Probing the ignition mechanism of aluminum nanothermites

Abstract

Nanothermites are defined as intimate mixtures of metal and metal oxidizer particles usually below 100 nm in diameter. They belong to a class of energetic materials which have been of recent interest due to their high amounts of stored energy, and their potential for future use in a variety of applications. Once ignited, nanothermites undergo self-sustaining reactions. Such reactions are very poorly understood due to the lack of proper diagnostic techniques replicating the heating rates in self-sustaining reactions.

We use a temperature jump (T-jump) technique by heating a thin platinum wire to study the nanothermite reactions at heating rates of 10 <super>5</super> K/s. First we study the ignition initiation mechanism in Al-CuO nanothermites and show that there is an inherent ignition delay, i.e., ignition occurs after the electric pulse is shut off. This ignition delay increases progressively as the oxide shell thickness is increased, suggesting that the reacting species have to move across the shell. T-jump time of flight mass spectrometry (T-jump TOFMS) is used qualitatively to support such a claim. Several nanothermites are also tested for their ignition temperature. The oxidizers were chosen based on their behavior towards heating. For several oxidizers (CuO, Fe₂O₃, KClO₄ etc.) ignition in the nanothermites is noticed to occur when the oxidizers release oxygen using T-jump TOFMS. Complementary electron microscopy techniques show that Al-CuO reactions can occur even in the absence of oxygen, via reactive sintering mechanism. Furthermore, electron microscopy techniques are used to show evidence of condensed phase initiation in other nanothermites.

The role of positive ions in correlation to ignition in nanothermites is also studied for selected nanothermites using the T-jump TOFMS. Positive ions are seen to be generated during the ignition interval and are found to consist primarily of Na<super>+</super> ions. A hypothesis for such observation is proposed and is seen to be consistent with molecular dynamics simulations from literature.