One of the challenges in the use of energetic nanoparticles within a polymer matrix is the difficulty in processing by traditional mixing methods. In this paper, electrospray deposition is employed to create high loadings of aluminum nanoparticles (Al-NPs) in polyvinylidene fluoride (PVDF) reactive composite films. The deposited films containing up to 50 wt% Al are found to be crack free and mechanically flexible. Thermochemical behavior characterized by thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis shows that the addition of Al-NPs sharply reduces the onset decomposition temperature due to a pre-ignition reaction occurring in the film. The combustion propagation velocity in air at three different mass loading of Al-NPs shows burning rates of 5, 16, and 23 cm s À 1 for loadings of 16.7,30, The results suggest electrospray deposition as a direct approach to make bulk polymer composites containing high metal particle mass loading and may be a prelude to 3D printing of rocket motors.

Energetic thin films with high mass loadings of nanosized components have been recently fabricated using electrospray deposition. These films are composed of aluminum nanoparticles (nAl) homogeneously dispersed in an energetic fluoropolymer binder, poly(vinylidene fluoride) (PVDF). The nascent oxide shell of the nAl has been previously shown to undergo a preignition reaction (PIR) with fluoropolymers such as polytetrafluoroethylene (PTFE). This work examines the PIR between alumina and PVDF to further explain the reaction mechanism of the Al/PVDF system. Temperature jump (T-jump) ignition experiments in air, argon, and vacuum environments showed that the nAl is fluorinated by gas phase species due to a decrease in reactivity in a vacuum. Thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC) was used to confirm the occurrence of a PIR, and gas phase products during the PIR and fluorination of nAl were investigated with temperature jump time-of-flight mass spectrometry (T-jump TOFMS). Results show a direct correlation between the amount of alumina in the PVDF film and the relative signal intensity of hydrogen fluoride release (HF). Although the PIR between alumina and PVDF plays an important role in the Al/PVDF reaction mechanism, burn speeds of Al/PVDF films containing additional pure alumina particles showed no burn speed enhancement.

The biological agents that can be weaponized, such as Bacillus anthracis, pose a considerable potential public threat. Bacterial spores, in particular, are highly stress resistant and cannot be completely neutralized by common bactericides. This paper reports on synthesis of metal iodate-based aluminized electrospray-assembled nanocomposites which neutralize spores through a combined thermal and chemical mechanism. Here metal iodates (Bi(IO3)3, Cu(IO3)2, and Fe(IO3)3) act as a strong oxidizer to nanoaluminum to yield a very exothermic and violent reaction, and simultaneously generate iodine as a long-lived bactericide. These microparticle-assembled nanocomposites when characterized in terms of reaction times and temporal pressure release show significantly improved reactivity. Furthermore, sporicidal performance superior to conventional metal-oxide-based thermites clearly shows the advantages of combining both a thermal and biocidal mechanism in spore neutralization.

Nanometallic fuels with high combustion enthalpy, such as aluminum, have been proposed as a potential fuel replacement for conventional metallic fuel to improve propellant performance in a variety of propulsive systems. Nevertheless, nanometallic fuels suffer from the processing challenges in polymer formulations such as increased viscosity and large agglomeration, which hinder their implementation. In this letter, we employ electrospray as a means to create a gel within a droplet, via a rapid, solvent evaporation-induced aggregation of aluminum nanoparticles, containing a small mass fraction of an energetic binder. The gelled aluminum microspheres were characterized and tested for their burning behavior by rapid wire heating ignition experiments. The gelled aluminum microspheres show enhanced combustion behavior compared to nanoaluminum, which possibly benefits from the nitrocellulose coating and the gelled microstructure, and is far superior to the corresponding dense micrometer-sized aluminum.

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Nanothermites offer high energy density and high burn rates, but are mechanistically only now being understood. One question of interest is how initiation occurs and how the ignition temperature is related to microscopic controlling parameters. In this study, we explored the potential role of oxygen ion transport in BiO as a controlling mechanism for condensed phase ignition reaction. Seven different doped δ-BiO were synthesized by aerosol spray pyrolysis. The ignition temperatures of Al/doped BiO, C/doped BiO and Ta/doped BiO were measured by temperature-jump/time-of-flight mass spectrometer coupled with a high-speed camera respectively. These results were then correlated to the corresponding oxygen ion conductivity (directly proportional to ion diffusivity) for these doped BiO measured by impedance spectroscopy. We find that ignition of thermite with doped BiO as oxidizer occurs at a critical oxygen ion conductivity (∼0.06 S cm) of doped BiO in the condensed-phase so long as the aluminum is in a molten state. These results suggest that oxygen ion transport limits the condensed state BiO oxidized thermite ignition. We also find that the larger oxygen vacancy concentration and the smaller metal-oxide bond energy in doped BiO, the lower the ignition temperature. The latter suggests that we can consider the possibility of manipulating microscopic properties within a crystal, to tune the resultant energetic properties.

This study investigates the ignition of nano-aluminum (n-Al) and n-Al based energetic materials (nanothermites) at varying O2 pressures (1–18 atm), aiming to differentiate the effects of free and bound oxygen on ignition and to assess if it is possible to identify a critical reaction condition for ignition independent of oxygen source. Ignition experiments were conducted by rapidly heating the samples on a fine Pt wire at a heating rate of ∼105 °C s−1 to determine the ignition time and temperature. The ignition temperature of n-Al was found to reduce as the O2 pressure increased, whereas the ignition temperatures of nanothermites (n-Al/Fe2O3, n-Al/Bi2O3, n-Al/K2SO4, and n-Al/K2S2O8) had different sensitivities to O2 pressure depending on the formulations. A phenomenological kinetic/transport model was evaluated to correlate the concentrations of oxygen both in condensed and gaseous phases, with the initiation rate of Al-O at ignition temperature. We found that a constant critical reaction rate (5 × 10−2 mol m−2 s−1) for ignition exists which is independent to ignition temperature, heating rate, and free vs bound oxygen. Since for both the thermite and the free O2 reaction the critical reaction rate for ignition is the same, the various ignition temperatures are simply reflecting the conditions when the critical reaction rate for thermal runaway is achieved.

The nano-Al/K2S2O8 thermite shows a low ignition temperature (600 °C), extensive O2/SO2 generation, as well as persistent combustion. An ignition mechanism involving gaseous oxygen was proposed for the nano-Al/K2S2O8 reaction.