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The Role of Metal Oxides in Nanothermite Reactions: Evidence of Condensed Phase Initiation

Abstract : This work is a culmination of several corresponding studies designed to probe the initiation and reaction of aluminum nanothermite systems. The main diagnostic tool used in this study is a Temperature-Jump/Time-of-Flight Mass Spectrometer (T-Jump/TOFMS), which uses a filament heating method capable of very high heating rates up to 106 degrees C/s, while spectra are simultaneously obtained at a time resolution of 100 s. Nanothermites of Al/CuO, Al/Fe2O3, Al/WO3, and Al/Bi2O3 were all tested with this system along with the neat Al and metal oxide powders. High speed imaging was also used to visually compare reaction rates of each sample showing that, contradictory to some previous works, Al/Bi2O3 reacts much faster than the other nanothermites. The nanothermites Al/CuO, Al/Fe2O3, and Al/WO3 showed a correlation of ignition with the neat oxide's decomposition to gas phase products. However, the Al/Bi2O3 sample clearly ignites at a temperature well below the decomposition of Bi2O3 to gaseous products, which strongly suggests a condensed phase initiation mechanism for Al/Bi2O3. To further investigate this mechanism, C/Bi2O3 was also tested as, unlike aluminum, carbon will remain in the solid phase in the temperature regime of our experiments. This work showed similar results as the Al/Bi2O3 where C/Bi2O3 clearly reacts before Bi2O3 decomposes. In a complimentary study a high-heating rate TEM grid was also used to probe the behavior of individual nanoparticles under these experimental conditions. While testing Bi2O3 it was seen that the material reacted with the carbon coating of the TEM grid, again displaying a condensed phase reaction. The TEM studies provide evidence that other nanothermites may follow this mechanism as well. Furthermore, a T-Jump/PMT setup was used to optically determine the burn times and ignition temperatures in filament heating experiments. This work provided high speed phase contrast imaging that offers further insight into naonthermic reactions.