Ultrafast Condensed-Phase Emission from Energetic Composites of Teflon and Nanoaluminum

Abstract

Time and wavelength-resolved spectroscopy was used to monitor optical emission from picosecond flash-heated nanoenergetic materials consisting of 50 nm diameter Al/Al(2)O(3) core-shell nanoparticles in a matrix of Teflon(AF) oxidizer. The Al/Teflon was confined between optical windows to emphasize condensed-phase emission rather than emission from gas-phase reaction products. The Al/Teflon emission is compared to control samples of bare Al nanoparticles or nanoparticles in a nominally inert polybutadiene matrix (Al/PB). In all three materials just two types of emission were observed, a broadband (BB) emission peaked at 320 nm stretching out past 700 nm, and a narrowband (NB) emission with approximately 30 nm wide bands at 310 and 400 nm, coincident with the most intense Al atomic emission lines. The BB emission is attributed to a laser-generated Al plasma and the NB emission to excited-state Al atoms in a dense medium. Compared to Al/PB, the plasma in Al/Teflon emits with higher intensity and longer duration. The Al/Teflon excited-state NB emission was also more intense. The energy release characterizing ignition in Al/Teflon could be monitored via the increased emission intensity of the BB and NB species. We find the ignition process can be described with a global time constant of 100 ps, about twice the approximately 50 ps time constant for the initiation process seen in earlier work (Zamkov, M. A.; Conner, R. W.; Dlott, D. D., J. Phys. Chem. C 2007, 111, 10278), where infrared spectroscopy was used to monitor the disappearance of C-F stretch transitions.