DIAGNOSTIC LIMITS OF PHENOMENOLOGICAL MODELS OF HETEROGENEOUS REACTIONS AND THERMAL-ANALYSIS KINETICS

Kinetic models of solid-state reactions are often based on a formal description of geometrically well defined bodies treated under strictly isothermal conditions; for real processes these prepositions are evidently incorrect. It can be equally useful to find an empirical function containing the smallest possible number of constants. In such a case the models of heterogeneous kinetics can be assumed as a distorted (fractal) case of the simpler homogeneous kinetics and mathematically treated by multiplying by an ''accommodation'' function. In addition, the conventional thermoanalytical (TA) studies apply intentionally the experimental conditions with constant heating and/or cooling where the model's validity must again be investigated. The general method of kinetic data evaluation is proposed to include two-step evaluation: first, determining the activation energy, E, from a set of TA curves at different heating rates, and second, using the pre-established E to search for the reaction mechanism by analyzing the entire course of the single TA curve. In this respect the possibility of a simultaneous determination of all kinetic data is discussed. The computer method is recommended to be based on the evaluation of two specific functions available for a direct derivation from experimental data.