Application of SCTA methods for kinetic analysis of solid state reactions and synthesis of materials

Abstract

The advantages of the sample controlled thermal analysis (SCTA) for performing the kinetic analysis of solid state reaction as compared with conventional non-isothermal methods order, diffusion and formation and growth of nuclei (AvraminErofeev) controlled reactions have been analysed. It has been shown that the shape of the α-T plots recorded under a strictly constant reaction rate (CRTA) is characteristic of the kinetic model obeyed by the reaction. Thus, it would be possible to discern among boundary n just from the inspection of the shape of a single CRTA plot. The α vs. T plots of en orderi reactions have a concave shape while the α-T plots corresponding to diffusion controlled processes are sigmoid shaped with an inflection point at a particular value of α characteristic of every particular diffusion model (D2 ,D 3 or D4). The α-T profiles of reactions fitting the AvraminErofeev kinetic model start with a rise in temperature until reaching the preset reaction rate. This step is immediately followed by a temperature fall (until reaching a determined value of the reacted fraction (αn) that is depending on the value of the n coefficient in the AvraminErofeev kinetic equation) and again a temperature rise once the corresponding αn value is attained. It has been shown that the value of αn exactly agree with the value of the reacted fraction at which the reaction rate reach a maximum under isothermal conditions. This fact suggests that the part of the α-T profile in which the curve back on itself would correspond to the acceleratory period in which the total surface area of the growing nuclei would be increasing leading to an acceleration of the reaction that would be offset by a diminution of the temperature. The later rising temperature stage would correspond to the decay period in which the growing nuclei overlap among themselves with the corresponding decrease of the interface reaction area that would lead to a decrease of the reaction rate that must be compensated by a rise in temperature in order to maintain the reaction rate constant. A comparison of (CRTA) and stepwise thermal analysis (SIA) has been carried out from simulated curves. It has been shown that in the case of en orderi and diffusion controlled reactions SIA curves approach to CRTA traces as the lower and upper reaction rate thresholds becomes closest. However, the shape of the SIA α-T plots of solid state reactions following an AvraminErofeev mechanism is quite different of