The paper investigates the validity of steady-state approximation for the case of constant rate thermal analysis experiments. It is shown that the approximation holds for the experiments run with a controlled rate of either the decomposition of the compound, or the production of gas.
A differential method is proposed which uses local heating rates to evaluate non-isothermal kinetic parameters. The method
allows to study the influence of the deviation of the true heating rate with respect to the programmed one on the values of
the kinetic parameters. For application, the kinetic parameters of the following solid-gas decomposition reaction were evaluated:
[Ni(NH3)6]Br2(s)→[Ni(NH3)2]Br2(s)+4NH3(g). The results obtained revealed significant differences between the values of the non-isothermal kinetic parameters obtained
by using local heating rates and those obtained by using the programmed heating rate. It was also demonstrated that the kinetic
equation which makes use of the local heating rates permits a better description of the experimental (α, t) data than the
kinetic equation which uses the programmed constant heating rate.
An improved version of the Coats-Redfern method of evaluating non-isothermal kinetic parameters is presented. The Coats-Redfern approximation of the temperature integral is replaced by a third-degree rational approximation, which is much more accurate. The kinetic parameters are evaluated iteratively by linear regression and, besides the correlation coefficient, the F test is suggested as a supplementary statistical criterion for selecting the most probable mechanism function. For applications, both non-isothermal data obtained by theoretical simulation and experimental data taken from the literature for the non-isothermal dehydration of Mg(OH)2 have been processed.
Authors:E. Urbanovici, H. A. Schneider, D. Brizzolara, and H. J. Cantow
A DSC study was carried out of the isothermal melt crystallization kinetics of poly(L-lactic acid), PLLA, at 110, 115, 120, 125 and 130‡C. The experimental data were evaluated within the framework of the well-known Avrami kinetic model and an extended model involving an additional third kinetic parameter . In order to perform the necessary numerical calculations, a number of functions built into the Mathematica® software system were used. The results showed that the isothermal melt crystallization kinetics of PLLA can be described adequately by both these kinetic models. It should also be stressed that the kinetic model of Urbanovici and Segal offers a better description of the experimental melt crystallization data of PLLA than the classical Avrami model.