(688d) Ignition of Fully-Dense Nanocomposite Thermite Powders By An Electric Spark

Electro-Static Discharge (ESD) ignition of nanocomposite thermite powders prepared by Arrested Reactive Milling was investigated.  Powders placed in a sample holder were subjected to an ESD produced by a discharging capacitor.  The optical emission produced by the ignited powders was monitored using several photomultipliers equipped with interference bandpass filters. Time-resolved current and voltage traces of the ESD were also measured.  Two ignition modes were detected: ignition of individual particles occurring during or immediately after the ESD, and powder ignition resulting in the combustion of a cloud of aerosolized particles, which was observed to begin after a substantial delay following the ESD pulse.  Two materials studied in this work were nanocomposite powders 2Al^.3CuO and 8Al^.MoO₃.  For each thermite, the bulk composition remained fixed, while the mixing morphology was varied by preparing samples using different milling times.  Experiments addressed the effects of material composition and morphology, energy input, powder layer thickness, and environment on the ignition mode observed and ignition delay for the case of powder ignition.  In addition, effect of sample aging on their ignition behavior was considered.  Finally, prepared thermites were blended with pure metal powders and ignition of the prepared blends was studied as well.  When samples were placed on a conductive surface as a powder monolayer, ESD resulted in ignition of individual particles.  Individual particle ignition was also observed in vacuum, for different powder layer thicknesses.  For experiments in air and in argon, when the powder thickness layer varied from about 50 to 500 µm, a delayed powder ignition mode was observed.  The ignition delays varied from 10 µs to several ms for different experimental conditions.  For the cases of powder ignition, it was observed that 2Al^.3CuO powders had shorter ignition delays compared to 8Al^.MoO₃.  For both materials, ignition delays decreased for the powders prepared using longer milling times, in which the mixing between aluminum and oxide was achieved at a finer scale.  The effect was especially pronounced for 8Al^.MoO₃.  For both materials, aged powders ignited after longer delays compared to the freshly prepared powders. Variation in the ESD energy and layer thickness (except for a monolayer) did not have a significant effect on the ignition behavior.  Blending nanocomposite thermites with pure Al and Ti powders reduced their ESD ignition sensitivity significantly.  When ignition was achieved, longer ignition delays were observed for the blended samples.  Results of these experiments are expected to be useful in developing a model describing ESD-induced ignition in nanocomposite thermite powders and accounting for transfer of ESD energy to the powder as well as initiation of exothermic chemical reactions leading to ignition.