To optimize the performance of hypergolic, ionic-liquid-based fuels, it is critical to understand the fundamental reaction mechanisms of ionic liquids (ILs) with the oxidizers. We consequently explored the reactions between a single levitated droplet of 1-butyl-3-methylimidazolium dicyanamide ([BMIM][DCA]), with and without hydrogen-capped boron nanoparticles, and the oxidizer nitrogen dioxide (NO). The apparatus consists of an ultrasonic levitator enclosed within a pressure-compatible process chamber interfaced to complementary Fourier-transform infrared (FTIR), Raman, and ultraviolet-visible spectroscopic probes. First, the vibrational modes for the Raman and FTIR spectra of unreacted [BMIM][DCA] are assigned. We subsequently investigated the new structure in the infrared and Raman spectra produced by the reaction of the IL with the oxidizer. The newly produced peaks are consistent with the formation of the functional groups of organic nitro-compounds including the organic nitrites (RONO), nitroamines (RR'NNO), aromatic nitro-compounds (ArNO), and carbonitrates (RR'C═NO), which suggests that the nitrogen or oxygen atom of the nitrogen dioxide reactant bonds to a carbon or nitrogen atom of [BMIM][DCA]. Comparison of the rate constants for the oxidation of pure and boron-doped [BMIM][DCA] at 300 K shows that the boron-doping reduces the reaction rate by a factor of approximately 2. These results are compared to the oxidation processes of 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT][DCA]) with nitrogen dioxide (NO) studied previously in our laboratory revealing that [BMIM][DCA] oxidizes faster than [MAT][DCA] by a factor of about 20. The present measurements are the first studies on the reaction rates for the oxidation of levitated ionic-liquid droplets.

Incorporating high energy nanoparticulate additives is considered as a means to enhance the performance of hypergolic ionic liquid (IL) propellants. In this manuscript, we demonstrate the energy content of borane–aluminum nanoparticles produced by ball milling and examine the ignition and combustion behavior of hypergolic ILs loaded with up to 30 weight percent (wt %) of the particles, as measured by two different methods. The goal is to better understand the effects of particle loading on hypergolic ignition and combustion mechanism. Bomb calorimetry and differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA) combined with X-ray diffraction (XRD) were used to determine the combustible energy content of particles with different capping layers and to examine the nature of the combustion products. Particles were found to have up to 97% of combustible metal content. Rapid scan-Fourier transform infrared spectroscopy (RS-FTIR) and photographic methods were used to examine ignition delay and the nature of the flames produced. The addition of borane–aluminum nanoparticles was found to have only small effects on ignition delay, but even small particle loadings were found to significantly change the flame structure and increase the duration of the combustion event. The effects of particle addition on specific impulse and density impulse were estimated.

Micrometer- and nanometer-scale aluminum (nAl) particles are often considered attractive choices for fuels in energetic materials. In general, reaction rates increase as particle size decreases because of the increased surface area and reduced diffusion lengths between reactants. The oxidation behavior for aluminum nanoparticles >10 nm in diameter has been widely studied, and so has the oxidation behavior of clusters <1 nm in diameter (primarily for catalysis applications). These two regimes exhibit vastly different reaction mechanisms, but there is no experimental work observing the oxidation behavior for intermediate size particles with diameters from 1 to 10 nm. The present study investigates this transition regime by producing unpassivated aluminum particles in this size range using superfluid helium droplet assembly (SHeDA) and then oxidizing the particles by rapidly transferring them from ultrahigh vacuum (UHV) to ambient air. Scanning transmission electron microscopy with energy dispersive spectroscopy (STEM/EDS) and X-ray photoelectron spectroscopy (XPS) showed that particles <4 nm in diameter vaporize upon oxidation while particles >4 nm in diameter do not. We have hypothesized that this is a critical diameter and is the threshold between the oxygen-etching mechanism of clusters and the heterogeneous oxidation of nanoparticles.

No abstract