Search Results

You are looking at 1 - 10 of 1,532 items for :

  • "activation energies" x
  • Refine by Access: All Content x
Clear All

Abstract  

The paper studies metastable phases of 4,4'-di-n-heptyloxyazoxybenzene (liquid crystal substance) using adiabatic calorimetry. The process of transformation between metastable and stable phases is described quantitatively. The conclusion concerns activation energies for metastable phase to stable phase transition.

Restricted access

Abstract  

Using dynamic mechanical analysis (DMA) we have studied the variation with the frequency of the dynamic mechanical properties (storages modulus,E'; loss modulus,E'' and loss tangent or tan σ) for a system containg a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC). These properties were measured both in the glass transition and β transition regions. An increase in frequency caused a shift of tan σ peak positions in both regions toward higher temperature. Finally, we report the activation energies of a DGEBA/1,3-BAC expoxy system for α and β transitions.

Restricted access

mass%Ag; ( c ) Cu–11 mass%Al– X mass%Ag The methods of Kissinger and Ozawa were used to study the influence of additions of 4, 6, 8, and 10 mass%Ag on the activation energy of the (α + γ 1 ) → β reverse eutectoid

Restricted access

]. This model (Kissinger) is also employed to study the thermal stability of current CdS/PMMA system in terms of activation energy which is given by the following relation: 2 where α is the heating rate, C is constant, and E t is the glass

Restricted access

Meaningful activation energies for complex systems II

Evaluation of the Friedman method when applied to multiple reactions, and comparison with the Ozawa-Flynn-Wall method

Journal of Thermal Analysis and Calorimetry
Author: D. R. Dowdy

The validity of the Friedman method is assessed for systems of overlapping reactions. By means of mathematical analysis and numerical examples it is shown that, in the case of competitive reactions, the method gives the true value of the instantaneous mean activation energy. However, some error may be incurred if this method is applied to systems of independent reactions. The relative accuracy of the Friedman and Ozawa-Flynn-Wall methods is discussed in respect of complex systems of reactions.

Restricted access

The measurement of meaningful activation energies

Using thermoanalytical methods. A tentative proposal

Journal of Thermal Analysis and Calorimetry
Authors: M. Reading, D. Dollimore, J. Rouquerol, and F. Rouquerol

The uncertainty surrounding the significance of the measured kinetic parameters of solid state decomposition reactions is discussed briefly. Some suggestions are made about what precautions should be taken in order to favour the measurement of undistorted results. Some criteria are proposed for deciding whether a measuredE value can be considered to have its usual meaning. The results of a series of experiments aimed at measuring the activation energy of the decomposition of calcium carbonate using a variety of methods, sample sizes and experimental conditions are presented. These results are compared with results found in the literature and it is concluded that it is possible to measure a reproducible value forE and it is tentatively proposed that this value is meaningful in terms of the energy barrier model of chemical reaction kinetics.

Restricted access

to calculate thermodynamic parameters such as change in activation energy for dipole orientation Δ G* , enthalpy Δ H* , entropy of activation Δ S* , and the other temperature dependent parameters such as relaxation time τ , the distribution parameter

Restricted access

Abstract  

The activation energies of the same process are often reported to have different values, which are usually explained by the differences in experimental conditions and sample characteristics. In addition to this type of uncertainty, which is associated with the process (ΔE process) there is an uncertainty related to the method of computation of the activation energy (ΔE method). For a method that uses fitting single heating rate data to various reaction models, the value of ΔE method) method is large enough to explain significant differences in the reported values of the activation energy. This uncertainty is significantly reduced by using multiple heating rate isoconversional methods, which may be recommended for obtaining reference values for the activation energy.

Restricted access

Summary  

Activation energy was experimentally determined using the curve-fitting, initial-rise and the peak-shape methods involving pulse annealing experiments in NaCl samples irradiated at 10, 20, 30 and 40 Gy beta-doses and infrared stimulated luminescence (IRSL) signal at a temperature range of 100-300 °C. It was observed that the activation energy for NaCl decreases as the dose increased. The results were compared to other studies and discussed.

Restricted access