An analysis is made of the method proposed by Gorbachev for the kinetic study of non-isothermal transformations using the KEKAM equationα=1−exp(−ktn). It is demonstrated that the procedure cannot be used to determine either the kinetic exponentn or the activation energyE.
The kinetic curves at infinite temperature for the solid-state reactions of the interface shrinkage type were drawn theoretically by taking account the particle size distribution in the sample mixture. The CRTA curves for the reactions with the particle size distribution can be drawn by utilizing the universal kinetic curves at infinite temperature. The proper kinetic treatment for the CRTA curves with the particle size distribution is discussed in connection with the property of the kinetic equation with respect to the particle size distribution. The present kinetic consideration is taken as a simulation for the reactions with a certain distribution in α among the reactant particles, produced preferably by the mass and heat transfer phenomena during the thermoanalytical measurements. The merit of the rate jump method by a single cyclic CRTA curve is also discussed on the basis of the present results.
A study has been carried out of the influence of sample dilution, the nature of the gas atmosphere, and the static or flowing conditions of this, on the DTA curves resulting from the thermal decomposition of solids. The results obtained seem to indicate that only the reversible reactions of solid thermal decomposition are seriously affected by such factors.
The computer kinetic analysis of simultaneously obtained TG and DTG curves of CaCO3 decomposition has been carried out. Ten different kinetic equations have been tested to decide the mechanism which drives the reaction. Either a two-thirds kinetic equation (phase boundary process) or a Jander equation (diffusion process) satisfactorily describe the kinetic data of both decomposition curves. From these results we conclude there is no chance of differentiating between these two mechanisms by only the kinetic analysis of TG and DTG curves separately.
Authors:J. M. Criado, F. Gonzalez, and M. Gonzalez
Kinetic analysis has been performed on TG and DTG diagrams of the forward reaction MnCO3⇌ MnO + CO2, recorded at different pressures of CO2 ranging form 2.6. 10−4 Pa to 26.6 kPa. The results obtained show that this reaction follows a first-order kinetic law, independently of the CO2 pressure used in carrying out the experiments. On the other hand, the activation energy increases on increase of the CO2 pressure, from 117 kJ/mol up to an asymptotic value of 292 kJ/mol at about 26.6 Pa. This finding cannot be explained by considering the influence of the reverse reaction of formation of MnCO3, for under the described experimental conditions the ratio
to zero. A mechanism that takes into account the adsorption of CO2 on the phase boundary has been proposed in order to interpret the results.
Summary The SCTA methods for the kinetic analysis of solid-state reactions have been reviewed. It has been shown that these methods present two important advantages with regards to the more conventional rising temperature experiments. Firstly, they have a higher resolution power for discriminating among the reaction kinetic models and, secondly, SCTA is a powerful tool for minimizing the influence of the experimental conditions on the forward reaction.
The reaction pathway of the thermal decomposition of synthetic brochantite, Cu4(OH)6SO4, to copper(II) oxide was investigated through the detailed kinetic characterization of the thermal dehydration and desulferation processes. The dehydration process was characterized by dividing into two overlapped kinetic processes with a possible formation of an intermediate compound, Cu4O(OH)4SO4. The dehydrated sample, Cu4O3SO4, was found first to be amorphous by means of XRD, followed by the crystallization to a mixture of CuO and CuO-CuSO4 at around 776 K. The specific surface area and the crystallization behaviour of the amorphous dehydrated compound depend largely on the dehydration conditions. The thermal desulferation process is influenced by the gross diffusion of the gaseous product SO3, which is governed by the advancement of the overall reaction interface from the top surface of the sample particle assemblage to the bottom.
A new equipment has been developed in order to apply the constant rate thermal analysis method (CRTA) to Temperature Programmed Oxidation (TPO) processes. The exhausted gases flow through an electrochemical oxygen sensor after leaving the reactor in order to monitor the oxygen consumption in the reaction. The control of the sample temperature is carried out by interfacing both the furnace and the electrical signal of the oxygen sensor to a PID controller that allows to monitor the sample temperature in such a way that the consumption of oxygen is a constant value previously selected by the user.
Authors:J. Criado, L. Pérez-Maqueda, M. Diánez, and P. Sánchez-Jiménez
The SCTA method implies to control the temperature in such a way that
the reaction rate changes with the time according to a function previously
defined by the user. Constant Rate Thermal Analysis (CRTA) is one of the most
commonly used SCTA methods and implies achieving a temperature profile at
which the reaction rate remains constant all over the process at a value previously
selected by the user. This method permits to minimize the influence of heat
and mass transfer phenomena on the forward reaction. The scope of this work
is to develop a universal CRTA temperature controller that could be adapted
to any thermoanalytical device. The thermoanalytical signal is programmed
to follow a preset linear trend by means of a conventional controller that
at the time controls a second conventional temperature programmer that forces
the temperature to change for achieving the trend programmed for the thermoanalytical
signal. Examples of the performance of this control system with a Thermobalance
and a Thermomechanical Analyser (TMA) are given.