Correct Activation Energy Research Articles

Influence of chemical composition on coarsening kinetics of coherent L12 precipitates in FCC complex concentrated alloys

In this study, we report the experimental coarsening kinetics at 850, 900 and 950 °C of four complex concentrated alloys in the Al–Ti–Cr–Fe–Co–Ni senary system with different chemical compositions but a similar γ’ (L12) volume fraction (∼35 % at 950 °C) in a face-centered cubic (γ, FCC) matrix. The selected alloys were specifically designed to investigate the influence of Fe additions and Ni–Co substitutions on Ostwald ripening kinetics. Atom Probe tomography (APT) was used to determine the compositions of the FCC and L12 phases, which agree very well with Calphad calculations at thermodynamic equilibrium. Thermo-kinetic modeling of L12 precipitation was carried out using the Prisma module developed by Thermo-Calc and compared with experimental results. Apparent activation energies were determined and discussed in light of diffusion-controlled coarsening models to identify the key parameters affecting Ostwald ripening. We suggest that the abnormally high apparent activation energies results from composition-dependent parameters. When the latter are accounted for, the corrected activation energies for coarsening are in better agreement with available diffusion data.

Simulation of thermoluminescence signals at very low dose rates and low doses: Implications for dosimetric applications

During luminescence dosimetry applications, samples are irradiated in the laboratory with irradiation dose rates of the order of 0.1 Gy/s. By contrast, samples in nature are irradiated with a dose rate many orders magnitude smaller, typically 1 mGy/year. In this paper, the effect of very low dose rates and also low doses on thermoluminescence (TL) signals is investigated using the basic one-trap one-recombination center model (OTOR). The simulations showed that at very low dose rates the assumptions of quasi-equilibrium conditions (QE) are violated. This violation of QE conditions results in a significant distortion of the shapes of TL glow curves, with the result that the peak shape methods of analysis fail to produce the correct activation energy E of the traps. At the same time, the estimated half-life of the electrons traps is found to increase significantly at very low doses. The dose response of the sample at very low dose rates is simulated for both the irradiation stage and the subsequent heating stage. The dose response of the integrated TL signal is found to coincide with the dose response during the irradiation stage. However, the TL dose response measured in terms of the peak height shows a significant under-response at very low doses, making the whole dose response curve superlinear before the onset of saturation. The peak height under-response is due to the clear violation of QE conditions at very low doses and dose rates. The reported distortion of the TL glow curves and the existence of superlinearity of the TL signals at low dose rates, have important implications for the analysis of TL signals. Researchers need to be aware of the possibility that both of these effects may be present, since they can influence the analysis of experimental data and can lead to the wrong conclusions in dosimetric applications.

A maturation scale for molecular simulation of kerogen thermal degradation

Correlating molecular dynamics (MD) simulations of thermal degradation of organic matter with natural processes in ancient sediments is vital for revealing the mechanisms of hydrocarbon generation at molecular and atomic levels. This study establishes a practical maturation scale for molecular modeling of the thermal degradation of kerogen. ReaxFF reactive MD simulations of the thermal evolution of typical kerogen types (Type I, II, and III) were carried out at temperatures ranging from 1200 K to 2600 K. The results demonstrate that the H/C and O/C atomic ratios of kerogen molecules decrease as simulation temperatures increase and that the evolutionary paths of the three kerogen types fit well with those shown in the van Krevelen diagram. The corrected activation energies of pyrolysis for these three typical kerogen types were found to be 61.1 kcal/mol, 53.5 kcal/mol and 52.8 kcal/mol, respectively, consistent with those obtained from both laboratory simulations and basin modeling. By comparison with the relationships between measured vitrinite reflectance values and the H/C atomic ratios of kerogen samples reported in the literature, a quantitative relationship was established between the vitrinite reflectance (%Ro) values of kerogen and simulation temperatures (T). The cross-plots of T vs %Ro for kerogen types II and III show that vitrinite reflectance exponentially increases with increasing simulation temperature. Two logarithmic correlation equations were constructed to match geological maturation levels to the corresponding levels in MD simulations of thermal degradation of types II and III kerogen. These maturation scales are generally in line with the simulation temperatures set for thermal degradation in kerogen pyrolysis in previous studies. This work has promising practical implications for research on the thermal evolution behavior of ancient sedimentary organic matter using ReaxFF MD simulation.

Non thermally-activated transients and buffer traps in GaN transistors with p-type gate: A new method for extracting the activation energy

This paper demonstrates that conventional drain current transient (DCT) measurements fail at identifying the correct activation energy of buffer defects in transistors with p-GaN gate. Based on combined pulsed and transient characterization, we demonstrate that (i) under off-state stress, the analysed devices suffer from moderate dynamic-Ron and from positive shift in the threshold voltage; (ii) the de-trapping kinetics, analysed by DCT, are unexpectedly not thermally-activated; (iii) de-trapping kinetics are significantly accelerated when measured at high gate voltage; (iv) we report a power law correlation between the gate leakage measured during the de-trapping phase and the time constant for recovery. Finally, (v) we propose a new characterization procedure, based on TLM structures, to overcome these issues, thus accurately investigating buffer-related effects.

A mechanistic study on the decomposition of a model bio-oil compound for hydrogen production over a stepped Ni surface: Formic acid

To obtain a clearer understanding of the catalytic bio-oil reforming mechanism, we performed density functional theory calculations on the decomposition of formic acid over a stepped Ni surface. Formic acid was selected as a model bio-oil compound. Fourteen elementary reactions were considered. All the zero-point-energy corrected activation energies and reaction energies were obtained. The kinetic parameters of the reaction rate constants and thermodynamic equilibrium constants in the temperature range of 300–1000 K were revealed. COOH formation and its subsequent dissociation are energetically easier than that of HCOO. When combined with kinetic modeling, the most preferable pathway for formic acid dissociation on the surface is HCOOH → COOH → CO. The rate-determining step is COOH dehydroxylation to CO, associating with an activation energy of 0.51 eV and a rate constant of 5.16 × 109 s−1 at 773 K. In addition, HCOO will be slowly accumulated on the surface as the temperature rises.

Tensile creep behavior of heat-treated TC11 titanium alloy at 450–550 °C

The effect of heat treatment on the tensile creep behavior of duplex titanium alloy Ti–6.3Al–1.6Zr–3.4Mo–0.3Si (TC11) at 500°C, 300MPa, as well as the tensile creep of heat-treated TC11-2 alloys over the temperature range 450–550°C at stress range 300–450MPa have been investigated in detail, by means of curve measurements and microstructure observation. At 500°C and 300MPa, the TC11 alloys with duplex microstructure exhibited less creep deformation and strain rates than basket-weave microstructure, and heat treatment at condition of 970°C/1h/AC+530°C/6h/AC has the best creep performance. TEM analysis indicates that the creep in duplex structure alloy is controlled by dislocation climb (Metal-type or Class II alloy creep), and changes to a jogged-screw creep model in basket-weave structure. For heat-treated TC11-2 alloy, the corrected activation energies between 65kJ/mol and 188kJ/mol, and stress exponents in the range 1–3.8 were obtained. At 450°C, small apparent stress exponent (equal ∼1) as well as low activation energies suggested that the creep is controlled by double mechanisms of Harper–Dorn dislocation creep and diffusion type creep. At 500°C and 550°C, the higher stress exponents and apparent activation energies indicated that creep is controlled by climb of dislocation. The second phases, mainly (Ti,Zr)5Si3, inhomogeneously precipitated from both primary α grain and α/β lath interface, which has limited effects on hindering the movement of dislocations during creep.

Clarifications regarding the use of model-fitting methods of kinetic analysis for determining the activation energy from a single non-isothermal curve

BackgroundThis paper provides some clarifications regarding the use of model-fitting methods of kinetic analysis for estimating the activation energy of a process, in response to some results recently published in Chemistry Central journal.FindingsThe model fitting methods of Arrhenius and Savata are used to determine the activation energy of a single simulated curve. It is shown that most kinetic models correctly fit the data, each providing a different value for the activation energy. Therefore it is not really possible to determine the correct activation energy from a single non-isothermal curve. On the other hand, when a set of curves are recorded under different heating schedules are used, the correct kinetic parameters can be clearly discerned.ConclusionsHere, it is shown that the activation energy and the kinetic model cannot be unambiguously determined from a single experimental curve recorded under non isothermal conditions. Thus, the use of a set of curves recorded under different heating schedules is mandatory if model-fitting methods are employed.

Theoretical studies on the effect of substituent in the proton transfer reaction of 4-substituted pyrazoles

Density functional theory (B3LYP and CAM-B3LYP) and MP2 methods have been employed to study of proton transfer in annular tautomerism and double proton transfer reaction of a series of pyrazoles substituted at the 4 position (NH2, OH, CH3, F, H, Cl, CN, NO2, CHO, NO). It was found that activation energy for monomers of pyrazole derivatives via tautomerism process is in the range of 47.8–55.5kcal/mol. In addition the pyrazole with electron releasing groups have lower activation energy than electron withdrawing ones. Moreover it was found that formation of dimers favored, and corrected interaction energies were found in the range of 11.4–12.0, 10.0–10.6 and 12.2–12.8kcal/mol using CAM-B3LYP, B3LYP and MP2 methods, using 6-311++G(d,p) basis function, respectively. The double proton transfer reaction was studied for dimers and the obtained results showed that corrected activation energies vary in the range of 13.0–15.0 and 11.4–12.6kcal/mol at DFT and MP2 levels of theory, respectively. A good correlation between B3LYP and CAM-B3LYP results for prediction of hydrogen bond strength was observed.

Evaluation of Kinetic Parameter Calculation Methods for Non-Isothermal Experiments in Case of Varying Activation Energy in Solid-State Transformations / Neizotermisko Eksperimentu Kinētisko Parametru Noteikšanas Metožu Izvērtēšana Mainīgas Aktivācijas Enerģijas Gadījumā Cietfāžu Pārvērtībām

Simulations of solid-state transformation kinetics were carried out calculating temperature and conversion degree for non-isothermal experiments with different heating rates. Simulations were divided in two parts: with constant and with variable activation energy. Simulations were analyzed with widely used model-based and model-free activation energy determination methods, frequency factor and kinetic model determination methods. Much of the attention was devoted to the calculation of kinetic models and frequency factors, as a more difficult and less developed step. For simulations where activation energy did not change all activation energy determination methods were found to give correct results. However, much attention should be devoted to frequency factor determination, because incorrect results would lead to problems in determination of kinetic models. For simulations where activation energy changes, correct activation energy can be determined only by differential methods or integral methods using numerical integration over small intervals. Isokinetic relationship coefficients b and c were more accurately determined with the average linear integral method. Correct kinetic model determination was possible only when coefficients b and c were accurate, and only by analyzing results of all available methods.

Monte Carlo Simulation of Linear Polymer Thermal Depolymerization under Isothermal and Dynamic Modes

Kinetics of linear polymer thermal depolymerization under isothermal and dynamic TGA modes was simulated by the Monte Carlo method. The simulation was carried out on model arrays having the same initial degree of polymerization and different width (polydispersity index, ) at three constant temperatures and five heating rates. Kinetics of the process in both modes is described by the Avrami equation, the exponent in which decreasing as the distribution width increases. Treatment of the model kinetic curves of degradation using the nonlinear regression method by the Avrami equation, under both isothermal and dynamic modes, gives correct activation energy and pre-exponential factor values independently of the initial PDI. Data obtained in the dynamic mode were also treated by two isoconversion methods, widely applied to kinetic analysis of TGA curves (Flynn-Wall-Ozawa method and Kissinger-Akahira-Sunose (KAS) method).

Creep Behavior of Fusion Zone and Base Metal of the Electron Beam Weldments of a Near-alpha Titanium Alloy

The high temperature creep behavior of fusion zone (FZ) and base metal (BM) of the electron beam weldments of a near-alpha titanium alloy Ti-60 has been investigated. While the BM shows a fully transformed, coarse primary beta grains with large colonies consisting of aligned alpha lamellar, the FZ exhibits thin martensitic alpha' platelets in the columnar beta grains. The creep results show that the steady state creep rates of FZ follow the power-law creep, with the stress exponents obtained in the range from 5.6 (550 degrees C) to 5.9 (600 degrees C), and corrected activation energies of 309-352 kJ/mol; the stress exponents of BM exhibit different values when the creep testing stress and temperature alternate. The values of 2.4-3.2 are obtained when the stresses are under 220 MPa or the temperature is 550 degrees C, also an exponent of 6.6 is achieved at stresses above 220 MPa at 600 degrees C. The corrected activation energies of BM corresponding to the stress exponents are 123-161 kJ/mol (n=2.4-3.2) and 344 kJ/mol (n=6.6). The creep mechanisms of FZ and BM have been discussed in light of the creep microstructures, activation energies and the stress exponents. The creep mechanisms of FZ is the diffusion controlled dislocation climb, the creep of BM is controlled by 'solute drag' creep and dislocation climb when the stress and temperature are different. Transmission electron microscopy (TEM) observations of the dislocation structures of crept specimens are presented to give some supports for the creep behavior of FZ and BM. In addition to the creep mechanism of dislocation movement, the interface sliding has been found to play an important role during creep of FZ.

Sugar radical formation by a proton coupled hole transfer in 2'-deoxyguanosine radical cation (2'-dG*+): a theoretical treatment.

Previous experimental and theoretical work has established that electronic excitation of a guanine cation radical in nucleosides or in DNA itself leads to sugar radical formation by deprotonation from the dexoxyribose sugar. In this work, we investigate a ground electronic state pathway for such sugar radical formation in a hydrated one electron oxidized 2'-deoxyguanosine (dG(*+) + 7H(2)O), using density functional theory (DFT) with the B3LYP functional and the 6-31G* basis set. We follow the stretching of the C(5')-H bond in dG(*+) to gain an understanding of the energy requirements to transfer the hole from the base to sugar ring and then to deprotonate to proton acceptor sites in solution and on the guanine ring. The geometries of reactant (dG(*+) + 7H(2)O), transition state (TS) for deprotonation of the C(5') site, and product (dG((*)C(5'), N(7)-H(+)) + 7H(2)O) were fully optimized. The zero point energy (ZPE) corrected activation energy (TS) for the proton transfer (PT) from C(5') is calculated to be 9.0 kcal/mol and is achieved by stretching the C(5')-H bond by 0.13 A from its equilibrium bond distance (1.099 A). Remarkably, this small bond stretch is sufficient to transfer the "hole" (positive charge and spin) from guanine to the C(5') site on the deoxyribose group. Beyond the TS, the proton (H(+)) spontaneously adds to water to form a hydronium ion (H(3)O(+)) as an intermediate. The proton subsequently transfers to the N(7) site of the guanine (product). The 9 kcal/mol barrier suggests slow thermal conversion of the cation radical to the sugar radical but also suggests that localized vibrational excitations would be sufficient to induce rapid sugar radical formation in DNA base cation radicals.

The Effects of Water on β-d-Xylose Condensation Reactions

Car-Parrinello-based ab initio molecular dynamics simulations (CPMD) combined with metadynamics (MTD) simulations were used to determine the reaction energetics for the beta-D-xylose condensation reaction to form beta-1,4-linked xylobiose in a dilute acid solution. Protonation of the hydroxyl group on the xylose molecule and the subsequent breaking of the C-O bond were found to be the rate-limiting step during the xylose condensation reaction. Water and water structure was found to play a critical role in these reactions due to the proton's high affinity for water molecules. The reaction free energy and reaction barrier were determined using CPMD-MTD. We found that solvent reorganization due to proton partial desolvation must be taken into account in order to obtain the correct reaction activation energy. Our calculated reaction free energy and reaction activation energy compare well with available experimental results.

Crystallization Kinetics for Synthesis Hyper-Structure in P<sub>2</sub>O<sub>5</sub>-ZnO-B<sub>2</sub>O<sub>3</sub>-BaO-Al<sub>2</sub>O<sub>3</sub>-TiO<sub>2</sub> Glass Ceramic Composite

The nucleation and crystallization kinetics of P2O5-B2O3-ZnO-BaO-Al2O3-TiO2 crystals in bulk glass in which this crystals were found to crystallize in the heating process of the glass, were studied by non-isothermal measurements using differential thermal analysis (DTA). A nucleation rate-temperature was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/Tp, or the height of the crystallization peak, (*T)p, as a function of nucleation temperature, Tn. The temperature where nucleation can occur for this glass ranges from 700°C to 890°C and the temperature for maximum nucleation is 760±5°C. The correct activation energy for crystallization, Ec, for this glass is the same for surface and/or bulk crystallization, and is 533±15°CkJ/mol. The analysis of the crystallization data with the Kissinger equation and the Marotta equation yields the correct value for Ec only crystal growth occurs on a fixed number of nuclei. The crystallization process of a sample heat treated at the temperature of the maximum nucleation rate was fitted to kinetic equations with an Avrami constant, n ≈2 and a dimensionality of crystal growth, m ≈2.

Modeling Proton Jumps in HY Zeolite: Effects of Acid Site Heterogeneity

We have computed the total mean rate coefficient for proton transfer in bare H−Y zeolite, for comparison with NMR experiments and previous calculations. We computed proton-transfer energies using two-layer ONIOM calculations on an 8T−53T cluster, where xT indicates x tetrahedral atoms. Rate coefficients were computed using truncated harmonic semiclassical transition state theory. The zero-point energy (ZPE) corrected proton site energies in H−Y (FAU structure) were found to be O3 (0 kJ mol-1), O1 O2 (16.1 kJ mol-1), and O4 (17.5 kJ mol-1), in quantitative agreement with previous calculations and in qualitative agreement with neutron diffraction occupancies. A new local minimum denoted O2* (31.4 kJ mol-1 relative to O3) was located, with a proton bound to O2 but pointing into the sodalite cage. Transition states between each pair of minima were fully characterized, yielding ZPE corrected activation energies ranging from 35.5 to 123.4 kJ mol-1. No correlation was found between barrier height and local structure; we considered 10 structural parameters including ring size, T−O−T angle, and nonbonded oxygen distance. Total mean rate coefficients were found to exhibit a strong non-Arrhenius temperature dependence, with apparent activation energies in the range of ca. 60−100 kJ mol-1 at high temperature, and ca. 3 kJ mol-1 at low temperature. This low-temperature value reflects thermally assisted tunneling to a site with a slightly higher energy. NMR experiments by Sarv et al. and Ernst et al. report apparent activation energies of 61 and 78 kJ mol-1, respectively, extracted from temperature ranges 298−658 and 610−640 K. Our theoretically computed apparent activation energies for these temperature ranges are 72 and 79 kJ mol-1, respectively, in quite good agreement with experiment.

EVALUATION OF MATHEMATICAL MODELS FOR PREDICTION OF THINLAYER DRYING OF BRAZILIAN LEMON-SCENTED VERBENA LEAVES (Lippia alba (MILL) N.E. BROWN)

The drying target is to reduce the moisture content and thereby increase the life time of products by limiting enzymatic and oxidative degradation. In addition, by reducing the amount of water, the percentages of active principles are increased in relation to the total mass. The aim of the current study was to determine the correct diffusion rate and activation energy by adapting the mathematical model according to the experimental data. Brazilian lemon-scented verbena leaves were harvested with a moisture content of around 85% wet basis (wb), and were then subjected to drying in a medicinal plant dryer at an air temperatures of 40, 50, 60, 70 and 80 ºC at 0.29±0.03 m.s-1. Approximately 400 g of leaves at a layer thickness of 0.15m were used with three tests performed at each temperature. Several models of drying kinetics were adapted for use with the experimental data, including Henderson & Pabis, Henderson & modified Pabis, Lewis, Midilli et al., Page, Thompson and Wang & Singh. The adjusted determination coefficients (R2), relative mean errors (P), estimated errors (SE) and residue distributions were all compared. The best models, which best represented the Brazilian lemon-scented verbena drying, were Midilli et al. and Page.

Thermal Kinetic Parameters of Thiamin in Wheat Flour at Temperatures Higher than 100°C

Kinetic parameters for thiamin degradation were obtained using 2 high-temperature heating methods: (1) atmospheric pressure (AP) with moisture correction and (2) controlled pressure (CP). At AP conditions, 33.3% dry basis(db) moisture wheat flour with 0.35% (db) thiamin was heated in thin steel cells isothermally at 145, 160, and 172 °C. To obtain the moisture correction factor, a constant-moisture study was conducted at 80 °C using 6 moisture contents (6.1% to 36.9%). At CP conditions, flour at 19%, 28.2%, and 33.3% (db) moisture in double-seamed cans was heated in a CP steam retort at 129 °C. For the AP method, the corrected activation energy for 33.3% moisture content was 129.5 kJ/g-mol and reaction rate at 80 °C was 3.48×10 - 4 min - 1 . Using the CP method, the activation energy and reaction rate were 121.0 kJ/g-mol and 9.69×10 - 5 min - 1 , respectively. Results obtained from 2 methods were not statistically different. These results illustrated that the correction method could be used as an alternative for researchers without access to controlled pressure equipment and transient heat transfer software.

Assessment of the suitability of using the composite G2, G3, and CBS-RAD methods for predicting activation energies

In this work, the accuracy of the G2, G3, and CBS-RAD methods for predicting activation barriers of ligand transfer reactions is investigated. We find that the zero point corrected G2 method has an RMS error of 3.82 kcal/mol for activation barriers. The G3 method has an RMS error of 4.16 kcal/mol. After adding thermal corrections to the G2 and zero point corrected results, the RMS error for the G2 method is 4.92 kcal/mol, while the error for the G3 method is 4.55 kcal/mol. In contrast, the CBS-RAD method has errors of 3.80 kcal/mol for zero point energy corrected activation energies, and 2.82 kcal/mol for thermally corrected results. The G3 method was found to require only 40% of the computational time required for the G2 method, making it an attractive alternative for predicting activation energies yielding errors of about 4 kcal/mol. The CBS-RAD method has a computational cost four times greater than that of the G2 method and gives an improvement of only about 1 kcal/mol.

The contribution of the tracer correlation factor to the diffusion activation energy in intermetallic compounds

In this paper, the fraction of the activation energy for tracer diffusion that resides in the correlation factor is addressed in the case of intermetallic compounds. We analysed the stoichiometric B2 structure with antistructural disorder. Two models—one, a mean-field treatment of the six-jump-cycle mechanism, the other, a detailed treatment of the six-jump-cycle mechanism—are considered. Monte Carlo computer simulation of an Ising-type alloy is used to provide an exact reference. It is shown that the detailed six-jump-cycle model provides the correct activation energy only at very low temperatures. On the other hand, the mean-field model provides a remarkably good description of the activation energy over a very wide temperature range. The findings are illustrated by application to CuZn.

KINETIC ANALYSIS OF NON-ISOTHERMAL DSC DATA Computer-aided test of its applicability

The applicability of the kinetic analysis of data obtained by non-isothermal differential scanning calorimetry (DSC) is discussed. The Johnson-Mehl-Avrami (JMA) model was used for the computer simulation of DSC traces subsequently analysed by common methods of kinetic analysis of non-isothermal data. For the temperature-independent kinetic exponent n of the JMA equation, the kinetic analysis was shown to provide correct results, e.g. a correct kinetic model and apparent activation energy. On the other hand, for the temperature-dependent kinetic exponent, there is a great possibility of erroneous determination of the correct kinetic model and apparent activation energy, especially at higher heating rates. Since the temperature dependence of n cannot be determined on the basis of non-isothermal DSC experiments, conclusions must be drawn with appropriate caution.