Kinetics and mechanism of linear crystal growth in As2Se3 glass

Ранее исследовано влияние малых добавок олова и свинца на кинетические параметры изотермической объемной кристаллизации стекол на основе As2Se3. Кинетика преимущественно поверхностной кристаллизация стекла As2Se3, используемого в электронной технике и волоконной ИК оптике, изучалась методами дифференциальной сканирующей калориметрии и микроскопии. Влияние величины переохлаждения ΔТ на механизм и кинетические параметры кристаллизации стекла As2Se3 изучено недостаточно. Автором статьи выполнен теоретический анализ экспериментальных зависимостей скорости линейного роста кристаллов As2Se3 от температуры и вязкости стекла As2Se3. На основе анализа существующих данных и приводимой работы показано, что в стекле As2Se3 при переохлаждениях ΔT = 30–135 вероятен дислокационный рост кристаллов в форме сферолитов. Для индивидуальных пластинчатых кристаллов As2Se3 с бездислокационными гранями внутри сферолитов при ΔT = 75–135° вероятен механизм роста с поверхностной двумерной нуклеацией, или 2Dsg–model. Скорость линейного роста кристаллов в стекле As2Se3 при 240−345 °C теоретически рассчитывается, если в формулу Тернбала–Коэна ввести вероятностный фактор f (ТΔT) роста на ступенях винтовых дислокаций, а свободную энергию активации роста кристаллов ΔGa'' (T) принять равной кинетическому барьеру при вязком течении стекла ΔGη # (T) в уравнении Эйринга. The influence of small additions of tin and lead on the kinetic parameters of isothermal bulk crystallization of glasses based on As2Se3. The kinetics of mainly surface crystallization of As2Se3 glass used in electronic engineering and fiber IR optics has been studied by differential scanning calorimetry and microscopy. The influence of the supercooling value ΔT on the mechanism and kinetic parameters of crystallization of As2Se3 glass is not sufficiently studied, The theoretical analysis of the experimental dependences of the linear growth rate of As2Se3 crystals on the temperature and viscosity of the As2Se3 glass has been performed. In the temperature range 240–345 °С (overcooling 135°–30°), a dislocation mechanism has appeared to be probable for the linear growth of spherulites and the surface crystallized layer in the bulk As2Se3 glass. For individual crystals with dislocation-free faces in As2Se3 spherulites, the lamellar (plate-like) growth mechanism with surface two dimensional nucleation (2Dsg model) appeared to be probable. The linear growth rate in glass As2Se3 at 240–345°C can be theoretically calculated, if we introduce the probability factor f (ТΔT) to the Turnball–Cohen formula of the growth on the steps of screw dislocations and assume the free activation energy for the crystal growth ΔGa'' (T) to be equal to the kinetic barrier at the viscous flow of the glass ΔGη # (T) in the Heiring equation.

An electron localization function analysis of the molecular mechanism and the C–O bond formation in the [3+2] cycloaddition reaction involving zwitterionic type between a nitrone and an electron deficient ethyne

The mechanistic nature of a [3+2] cycloaddition reaction involving zwitterionic species has been investigated, and the changes of electron density related to the O–C and C–C bond formation along the intrinsic reaction coordinate have been characterized. This polar [3+2] cycloaddition reaction, which takes place through a non-concerted two-stage one-step mechanism, proceeds with a moderate Gibbs free activation energy of 21 kcal mol−1. The reaction begins by the creation of a pseudoradical centre at the central carbon, first on the dimethyl acetylenedicarboxylate, and second on the nitrone framework. This immediately favours the formation of the first O–C single bond by donation of some electron density of the oxygen atom lone pairs, which represents the most attractive centre in this cycloaddition reaction.

Cope Rearrangement in Bicyclo[5.1.0]octa-2,5-diene and its Mono- and Di-Hetero Analogues: A DFT Study

The Cope rearrangements of bicyclo[5.1.0]octa-2,5-diene and its 4-hetero-(aza/oxa/phospha) and 4,8-dihetero analogues were investigated using density functional theory at the B3LYP/6–31+G* level in gas phase. The rearrangements of bicyclo[5.1.0]octa-2,5-diene and its symmetrical 4,8-dihetero analogues followed a concerted mechanism involving synchronous transition states. In other cases, although a concerted mechanism was observed, asynchronous transition states were involved. In the case of bicyclo[5.1.0]octa-2,5-diene, a degenerate Cope rearrangement was expected to occur at room temperature (25°C) due to a low free activation energy (ΔG‡ = 14.46 kcal mol–1). However, under similar conditions, the rearrangement of 4,8-dioxabicyclo[5.1.0]octa-2,5-diene was much slower (ΔG‡ = 23.85 kcal mol–1) and the 4,8-diaza- and diphospha analogues did not undergo Cope rearrangement. The Cope rearrangements of 4-phospha-, 8-aza-, 8-aza-4-oxa-, 8-aza-4-phospha-, and 8-oxa-4-phospha-bicyclo[5.1.0]octa-2,5-dienes were exergonic and were expected to occur spontaneously to form the corresponding products. In contrast, rearrangement of 8-oxabicyclo[5.1.0]octa-2,5-diene, though exergonic, was accompanied by a decrease in entropy, due to which Cope rearrangement would occur much more slowly and a mixture of both valence isomers would be formed. The Cope rearrangements of 4-aza-, 4-oxa-, 4-aza-8-oxa-, 8-phospha-, 4-aza-8-phospha-, 4-oxa-8-phospha-, and 4,8-diphospha-bicyclo[5.1.0]octa-2,5-dienes were endergonic; consequently either no Cope rearrangement would take place or it would occur sluggishly.

Relaxation mapping analysis with an hyperbolic heating rate

ABSTRACTThermally stimulated discharge currents (TSDC) together with the Relaxation Map Analysis (RMA) method is a convenient method to study relaxation processes in complex materials. In the RMA technique, one performs repeated TSDC runs while selecting polarization states through the use of a fixed polarization time schedule and a sucessively higher polarization temperature. Each peak has an associated polarization strength and a pair of activation parameters which can be the Gibbs free activation energy and the activation entropy. In the present paper we propose a different approach to RMA. It consists of a global heating of the sample in an hyperbolic manner to determine the activation parameters as a function of temperature. This determination is done by first selecting a value for the activation entropy for each temperature and then calculating the implied activation enthalpy or Gibbs free energy.We have reasoned that normal relaxation processes usually proceed with a null or close to zero activation entropy and that only relaxations associated to the glass transitionand/or cooperative phenomena involve large values of the activation entropy. In those cases and based on the curvature of the TSDC curve one can select a minimum activation entropy value which will fit the TSDC curve at a particular temperature. Using this data one determines the polarization strength associated to each pair of activation parameters. Results for the TSDC relaxation parameters spectra of a liquid crystal polymer will be presented together with comparisons between the thermal sampling method and the proposed method.

Investigation of the effect of organic solvents on kinetic parameters in metal catalyzed reactions

The effects of acetone and acetonitrile on the kinetic parameters of azorubin S oxidation by hydrogen peroxide catalyzed by manganese(II), pyrocatechol violet oxidation by hydrogen peroxide catalyzed by copper(II), and carminic acid oxidation by hydrogen peroxide catalyzed by copper(II) and activated by bifenox, were examined. It was established that the examined solvents exhibit various effects on the kinetic parameters of the above said reactions. In all instances a change in the solvent concentration affects both the enthalpy and the entropy contributions to the free activation energy during the transition of the system into the active state, as well as the constant of the active complex formed at this point.

Characterization of Fracture in Asphaltic Mixes Based on a Molecular Approach

In contradiction to the fracture mechanics approach, the fundamental absolute rate theory (ART) model describes the macroscopic behavior of processes on the molecular level. Attention is given to the theoretical background of the ART and its application to describing the crack growth process in asphalt concrete mixes. From the analyses, it is concluded that in the crack propagation phase, three parallel processes are distinguished: cohesive crack growth, adhesive crack growth, and a crack-retardant process. In the crack initiation phase, only cohesive crack growth is present. The difference between the mixes is specified by the free activation energy Qa: increase in crack growth resistance increases the Qa-value. This Qa-value, determined from mechanical tests, is close to the energy that bonds the bitumen to the aggregate, determined from chemical tests.

A study of internal rotation in 2-furaldehyde through 13C relaxation times in the rotating frame

Carbon-13 relaxation times in the rotating frame were used to study the exchange between unequally populated rotamers of 2-furaldehyde. Calculated free activation energy and activation enthalpy are in good agreement with the results of 1H line shape analysis. Accuracy and reliability of this method in comparison with the line shape analysis is discussed.