Temperature Dependence Of Constants Research Articles

Analysis of defects dominating carrier recombination in CeO2 single crystal for photocatalytic applications

In photocatalytic water splitting and CO2 reduction, recently, cerium oxide (CeO2) has been widely investigated. Defects that can directly dominate carrier recombination are essential for the photocatalytic performance of CeO2. In this study, several photoluminescence (PL) peaks are observed on the (100) face of an undoped CeO2 single crystal, indicating the presence of defects. Moreover, we characterize carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) measurements. The temperature dependence of decay curves is the result of carrier trapping and emission at deep levels. These decay curves are observed separately using a 565 nm band-pass filter (BPF) based on the PL spectra. The trap energy level ( ET ) and majority carrier capture cross-section ( σT ) of each defect are also analyzed using rate equations to fit the experimental results. The temperature-dependent time constants are well reproduced by a recombination model using hole traps HTinfrared and HTvisible at ET of 0.76 and 0.55 eV from the conduction or valence band, with estimated σT of the order of 10−21 and 10−22 cm2 for without and with the BPF, respectively. These findings regarding the presence of multiple defects in a single crystal indicate the necessity to focus on defect control.

Faraday isolator with compensation depolarization caused by Verdet constant temperature dependence

Faraday isolator with compensation depolarization caused by Verdet constant temperature dependence

Accurate Surface and Finite-Temperature Bulk Properties of Lithium Metal at Large Scales Using Machine Learning Interaction Potentials.

The properties of lithium metal are key parameters in the design of lithium-ion and lithium-metal batteries. They are difficult to probe experimentally due to the high reactivity and low melting point of lithium as well as the microscopic scales at which lithium exists in batteries where it is found to have enhanced strength, with implications for dendrite suppression strategies. Computationally, there is a lack of empirical potentials that are consistently quantitatively accurate across all properties, and ab initio calculations are too costly. In this work, we train a machine learning interaction potential on density functional theory (DFT) data to state-of-the-art accuracy in reproducing experimental and ab initio results across a wide range of simulations at large length and time scales. We accurately predict thermodynamic properties, phonon spectra, temperature dependence of elastic constants, and various surface properties inaccessible using DFT. We establish that there exists a weak Bell-Evans-Polanyi relation correlating the self-adsorption energy and the minimum surface diffusion barrier for high Miller index facets.

Antiproliferative activity of platinum(II) and copper(II) complexes containing novel biquinoxaline ligands.

Nowadays, cancer represents one of the major causes of death in humans worldwide, which renders the quest for new and improved antineoplastic agents to become an urgent issue in the field of biomedicine and human health. The present research focuses on the synthesis of 2,3,2',3'-tetra(pyridin-2-yl)-6,6'-biquinoxaline) and (2,3,2',3'-tetra(thiophen-2-yl)-6,6'-biquinoxaline) containing copper(II) and platinum(II) compounds as prodrug candidates. The binding interaction of these compounds with calf thymus DNA (CT-DNA) and human serum albumin were assessed with UV titration, thermal decomposition, viscometric, and fluorometric methods. The thermodynamical parameters and the temperature-dependent binding constant (K'b) values point out to spontaneous interactions between the complexes and CT-DNA via the van der Waals interactions and/or hydrogen bonding, except Cu(ttbq)Cl2 for which electrostatic interaction was proposed. The antitumor activity of the complexes against several human glioblastomata, lung, breast, cervix, and prostate cell lines were investigated by examining cell viability, oxidative stress, apoptosis-terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling, in vitro migration and invasion, in vitro-comet DNA damage, and plasmid DNA interaction assays. The U87 and HeLa cells were investigated as the cancer cells most sensitive to our complexes. The exerted cytotoxic effect of complexes was attributed to the formation of the reactive oxygen species in vitro. It is clearly demonstrated that Cu(ttbq)Cl2, Pt(ttbq)Cl2, and Pt(tpbq)Cl2 have the highest DNA degradation potential and anticancer effect among the tested complexes by leading apoptosis. The wound healing and invasion analysis results also supported the higher anticancer activity of these two compounds.

The thermoelastic properties of monolayer covalent organic frameworks studied by machine-learning molecular dynamics.

Two-dimensional (2D) covalent organic frameworks (COFs) are emerging as promising 2D polymeric materials with broad applications owing to their unique properties, among which the mechanical properties are quite important for various applications. However, the mechanical properties of 2D COFs have not been systematically studied yet. Herein, a machine-learned neuroevolution potential (NEP) was developed to study the elastic properties of two representative monolayer 2D COFs, namely COF-1 and COF-5. The trained NEP enables one to study the elastic properties of 2D COFs in realistic situations (e.g., finite size and temperature) and possesses greatly improved computational efficiency when compared with density functional theory calculations. With the aid of the obtained NEP, molecular dynamics (MD) simulations together with a strain-fluctuation method were employed to evaluate the elastic constants of the considered 2D COFs at different temperatures. The elastic constants of COF-1 and COF-5 monolayers were found to decrease with an increase in the temperature, though they were almost isotropic irrespective of the temperature. The thermally induced softening of 2D COFs below a critical temperature was observed, which is mainly attributed to their inherent ripple configurations at finite temperatures, while above the critical temperature, the damping effect of anharmonic vibrations became the dominant factor. Based on the proposed mechanisms, analytical models were developed for capturing the temperature dependence of elastic constants, which were found to agree with the MD simulation results well. This work provides an in-depth insight into the thermoelastic properties of monolayer COFs, which can guide the development of 2D COF materials with tailored mechanical behaviors for enhancing their performance in various applications.

Kinetic study of the CN + C2H6 hydrogen abstraction reaction based on an analytical potential energy surface.

The temperature dependence of the thermal rate constants and kinetic isotope effects (KIE) of the CN + C2H6 gas-phase hydrogen abstraction reaction was theoretically determined within the 25-1000 K temperature range, i.e., from very low- to high-temperature regimes. Based on a recently developed full-dimensional analytical potential energy surface fitted to highly accurate explicitly correlated ab initio calculations, three different kinetic theories were used: canonical variational transition state theory (CVT), quasiclassical trajectory theory (QCT), and ring polymer molecular dynamics (RPMD) method for the computation of rate constants. We found that the thermal rate constants obtained with the three theories show a V-shaped temperature dependence, with a pronounced minimum near 200 K, qualitatively reproducing the experimental measurements. Among the three methods used in this work, the QCT and RPMD methods have the best agreement with the experiment at low and high temperatures, respectively, while the CVT model shows the largest discrepancies. The significant increase in the rate constant at very low temperatures in this very exothermic and practically barrierless reaction could be attributed to the large value of the impact parameter, possibly ruling out the role of the tunneling effect and the intermediate complexes in the entrance channel. The theoretical H/D KIE depicted a "normal" behaviour, i.e., values greater than unity, emulating the experimental measurements and improving previous theoretical results. Finally, the discrepancies between theory and experiments were analysed as a function of several factors, such as limitations of the kinetics theories and the potential energy surface, as well as the uncertainties in the experimental measurements.

Temperature-dependent interatomic force constants and phonon coherent resonance contribution in quaternary non-centrosymmetric chalcogenides BaAg2SnSe4

Phonon heat transfer by thermoelectric modules undoubtedly plays a pivotal role for the future waste heat recovery and utilization. BaAg2SnSe4, a prototypical glasslike compound among quaternary non-centrosymmetric chalcogenides, demonstrates remarkable thermoelectric performance due to its exceptionally low lattice thermal conductivity (κL). Even with a comprehensive grasp of the underlying cause behind ultra-low κL and the subsequent heightened thermoelectric conversion efficiency, the temperature-dependent lattice dynamics and glass-like attributes governing κL continue to pose unresolved questions. In this study, we investigate the phonon thermal transport properties of quaternary BaAg2SnSe4 by incorporating the influence of phonon coherent resonance, thereby departing from the conventional Peierls's framework. Based on the temperature dependence of the interatomic force constants and coherences’ contribution to total lattice thermal conductivity (κL(C)), we intriguingly observe that the particle-like driven lattice thermal conductivity (κL(P)) and κL(C) have the quite significant and dominant role for describing ultralow phonon transport behaviors in the middle-high-temperature regions. Specially, enhanced phonon anharmonicity coupled with excited coherent phonons due to increasing temperature yields two close values, with 700 K values of 0.16 and 0.13 W/mK predicted for BaAg2SnSe4, respectively, accelerating its application in thermoelectric field. Furthermore, we also find that the off-diagonal terms of heat flux operators in κL exist distinction for different phonons, with zero contribution modes exhibiting temperature-independent phonon coherence effect. Temperature response of the force constants tensors and coherent resonance proposed in our work can be employed as a powerful tool, which would substantially be expected to inspire a broad revisiting phonon transport study in various device applications, including advanced thermal barrier coatings and superior thermoelectrics.

A prediction of the thermodynamic, thermophysical, and mechanical properties of CrTaO4 from first principles

AbstractIt is reported that the self‐forming CrTaO4 oxide scale can protect refractory high‐entropy alloys from oxidation, superior to Cr2O3. In this paper, the phase stability, mechanical, and thermal properties of three polymorphous phases of CrTaO4 are systematically investigated from first‐principles density functional theory calculations. The mechanical properties predicted using the strain–energy methods indicated that all three phases are mechanically stable. The temperature dependence of elastic constants and polycrystalline moduli of three phases demonstrated the thermal softening as temperature increase. The Helmholtz‐free energies as a function of volume and temperature are derived from phonon dispersions within the quasi‐harmonic approximation at six strained volumes. The calculated apparent bulk coefficients of thermal expansion of these three phases are evaluated, the highest value approximately 13.4× 10−6 K−1 within a temperature range of 500–2000 K for the rutile I41md phase. The lattice thermal conductivity calculated by the Debye–Callaway model suggested that the rutile type I41md phase has the lowest value of approximately 2.1 W/m/K at 1800 K. The other two phases, C2/m and P2/c, exhibit higher values due to relatively lower Grüneisen parameters and larger phonon velocities. The melting point of CrTaO4 is predicted to be between 1975 and 2449 K using ab initio molecular dynamics simulations. This work provides a comprehensive theoretical understanding of the thermodynamic, mechanical, and thermal properties for the new material CrTaO4 and serves as an example of a viable computational design strategy for improved oxidation resistance of refractory alloys at high temperatures.

Kinetics-Based Prediction for Sintering of Pd/CeO2–ZrO2–Al2O3 Three-Way Catalysts during Engine-Bench Aging

The thermal deactivation of Pd/CeO2–ZrO2–Al2O3 (Pd/CZA) three-way catalysts (TWCs) was studied using a full-sized washcoated monolithic honeycomb after engine-bench aging at 600–1000 °C under a stoichiometric-lean-rich cycling gas condition. This was followed by chassis dynamometer driving tests. The nanoscale structure of these aged catalysts was characterized by gas adsorption, X-ray diffraction, and electron microscopy imaging to elucidate the thermal deactivation mechanism. Aging at high temperatures and for long periods facilitated the Pd particle growth via surface migration and coalescence, which caused a drop in the number of active sites and thus the catalytic activity. The detailed particle growth kinetics was analyzed using simulated laboratory-scale aging of Pd/CZA powder samples at different temperatures (700–1000 °C) and periods of time (0–40 h). The as-obtained Pd particle size vs time data were successfully fitted by applying the Finke–Watzky model with two-step agglomeration, which provided the rate constants for the initial slow growth and the subsequent faster growth. The temperature dependence of the as-obtained rate constants showed simple Arrhenius-type behavior, which was expressed using the optimized frequency factor and activation energy for the Pd particle growth. Furthermore, these results were roughly consistent with the data obtained by the engine-bench aging test for the monolithic honeycomb catalysts. The sintering kinetic model developed in the present study will be useful for the prediction of TWC deactivation and lifetime.

Electrical and optical properties of a lead-free complex double perovskite BaNaFeMoO6: Photovoltaic and thermistor applications

In this communication, synthesis (solid-state reaction) and characterizations of a complex double perovskite BaNaFeMoO6 are reported. X-ray diffraction analysis (structure and lattice parameters) adopts a monoclinic structure while Raman spectroscopy provides information about the vibrational characteristics. Homogenous distribution of the grains is observed from the scanning electron microscope (SEM) whereas energy dispersive X-ray analysis (EDX) supports compositional consistency. Ultraviolet–visible (UV–vis) data provides a direct energy bandgap of 2.75 eV, which may be suitable for photovoltaic applications. The analysis of Maxwell-Wagner type of dielectric dispersion, resistance, relaxation, and transport mechanism are carried out using dielectric, impedance, modulus, and conductivity data recorded within an available range of frequency (1 kHz – 1 MHz) and temperature (25 °C – 500 °C). Both the theoretical fit of the Nyquist data using ZSIMPWIN software as well as temperature-dependent conductivity support a negative temperature coefficient of resistance (NTCR) nature. The scaling nature of modulus data supports the non-Debye type of relaxation. The obtained temperature-dependent thermistor constant (β), sensitivity factor (α), and activation energy make the prepared material suitable for thermistor-related device application. The study of polarization–electric field (P-E) loop suggests the possibility of ferroelectric nature in the reported material.

1,4-Bis(trimethylsilyl)piperazine—Thermal Properties and Application as CVD Precursor

We report an investigation into 1,4-Bis-N,N-(trimethylsilyl)piperazine (BTMSP) as a novel precursor for the synthesis of silicon carbonitride films by chemical vapor deposition (CVD). The thermal stability, temperature dependence of vapor pressure and thermodynamic constants of the evaporation process of BTMSP were determined by static tensimetry with a glass membrane zero manometer. The transformation of the compound in low-power (25 W) plasma conditions was investigated by optical emission spectroscopy. It was shown that BTMSP undergoes destruction, accompanied by H and CH elimination and CN formation. SiCN(H) films were deposited in a hot-wall plasma-enhanced CVD reactor. The optical properties of the films were studied by spectral ellipsometry (refractive index: 1.5–2.2; absorption coefficient: 0–0.12) and UV–Vis spectroscopy (transmittance: up to 95%; optical bandgap: 1.6–4.9 eV). Information on the aging behavior of the films is also provided. The transformation of the films occurred through water adsorption and the formation of Si–O bonds with the degradation of Si–H, N–H and Si–CHx–Si bonds.

Revisiting thermal transport in single-layer graphene: On the applicability of thermal snapshot interatomic force constant extraction methodology for layered materials

The thermal conductivities of single-/bi-layer graphene and bulk-graphite are obtained using the Boltzmann transport equation (BTE) framework by accounting for three-phonon and four-phonon scatterings. For single-layer graphene, the thermal conductivity and interatomic force constants obtained using temperature-independent finite-difference, and temperature-dependent molecular dynamics-based approaches agree with each other. The use of the thermal snapshot approach to get temperature-dependent force constants results in a non-physical description of interatomic distances for single-layer graphene. The predicted thermal conductivity at room temperature using finite-difference based force constants is 800 W/m K, which is a severe under-prediction of experimentally measured values. For bi-layer graphene and bulk graphite, the thermal snapshot methodology is applicable and thermal conductivity changes by 25% and 5% with temperature-dependent force constants. The effect of four-phonon scattering is less than 10% on the predicted thermal conductivity of bi-layer graphene and graphite, and the obtained thermal conductivities using thermal snapshot methodology are in agreement with the literature. The limitation in the prediction of thermal conductivity of single-layer graphene via the BTE approach stems from non-accountability of temperature-dependence in finite-difference based force constants and non-physical description of interatomic bonds in thermal snapshot based force constants extraction for planar 2-atoms unitcell of single layer graphene.

Dissociation Constants of Amine Solvents Used in CO2 Capture: Titrimetric Estimation and Density Functional Theory Calculation

Amine solvents are basic in nature and used for the absorption of acidic CO2. The dissociation constant of protonated amine solvents significantly influences their CO2 absorption, and it varies with temperature. It is thus important to estimate the temperature-dependent dissociation constant of new protonated amine solvents before using them for CO2 absorption purposes. In the present work, the procedures and protocols for estimating the dissociation constant (Ka) of protonated amine solvents using the potentiometric titration method are optimized in the laboratory. The validation of the optimized potentiometric methodologies is confirmed by estimating precisely the reported −ln Ka values of two different amine solvents, namely, monoethanolamine (MEA) and 2-amino-2-methyl-1-propenol (AMP). Subsequently, the −ln Ka of the new amine solvent bis(3-aminopropyl)amine (APA) is determined by the potentiometric titration method at different temperatures (298–318 K). The −ln Ka value estimated for APA at 298 K is 21.73, which is comparable to those of amine solvents known for efficient CO2 capture application. Electronic structure calculations are carried out to explain the influence of the methyl group (−CH3) and alcoholic group (−OH) on the −ln Ka values of the studied amine solvents. The scaffolds for the studied amine solvents are optimized by density functional theory (DFT). Natural bond orbital (NBO) calculations are performed to estimate the bond lengths as well as charge distributions on nitrogen and other elements in the optimized geometries of amine solvents. The experimentally obtained −ln Ka values of the studied amines are validated through DFT calculations.

Low-temperature and elastic properties of the 1/1 Tsai-type quasicrystal approximant GdCd6 investigated by ultrasonic measurements

Low-temperature and elastic properties of the quasicrystal approximant GdCd6 have been investigated by means of an ultrasonic measurement. Salient elastic anomalies were observed in the temperature and magnetic field dependence of the principal elastic constants, most probably being associated with the successive multiple magnetic phase transitions showing up at low temperatures. Based on the experimental data, a magnetic field vs temperature phase diagram was constructed. In a zero magnetic field, at least four phases appear to exist. However, the phase diagram both for the magnetic field applied for H// and H// is richer, and interestingly some phase boundaries correspond to the development of field-induced phases, leading to the intricate magnetic phase diagram possibly with multiple ordered phases. We discuss the elastic and electronic properties at low temperatures, and also the nature of the multiple magnetic phase transitions of GdCd6.

Permittivity Threshold and Thermodynamics of Integer Charge-Transfer Complexation for an Organic Donor–Acceptor Pair

Molecular charge doping involves the formation of donor-acceptor charge-transfer complexes (CTCs) through integer or partial electron transfer; understanding how local chemical environment impacts complexation is important for controlling the properties of organic materials. We present steady-state and temperature-dependent spectroscopic investigations of the p-dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) complexed with the electron donor and hole transport material N,N'-diphenyl-N,N'-di-p-tolylbenzene-1,4-diamine (MPDA). Equilibrium formation constants (KCT) were determined for donor-acceptor pairs dissolved in a series of solvents covering a range of values of permittivity. A threshold for highly favorable complex formation was observed to occur at ϵ ∼ 8-9, with large (>104) and small (<103) values of KCT obtained in solvents of higher and lower permittivity, respectively, but with chloroform (ϵ = 4.81) exhibiting an anomalously high formation constant. Temperature-dependent formation constants were determined in order to evaluate the thermodynamics of complex formation. In 1,2-dichloroethane (ϵ = 10.36) and chlorobenzene (ϵ = 5.62), complex formation is both enthalpically and entropically favorable, with higher enthalpic and entropic stabilization in the solvent with higher permittivity. Complexation in chloroform is exothermic and entropically disfavored, indicating that specific, inner-shell solvent-solute interactions stabilize the charge-separated complex and result in a net increase in local solution structure. Our results provide insight into how modification to the chemical environment may be utilized to support stable integer charge transfer for molecular doping applications and requiring only modest changes in local permittivity.

Phononic thermal rectifier: a new proposition

We put forward a new proposition for achieving a phonon-based thermal rectifier at the nanoscale level, considering a one-dimensional phononic system. The central mechanism relies on the temperature-dependent spring constant along with the presence of mass modulation which essentially breaks the symmetry in the system and thus leads to different thermal currents in two bias polarities. Along with thermal rectification, the phenomenon of negative differential thermal resistance (NDTR) is also observed. The NDTR effect is utilized further to have the NOT gate operation. The present analysis may open up new possibilities for designing efficient thermal rectifiers and logic gates.

Phase-Transition Behaviors of Crystalline Hypophosphorous Acid and a Feasible Incommensurate Wave-like Array of the Proton Displacements from Each Center of Hydrogen Bonds along the 1-Dimensional Chain

Crystalline hypophosphorous acid, composed of strongly hydrogen-bonded chains, was investigated by adiabatic calorimetry in a temperature range between 20 and 306 K and by dielectrometry as a supplemental means. The heat-capacity curve showed subsequent phase transitions: a second-order one at 210.4 K followed by a first-order one at 159 K and the other two ones at 149 and 139.4 K with decreasing temperature. The total entropy of the transitions was assessed to be 0.35 J K–1 mol–1 by using the baseline heat capacities that were determined by means of a real-coded genetic algorithm. The small value was interpreted as the transitions were associated with displacements of the protons forming hydrogen bonds: namely, while they are located at the center between two oxygen atoms above 210.4 K, the protons are displaced close to one of the oxygen atoms with the bond length enlarged below it. Temperature dependence of dielectric constants displayed no sign, around 210 K, indicating the existence of (anti-)ferroelectricity. It was thus expected that the manner of off-center proton displacements along each hydrogen-bonded chain reveals a wave-like and incommensurate appearance below 210.4 K, referred to as the normal crystal lattice above it, and following locking to a wave-like but superlattice commensurate one at 159 K.

Thermodynamics of hydrogen storage: Equilibrium study of the LOHC system indole/octahydroindole

Reversible hydrogenation of indole or its derivatives could be an attractive hydrogen storage technology. Particularly for hydrogen release, the reaction equilibrium is a crucial factor determining the conditions in the respective process. This study is supposed to provide inside into this reaction equilibrium. An experimental study of the chemical equilibrium of hydrogenation/dehydrogenation reactions was performed in a stirred autoclave with n–hexane as a solvent by varying the temperature, pressure, and hydrogen:feedstock ratio. Based on the compositions in stationary state, equilibrium constants have been derived. Furthermore, thermodynamic properties of chemical reactions were derived from the equilibrium constant temperature dependence. This temperature dependence reveals that the enthalpy of reaction for the dehydrogenation of the six-membered ring is significantly larger than in the nitrogen containing five-membered ring. To get deeper insides, the molar enthalpies of formation of intermediates have been calculated using high-level quantum chemical methods. This analysis supports the observations from the equilibrium reactions. Additionally, it show that the heat of reaction for the endothermal dehydrogenation is about 2 kJ mol (H2)–1 smaller in the liquid phase than in case of a gas phase reaction.

Features of the Formation of Conductive Channels in Memristors Based on Solid Electrolytes

Experimental data on measurement of the resistance and electrical conductivity in the low-resistance mode of operation of a germanium-selenide-based memristor with a self-forming conductive channel in the form of a silver filament are obtained in the range of operating frequencies and temperatures. In the frequency experiment, the influence of the switching frequency is tested at room temperature in the range of 1–10 000 Hz. The main result of the experiment is the identification of a linear relationship between the electrical conductivity and the memristor operating cycle time in semilogarithmic coordinates, which made it possible to introduce a temperature-dependent kinetic constant. This experimental fact made it possible to establish the main parameter affecting the shape of the current–voltage characteristics, namely the thickness of the conductive channel.

Phononic heat conductance of gold atomic contacts: Coherent versus incoherent transport

We present here a theoretical method to determine the phononic contribution to the thermal conductance of nanoscale systems in the phase-coherent regime. Our approach makes use of classical molecular dynamics (MD) simulations to calculate the temperature-dependent dynamical matrix, and the phononic heat conductance is subsequently computed within the Landauer-B\"uttiker formalism with the help of nonequilibrium Green's function techniques. Tailored to nanostructures, crucial steps of force constant and heat transport calculations are performed directly in real space. As compared to conventional density functional theory (DFT) approaches, the advantage of our method is two-fold. First, interatomic interactions can be described with the method of choice. Semiempirical potentials may lead to large computational speedups, enabling the study of much larger systems. Second, the method naturally takes into account the temperature dependence of atomic force constants, an aspect that is ignored in typical static DFT-based calculations. We illustrate our method by analyzing the temperature dependence of the phononic thermal conductance of gold (Au) chains with lengths ranging from 1 to 12 atoms. Moreover, in order to evaluate the importance of anharmonic effects in these atomic-scale wires, we compare the phase-coherent approach with nonequilibrium MD (NEMD) simulations. We find that the predictions of the phase-coherent method and the classical NEMD approach largely agree above the Debye temperature for all studied chain lengths, which shows that heat transport is coherent and that our phase-coherent approach is well suited for such nanostructures.