The oxidation kinetics study of ultrafine iron powders by thermogravimetric analysis

The oxidation kinetics of ultrafine metallic iron powder to hematite (α-[Fe.sub.2][O.sub.3]) up to temperatures 800°C were studied in air using non-isothermal and isothermal thermogravimetric (TG) analysis. The powders with average particles size of 90, 200, and 350 nm were made by the electric explosion of wire. It was observed that the reactivity of the iron powder is increased with the decreasing particle size of powder. The experimental TG curves clearly suggest a multi-step process for the oxidation, and therefore a model-fitting kinetic analysis based on multivariate non-linear regressions was conducted. The complex reaction can be best described with a three-step reaction scheme consisting of two concurrent and one parallel reaction step. In one reaction pathway Fe is oxidized to α-[Fe.sub.2][O.sub.3]. The other pathway is described by the oxidation of Fe to magnetite ([Fe.sub.3][O.sub.4]). At higher temperatures the formed [Fe.sub.3][O.sub.4] is further oxidized in a α-[Fe.sub.2][O.sub.3].It is established that the best fitting three-step mechanism employed a branching set of n-order equations for each step.