On the Origin of Abrupt Exothermic Reactions Induced by Controlled Ball Milling of Graphite Powder in Inert Atmosphere

Understanding the mechanism behind unanticipated and sudden exothermic reactions observed during the controlled ball milling of graphite powder in an inert Helium (He) atmosphere was investigated. High energy ball milling using a Uni-ball-mill and with magnetic control of the ball movement and collision forces was performed. Detailed characterisation of the milled products at different intervals of milling using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) with selected area electron diffraction (SAED), and laser-Raman spectroscopy was performed.,The experimental results revealed that, after shorter milling times, there were heavily deformed graphitic particles and accumulation of iron-based impurities found as Cr0.19Fe0.7Ni0.11 was observed, possibly due to contamination from the milling container and balls. After higher milling times, highly exothermic reactions were observed to occurred spontaneously during powder discharge, even within controlled environment of air-tight glove box. This instantaneous reaction resulted in the formation of iron oxide (Fe2O3) and iron-rich oxide intertwined with stainless steel particles, in addition to residual graphitic nanoparticles.,The origin of these reactions is likely attributed to the presence of highly surface-reactive iron nanoparticles that was created through continuous contaminations from milling balls and product size reduction during the high energy ball milling. Presumably, these nanoparticles may act as thermal trigger upon exposure to trace oxygen during immediate powder discharge, resulting to sudden reaction of milling products. This study details the microstructural evolution with additional phase formation during exothermic reactions and discusses the best strategies to prevent such occurrence during high energy ball milling.