Some possible elementary reactions are not included in the classical mathematical models of thermal decomposition. For example, we can assume that in the thermal decompositions of simple carbonates a proportion of the O2− ions produced on the reaction interface can migrate into the interior of the reactant phase, since at this temperature there is some probability of CO2 exchange between an O2− and a neighbouring CO32− ion. A similar diffusion-type process can be assumed in a wide class of decomposition reactions. The present state of computer science makes it possible to show by mathematical modelling how this migration influences the TG curves of the simplest contracting-sphere-type reactions. The resulting extended contracting-sphere model can provide the induction and the acceleration period of the TG curves.
Authors:G. Várhegyi, T. Székely, F. Till, E. Jakab and P. Szabó
It has been known from the very beginning of the thermal analysis that the transport processes can significantly influence the thermoanalytical results. In this paper, three characteristic examples are given to show that this problem is more complex and arises more frequently than it is generally believed. The studied reactions are the carbon monoxide evolution from calcium oxalate, the thermal degradation of polytetrafluoroethylene, and the thermal decomposition of azodicarbonamide. TG and DSC experiments were carried out with sample masses varying between 0.05 and 8 mg. The necessity of the development of new kinetic models is concluded.
Authors:I. Pap, Yu. Lajner, I. Bertóti, G. Mink and T. Székely
Alumina samples were produced by the thermal decomposition of hydrates of aluminium sulphate, nitrate and chloride. The kinetics of the chlorination were studied by means of isothermal TG measurements, using COCl2 as chlorinating agent. Apparent activation energies were determined in the temperature ranges where the reaction was controlled by chemical processes. The reaction rates related to unit surface area were similar for all the samples, and the isothermal TG curves could be described by the models of contracting cylinders and spheres.
Authors:I. Pap, J. Szépvölgyi, I. Bertóti, G. Mink and T. Székely
The reduction kinetics of a pure and a Ca-added hematite, as well as of two iron oxide-containing metallurgical wastes, were studied by means of isothermal TG measurements, using H2 or CO as reducing agents.