Authors:A. Małecki, B. Prochowska-Klisch, and K. Wojciechowski
A detailed analysis is presented of the applicability of several dependences commonly used for the determination of activation
energies from non-isothermal measurements. Reactions proceeding according to different kinetic equations are simulated and
the validity of the activation energy values obtained is discussed. The general conclusion is drawn that none of the examined
dependences should be used to determine the activation energy. For a rough estimation of activation energy, the Kissinger
equation can be applied according to Ockham's razor.
The integral methods, which are obtained from the various approximations for the temperature integral, have been extensively
used in the non-isothermal kinetic analysis. In order to obtain the precision of the integral methods for the determination
of the activation energy, several authors have calculated the relative errors of the activation energy obtained from the integral
methods. However, in their calculations, the temperature integral at the starting temperature was neglected. In this work,
we have performed a systematic analysis of the precision of the activation energy calculated by the integral methods without
doing any simplifications.
The results have shown that the relative error involved in the activation energy determined from the integral methods depends
on two dimensionless quantities: the normalized temperature θ=T/T0, and the dimensionless activation energy x0=E/RT0 (where E is the activation energy, T is the temperature, T0 is the starting temperature, R is the gas constant).
A nonlinear algorithm has been suggested to increase the accuracy of evaluating the activation energy by the integral isoconversional method. A minor modification of the algorithm has made it possible to adapt the isoconversional method for an arbitrary variation of the temperature. This advanced isoconversional method allows for trustworthy estimates of the activation energy when the thermal effect of a reaction makes the temperature of a sample deviate from a prescribed heating program.
The response of a chemical reaction to temperature modulation has been examined experimentally in an epoxy thermosetting system.
The kinetic response appears in the imaginary part of the complex heat capacity determined by TMDSC. From the imaginary part
and the ‘non-reversing’ heat flow of reaction, the activation energy has been determined. The value of the activation energy
obtained is in good agreement with the value determined from Kissinger's plot utilizing the peak temperatures of the exothermic
reaction with different heating rates.
Authors:R. Pitchimani, W. Zheng, S. Simon, L. Hope-Weeks, A. Burnham, and B. Weeks
Pentaerythritol tetranitrate (PETN) powders are used to initiate other explosives. During long-term storage, changes in powder
properties can cause changes in the initiation performance. Changes in the morphology and surface area of aging powders are
observed due to sublimation and growth of PETN crystals through coarsening mechanisms, (e.g. Ostwald ripening, sintering,
etc.). In order to alleviate the sublimation of PETN crystals under service conditions, stabilization methods such as thermal
cycling and doping with certain impurities during or after the crystallization of PETN have been proposed.
In this report we present our work on the effect of impurities on the morphology and activation energy of the PETN crystals.
The pure and impurity doped crystals of PETN were grown from supersaturated acetone solution by solvent evaporation technique
at room temperature. The difference in the morphology of the impurity-doped PETN crystal compared to pure crystal was examined
by optical microscopy. The changes in the activation energies and the evaporation rates are determined by thermogravimetry
(TG). Our activation energies of evaporation agree with earlier reported enthalpies of vaporization. The morphology and activation
energy of PETN crystals doped with Ca, Na, and Fe cations are similar to that for pure PETN crystal, whereas the Zn-ion-doped
PETN crystals have different morphology and decreased activation energy.
Authors:G. Papp, Beáta Bugyi, Z. Ujfalusi, Sz. Halasi, and J. Orbán
The effect of pH was characterised on the thermal stability of magnesium saturated skeletal and cardiac α-actin isoforms with
differential scanning calorimetry (DSC) at pH 7.0 and 8.0. The calorimetric curves were further analysed to calculate the
enthalpy and transition entropy changes. The activation energy was also determined to describe the energy consumption of the
initiation of the thermal denaturation process. Although the difference in Tmvalues is too small to interpret the difference between the a-actin isoforms, the values of the activation energy indicated
that the α-skeletal actin is probably more stable compared to the α-cardiac actin. The difference in the activation energies
indicated that lowering the pH can produce a more stable protein matrix in both cases of the isoforms. The larger range of
the difference in the values of the activation energies suggested that the α-cardiac actin is probably more sensitive to the
change of the pH compared to the α -skeletal actin.