Atomic insights into micro-explosion from iron particle with carbon impurity

Iron (Fe) has recently emerged as a promising carbon-free energy carrier, but the mechanisms governing its oxidation including the experimentally observed micro-explosion phenomenon, remain less understood. Rather than directly comparing with experimental quantities, this study uses validated reactive force field (ReaxFF) molecular dynamics simulations to reveal, for the first time, the atomistic mechanisms of micro-explosion in iron particles with carbon impurities and to characterise the properties of the released gases. Simulations of pure Fe and C particles in oxygen environments show that oxygen adsorption preferentially occurs on iron surfaces, while carbon remains largely inert. In Fe–C particles, carbon distributions (clustered or random) remain thermally stable prior to significant iron oxidation. Simulations of iron oxides with varying oxidation states, carbon impurity levels, distributions, and temperatures show that gas generation and micro-explosion are governed by the oxidation degree of iron. Micro-explosion, defined as rapid gas accumulation and shell rupture, occurs only in Fe₂O₃-C particles, and not in FeO–C or Fe₃O₄-C systems. In Fe₂O₃-C, representing highly localised oxidation, micro-explosion occurs only for clustered carbon distributions with concentrations exceeding 4% (atomic ratio), serving as a relative indicator rather than a predictive threshold. Random distributions instead lead to gradual gas release without shell rupture. The intensity of micro-explosion increases with carbon content and temperature, accompanied by a rapid rise in carbon kinetic energy. Generated CO and CO₂ may also be partially re-adsorbed before release into the surrounding environment. These findings provide atomistic insights into gas generation, accumulation, and release during iron oxidation, offering mechanistic understanding of micro-explosion phenomena. Novelty and significance statement: This work presents the first atomistic study of micro-explosions in iron particles with carbon impurities during oxidation using ReaxFF molecular dynamics simulations. Although micro-explosions have been observed experimentally in iron-based energy carriers, their underlying mechanisms and the identity of the released gases remain unclear. Rather than aiming for direct quantitative comparison with experiments, this study focuses on revealing the governing mechanisms at the atomic scale. The results establish a direct link between carbon impurity level, spatial distribution, and micro-explosion behaviour. Reactions between carbon and lattice oxygen in highly oxidised iron (Fe₂O₃) generate CO and CO₂, leading to internal gas accumulation and pressure-driven shell rupture. Micro-explosion occurs only for clustered carbon distributions and sufficiently high impurity levels, highlighting the importance of local structure and oxidation state. These findings provide a mechanistic framework for impurity-driven micro-explosion, advancing understanding for iron-based energy carriers.