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Aluminum nanoparticles are being considered as a possible fuel in advanced energetic materials application. Of considerable interest therefore is a knowledge of just how reactive these materials are, and what the effect of size on reactivity is. In this paper we describe results of size resolved oxidation rate using a recently developed quantitative single particle mass spectrometer (SPMS). Aluminum nanoparticles used were either generated by DC Arc discharge or laser ablation, or by use of commercial aluminum nanopowders. These particles were oxidized in an aerosol flow reactor in air for specified various temperatures (25-1100 degrees C), and subsequently sampled by the SPMS. The mass spectra obtained were used to quantitatively determine the elemental composition of individual particles and their size. We found that the reactivity of aluminum nanoparticles is enhanced with decreasing primary particle size. Aluminum nanoparticles produced from the DC Arc, which produced the smallest primary particle size (approximately 19 nm), were found to be the most reactive (approximately 68% aluminum nanoparticles completely oxidized to aluminum oxide at 900 degrees C). In contrast, nanopowders with primary particle size greater than approximately 50 nm were not fully oxidized even at 1100 degrees C (approximately 4%). The absolute rates observed were found to be consistent with an oxide diffusion controlled rate-limiting step. We also determined the size-dependent diffusion-limited rate constants and Arrehenius parameters (activation energy and pre-exponential factor). We found that as the particle size decreases, the rate constant increases and the activation energy decreases. This work provides a quantification of the known pyrophoric nature of fine metal particles.

Aluminum nanoparticles have increasingly gained attention because of their potential incorporation in explosive and propellant mixtures. This letter reports on a qualitative study on the oxidation of aluminum nanoparticles containing a passivating oxide coating. Hot-stage transmission electron microscopy (TEM) studies were performed to understand the stability of the oxide coating in nanoaluminum, and oxidation was investigated using a single particle mass spectrometer (SPMS). We find that the oxidation of oxide-coated nanoaluminum coincides with and therefore is presumably initiated by melting of the aluminum core and subsequent mechanical rupture of the oxide coating.

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Nanoscaled nickel particles have attracted interest for their potential use as a fuel in energetic materials. In this work, we combined two ion-mobility spectrometry approaches, tandem differential mobility analysis (TDMA) and tandem differential mobility-particle mass analysis (DMA-APM), to study the size-resolved reactivity of nickel nanoparticles. Nickel nanoparticles were generated in situ by using gas-phase thermal pyrolysis of nickel carbonyl. Four particle sizes (40, 62, 81, and 96 nm, mobility size) were then selected by using a differential mobility analyzer. These particles were sequentially oxidized in a flow reactor at various temperatures (25-1100 °C). The size and mass change of the size-selected and -reacted particles were then measured by a second DMA, or an APM. We found that both particle size and mass were increased as the temperature increased. However, at higher temperature (600-1100 °C), a different mass and size change behavior was observed that could be attributed to a phase transition between NiO and Ni 2 O 3 . A shrinking core model employed to extract the size-resolved kinetic parameters shows that the activation energy for oxidation decreased with decreasing particle size. The burning time power dependence on particle size was found to be less than 2 and nickel particles were found to be kinetically more active than aluminum.

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Mixtures of fuel and oxidizers with particle sizes in the nanometer range have been widely used for energy intensive applications like propellants and explosives. Nano- Al is invariably used as fuel, while a host of metal oxide nanoparticles are used as oxidizers. This article aims at understanding and tuning the reactivity of these nano-energetic materials. The first part of this article discusses the oxidative reactivity of aluminum nanoparticles as measured experimentally using single-particle mass-spectrometer (SPMS) and microscopy and then modeled. Experimental evidence suggests that outward diffusion of aluminum is an important phenomenon in the oxidation of aluminum nanoparticle. Also melting of the aluminum core is necessary for the reaction to take place vigorously. In the second part of the paper we discuss the formation of novel oxidizers. A super-reactive formulation of Al/KMnO4 has been developed which is shown to be orders of magnitude more reactive than the traditional formulations of Al/Fe2O3, Al/MoO3 and Al/CuO. We demonstrate the formation of novel composite oxidizers to tune the reactivity of the Al/Metal oxide system.