Measuring Fast and Slow Energy Release from Aluminum Powders

Because of its high specific energy density (31 kJ/g) and widespread availability, micron-sized aluminum (Al) powders have been used in energetics applications, primarily for blast enhancement on extended timescales. A key goal in energetic materials research is to accelerate the reaction of metals during an explosion so that the detonation performance of the explosive is enhanced. Nano-sized Al particles have the potential to react faster than micron-Al, but suffer from issues such as the formation of a native oxide layer which delays reaction and strong agglomeration of the particles resulting in incomplete combustion. The mechanisms and timescale of energy release from Al at very high heating rates (\textasciitilde 10$^{\mathrm{13}}$ K/s) comparable to those behind a detonation front are of significant interest for energetic applications. For the first time, we have systematically investigated the fast (microsecond-timescale) energy release of Al following laser-induced breakdown ignition. A ns-pulsed laser was used to ignite 9 different Al powders ranging in size from 20 nm to 30 $\mu $m. A wide variety of diagnostics including the detection of time-resolved AlO emission with a PMT and integrated combustion emission with a photodiode, high-resolution spectroscopy of the laser-induced plasma and subsequent combustion events, and high-speed imaging to measure the laser-induced shock velocities were employed to understand the effect of particle size and active aluminum content on the rate of energy release.