Micron and nano iron particles: Experimental investigation of size dependent combustion in ethanol slurries and bimetallic thermite

Abstract

The combustion dynamics and characteristics of dense iron–ethanol slurry droplets containing 30 wt% Fe (in both micron- and nano-sized forms) and Fe-doped Al/Fe₂O₃ thermites containing 5 wt% Fe (in both micron- and nano-sized forms) are investigated as a function of iron particle size. Single droplet experiments with high-speed imaging and in-situ thermocouple measurements are combined with TGA–DSC and SEM/EDS to link burning behavior to aggregate morphology. In ethanol, micron Fe (10 µm) settles toward the droplet periphery (terminal velocity ∼0.06 mm/s), forming a porous shell that improves O₂ access, raises the peak internal temperature to ∼326 °C, and increases the d²-law burning rate to 0.76 mm²/s (+10%) while shortening the total burn time. Nano Fe (40 nm) remains more uniformly dispersed (terminal velocity ∼0.04 mm/s) and forms smoother, less-porous aggregates; despite a higher peak temperature (∼266.5 °C) and frequent small disruptive events, the burning rate decreases to 0.63 mm²/s (–9%) and the total burn time contracts through secondary atomization. These outcomes show that aggregate architecture and settling, not surface area alone, govern heat/mass transfer and apparent burning rate in highly loaded metal fuel droplets. Translating these insights to thermites, adding 5 wt% nano-Fe triggers earlier ignition and rapid energy release, whereas 5 wt% micron-Fe yields more completely reduced, Fe-rich, semi-spherical residues, indicating locally more complete conversion. The work establishes a transport-to-morphology-to-combustion framework for designing metalized fuels and tailoring ignition and energy release in aluminothermic composites.