Correlation of optical emission and pressure generated upon ignition of fully-dense nanocomposite thermite powders

Abstract

Ignition was studied experimentally for four nanocomposite materials prepared by Arrested Reactive Milling (2Al·3CuO, 2.35Al·Bi2O3, 4Al·Fe2O3, and 8Al·MoO3). Thin layers of the prepared powders were coated on an electrically heated Ni–Cr filament and ignited at heating rates between 200 and 17,000K/s in a miniature vacuum chamber. The ignition was monitored based on both photodiode and pressure transducer signals recorded simultaneously. Ignition temperatures were found to be weakly dependent of the heating rate and similar for all thermites (in the range of ∼800–950K) except for that using Bi2O3 as an oxidizer, which ignited at lower temperatures. Both maximum pressure and the rate of pressure rise (dP/dt) decreased with increasing heating rate. The onsets of pressure pulses produced by ignited thermites occurred prior to those of the respective emission pulses, with the effect increasing at higher heating rates. It was observed that 2.35Al·Bi2O3 did not ignite at heating rates above 5000K/s in vacuum, while it readily ignited when heated in air. A qualitatively similar observation was made for 8Al·MoO3, which could not be ignited in vacuum when heating rates reached 13,000K/s. The results are interpreted to suggest that the low-temperature redox reactions occurring prior to ignition modify the oxidizers, making them less stable and prone to rapid decomposition upon further heating. Ignition, triggered by a change in transport properties of the growing interfacial Al2O3 layer is not directly affected by the oxygen release from the decomposing oxides. Both oxygen release and growth of the interfacial Al2O3 layer are directly affected by the rates of low-temperature redox reactions preceding ignition.