Fast spectroscopy of laser-initiated nanoenergetic materials

Nanoenergetic materials consisting of aluminum nanoparticle (50-200 nm) aggregates termed ALEX plus nitrocellulose (NC) oxidizer are studied by ultrafast spectroscopy following 100 ps laser flash-heating. Thermal conduction calculations are used to estimate the initial ALEX temperature as a function of laser fluence, and to show that initiation and ignition occurs as a result of the reaction between nearly uniformly heated ALEX particles and cold NC. The onset of light emission and At oxidation occurs near the melting point of At, and fast violent ignition reactions begin near the vaporization transition. At lower fluences and lower ALEX concentrations, hot spots consisting of an ALEX aggregate and nearby NC ignite and then die out. At higher fluences and higher ALEX concentrations, the ignition reaction propagates from the hot spots throughout the entire laser-heated region. The nanosecond-duration burst of light accompanying ignition triggered by vaporizing ALEX consists of a continuum peaked near 500 nm plus features associated with electronically excited At atoms and hot excited AlO. The time for energy release with high-temperature initiation and minimal contribution from mass transport is similar to2 ns. When the ignition reaction also propagates through the sample, the time for energy release is increased. Coherent anti-Stokes Raman (CARS) measurements of nitro consumption via the disappearance of ONO2 (nitrato) groups of the NC due to At attack on the oxidizer show two distinct phases of similar to300 ps and similar to2 ns, associated with hot spot initiation and propagation, respectively. The 300 ps time constant for nitro consumption adjacent to an At particle is significantly shorter than the 2 ns time constant for energy release. The rate-limiting step in energy release is neither nanoparticle depassivation nor oxidization chemistry, but rather is nonradiative relaxation of hot molecules produced by At oxidation.