Sensitive nanoenergetic powders, such as nanothermites, have traditionally been processed by ultrasonic mixing of very low solids loaded suspensions in organic solvents, which has restricted their use and application due to high solvent content and associated handling issues. In this work, we report on the performance and mixing quality of nanothermite mixtures prepared in a LabRAM resonant mixer at high solids loadings as compared to ultrasonic mixing. Specifically, the aluminum‐bismuth(III) oxide (Al/Bi2O3) system processed in the polar solvent N,N‐dimethylformamide (DMF) was investigated. It was found that the performance and overall quality of mixing was strongly correlated to the volumetric solids loading during processing; increasing volumetric solids loading decreases separation of particles, leading to more particle interaction and more intimate mixing. The measured performance of this system processed at 30 vol‐% was similar to traditionally ultrasonicated mixtures. Increasing the solids loading above 30 vol‐% yielded diminishing returns in performance and may introduce additional safety concerns since dry powders are very sensitive to electrostatic discharge. This mixing approach uses significantly less solvent than traditional ultrasonic mixing, results in a higher density final material, and is amenable to scaling. In addition, solvent wetted nanothermite mixed at 30 vol‐% solids loading can be mixed and deposited from a single applicator and was observed to be over five orders of magnitude less sensitive to electrostatic discharge than dry powders. This relative insensitivity enables the safe deposition of high density nanothermite ink onto devices.

Hydrochloric acid (HCl) pollution from perchlorate based propellants is well known for both launch site contamination, as well as the possible ozone layer depletion effects. Past efforts in developing environmentally cleaner solid propellants by scavenging the chlorine ion have focused on replacing a portion of the chorine-containing oxidant (i.e., ammonium perchlorate) with an alkali metal nitrate. The alkali metal (e.g., Li or Na) in the nitrate reacts with the chlorine ion to form an alkali metal chloride (i.e., a salt instead of HCl). While this technique can potentially reduce HCl formation, it also results in reduced ideal specific impulse (ISP). Here, we show using thermochemical calculations that using aluminum-lithium (Al-Li) alloy can reduce HCl formation by more than 95% (with lithium contents ≥15 mass%) and increase the ideal ISP by ∼7s compared to neat aluminum (using 80/20 mass% Al-Li alloy). Two solid propellants were formulated using 80/20 Al-Li alloy or neat aluminum as fuel additives. The halide scavenging effect of Al-Li propellants was verified using wet bomb combustion experiments (75.5±4.8% reduction in pH, ∝ [HCl], when compared to neat aluminum). Additionally, no measurable HCl evolution was detected using differential scanning calorimetry coupled with thermogravimetric analysis, mass spectrometry, and Fourier transform infrared absorption.

The microexplosive nature of multicomponent liquid fuels has been both studied and fielded to decrease droplet residence times and increase completeness of combustion. However, little work has focused on investigating microexplosive metal fuels to enhance the metal fuel combustion efficiency in traditional energetic material formulations. Microscopic surface videography was performed on two solid propellant formulations, one using aluminum (baseline) and the other with 80/20 wt.% Al-Li alloy as fuel additives. It was observed that the propellant combustion with neat aluminum formed large molten droplets at the surface as aluminum particles agglomerate, which is a well-known problem with aluminized propellants. In contrast, the Al-Li propellant formed an Al-Li melt-layer on the propellant surface during combustion. Droplets were ejected from the surface melt-layer through dispersive boiling. Above the surface, further dispersive boiling is observed from the ejected droplets and droplet-shattering microexplosions are also observed. These dynamics are thought to be a result of a large disparity in volatility (i.e., boiling points) between the metals in the molten alloy and the large Lewis number in the droplet, so that superheating occurs before the more volatile component (here Li) can diffuse to the surface. A Lewis number of 7440 was estimated for molten 80/20 wt.% Al-Li alloy, which is nearly three orders of magnitude larger than typical multicomponent liquid hydrocarbon droplets that microexplode, suggesting a higher propensity for molten droplet microexplosions. This would also indicate that a smaller amount of the volatile component might be necessary for microexplosions and dispersive boiling than observed for liquid hydrocarbon fuels. These dynamics are important for metal fuel applications, because injectors cannot be used to decrease droplet size in a metallized energetic material formulation.

Due to its thin passivation layer, potentially good aging characteristics, and ease of surface functionalization nanoscale silicon (Si) may offer some advantages over nanoaluminum as a reactive fuel in nanoenergetic compositions, particularly with fluorine‐based oxidizers. Currently, Si nanopowder can be quite expensive and the quality of commercial powders has been found to vary drastically. As a result limited efforts have focused on the role of specific surface area, active content, morphology, and dominant particle size of the powder have on the combustion performance. In this work we report the effect of such characteristics on the combustion of silicon (Si)/polytetrafluoroethylene (Teflon)/FC‐2175 (Viton) (SiTV) nanoenergetics. A cost effective combustion synthesis route, salt assisted combustion synthesis, was used to produce several Si powders and these were directly compared to commercial nanoscale Si powders. Reactive mixtures of SiTV were burned at atmospheric conditions and burning rates, combustion temperatures, spectral intensities, and effective plume emissivities were measured. Measured combustion temperatures ranged from 1664 to 2380 K and were limited by Si powder active content. This was found to drive plume emissivity and maximum spectral intensity, which had values ranging from 0.10 to 0.55 for effective plume emissivity and 17.6 to 48.1 kW m−2 sr−1 μm−1 for maximum spectral intensity. Burning rates ranged from 0.7 to 3.4 mm s−1 and were found to be dependent on the dominant particle size of the powder. Powders synthesized with salt assisted combustion resulted in comparable burning rate, plume emissivity and maximum spectral intensity to porous Si powder (Vesta Ceramics).

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The burning rate of solid propellants is an important factor for optimizing rocket motors and improving performance. The enhanced burning rate can increase thrust and reduce a propulsion system's overall size and weight. In this study, a novel nano-aluminum/THV composite additive was prepared and introduced into a solid ammonium perchlorate/polybutadiene composite solid rocket propellant to enhance its burning rate. The morphology of the composite particle additive and its effects on combustion were characterized. The use of small quantities (< 15 wt.%) of the additive resulted in a burning rate enhancement of up to 2.1 times that of the conventional coarse aluminized propellant with a specific impulse loss of only 3 seconds, and as much as 4.7 times enhancement with a predicted loss of 22 seconds in theoretical specific impulse. Some of this loss may be recovered by the improved combustion efficiency in smaller rocket motors because the additive was shown to significantly reduce the aluminum agglomeration at the propellant burning surface and reduce the size of reaction products which may reduce two-phase flow losses. The additive also provides wide burning rate tailorability, favorable for motor, grain, and thrust curve design. The burning rate enhancement mechanism is thought to be a physical cratering mechanism governed by the burning rate disparity between the binder/oxidizer system and the nano-aluminum/fluoropolymer additive and not a chemical catalytic effect.