Kinetics of Consecutive Reactions in the Solid State: Thermal Decomposition of Oxalates

Abstract

The thermal and kinetic aspects of solid state decomposition reactions can be complex and posses a large number of parameters to investigate. The determination of the activation energies for the several intermediates in the thermal decomposition is particularly sensitive to the overlap in thermal decomposition traces. To overcome this problem we have undertaken the detailed kinetic analysis of the well-known thermal decomposition of calcium and strontium oxalates and mixtures of both. Indeed, their small difference in decomposition temperatures, from oxalate to carbonate, allowed us to simulate overlapping TGA signals and to use these data to test the validity of the experimental conditions and mathematical methods used to derive calculated kinetic values. Knowledge of kinetic parameters, such as the reaction rate and activation energy, is one of the keys to determine the reaction mechanisms in solid phases. When changes in the mechanisms are observed, this can lead to a unique characteristic and hence a better knowledge of the materials. Besides this, there are also more practical reasons to know the reaction rates and their temperature dependence. The industry needs measurements of those parameters for the accurate design of installations and treatment conditions, because augmentation of temperature or elongation of reaction time means more costs. Using an appropriate mathematical expression, the TA-experiments can be applied for the modeling of industrial thermal processes. The results of the kinetic investigation of thermoanalytical reactions in the solid state can also be applied to problems as useful lifetime of certain components, oxidative and thermal stability and quality control. During our prior thermoanalytical studies of the decomposition kinetics of nitrate precursors for the synthesis of ceramic high temperature superconductors (HTSC) we were confronted with the ubiquitous presence of consecutive reactions in the solid state. Although spray drying of aqueous nitrate solutions was demonstrated to result in very reactive precursor materials, the determination of activation energies for the several intermediates in the thermal decomposition is particularly sensitive to the overlap in thermal decomposition traces. The thermal and kinetic aspects of these decompositions are complex and possess a large number of parameters to investigate.