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 (approximate to 10 mu s). 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 (approximate to 10(5) K/s) to ignite the sample. The subsequent products were captured/quenched very rapidly (approximate to 500 mu s) 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 approximate to 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 (approximate to 15 mg). The results suggest that thermites that produce sufficient amounts of gaseous products generate smaller product particles and achieve higher extents of completion. Published by AIP Publishing.