Reaction/crystallization kinetics studied via in situ XRD: experimental conditions versus methods of kinetic analysis

Abstract

Theoretically simulated kinetic data were used to evaluate the errors associated with the common issue of evaluating the in situ non-isothermal X-ray diffraction data, where the complex multi-step temperature program (alternating the non-isothermal heating steps with isothermal steps during which the diffraction patterns are collected) is for the purposes of evaluation replaced by a simple non-isothermal heating performed at the reduced/effective heating rate. The kinetic analysis has shown that, in general, best results are provided by the non-linear optimisation methods simultaneously evaluating the data-curves obtained for all the different heating rates. For the nucleation growth (KMJMA) kinetics the distortive influence of the temperature program parameters increases as follows: heating rate during non-isothermal segments < duration of the isothermal segment < temperature interval between the isothermal segments. The non-optimisation methods of kinetic analysis (integral isoconversional methods for evaluation of activation energy E and master plots for determining the appropriate kinetic model) were found to perform inaccurately, with large degree of randomness based on the selection of starting temperature, and are not recommended for evaluation of the in situ XRD data - the only exception seem to be the differential isoconversional methods that provided accurate E values. Generalisation of the present conclusions for all KMJMA processes is suggested and discussed.