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This work investigates the reaction mechanism of metastable intermolecular composites by collecting simultaneous pressure and optical signals during combustion in a constant-volume pressure cell. Nanoaluminum and three different oxidizers are studied: CuO, SnO 2 , and Fe 2 O 3 . In addition, these mixtures are blended with varying amounts of WO 3 as a means to perturb the gas release in the system. The mixtures with CuO and SnO 2 exhibit pressure signals that peak on timescales faster than the optical signal, whereas the mixtures containing Fe 2 O 3 do not show this behavior. The burn time is found to be relatively constant for both CuO and SnO 2 , even when a large amount of WO 3 is added. For Fe 2 O 3 , the burn time decreases as WO 3 is added, and the temperature increases. The results are consistent with the idea that oxidizers such as CuO and SnO 2 decompose and release gaseous oxidizers fast, relative to the burning, and this is experimentally seen by an initial pressure rise followed by a prolonged optical emission. In this case, the burning is rate limited by the aluminum, and it is speculated to be similar to the burning of aluminum in a pressurized oxygenated environment. For the Fe 2 O 3 system, the pressure and optical signals occur concurrently, indicating that the oxidizer decomposition is the rate-limiting step.

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Articles you may be interested inStructural and dielectric properties of laser ablated BaTiO3 films deposited over electrophoretically dispersed CoFe2O4 grains A pressure-driven flow analysis of gas trapping behavior in nanocomposite thermite films In situ microscopy of rapidly heated nano-Al and nano-Al / WO 3 thermites

Solid−solid reactions at the nanoscale between a metal passivated with a nascent oxide and another metal oxide can result in a very violent reaction. This begs the question as to what mechanism is responsible for such a rapid reaction. The ignition of nanoscale Al/CuO thermites with different aluminum oxide shell thicknesses were investigated on a fast heated (∼105 K/s) platinum wire. Ramping the wire temperature to ∼1250 K and then shutting off the voltage pulse result in ignition well after the pulse is turned off; i.e., an ignition delay is observed. The delay is used as a probe to extract the effective diffusion coefficient of the diffusing species, which is confirmed by fast time-resolved mass spectrometry. The results of this study are consistent with a diffusion controlled ignition mechanism.

The oxidation mechanism of nanoaluminum particles, nominally employed as fuel component, is still an unsettled problem, because of the complex nature of thermomechanical properties of the oxide shell surrounding the elemental core. Although mechanical breakage of the alumina shell upon or after melting of aluminum core has been thought to play a key role in the combustion of aluminum nanoparticles, there has been little direct evidence. In this study, the microstructural behaviors of Al core and alumina shell lattices were investigated with increasing temperatures. Three in situ techniques, high-temperature X-ray diffraction analysis, hot-stage transmission electron microscopy, and high-resolution transmission electron microscopy for heat-treated samples, were employed to probe the thermal behaviors of aluminum and alumina lattices before and after melting of the aluminum core. High-temperature X-ray diffraction analysis revealed that nano aluminum lattice was initially expanded under tension at room temperature, and then when heated passed through a zero-strain state at ∼300 °C. Upon further heating above the bulk melting temperature of aluminum, the aluminum lattice expanded under almost no constraint. This interesting observation, which is contrary to almost all of the previous results and models, was ascribed to the inhomogeneous (localized) crystalline phase transformation of amorphous alumina. High-resolution transmission electron microscopy and in situ hot-stage transmission electron microscopy evidenced localized phase transformation accompanied by a significant shell thickening, presumably resulting from diffusion processes of Al cations and O anions, which is to absorb the pressure built in aluminum core, by creating a more ductile shell.

In this work, high‐oxygen‐content strong oxidizer perchlorate salts were successfully incorporated into current nanothermite composite formulations. The perchlorates were encapsulated within mild oxidizer particles through a series of thermal decomposition, melting, phase segregation, and recrystallization processes, which occurred within confined aerosol droplets. This approach enables the use of hygroscopic materials by stabilizing them within a matrix. Several samples, including Fe2O3/KClO4, CuO/KClO4 and Fe2O3/NH4ClO4 composite oxidizer particles, have been created. The results show that these composite systems significantly outperform the single metal oxide system in both pressurization rate and peak pressure. The ignition temperatures for these mixtures are significantly lower than those of the metal oxides alone, and time‐resolved mass spectrometry shows that O2 release from the oxidizer also occurs at a lower temperature and with high flux. The results are consistent with O2 release being the controlling factor in determining the ignition temperature. High‐speed imaging clearly shows a much more violent reaction. The results suggest that a strategy of encapsulating a very strong oxidizer, which may not be environmentally compatible, within a more stable weak oxidizer offers the opportunity to both tune reactivity and employ materials that previously could not be considered.

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