Evaluating free vs bound oxygen on ignition of nano-aluminum based energetics leads to a critical reaction rate criterion

摘要

This study investigates the ignition of nano-aluminum (n-Al) and n-Al based energetic materials (nanothermites) at varying O_2 pressures (1-18 atm), aiming to differentiate the effects of free and bound oxygen on ignition and to assess if it is possible to identify a critical reaction condition for ignition independent of oxygen source. Ignition experiments were conducted by rapidly heating the samples on a fine Pt wire at a heating rate of ～10~5 ℃ s~(-1) to determine the ignition time and temperature. The ignition temperature of n-Al was found to reduce as the O_2 pressure increased, whereas the ignition temperatures of nanothermites (n-Al/Fe_2O_3, n-Al/Bi_2O_3, n-Al/K_2SO_4, and n-Al/K_2S_2O_8) had different sensitivities to O_2 pressure depending on the formulations. A phenome-nological kinetic/transport model was evaluated to correlate the concentrations of oxygen both in condensed and gaseous phases, with the initiation rate of Al-O at ignition temperature. We found that a constant critical reaction rate (5 × 10~(-2)mol m~(-2) s~(-1)) for ignition exists which is independent to ignition temperature, heating rate, and free vs bound oxygen. Since for both the thermite and the free O_2 reaction the critical reaction rate for ignition is the same, the various ignition temperatures are simply reflecting the conditions when the critical reaction rate for thermal runaway is achieved.