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 micros). This article examines the extent of 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 (≈ 10⁵ K/s) to ignite the sample. The subsequent products were captured/quenched very rapidly (≈ 500 micros) in order to preserve the chemistry/morphology during initiation and subsequent reaction and were quantitatively analyzed using electron microscopy (EM), focused ion beam (FIB) cross-sectioning followed by energy dispersive X-ray spectroscopy (EDX). Elemental examination of the cross-section of the quenched particles show oxygen predominantly localized in the regions containing aluminum, implying the occurrence of redox reaction. The Al/CuO system, which has simultaneous gaseous oxygen release and ignition (T_Ignition ≈ T_{Oxygen Release} ), shows 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 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.