Strong Temperature Dependence Research Articles

Numerical Modeling of Temperature Effect on Tensile Strength of Granitic Rock

The aim of this paper is to numerically predict the temperature effect on the tensile strength of granitic rock. To this end, a numerical approach based on the embedded discontinuity finite elements is developed. The underlying thermo-mechanical problem is solved with a staggered method marching explicitly in time while using extreme mass scaling, allowed by the quasi-static nature of the slow heating of a rock sample to a uniform target temperature, to increase the critical time step. Linear triangle elements are used to implement the embedded discontinuity kinematics with two intersecting cracks in a single element. It is assumed that the quartz mineral, with its strong and anomalous temperature dependence upon approaching the α-β transition at the Curie point (~573 °C), in granitic rock is the major factor resulting in thermal cracking and the consequent degradation of tensile strength. Accordingly, only the thermal expansion coefficient of quartz depends on temperature in the present approach. Moreover, numerically, the rock is taken as isotropic except for the tensile strength, which is unique for each mineral in a rock. In the numerical simulations mimicking the experimental setup on granitic numerical rock samples consisting of quartz, feldspar and biotite minerals, the sample is first heated slowly to a target temperature below the Curie point. Then, a uniaxial tension test is numerically performed on the cooled down sample. The simulations demonstrate the validity of the proposed approach as the experimental deterioration of the tensile strength of the rock is predicted with agreeable accuracy.

The temperature dependence of the frequency of longitudinal excitations in liquid along isobars: Simple liquid and water

Using molecular simulation, we studied the spectra of longitudinal and transverse excitations of water (the SPC/E model) and simple liquid (the Lennard-Jones system) along isobars. Comparison of the results for water and simple liquid has shown that the frequency of longitudinal excitations of water demonstrates strong temperature dependence at low temperatures and comes to saturation at high ones. The low temperature regime is in qualitative agreement with the published experimental spectra of longitudinal excitations of water along an isobar. The high temperature regime has not been studied experimentally up to now.

Quantum oscillations in the field-induced ferromagnetic state of MnBi2−xSbxTe4

The intrinsic antiferromagnetic topological insulator MnBi$_{2}$Te$_{4}$ undergoes a metamagnetic transition in a c-axis magnetic field. It has been predicted that ferromagnetic MnBi$_{2}$Te$_{4}$ is an ideal Weyl semimetal with a single pair of Weyl nodes. Here we report measurements of quantum oscillations detected in the field-induced ferromagnetic phase of MnBi$_{2-x}$Sb$_{x}$Te$_{4}$, where Sb substitution tunes the majority carriers from electrons to holes. Single frequency Shubnikov-de Haas oscillations were observed in a wide range of Sb concentrations (0.54 $\leq$ x $\leq$ 1.21). The evolution of the oscillation frequency and the effective mass shows reasonable agreement with the Weyl semimetal band-structure of ferromagnetic MnBi$_{2}$Te$_{4}$ predicted by density functional calculations. Intriguingly, the quantum oscillation frequency shows a strong temperature dependence, indicating that the electronic structure sensitively depends on magnetism.

Fluorescence Characteristics of CdTe Quantum Dot Colloids below the Freezing Point

In this paper, water-soluble CdTe quantum dots (QDs) with different sizes from 3 to 3.9 nm are synthesized by the aqueous phase method using glutathione as the stabilizer, and their fluorescence properties below the freezing point are studied. The results show that the QD dispersion system still maintains the liquid state at −30 °C when the volume fraction of ethanol is 30%, and its fluorescence has a strong temperature dependence below the freezing point; as the temperature decreases from 0 to −30 °C, the fluorescence intensity increases about 62–82%, and the fluorescence emission peak also blue-shifts (5–10 nm). After cycles of heating (0 °C)–cooling (−30 °C), the fluorescence intensity can still be recovered, indicating a strong fluorescence reversibility. This work provides help for expanding the application of QDs in the fields of biomedicine.

Transport Modeling of Locally Photogenerated Excitons in Halide Perovskites.

Excitons have fundamental impacts on optoelectronic properties of semiconductors. Halide perovskites, with long carrier lifetimes and ionic crystal structures, may support highly mobile excitons because the dipolar nature of excitons suppresses phonon scattering. Inspired by recent experimental progress, we perform device modeling to rigorously analyze exciton formation and transport in methylammonium lead triiodide under local photoexcitation by using a finite element method. Mobile excitons, coexisting with free carriers, can dominate photocurrent generation at low temperatures. The simulation results are in excellent agreement with the experimentally observed strong temperature and gate dependence of carrier diffusion. This work signifies that efficient exciton transport can substantially influence charge transport in the family of perovskite materials.

Thermoelectric materials with crystal-amorphicity duality induced by large atomic size mismatch

Summary Discovering novel materials and attaining higher performance are the eternal pursuit of thermoelectric materials research. Here, we report a material series, (Cu1−xAgx)2(Te1−ySy) (0.16 ≤ x ≤ 0.24, 0.16 ≤ y ≤ 0.24), which adopts a complex orthorhombic structure differing from any known crystal structure of (Cu/Ag)2(S/Te). This material series is featured by the crystal-amorphicity duality induced by the large anionic size mismatch: a crystalline sublattice of highly size-mismatched anions Te/S coexists with an amorphous-like sublattice of cations Cu/Ag. In the context of structure-property correlation, the crystal-amorphicity duality gave rise to not only interesting electrical properties but also exceptionally low lattice thermal conductivities from 300 to 1,000 K. A state-of-the-art figure of merit zT of 2.0 is obtained in the x = y = 0.22 sample at 1,000 K. These results give insights into crystal-amorphicity duality as a paradigm-shifting materials design approach to develop high-performance thermoelectric materials.

Revealing Conformational Transition Dynamics of Photosynthetic Proteins in Single-Molecule Electrical Circuits.

The function of proteins depends on their structural flexibility and conformational change. By utilizing silicon-nanowire-based single-molecule electrical circuits, here we present a label-free real-time measurement method that can directly monitor conformational changes of a photosynthetic LH1-RC complex, reaching the ultimate goal of analytic chemistry. These results manifest that the conformation of the LH1-RC complex vibrates among four conformations with strong temperature dependence. At the optimal temperature, States 2 and 3 occupy the main conformations of the LH1-RC complex, and its conformational variation mostly emerges as anharmonic vibration modes, which contributes to photon acquisition and heat transmission. The influence of light activation on occurrence percentage is observed, resulting from light-driven quivering of pigments. Therefore, this avenue proves to be an efficient platform for revealing the fundamental mechanisms of various biological processes in vitro.

Identifying the structural relaxation dynamics in a strongly asymmetric binary glass former.

Here, we provide calorimetric and dielectric studies in asymmetric binary mixtures constituted by 2-picoline and triphenylethylene. Extreme broadening of the calorimetric glass transition is observed in the mixtures, which is accompanied by a large mismatch of the glass transition temperatures defined by the two techniques. As large broadening in the relaxation dispersion is identified in the mixtures of intermediate concentrations, strong temperature dependence of the relaxation dispersion is detected. The relation between the stretching exponent and non-linear factor derived from the Tool-Narayanaswamy-Moynihan-Hodge model shows a remarkable shift from the one established by pure molecular glass formers and symmetric mixtures. The unusual behaviors suggest an extreme dynamical decoupling mode imposed by the occurrence of strong concentration fluctuation.

Fermi surface in La-based cuprate superconductors from Compton scattering imaging

Compton scattering provides invaluable information on the underlying Fermi surface (FS) and is a powerful tool complementary to angle-resolved photoemission spectroscopy and quantum oscillation measurements. Here we perform high-resolution Compton scattering measurements for La2−xSrxCuO4 with x = 0.08 (Tc = 20 K) at 300 K and 150 K, and image the momentum distribution function in the two-dimensional Brillouin zone. We find that the observed images cannot be reconciled with the conventional hole-like FS believed so far. Instead, our data imply that the FS is strongly deformed by the underlying nematicity in each CuO2 plane, but the bulk FSs recover the fourfold symmetry. We also find an unusually strong temperature dependence of the momentum distribution function, which may originate from the pseudogap formation in the presence of the reconstructed FSs due to the underlying nematicity. Additional measurements for x = 0.15 and 0.30 at 300 K suggest similar FS deformation with weaker nematicity, which nearly vanishes at x = 0.30.

Temperature dependence of optical centers in Ib diamond characterized by photoluminescence spectra

The optical properties of defects in Ib diamond prepared by Chemical Vapor Deposition (CVD) and High Pressure High Temperature (HPHT) methods are studied by photoluminescence in a temperature range of 77– 297 K. The temperature dependence of color centers in diamond of 2.65 eV center, NV0, NV-, SiV-, 3H is studied. The zero-phonon line (ZPL) shift, intensity and full width at half maxima (FWHM) have a strong temperature dependence. The study shows that, due to the expansion of the lattice and the enhancement of the electron-phonon coupling strength, the ZPL shift can be described by the modified Varshni model which shows great potential in temperature sensing. The FWHM of ZPL matches a cubic function of temperature, this model can be employed to well describe the broadening of ZPLs in the defect-rich diamond. And the thermal quenching of absolute PL intensity can be described by the Arrhenius equation. Meanwhile, the difference of variable PL signal between CVD and HPHT samples is analyzed.

Electronic transport properties and hydrostatic pressure effect of FeSe0.67Te0.33 single crystals free of phase separation

FeSe1−x Te x superconductors manifest some intriguing electronic properties depending on the value of x. In FeSe single crystal, the nematic phase and Dirac band structure have been observed, while topological surface superconductivity (SC) with the Majorana bound state was found in the crystal of x∼ 0.55. Therefore, the electronic properties of single crystals with 0 < x ⩽ 0.5 are crucial for probing the evolution of those intriguing properties as well as their relations. However, this study is still left blank due to the lack of single crystals because of phase separation. Here, we report the synthesis, magnetization, electronic transport properties, and hydrostatic pressure effect of FeSe0.67Te0.33 single crystals free of phase separation. A structural (nematic) transition is visible at T s = 39 K, below which the resistivity exhibits a Fermi-liquid behavior. Analysis of the upper critical fields suggests that spin-paramagnetic effect should be taken into account for both H || c axis and H || ab plane. A crossover from the low-H quadratic to the high-H quasi-linear behavior is observed in the magnetoresistance, signifying the possible existence of Dirac-cone state. Besides, the strong temperature dependence of Hall coefficient, violation of (modified) Kohler’s rule, and two-band model analysis indicate the multiband effects in FeSe0.67Te0.33 single crystals. Hydrostatic pressure measurements reveal that T s is quickly suppressed with pressure while T c is monotonically increased up to 2.31 GPa, indicating the competition between nematicity and SC. No signature of magnetic order that has been detected in FeSe1−x S x is observed. Our findings fill up the blank of the knowledge on the basic properties of FeSe1−x Te x system with low-Te concentrations.

Magnetic Properties and Environmental Temperature Effects on Battery Performance of Na0.67Mn0.5Fe0.5O2

Herein, a modified solid state synthesis of Na0.67Mn0.5Fe0.5O2 and the results of a detailed investigation of the structural and magnetic properties via Fourier transform infrared spectroscopy (FTIR), X‐ray diffraction (XRD), X‐ray absorption spectroscopy (XAS), scanning electron microscopy (SEM), and energy dispersive X‐ray (EDX) analysis are reported. The magnetic properties of Na0.67Mn0.5Fe0.5O2 do not fit the Curie–Weiss law and a model regarding the spin configuration of the Mn and Fe ions and a possible ferrimagnetic order is suggested. Electrochemical measurements and ex situ structural analysis of the cathode material confirm the reversible structural transitions for the cells charged up to 4.0 V. Environmental temperature–dependent electrochemical measurements reveal a strong temperature dependence of both, the initial capacity and the capacity retention. Ex situ SEM, FTIR, and XRD studies on the battery membrane verify the formation of a Na2CO3 phase on the membrane, which blocks the Na ion diffusion through membrane pores and is responsible for the capacity fade for this compound.

First report of energy transfer from uranyl to Mn4+ in K3(UO2)F5:Mn4+

This work relates to the synthesis and optical characterization of a novel Mn4+ activated luminescent material with the composition K3(UO2)F5:Mn4+. This is the first report of a Mn4+ activated compound comprising the uranyl cation UO22+. A highly efficient energy transfer from uranyl to Mn4+ is observed, which enables the phosphor to be excited in a broad spectral range with a large absorption cross section. Simulated spectra show that the phosphor is suitable for use in full-conversion (In,Ga)N LEDs for high color rendering index/high luminous efficacy applications. The ratio between the emission of uranyl and Mn4+ exhibits a strong temperature dependence which is linked to an underlying phonon-assisted energy transfer.

Temperature dependence of tropospheric ozone under NOx reductions over Germany

Due to the strong temperature dependence of surface ozone concentrations (O3), future warmer conditions may worsen ozone pollution levels despite continued efforts on emission controls of ozone precursors. Using long-term measurements of hourly O3 concentrations co-located with NOx concentrations in stations distributed throughout Germany, we assess changes in the climate penalty in summertime, defined as the slope of ozone-temperature relationship during the period 1999–2018. We find a stronger temperature sensitivity in the urban stations over the southwestern regions, especially in the first period of the study (1999–2008). We show a decrease in the climate penalty in most of stations during the second period of the study (2009–2018), with some exceptions (e.g. Berlin) where the climate penalty did not show significant changes. A key motivation of this study is to provide further insights into the impacts of NOx reductions in the O3-temperature relationship. For that, we propose a statisti-cal approach based on Generalized Additive Models (GAMs) to describe ozone production rates, inferred from hourly observations, as a function of NOx and temperature, among other variables relevant during the O3 production. The GAMs confirm lower O3 production rates during the second period (2009–2018) at most of the stations and a decreasing sensitivity to temperature. We observe that a large number of stations are transitioning to NOx-limited chemistry, consistent with a decreasing temperature dependence of O3 at moderate-high temperatures as a result of sustained NOx reductions. Moreover, the GAMs results showed changes in the shape of the function representing the O3-temperature relationship when comparing the first and second period, which suggest changes in VOC influencing the temperature dependence of O3. From these results, we infer effective VOC reductions over time that have also contributed to the observed decrease of O3 production rates. Thus, our analysis indicates that emissions reductions have been effective in a number of stations, particularly in the southwestern regions. However, we notice that in a few stations (e.g. Berlin) additional emission reductions should be required to effectively mitigate the temperature dependence of O3.

Temperature dependence of contact quality inducing suppression of stick–slip friction

Atomic-scale stick–slip friction of monolayer graphene on a copper substrate is computationally studied at a wide range of temperatures, which reveals strong temperature dependence of friction force and friction mode. The increase in temperature distorts regular stick–slip behavior and causes a nonlinear decrease of friction force, demonstrating a friction behavior transition from athermal friction to thermally activated. By analyzing atomic morphologies, the true contact area, defining the number of atoms interacting across the interface, shows a weak temperature dependence, yet the quality of interfacial contact substantially varies with temperature. Spatial distributions of atomic friction force uncover the significant effect of Moiré superlattices, that act as strong pinning sites at low temperatures, partly change to pushing due to nonconcurrent atomic jumps at high temperatures, which leads to a chaotic friction mode with reduced lateral force. Additionally, we demonstrate the role of superlattices in strengthening friction and dictating the periodic stick–slip motion of interfacial sliding.

Electron mobility in monolayer WS2 encapsulated in hexagonal boron-nitride

We report electron transport measurements in dual-gated monolayer WS2 encapsulated in hexagonal boron-nitride. Using gated Ohmic contacts that operate from room temperature down to 1.5 K, we measure the intrinsic conductivity and carrier density as a function of temperature and gate bias. Intrinsic electron mobilities of 100 cm2/(V s) at room temperature and 2000 cm2/(V s) at 1.5 K are achieved. The mobility shows a strong temperature dependence at high temperatures, consistent with phonon scattering dominated carrier transport. At low temperature, the mobility saturates due to impurity and long-range Coulomb scattering. First-principles calculations of phonon scattering in monolayer WS2 are in good agreement with the experimental results, showing we approach the intrinsic limit of transport in these two-dimensional layers.

Mathematical Modeling of Outdoor Natural Weathering of Polycarbonate: Regional Characteristics of Degradation Behaviors.

Many natural exposure sites have been developed to ensure the reliability of materials intended for outdoor use. However, the effects of local climate on aging have not been completely understood. This study aimed to elucidate the regional characteristics of natural aging. Non-stabilized and stabilized polycarbonates were monitored in terms of their appearance (yellowing and loss of gloss) during natural weathering at five exposure sites (Tokyo, Kagoshima, Okinawa, Florida, and Arizona) in conjunction with climate fluctuation for up to 24 months. Three approaches were employed to characterize the natural aging behaviors: (i) modeling the rate function of degradation, (ii) evaluating the contribution ratio of individual degradational factors, and (iii) estimating the “synchronicity” by cross-correlation analysis with the climate dataset. The aging rates were the highest in Arizona and lowest in Kagoshima among the five exposure sites. First, prediction curves were constructed from the degradation rate function (variables: UV irradiation, temperature, and humidity), and these curves were found to agree well with the measured aging behaviors. Second, the exposure data in Arizona demonstrated strong temperature dependence, while those in Okinawa and Florida had stronger dependence on UV irradiation compared to other sites. Lastly, the synchronicity between UV irradiation and temperature was the highest in Arizona and lowest in Kagoshima, which can explain the significantly faster deterioration in Arizona and the slow deterioration in Kagoshima.

Thermo-Optical Effects in Plasmonic Metal Nanostructures

The effects of the temperature on the surface plasmon resonance (SPR) in noble metal nanoparticles at various temperatures ranging from 77 to 1190 K are reviewed. A temperature increase results in an appreciable red shift and leads to a broadening of the SPR in the nanoparticles (NPs). This observed thermal expansion along with an increase in the electron-phonon scattering rate with rising temperature emerge as the dominant physical mechanisms producing the red shift and broadening of the SPR. Strong temperature dependence of surface plasmon enhanced photoluminescence from silver (Ag) and copper (Cu) NPs is observed. The quantum photoluminescence yield of Ag nanoparticles decreases as the temperature increases, due to a decrease in the plasmon enhancement resulting from an increase in the electron-phonon scattering rate. An anomalous temperature dependence of the photoluminescence from Cu nanoparticles was also observed; the quantum yield of photoluminescence increases with the temperature. The interplay between the SPR and the interband transitions plays a critical role in this effect. The surface-plasmon involved laser heating of a dense 2D layer of gold (Au) NPs and of Au NPs in water colloids is also examined. A strong increase in the Au NP temperature occurs, when the laser frequency approaches the SPR. This finding supports the resonant plasmonic character of the laser heating of metal NPs. The sharp blue shift of the surface plasmon resonance in colloidal Au NPs at temperatures exceeding the water boiling point indicates the vapor-bubble formation near the surface of the NPs.

Weibull Modulus of Cleavage Fracture Toughness of Ferritic Steels

The ordinary Weibull distribution function has been commonly accepted for empirical characterization of cleavage fracture toughness of nuclear reactor and containment pressure vessel steels. However, this method lacks a fundamental basis. This work adopts the standardized Weibull distribution function to analyze cleavage fracture toughness of ferritic steels measured from different sized fracture mechanics specimens at different temperatures to estimate the Weibull modulus. The toughness data of five different nuclear reactor and containment vessel steels are analyzed. The estimations obtained the Weibull modulus (m) in the range of 1.83 to 2.55 and strong temperature dependence of the threshold cleavage fracture toughness Kmin, as opposed to the constant values of mK = 4 and Kmin = 20 MPam1/2 given in ASTM E1921-19. The goodness of fit test by the one-sample Kolmogorov–Smirnov (K–S) test validated Weibull distribution function for describing the toughness distribution.

Extremely large linear magnetoresistance in antimony crystal

In this article, we report the observation of extremely large non-saturating linear magnetoresistance (MR) in antimony (Sb) crystal. An extremely large magnetoresistance of 43 000% at 2 K and large unsaturating MR ∼70% at room temperature is observed at the magnetic field of 9 T. Hall measurements reveal a very high mobility ∼3.8 × 104 cm2 V s−1 of charge carriers and strong temperature dependence of carrier concentration and mobility. The respective scaling of MR and crossover field (B c ) from quadratic to linear MR with mobility and inverse of mobility describes the classical origin of large linear MR in this crystal as suggested by Parish and Littlewood model for disordered systems.