Fast Spectroscopy of Laser-Initiated Nanoenergetic Materials

Abstract

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 Al oxidation occurs near the melting point of Al, 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 Al atoms and hot excited AlO. The time for energy release with high-temperature initiation and minimal contribution from mass transport is ∼2 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 Al attack on the oxidizer show two distinct phases of ∼300 ps and ∼2 ns, associated with hot spot initiation and propagation, respectively. The 300 ps time constant for nitro consumption adjacent to an Al 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 Al oxidation.

Spatially and temporally resolved temperature and shock-speed measurements behind a laser-induced blast wave of energetic nanoparticles

“…20 In contrast, the initial flash-heating process under low-laser-fluence ($0.3 J/cm 2 ) conditions used here is expected to produce significantly less initial heating of the absorbing aluminum nanoparticle, with temperatures ranging between the melting point (933 K) and the boiling point (2740 K) of aluminum. 3,25 The presumed mechanism for reaction of these heated aluminum nanoparticles then follows a sequence involving the breakdown of the surrounding passivation layer, the initiation of a reaction sequence between the heated aluminum and the surrounding solid-oxidant substrate material, and subsequent sustained solid-phase exothermic reactions that produce heat and light and eject aluminum nanoparticles into the surrounding environment to react with oxidants present in the gas-phase mixture. 3 At longer time delays, this explosive ejection of material from the sample surface is observed-as was the case for all experimental samples studied here-as an expanding dark plume emanating from the sample surface.…”

“…3,25 The presumed mechanism for reaction of these heated aluminum nanoparticles then follows a sequence involving the breakdown of the surrounding passivation layer, the initiation of a reaction sequence between the heated aluminum and the surrounding solid-oxidant substrate material, and subsequent sustained solid-phase exothermic reactions that produce heat and light and eject aluminum nanoparticles into the surrounding environment to react with oxidants present in the gas-phase mixture. 3 At longer time delays, this explosive ejection of material from the sample surface is observed-as was the case for all experimental samples studied here-as an expanding dark plume emanating from the sample surface. Mie-scattering measurements, as depicted in Fig.…”

“…(3), with q 0 ¼ 1:20 kg=m 3 and c ¼ 1:4 for room-temperature air; the values used for the solid-phase components were estimated to be q s ¼ 1750 kg=m 3 and R s ¼ 1:0 Â 10 À4 m, and it was assumed that the combustion-reaction heat-capacity ratio was comparable to that of room-temperature air (i.e., c ¼ 1:4). Under these conditions, K 1 ( K 2 r 3 over the observed timescales and corresponding shock-wave radii, and the calculated released energy over the observed timescale takes the approximate form…”

“…1 In particular, aluminum and boron exhibit considerable promise as high-energy-density species in the presence of oxidizers, 2 and the increasingly economical availability of aluminum nanoparticles, along with a variety of readily tunable physical properties, such as particle diameter and composition of passivation layer, make aluminum nanoparticles particularly attractive for further study. 1,[3][4][5][6][7] When used as an additive in solid-rocket propellants or high explosives, aluminum particles are rapidly ejected into the gas phase following the initial impulsive combustion event; these particles then continue to react with oxidants present in that gas-phase mixture produced by the initial impulsive event. 8 The dynamics of aluminum-particle reactions in such gas-phase environments have been studied in depth by several groups, and a notable dependence of these reaction dynamics on particle size has been observed.…”

Laser-Induced Breakdown Spectroscopy: Capabilities and Applications

“…It is well known that two-component explosives consisting of metal particle fuels and oxidizers can produce more than twice as much energy as high performance molecular explosives alone (94). In recent years it has been suggested that nanometer-scale metal particles would provide faster energy release and better control over material properties, but the development of nanoenergetic materials has been hampered by the lack of fundamental knowledge of the chemical dynamics involved.…”