Dependence Of Activation Energy Research Articles

Zwitterionic Bergman cyclization triggered polymerization gives access to metal-graphene nanoribbons using a boron metal couple

With the rapid growth in artificial intelligence, designing high-speed and low-power semiconducting materials is of utmost importance. This investigation provides a theoretical basis to access covalently bonded transition metal-graphene nanoribbon (TM-GNR) hybrid semiconductors whose DFT-computed bandgaps were much narrower than the commonly used pentacene. Systematic optimization of substrates containing remotely placed boryl groups and the transition metals produced the zwitterions via ionic Bergman cyclization (i-BC) and unlocked the polymerization of metal-substituted polyenynes. Aside from i-BC, the subsequent steps were barrierless, which involved structureless transition regions. Multivariate analysis revealed the strong dependence of activation energy and the cyclization mode on the electronic nature of boron and Au(I). Consequently, three regions corresponding to radical Bergman (r-BC), ionic Bergman (i-BC), and ionic Schreiner-Pascal (i-SP) cyclizations were identified. The boundaries between these regions corresponded to the mechanistic shift induced by the three-center-three-electron (3c-3e) hydrogen bond, three-center-four-electron (3c-4e) hydrogen bond, and vacant p-orbital on boron. The ideal combination for cascade polymerization was observed near the boundary between i-BC and i-SP.

Crystallization kinetics of Hf28Be18Ti17Zr17Cu7.5Ni12.5 high-entropy bulk metallic glass

Crystallization kinetics of Hf28Be18Ti17Zr17Cu7.5Ni12.5 high-entropy bulk metallic glass (HE-BMG) in the non-isothermal and isothermal conditions were investigated by differential scanning calorimetry (DSC). In non-isothermal crystallization condition, DSC traces exhibited three crystallization processes. The apparent activation energies of peak crystallization temperatures were calculated by Kissinger methods and found to be Ep1 > Ep2 > Ep3. The dependence of local activation energy on crystallized volume fraction was summarized by the Kissinger-Akahira-Sunose (KAS) method. The non-isothermal crystallization reaction was described mathematically using the Šesták-Berggren model. In the isothermal crystallization condition, with the increase of annealing temperature, the incubation time and crystallization time become shorter and the crystallization peak becomes higher. All Avrami indices were less than 2.5, indicating that crystallization of glassy alloy is dominated by a diffusion-controlled three-dimensional growth with a decreasing nucleation rate.

Cure Kinetics of Commercial Epoxy-Amine Products with Iso-Conversional Methods

The dependence of the apparent activation energy for the epoxy-amine reaction on the degree of conversion can be obtained by applying iso-conversional methods to the non-isothermal cure data obtained by using differential scanning calorimetry (DSC). The application of three iso-conversional methods has been utilized for the analysis of non-isothermal DSC cure data for three commercial high solids epoxy-amine coatings. The average apparent activation energy for cure of the fully formulated commercial product(s) is very similar to that previously reported for the epoxy-amine clear coats, indicating that the presence of additives does not influence the epoxy-amine apparent activation energy. Among the methods tested, Friedman’s method performed the best in fitting the experimental DSC data. In addition, all three methods underpredict the experimental isothermal cure data for three commercial products at two different cure conditions (i.e., 23 °C/50% RH and 40 °C/70% RH), showing that the non-isothermal DSC experiments cannot capture the catalytic effect of water on the curing reaction of epoxy-amine coatings. Furthermore, for high-solids epoxy-amine products, at least 60% conversion is required to achieve the time when the applied coating will not show any tackiness (i.e., the T2 time measured using the Beck Koller method).

Kinetic studies of polystyrene using non - isothermal degradation by TGA

The kinetic analysis was carried out for thermal degradation by using of Thermo gravimetric analysis TGA analysis of polystyrene (PS 120) produced by Sabic company , using models of Flynn-Wall-Ozawa , Friedman, Kissinger , Coats-Redfern, dynamic, and Horowitz-Metzger methods. Activation energy was determined . The experiments were conducted at five heating rates of 5,10,15,20,25 Co/min. The temperature range of 30 - 775 Co under helium as inert gas was used with flow rate 20 ml/min . The obtained activation energy dependences are interrupted, the thermal degradation which follows random scission pathway, average activation energy obtained was 230 KJ/mol . This value was compared with published data and was found in a good agreement.

Development of a new approach for the kinetic modeling of the lithium reactive hydride composite (Li-RHC) for hydrogen storage under desorption conditions

Among some promising candidates for high-capacity energy and hydrogen storage is the Lithium-Boron Reactive Hydride Composite System (Li-RHC: 2 LiH + MgB2/2 LiBH4 + MgH2). This system desorbs hydrogen only at relatively high temperatures and presents a two-step series of reactions occurring in different time scales: first, MgH2 desorbs, followed by LiBH4. Hitherto, the dehydrogenation kinetic behavior of such a system has been described for different temperatures at specific values of operative pressure. However, a comprehensive model representing its dehydrogenation kinetic behavior under different operative conditions has not yet been developed. Herein, the separable variable method is applied to develop a comprehensive kinetic model, including the two-step dehydrogenation series reaction. The MgH2 decomposition is described with the one-dimensional interface-controlled reaction rate Johnson-Mehl-Avrami-Erofeyev-Kholmogorov (JMAEK) with a (Pequilibrium/Poperative) pressure functionality and an Arrhenius temperature dependence activation energy of 63 ± 3 kJ/mol H2. The LiBH4 decomposition is modeled applying the autocatalytic Prout-Tompkins model. A novel approach to describe the Prout-Tompkins t0 parameter as a function of the operative temperature and pressure model is proposed. This second reaction step presented a (Pequilibrium – Poperative/Pequilibrium)2 pressure dependence and an Arrhenius temperature dependence with activation energy 94 ± 13 kJ/mol H2. The proposed approach is experimentally and computationally validated, successfully describing the decomposition kinetic behavior of MgH2 and LiBH4 under three-phase gas, liquid and solid environment and shows good agreement between experimental and modeled curves.

Kinetic analysis of non-isothermal volume melting processes by differential scanning calorimetry

The onset of melting of standard samples, ascribed to surface melting, is generally used for calibration of calorimeters. However, in non-isothermal conditions, nucleation-driven volume melting, which is thermally activated, takes place. In this work, we propose an approximation in the frame of the classical nucleation and growth transformation kinetics to extend to non-isothermal regimes the analysis of processes governed by constant nucleation and interface controlled growth. The approximation allows both to observe the temperature dependence of nucleation activation energy with the overheating and to obtain the surface energy between the liquid nucleus and the surrounding solid phase for pure indium and lead (~ 10 mJ m−2) and for a Fe70B5C5Si3Al5Ga2P10 bulk metallic glass eutectic composition (~ 50 mJ m−2). These values are about 50% lower than the theoretical ones for homogeneous nucleation, which can be ascribed to the random heterogeneous nucleation occurring at the crystals boundaries.

Potential Driven Volume Diffusion in Nanoporous AgBi at Room Temperature

AbstractVolume diffusion is crucial for crystal growth and interface manipulation in electron devices and catalysts. Its kinetics only becomes vigorous at high temperature, in which case grain boundary diffusion and surface diffusion dominate usually. In this study, volume diffusion of Bi atoms in nanoporous AgBi can be driven by electrochemical potential at room temperature due to ligament expansion. Bi diffusion induces conformal covering of amorphous Bi2O3 shell onto the spatially interconnected skeletons without ligament coarsening. Growth kinetics of amorphous shell demonstrates that the potential dependence of Bi volume diffusion activation energy corresponds to 0.932 eV V−1. Compared with temperature dependence, potential supply is confirmed to be more feasible to modulate volume diffusion rate and shell thickness, and Bi enrichment increases 3.4 times when potential raises from −0.85 to −1.05 V. Potential driven volume diffusion at room temperature reported here knobs one door to manipulate local composition of functional nanostructures and nano‐devices at atomic scale.

Kinetic modelling of mixed plastic waste pyrolysis

The study of plastic waste thermal decomposition and kinetics is a crucial step for optimal reactor design and the scale-up of pyrolysis technologies for chemical recycling. Kinetic parameters of individual and mixed plastic waste samples that resemble the main components of plastic waste streams (polyolefins and polyesters) were determined in this study using thermogravimetric analysis. Strong dependencies of activation energy on conversion were found for the mixtures, indicating that the process occurs in multiple steps and there are strong interactions between plastic components, lowering their activation energy during the decomposition. PP and LDPE decomposed via two separate steps whereas others decomposed via a single step process. For plastic mixtures, three dominant partially overlapping steps were identified. Kinetic deconvolution analysis improved model accuracy of mixtures and enabled the study of kinetic contributions and behaviour of individual steps. The models were then applied to predict experimental data at various heating conditions and plastic compositions.

Influence of Laser Irradiation on the Mechanism of Viscous Flow of Aqueous Sodium Chloride Solution

The effect of 632.8-nm radiation of a helium–neon laser on the viscosity of an aqueous sodium chloride solution has been studied. It has been shown that irradiation changes the nature and numerical values of the concentration dependence of viscosity. The concentration dependences of the viscous-flow activation energy for unirradiated and irradiated aqueous sodium chloride solutions have been determined. The influence of irradiation on the parameters of the power-law dependence of the solution viscosity on concentration has been analyzed, and the temperature dependences of these parameters have been determined. It has been concluded that laser irradiation alters the structural organization of the aqueous system and reduces

Sulfur-deficient edges as active sites for hydrogen evolution on MoS2.

A grand-canonical approach is employed to calculate the voltage-dependent activation energy and estimate the kinetics of the hydrogen evolution reaction (HER) on intrinsic sites of MoS2, including edges of varying S-coverage as well as S-vacancies on the basal plane. Certain edge configurations are found to be vastly more active than others, namely S-deficient edges on the Mo-termination where, in the fully S-depleted case, HER can proceed with activation energy below 0.5 eV at an electrode potential of 0 V vs. SHE. There is a clear distinction between the performance of Mo-rich and S-rich adsorption sites, as HER at the latter sites is characterized by large (generally above 1.5 eV) Heyrovsky and Tafel energy barriers despite near-thermoneutral hydrogen adsorption energy. Thus, exposing Mo-atoms on the edges to which hydrogen can directly bind is crucial for efficient hydrogen evolution. While S-vacancies on the basal plane do expose Mo-rich sites, the energy barriers are still significant due to high coordination of the Mo atoms. Kinetic modelling based on the voltage-dependent reaction energetics gives a theoretical overpotential of 0.25 V and 1.09 V for the Mo-edge with no S atoms and the weakly sulfur-deficient (2% S-vacancies) basal plane, respectively, with Volmer-Heyrovsky being the dominant pathway. These values coincide well with reported experimentally measured values of the overpotential for the edges and basal plane. For the partly Mo-exposed edges, the calculated overpotential is 0.6-0.7 V while edges with only S-sites give overpotential exceeding that of the basal plane. These results show that the overpotential systematically decreases with increased sulfur-deficiency and reduced Mo-coordination. The fundamental difference between Mo- and S-rich sites suggests that catalyst design of transition metal dichalcogenides should be focused on facilitating and modifying the metal sites, rather than activating the chalcogen sites.

Exploring kinetic and thermodynamic mechanisms of switchgrass pyrolysis using iterative linear integral isoconversional method and master plots approach

This study aimed to explore the kinetic mechanism of switchgrass pyrolysis through isoconversional kinetic analysis and master plots approach. The pyrolysis kinetics of switchgrass was experimentally studied by thermogravimetric analysis at 5, 10, 20, and 40 K min−1. The experimental kinetic data of switchgrass pyrolysis was processed by the iterative linear integral isoconversional method and the obtained effective activation energies varied significantly with conversion: from 190.3 to 216.7, 217.3 ± 0.9 and 219.4 to 301.4 kJ·mol−1 in the conversion range of 0.05 – 0.45, 0.45 – 0.75 and 0.75 – 0.95, respectively. The significant conversion dependence of activation energies indicated switchgrass pyrolysis was a kinetically complex process, which was caused by the overlapping decomposition reactions of lignocellulosic components contained in switchgrass. Based on the master plots approach and the general empirical reaction model, the pyrolysis kinetic process of switchgrass pyrolysis followed the reaction model: f(α) = α-0.307·(1–1.020α)2.276. The frequency factor of switchgrass pyrolysis varied from 4.2 × 1012 to 3.9 × 1018 s−1 in the conversion range of 0.05 – 0.95, and a kinetic compensation effect between the frequency factor and activation energy was discovered. Based on the kinetic results, the changes in Gibbs free energy, enthalpy and entropy for switchgrass pyrolysis were calculated. It is expected that this study can provide a methodological guideline for the kinetics and thermodynamics of lignocellulosic biomass pyrolysis.

A perspective on the activation energy dependence of the Fischer-Tropsch synthesis reaction mechanism

The conversion of syngas to hydrocarbons is achieved through different mechanisms based on the type of perceived monomer and chain initiator, with preference given to two key mechanisms in the explanation of the conversion process. Concerted efforts at explaining the Fischer-Tropsch Synthesis (FTS) mechanism remain unresolved. By graphing the energy of a reaction using the Arrhenius equation, changes occurring in a reaction for temperature variation can produce valuable information on the reaction. This work uses the activation energy (Ea) deduced from the Arrhenius equation to determine the coexistence nature of the different mechanisms for the FTS reaction. The work shows the reaction mechanisms are not competing, but rather complement one another during FTS reaction to yield different products.

Insights into the Effect of Pb on Formic Acid Electro-Oxidation on Pt

Formic acid is a promising energy carrier and electrochemical formic acid oxidation can serve as an important reaction within fuel cells. Although Pt remains the best-known catalyst, but the major currents contributing pathways is indirect oxidation pathways, which leads to poor activities at low potentials. The surface adatoms like Pb have been known to promote the direct oxidation pathways significantly. We have carried out theoretical calculations, and experiments to probe the reason for this enhancement. Theoretical results show an upshift of Pt energy levels above the Fermi energy upon adsorption of Pb on the Pt surface, which influences the binding energy of the adsorbates. Reaction free energy calculations of typical intermediates from water (H2O) and formic acid (HCOOH) molecule indicate that Pb adsorption suppresses the adsorption of Hydrogen atoms at potentials typical of underpotential deposition on Pt electrode. This results in larger number of vacant sites on the Pt surface which is now able to carry out formic acid oxidation to CO2. The calculations were supported by electrochemical experiments of underpotentially deposited hydrogen and potential dependent apparent activation energies. Figure 1

Fluoroalkylated-GAP copolymers (GAP-FP) as promising energetic binders

The development of a novel energetic block copolymer of glycidyl azide polymer (GAP) and fluoropolymer (FP) using a boron trifluoride-tetrahydrofuran complex/diol initiator system is reported herein. Well-defined compositions of the GAP-FP copolymers in two different GAP to FP ratios (1:1 and 1:3) were synthesized by tailoring the desired molecular weights of each block in the copolymer, demonstrating the synthetic versatility of such a copolymer system. The resultant GAP-FP copolymers represent a unique hybrid binder system with tunable energy releasing and oxidizing potentials intended for metallized formulations. Thermogravimetric analysis showed that the carbonaceous residue usually formed from the decomposition of GAP could be reduced significantly by the copolymerization with FP. Isoconversional method of kinetic analysis of the GAP-FP copolymers revealed an increasing dependence of the effective activation energy on the extent of conversion. The increasing dependence suggested a mechanism of the competing reactions that were found to be between the reactions of FP triggered oxidation and intermolecular crosslinking of the polyimine intermediates formed from GAP decomposition that ultimately resulted in the reduction of the carbonaceous residue.

Activation energy dependence on doping concentration in PVA-MgCl2 composites

Solid polymer electrolytes are fast replacing the liquid ones because of their numerous advantages such as high energy density, long cycling life, fast charging, low leakage currents and easy fabrication in a variety of shapes. Although most of the reports are based on Li-ion batteries, focus is slowly shifting to develop electrolytes with Group II elements such as Magnesium and Calcium because of their easy availability and moderate ionic activity.In this work, we have complexed polymer polyvinyl alcohol with Magnesium Chloride (MgCl2·6H2O) at different weight percentages. DC conductivity studies were conducted using a two probe setup. It is seen that the activation energy of the system depends on ionic conductivity which in turn depend on the percolating interfacial effect, i.e. anions adsorb on the surface of fillers (and then break up the ion pair and lead to increased interfacial ionic conductivity.This increased conductivity is partly due to the doping and partly due to midgap energy levels developing due to formation of polarons and bipolarons in the composite gel polymer electrolyte .

Dependences of ionic conductivity and activation energy on germanium content in superionic Li1.4Al0.4GexTi(1.6−x)(PO4)3 solid electrolytes

• Superionic Li 1.4 Al 0.4 Ge x Ti 1.6−x (PO 4 ) 3 solid electrolyte is successfully synthesized by a facile solution-based method. • The dependence of ionic conductivity and activation energy on Ge content is thoroughly investigated. • The synthesized solid electrolyte has displayed a high conductivity of 1.05 × 10 −3 S/cm. Herein, to improve the ionic conductivities of Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 (LATP) solid-state electrolytes and establish the optimal Ge content to yield a high lithium-ion conductivity, Ti 4+ is partially substituted with Ge 4+ to yield Li 1.4 Al 0.4 Ge x Ti (1.6−x) (PO 4 ) 3 (LAGTP, x = 0.1, 0.2, 0.3, and 0.4). The LAGTP solid electrolytes are synthesized using a facile solution-based method. Further, the obtained material is calcined and sintered at 800 and 900 °C, respectively. The structural properties of the LAGTP ceramic powders are thoroughly investigated, and the dependences of the ionic conductivity and activation energy on Ge content are also explored. The LAGTP (x = 0.1) solid electrolyte exhibits the highest total ionic conductivity (1.05 × 10 –3 S/cm) at 25 °C and the lowest activation energy (0.237 eV), which are far superior to those of the conventional LATP solid electrolytes. Therefore, LAGTP solid electrolytes are potential candidates for use in developing safer high-performance all-solid-state batteries.

Kinetics of benzoxazine and epoxy oligomer copolymerization

This work is devoted to the non-isothermal kinetic analysis of the curing process of benzoxazine - epoxy alloys in a wide range of ratios by the DSC method. Compositions consisted of benzoxazine (BA-mt) based on bisphenol A and m-toluidine and epoxy resin based on bisphenol A (KER 828). The dependencies of the isoconversional activation energy on the extent of conversion for all systems were obtained according to the methods developed by Vyazovkin. These dependencies indicated a complex multi-step curing mechanism. Based on the calculated values ​​of the isoconversional activation energy dependencies, some isothermal predictions were made. It is shown that in order to derive an optimal mathematical model that describes well the real process under isothermal conditions, it is necessary to take into account diffusion limitations, which may be associated with the process of vitrification of systems during the curing process. The possibility of vitrification of systems was proved in terms of oscillatory rheology. In general, it was shown that the addition of epoxy oligomer to the benzoxazine resin significantly slowed down the curing process. However, we suppose that cure for all systems under study was complete, that was confirmed by FTIR data. The kinetic model for the curing process of BA-mt benzoxazine was derived.

Superconducting and structural properties of the noncentrosymmetric Re6Hf superconductor under high pressure

We report the effect of high pressure on the superconducting, vortex pinning, and structural properties of a polycrystalline non-centrosymmetric superconductor Re6Hf. The superconducting transition temperature, Tc, reveals a modest decrease as pressure P increases with a slope -0.046 K/GPa (-0.065 K/GPa) estimated from resistivity measurements up to 8 GPa (magnetization measurement ~ 1.1 GPa). Structural analysis up to ~18 GPa reveals monotonic decreases of lattice constant without undergoing any structural transition and a high value of bulk modulus B0= 333.63 GPa, indicating the stability of the structure. Furthermore, the upper critical field and lower critical field at absolute temperature (Hc2(0) & Hc1(0)) decreases slightly from the ambient pressure value as pressure increases up to 2.5 GPa. In addition, up to P ~ 2.5 GPa using thermally activated flux flow of vortices revealed a double linearity field dependence of activation energy of vortices, confirming the coexistence of single and collective pinning vortex states. Moreover, analysis of critical current density using the collective pinning theory showed the transformation of {\delta}Tc to {\delta}l pinning as pressure increases, possibly due to migration of grain boundaries. Besides, the band structure calculations using density functional theory show that density of states decreases modestly with pressure, which may be a possible reason for such a small decrease in Tc by pressure.

Temperature dependent activation energies of exciton states on a single GaAs laterally coupled-quantum-dot molecule and quantum dot

The objective of this study was to perform spectroscopy measurements of a single GaAs laterally coupled quantum dot molecule and quantum dot. Compared to photoluminescence spectra between a single GaAs laterally coupled quantum dot molecule and a single quantum dot, the presence of optical coupling was confirmed through dipole-dipole interactions in a single coupled quantum dot molecule. Based on temperature dependence measurements, weak exciton-phonon interactions (25 meV) were observed from the ground exciton state of a single GaAs laterally coupled quantum dot molecule in comparison with a strong exciton-phonon interaction (49 meV) from the ground exciton state of a single GaAs QD. Such weak interactions were due to reduced confinement at exciton states in a single laterally coupled quantum dot molecule. In addition, weaker activation energy (∼15 meV) was observed in terms of confined electrons in a single laterally coupled quantum dot molecule, where a confinement effect became reduced due to enlarged dimensional area in one direction along [1 1‾ 0].

Water self-diffusion in C-S-H: Effect of confinement and temperature studied by molecular dynamics

Despite the recent advances in the understanding of diffusion processes in C-S-H, there are still open questions regarding the pore-size dependence of diffusion and associated activation energy, and the transition from glassy to Fickean dominated dynamics. We perform molecular dynamics simulations in C-S-H with varying pore sizes (spanning interlayer and gel pore sizes) to investigate the pore size and temperature dependence of water and ion diffusion and activation energy. We quantify the finite-size artifacts from these simulations. The transition from glassy to Fickean-dominated dynamics occurs at an interlayer distance of 2.6 nm. In the Fickean regime, an expression using the self-diffusion of bulk water and a single activation energy captures the pore size and temperature dependence of the water self-diffusion in C-S-H. Finally, the anisotropic self-diffusion obtained from the molecular scale is used as input in mean-field homogenization to get gel diffusivity.