cp shows the normal sigmoidal temperature dependence with no evidence of transitions and other anomalies, and which was fitted using Debye function from 0 K to 77 K and using Maier-Kelley type expression from 77 K to 450 K. The entropy, enthalpy and free-energy values were derived with integration of the heat capacities based on the relations of thermodynamic properties with Cp.

Effect of atomic substitution and structure on thermal conductivity in monolayers H-MN and T-MN (M = B, Al, Ga).

Finding materials with suitable thermal conductivity (κ) is crucial for improving energy efficiency, reducing carbon emissions, and achieving sustainability. Atomic substitution and structural adjustments are commonly used methods. By comparing the κ of two different structures of two-dimensional (2D) IIIA-nitrides and their corresponding carbides, we explored whether atomic substitution has the same impact on κ in different structures. All eight materials exhibit normal temperature dependence, with κ decreasing as the temperature rises. Both structures are single atomic layers of 2D materials, forming M-N bonds, with the difference being that H-MN consists of hexagonal rings, while T-MN consists of tetragonal and octagonal rings. 2D IIIA-nitrides provide a good illustration of the impact of atomic substitution and structure on κ. On a logarithmic scale of κ, it approximates two parallel lines, indicating that different structures exhibit similar trends of κ reduction under the same conditions of atomic substitution. We analyzed the mechanisms behind the decreasing trend in κ from a phonon mode perspective. The main reason for the decrease in κ is that heavier atoms lower lattice vibrations, reducing phonon frequencies. Electronegativity increases, altering bonding characteristics and increasing anharmonicity. Reduced symmetry in complex structures decreases phonon group velocities and enhances phonon anharmonicity, leading to decreased phonon lifetimes. It's noteworthy that we found that atomic substitution and structure significantly affect hydrodynamic phonon transport as well. Both complex structures and atomic substitution simultaneously reduce the effects of hydrodynamic phonon transport. By comparing the impact of κ on two different structures of 2D IIIA-nitrides and their corresponding carbides, we have deepened our understanding of phonon transport in 2D materials. Heavier atomic substitution and more complex structures result in reduced κ and decreased hydrodynamic phonon transport effects. This research is likely to have a significant impact on the study of micro- and nanoscale heat transfer, including the design of materials with specific heat transfer properties for future applications.

The origin of interlayer-induced significant enhancement of EQE in CzDBA-based OLEDs studied by magneto-electroluminescence

Over twelve-fold enhancement of external quantum efficiency (EQE) is observed in 9,10-bis(4-(9H-carbazol-9-yl)-2,6-dimethylphenyl)-9,10-diboraanthracene (CzDBA)-based organic light-emitting diodes (OLEDs) with an interlayer between the hole-transporting layer (HTL) and the emission layer, where the CzDBA emitter is a typically donor–acceptor–donor (D–A–D)-type thermally activated delayed fluorescence material. Analyses of the fingerprint magneto-electroluminescence traces indicate that the interlayer ensures the charge balance of the emission layer in devices, avoiding triplet-charge annihilation and contributing to the enhancement of EQE. Additionally, experimental results also show that intersystem crossing (ISC) and reverse ISC (RISC) processes coexist in the device with an interlayer. Notably, ISC boosts with increasing bias currents and working temperatures, respectively, exhibiting abnormal current and normal temperature dependences. This abnormal phenomenon is caused by the weakened RISC between charge-transfer states of CzDBA molecules at large bias currents. More interestingly, as bias currents increase, ISC in the device without an interlayer first exhibits normal current dependences and then turns into an abnormal one, which may attribute to the competitive effects of exciplex at the HTL/CzDBA interface and excited states of CzDBA molecules. Our findings not only unravel the underlying mechanisms in D–A–D-type molecules but also provide ideas for designing highly efficient devices.

Assessment of concrete strength development in winter

The study attempts to demonstrate that it is possible to apply classical equations of chemical reactions for assessing the strength of concrete, cured in winter. This assumption differs from the existing classical theories of strength development in concrete, based on physical approaches. The graphs of the known normal temperature dependences of the chemical reaction rate and the increase in substance concentration in time allowed us to confirm the possibility for further research of strength development in concrete from the standpoint of chemical theories. For these purposes, the Van’t Hoff and Arrhenius equations were considered. To compare the results of theoretical studies with real constructions, we conducted an extensive experiment that involved a testing machine and a temperature chamber to assess the actual development of concrete strength in time at different temperatures (5, 20, 40, and 60°C). The temperature graphs of hardening of B40 concrete were plotted. Both equations showed identical results in calculating the increase in the reaction rate with the increase in concrete temperature. The research results allowed us to conclude that classical chemical theories used to assess the dependence of the reaction rate on temperature can be applied for calculating the concrete strength in winter.

Assessment of concrete strength development in winter

The study attempts to demonstrate that it is possible to apply classical equations of chemical reactions for assessing the strength of concrete, cured in winter. This assumption differs from the existing classical theories of strength development in concrete, based on physical approaches. The graphs of the known normal temperature dependences of the chemical reaction rate and the increase in substance concentration in time allowed us to confirm the possibility for further research of strength development in concrete from the standpoint of chemical theories. For these purposes, the Van’t Hoff and Arrhenius equations were considered. To compare the results of theoretical studies with real constructions, we conducted an extensive experiment that involved a testing machine and a temperature chamber to assess the actual development of concrete strength in time at different temperatures (5, 20, 40, and 60°C). The temperature graphs of hardening of B40 concrete were plotted. Both equations showed identical results in calculating the increase in the reaction rate with the increase in concrete temperature. The research results allowed us to conclude that classical chemical theories used to assess the dependence of the reaction rate on temperature can be applied for calculating the concrete strength in winter.

Thermal, magnetic field- and stress-induced transformation in Heusler-type Co-Cr-Al-Si shape memory alloys

The phase diagram, including the martensitic and magnetic transformations, was determined by thermoanalysis and thermomagnetization measurements in the CoxCr79-xAl10.5Si10.5 section of the Co-based Heusler shape memory alloy. Magnetization measurements under pulsed magnetic fields were conducted and magnetic field-induced reverse martensitic transformation was clearly observed at room temperature. A superelastic behavior was obtained in the Co55.7Cr23.3Al10.5Si10.5 alloy in the temperature range of 198 to 423 K. The critical stress for martensitic transformation was found to show a negative temperature dependence at temperatures lower than approximately 310 K due to the magnetic effect despite the normal temperature dependence at higher temperatures.

Enhanced diffusion with abnormal temperature dependence in underdamped space-periodic systems subject to time-periodic driving.

We present a study of the diffusion enhancement of underdamped Brownian particles in a one-dimensional symmetric space-periodic potential due to external symmetric time-periodic driving with zero mean. We show that the diffusivity can be enhanced by many orders of magnitude at an appropriate choice of the driving amplitude and frequency. The diffusivity demonstrates abnormal (decreasing) temperature dependence at the driving amplitudes exceeding a certain value. At any fixed driving frequency Ω normal temperature dependence of the diffusivity is restored at low enough temperatures, T<T_{TAD}(Ω)-in contrast with the problem with constant external driving. At fixed temperature at small driving frequency the diffusivity either slowly decreases with Ω, or (at stronger driving) goes through a maximum near Ω_{2}, the reciprocal superdiffusion regime termination time. At high frequencies, between Ω_{2} and a fraction of the oscillation frequency at the potential minimum, the diffusivity is shown to decrease with Ω according to a power law, with the exponent related to the transient superdiffusion exponent. This behavior is found similar for the cases of sinusoidal in time and piecewise constant periodic ("square") driving.

Evaluation of High-temperature Deformation Behavior of Ni-based Alloy by Compression Test

An experimental procedure for the hot compression test for Ni-based alloy was proposed and evaluated in order to obtain an appropriate stress-strain curve for numerical analysis. The effect of various parameters, i.e., top and bottom shapes of specimen, lubricant and jig/tool, on the deformation tendency was systematically studied. Ni-based alloy (CMSX-4), which has a higher strength than the specimen, was used as die for preventing temperature gradient. This improved the uniformity of temperature distribution in the specimen. The combination of a specimen having concentric circles on the end surface and a glass sheet as lubricant was the most appropriate to gain stable and low-friction condition of the specimen end. The proposed procedure of the hot compression test is applicable for the temperature range of 1173－1423 K and strain rate range of 0.01－5 s-1. The effectiveness of the obtained stress-strain curve was discussed from the viewpoint of the value and tendency of characteristic features observed in the flow stress curve. The maximum stress exhibited normal temperature and strain rate dependence. Related activation energy for deformation was similar in value to those in other reports.

Broadband light-scattering spectroscopy on fractal and non-fractal relaxors

We show the quasi-elastic light scattering (QELS) spectra of two groups of relaxors: the first group includes relaxors that exhibit glasslike low-temperature thermal conductivity and heat capacity, namely, Pb(Mg1/3Nb2/3)O3 (PMN), (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN–xPT), Pb(Zr1/3Nb2/3)O3 (PZN), and (Na1/2Bi1/2)TiO3 (NBT). The other group consists of relaxors exhibiting a normal (crystal) temperature dependence of the thermal conductivity and heat capacity, namely, K1−xLixTaO3 (KLT) and KTa1−xNbxO3 (KTN). The crystals of the first group yielded self-similar (power-law) QELS spectra, indicating the existence of fractal networks/clusters of polar nanoregions, while those of the second group did not show any self-similarity in the QELS spectra. These results imply that the glasslike low-temperature thermal conductivity and heat capacity in relaxors can be attributed to the vibrational modes specific to fractal networks/clusters formed by polar nanoregions.

Interfacial effects on magnetic interlayer coupling between perpendicular Co/Ni multilayers across Ru spacer

The interlayer exchange coupling properties in synthetic antiferromagnetic (AF) structures based on Co/Ni perpendicular multilayers were investigated by varying the Ru spacer thickness (tRu) and the ferromagnetic (FM) layer material at the FM/Ru interfaces. Samples of two Co layers adjacent to Ru own much stronger AF coupling field (HAF) and show two HAF peaks located at tRu = 0.47 and 1.08 nm. If the interfacial Co layers are substituted by Ni, the HAF at the 1st peak decreases much more considerably, becomes lower than the 2nd one and even disappears due to strong pinhole-induced FM coupling. Such pinhole effect makes HAF of tRu &amp;lt; 0.85 nm remains unchanged or even decrease with decreasing temperatures. By tuning the interfacial Co layer thickness, pinhole effect has been suppressed as a result of greatly enhanced HAF, leading to a normal temperature dependence of Ruderman-Kittel-Kasuya-Yosida-type HAF even at thin tRu cases, i.e., increase in HAF with temperature decreasing.

Plastic deformation of single crystals of Pt3Al with the L12 structure

The plastic deformation behaviour of single crystals of Pt3Al with the L12 structure having an off-stoichiometric composition of Pt–27 at% Al has been investigated in compression from 77 to 1073 K. The L12 structure is not stable below around 220 K, transforming into either a D0c or D0c′ structure. Slip occurs along ⟨1 1 0⟩ both on (0 0 1) and on (1 1 1) with slip on (0 0 1) being the primary slip system, which operates for most crystal orientations except for near [0 0 1], accompanied by a considerably lower CRSS (critical resolved shear stress). The CRSS tends to decrease gradually with increasing temperature for both slip in the temperature range where the L12 phase is stable, except for a moderate increase in CRSS observed above 673 K for slip on (0 0 1). Dislocations with b = [] dissociate into two collinear superpartials with b = 1/2[] separated by an APB on the corresponding slip plane for both slip on (0 0 1) and (1 1 1). For slip on (1 1 1), dislocations tend to align along their screw orientation at room temperature, suggesting the high Peierls stress for their motion. The possibility of showing the normal (large negative) temperature dependence of CRSS at low temperatures as well as the reason for the absence of the anomalous (positive) temperature dependence of CRSS for slip on (1 1 1) at high temperatures is discussed.

The coercivity and domain structure of Sm(CobalFe0.1CuxZr0.033)6.9 (x = 0.07, 0.10, 0.13) high temperature permanent magnets

The domain structures have been found to be different in the Sm(CobalFe0.1CuxZr0.033)6.9 magnets (x = 0.07, 0.10, 0.13) with the abnormal and normal temperature dependence of coercivity by quenching at 400 °C and 600 °C, respectively. In the magnets with abnormal temperature dependence of coercivity, the domain structure shows more like the strip domain which is typical domain shape of the ferromagnets of easy-axis anisotropy, while the domain structure becomes narrower and shows more additional domains in the magnets with normal temperature dependence of coercivity. The difference of the domain structure can be attributed to whether the Cu is homogeneous in the 1:5 cell boundary phase, and this will lead to different domain wall pinning and different temperature dependence of coercivity in the magnets according to the noncontinuous domain wall pinning model. As the Cu is nearly homogeneous in the 1:5 cell boundary phase, the domain wall should be pinned near the interface between 2:17 cell phase and 1:5 cell boundary phase, and the coercivity shows abnormal temperature dependence. On the contrary, the domain wall should be pinned into the 1:5 cell boundary phase, where a gradient of Cu content exists, leading to the normal temperature of coercivity.

Conversion and Origin of Normal and Abnormal Temperature Dependences of Kinetic Isotope Effect in Hydride Transfer Reactions

The effects of substituents on the temperature dependences of kinetic isotope effect (KIE) for the reactions of the hydride transfer from the substituted 5-methyl-6-phenyl-5,6-dihydrophenanthridine (G-PDH) to thioxanthylium (TX(+)) in acetonitrile were examined, and the results show that the temperature dependences of KIE for the hydride transfer reactions can be converted by adjusting the nature of the substituents in the molecule of the hydride donor. In general, electron-withdrawing groups can make the KIE to have normal temperature dependence, but electron-donating groups can make the KIE to have abnormal temperature dependence. Thermodynamic analysis on the possible pathways of the hydride transfer from G-PDH to TX(+) in acetonitrile suggests that the transfers of the hydride anion in the reactions are all carried out by the concerted one-step mechanism whether the substituent is an electron-withdrawing group or an electron-donating group. But the examination of Hammett-type free energy analysis on the hydride transfer reactions supports that the concerted one-step hydride transfer is not due to an elementary chemical reaction. The experimental values of KIE at different temperatures for the hydride transfer reactions were modeled by using a kinetic equation formed according to a multistage mechanism of the hydride transfer including a returnable charge-transfer complex as the reaction intermediate; the real mechanism of the hydride transfer and the root that why the temperature dependences of KIE can be converted as the nature of the substituents are changed were discovered.

Temperature Dependence of the Dielectric Permittivity of Acetic Acid, Propionic Acid and Their Methyl Esters: A Molecular Dynamics Simulation Study

For most liquids, the static relative dielectric permittivity is a decreasing function of temperature, because enhanced thermal motion reduces the ability of the molecular dipoles to orient under the effect of an external electric field. Monocarboxylic fatty acids ranging from acetic to octanoic acid represent an exception to this general rule. Close to room temperature, their dielectric permittivity increases slightly with increasing temperature. Herein, the causes for this anomaly are investigated based on molecular dynamics simulations of acetic and propionic acids at different temperatures in the interval 283-363 K, using the GROMOS 53A6(OXY) force field. The corresponding methyl esters are also considered for comparison. The dielectric permittivity is calculated using either the box-dipole fluctuation (BDF) or the external electric field (EEF) methods. The normal and anomalous temperature dependences of the permittivity for the esters and acids, respectively, are reproduced. Furthermore, in the EEF approach, the response of the acids to an applied field of increasing strength is found to present two successive linear regimes before reaching saturation. The low-field permittivity ε, comparable to that obtained using the BDF approach, increases with increasing temperature. The higher-field permittivity ε' is slightly larger, and decreases with increasing temperature. Further analyses of the simulations in terms of radial distribution functions, hydrogen-bonded structures, and diffusion properties suggest that increasing the temperature or the applied field strength both promote a relative population shift from cyclic (mainly dimeric) to extended (chain-like) hydrogen-bonded structures. The lower effective dipole moment associated with the former structures compared to the latter ones provides an explanation for the peculiar dielectric properties of the two acids compared to their methyl esters.

English

Treatment wetlands (TWs) are widely used for treating domestic, agricultural, and industrial wastewater, stormwater runoff, and acid mine drainage; natural wetlands are also exposed to these pollutant sources. Currently, few plant species are used in the majority of TWs, and these are often non-native and/or weedy. We are working to identify native species for year-round use in cold-region TWs, particularly the Rocky Mountain region, and explore the basis of differences in performance. In studies presented here, we evaluated chemical oxygen demand (COD) removal from simulated wastewater in microcosms planted with monocultures of 19 species. Experiments were conducted over one year at seasonal temperatures of 4-24°C. With some species and in unplanted controls, COD removal declined at cold temperatures during dormancy, as expected with normal temperature dependence of microbial processes. However, COD removal was constant across seasons with the majority of species. Average COD removal exceeded 90% for Carex aquatilis, C. bebbii, C. praegracilis, C. utriculata, Schoenoplectus acutus, Juncus arcticus, J. torreyi, and Deschampsia cespitosa; of these, only S. acutus is widely used. In contrast, the widely used (and frequently invasive) species T. latifolia, P. australis, and P. arundinacea were somewhat less effective, with average COD removals of 84%, 74%, and 83%, respectively. Redox, sulfate, and root oxygen loss measurements suggest that plant-mediated oxygen transfer may explain the ability of some species to offset the effect of temperature on microbial processes and maintain high COD removal in all seasons. Results indicate that many non-weedy, regionally native species may be candidates for use in TWs or for rehabilitation of natural wetlands exposed to certain pollutants. In addition to the species we studied, other Obligate Wetland and Facultative Wetland species of the Cyperaceae and Juncaceae merit investigation.

From Powder to Solution: Hydration Dependence of Human Hemoglobin Dynamics Correlated to Body Temperature

A transition in hemoglobin (Hb), involving partial unfolding and aggregation, has been shown previously by various biophysical methods. The correlation between the transition temperature and body temperature for Hb from different species, suggested that it might be significant for biological function. To focus on such biologically relevant human Hb dynamics, we studied the protein internal picosecond motions as a response to hydration, by elastic and quasielastic neutron scattering. Rates of fast diffusive motions were found to be significantly enhanced with increasing hydration from fully hydrated powder to concentrated Hb solution. In concentrated protein solution, the data showed that amino acid side chains can explore larger volumes above body temperature than expected from normal temperature dependence. The body temperature transition in protein dynamics was absent in fully hydrated powder, indicating that picosecond protein dynamics responsible for the transition is activated only at a sufficient level of hydration. A collateral result from the study is that fully hydrated protein powder samples do not accurately describe all aspects of protein picosecond dynamics that might be necessary for biological function.

Kinetics and Thermodynamics of Glucose Isomerase Crystallization

A quantitative study using laser confocal microscopy combined with differential interference microscopy on the kinetics and thermodynamics of the crystallization of glucose isomerase is presented. Fundamental crystallization parameters are determined from the kinetics of step advancement and rates of two-dimensional (2D) nucleation. The ruling mass transfer pathway and accompanying activation barriers are discussed. In brief, the solubility exhibits normal temperature dependence and the crystallization enthalpy is the thermodynamic driving force. The diminishing entropic cost for higher PEG concentrations is attributed to water structuring and a decrease in water activity. The prominent step generation mechanism is homogeneous 2D nucleation for high supersaturations. At low driving forces 2D nucleation occurs on anomalously hyperactive sites and the step edge free energies for homogeneous and heterogeneous nucleation are determined. The number of nucleation centers for both mechanisms are estimated and fr...

Hemoglobin Dynamics in Red Blood Cells: Correlation to Body Temperature

A transition in hemoglobin behavior at close to body temperature has been discovered recently by micropipette aspiration experiments on single red blood cells (RBCs) and circular dichroism spectroscopy on hemoglobin solutions. The transition temperature was directly correlated to the body temperatures of a variety of species. In an exploration of the molecular basis for the transition, we present neutron scattering measurements of the temperature dependence of hemoglobin dynamics in whole human RBCs in vivo. The data reveal a change in the geometry of internal protein motions at 36.9°C, at human body temperature. Above that temperature, amino acid side-chain motions occupy larger volumes than expected from normal temperature dependence, indicating partial unfolding of the protein. Global protein diffusion in RBCs was also measured and the findings compared favorably with theoretical predictions for short-time self-diffusion of noncharged hard-sphere colloids. The results demonstrated that changes in molecular dynamics in the picosecond time range and angstrom length scale might well be connected to a macroscopic effect on whole RBCs that occurs at body temperature.