Incomplete reactions in nanothermite composites

Exothermic reactions between oxophilic metals and transition/post transition metal-oxides have been well documented owing to their fast reaction time scales (≈10 μs). This article examines the extent of the reaction in nano-aluminum based thermite systems through a forensic inspection of the products formed during reaction. Three nanothermite systems (Al/CuO, Al/Bi₂O₃, and Al/WO₃) were selected owing to their diverse combustion characteristics, thereby providing sufficient generality and breadth to the analysis. Microgram quantities of the sample were coated onto a fine platinum wire, which was resistively heated at high heating rates (≈10⁵K/s) to ignite the sample. The subsequent products were captured/quenched very rapidly (≈500 μs) in order to preserve the chemistry/morphology during initiation and subsequent reaction and were quantitatively analyzed using electron microscopy and focused ion beam cross-sectioning followed by energy dispersive X-ray spectroscopy. Elemental examination of the cross-section of the quenched particles shows that oxygen is predominantly localized in the regions containing aluminum, implying the occurrence of the redox reaction. The Al/CuO system, which has simultaneous gaseous oxygen release and ignition (T_Ignition ≈ T_{Oxygen Release}), shows a substantially lower oxygen content within the product particles as opposed to Al/Bi₂O₃ and Al/WO₃ thermites, which are postulated to undergo a condensed phase reaction (T_Ignition ≪ T_{Oxygen Release}). An effective Al:O composition for the interior section was obtained for all the mixtures, with the smaller particles generally showing a higher oxygen content than the larger ones. The observed results were further corroborated with the reaction temperature, obtained using a high-speed spectro-pyrometer, and bomb calorimetry conducted on larger samples (≈15 mg). The results suggest that thermites that produce sufficient amounts of gaseous products generate smaller product particles and achieve higher extents of completion.