The production of the next generation of hypergolic, ionic-liquid-based fuels requires an understanding of the reaction mechanisms between the ionic liquid and oxidizer. We probed reactions between a levitated droplet of 1-methyl-4-amino-1,2,4-triazolium dicyanamide ([MAT][DCA]), with and without hydrogen-capped boron nanoparticles, and the nitrogen dioxide (NO) oxidizer. The apparatus exploits an ultrasonic levitator enclosed within a pressure-compatible process chamber equipped with complementary Raman, ultraviolet-visible, and Fourier-transform infrared (FTIR) spectroscopic probes. Vibrational modes were first assigned to the FTIR and Raman spectra of droplets levitated in argon. Spectra were subsequently collected for pure and boron-doped [MAT][DCA] exposed to nitrogen dioxide. By comparison with electronic structure calculations, some of the newly formed modes suggest that the N atom of the NO molecule bonds to a terminal N on the dicyanamide anion yielding [ON-NCNCN]. This represents the first spectroscopic evidence of a key reaction intermediate in the oxidation of levitated ionic liquid droplets.

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.

Addition of high-energy-density materials such as aluminum (Al) microparticles or nanoparticles to liquid propellants potentially improves performance of the fuel. We report on the effects of untreated, prestressed, and superquenched aluminum particles with diameters of 100 nm, 250 nm, 500 nm, 1.6 μm, and 8.8 μm on the combustion of JP-10 droplets acoustically levitated in an oxygen–argon atmosphere. Ignition was initiated by a carbon dioxide laser, and the resulting oxidation processes were traced by Raman, Fourier-transform infrared (FTIR), and ultraviolet–visible (UV–vis) spectroscopies together with high-speed optical and IR thermal-imaging cameras. The UV–vis emission spectra reveal that the key reactive radical intermediates hydroxyl (OH), methylidyne (CH), dicarbon (C2), aluminum monoxide (AlO), and aluminum monohydride (AlH) were formed in addition to atomic aluminum (Al) and the final oxidation products of JP-10, namely, water (H2O) and carbon dioxide (CO2). The Al particles facilitated ignition of the JP-10 droplets and produced higher temperatures in the combustion process of up to typically 2600 K. The effect of the Al particles on the ignition and maximum flame temperatures increased as the diameters reduced. The different stress treatments did not produce observable changes for the ignition or combustion of the droplets, which indicates that the liquid propellant was not significantly affected by manipulating the mechanical properties of the fuel particle additive. The initiation and enhancement of the combustion were a consequence of forming highly reactive atomic oxygen (O) and aluminum monoxide (AlO) radicals in the reaction of aluminum atoms with molecular oxygen in the gas phase. These radicals initiate the degradation of JP-10 via atomic hydrogen abstraction forming the hydroxyl (OH) and aluminum hydroxide (AlOH) radicals in reactions which are mainly exothermic by up to 68 kJ mol–1. In contrast, hydrogen abstractions from JP-10 by molecular oxygen or atomic aluminum are strongly endothermic by up to 236 kJ mol–1, thus making these reactions less competitive. The generation of C10H15 hydrocarbon radicals from the JP-10 initiates successive oxidations and chain reactions with molecular oxygen leading eventually to carbon dioxide and water. These combined experimental results provide insight into how aluminum particles facilitate the oxidation and reaction mechanisms of JP-10 droplets.

We describe an original apparatus comprising of an acoustic levitator enclosed within a pressure-compatible process chamber. To characterize any chemical and physical modifications of the levitated particle, the chamber is interfaced to complimentary, high-sensitivity Raman (4390-170 cm(-1)), and Fourier transform infrared (FTIR) (10,000-500 cm(-1)) spectroscopic probes. The temperature of the levitated particle can be accurately controlled by heating using a carbon dioxide laser emitting at 10.6 μm. The advantages of levitating a small particle combined with the two spectroscopic probes, process chamber, and infrared laser heating makes novel experiments possible relevant to the fields of, for example, planetary science, astrobiology, and combustion chemistry. We demonstrate that this apparatus is well suited to study the dehydration of a variety of particles including minerals and biological samples; and offers the possibility of investigating combustion processes involving micrometer-sized particles such as graphite. Furthermore, we show that the FTIR spectrometer enables the study of chemical reactions on the surfaces of porous samples and scientifically and technologically relevant, micrometer-thick levitated sheets. The FTIR spectrometer can also be used to investigate non-resonant and resonant scattering from small, irregularly-shaped particles across the mid-infrared range from 2.5 μm to 25 μm, which is relevant to scattering from interplanetary dust and biological, micrometer-sized samples but cannot be accurately modelled using Mie theory.