Oxidation kinetics of magnesium particles determined by isothermal and non-isothermal methods of thermogravimetric analysis

Abstract

Knowledge of the oxidation kinetics of magnesium (Mg) particles is important for the development of advanced propulsion and power systems based on combustion of metals. However, reports on the high-temperature oxidation of Mg powders in oxygen are contradictory. In the present work, the oxidation of spherical and non-spherical (flakes) Mg particles in O2 flow was investigated using isothermal and non-isothermal thermogravimetry. Three fractions of the spherical powder were tested, with average sizes about 30, 60, and 300 μm. The oxidation of all powders was complete at temperatures lower than the melting point of Mg, 650 °C. The Friedman analysis of non-isothermal and isothermal thermogravimetric (TG) data has shown that the activation energy exhibits only small fluctuations during a large part of the oxidation process, which indicates that the process can be modeled as a single-step reaction. Model-based analysis has shown that the Avrami-Erofeev model of simultaneous nucleation and growth provides the best fit with the experimental isothermal and non-isothermal TG curves. Scanning electron microscopy of the oxidized spherical particles has shown submicron grains, which is consistent with the Avrami-Erofeev model. The Mampel-Delmon analysis of the isothermal TG data has shown that for the 30 and 60 μm particles, the nucleation is relatively slow, leading to the sigmoidal TG curve, described by the three-dimensional Avrami-Erofeev equation. An increase in the particle size decreases the dimension of this equation, and for the 300 μm particles and flakes, the entire process can be considered as a growth of a non-protective oxide layer. The apparent activation energy of Mg oxidation is likely in the range of 200 – 230 kJ/mol, independently of the particle size and shape.