Molecular aluminum and nitrocellulose composite additives for burn rate enhancement of liquid propellants

Hydrocarbon fuel additives are frequently investigated to change the combustion dynamics, chemical distribution, and/or product integrity while adding metal or metal oxide nanoparticles has been shown to increase the volumetric energy density, decrease ignition delay, increase heat of combustion, or catalyze fuel decomposition in recent research. Energetic metal nanoparticles however are prone to aggregation, which occurs at an increased rate near the regressing surface of a burning liquid droplet and can form a transport-inhibiting shell and decrease the droplet burning rate. As a soluble alternative to nanoparticle energetic additives, here we employ a novel aluminum-based molecular additive, Al(I) tetrameric cluster [AlBrNEt3]4 (Et = C2H5), to a hydrocarbon fuel and evaluate the resultant single-droplet combustion properties. Results show the [AlBrNEt3]4 additive to increase the burn rate constant of a toluene-diethyl ether fuel mixture by ∼20% in a room temperature oxygen environment with only 39mM of active aluminum additive (0.16 wt% limited by additive solubility). In comparison, a roughly similar addition of nanoaluminum (nAl) particulate shows no discernable difference in burn properties of the same hydrocarbon fuel. High speed video shows the [AlBrNEt3]4 to induce microexplosive gas release events during the last ∼30% of the droplet combustion time. We attribute this to HBr gas release based on results of Time-of-Flight Mass Spectrometry (TOFMS) experiments of the [AlBrNEt3]4. A possible mechanism of burn rate enhancement is presented that is consistent with microexplosion observations and TOFMS results. For cases when higher energetic material loadings are desired, Nitrocellulose (NC) is also investigated as a companion additive for particulate nAl. Up to a 12.1% decrease in the burn rate constant of Kerosene droplets when 6.1 wt% nanoaluminum (nAl) particles are added (the maximum stable loading) is observed; however, addition of NC particles diminishes or fully counteracts the burn rate decreases and provides means of tuning the burn rate constant higher than that of pure Kerosene (maximum 13.8% increase over control with 2.3 wt% nAl added). To achieve stable nanofuels at higher particle loadings up to 12.3 wt% nAl, NC and nAl were electrosprayed into composite mesoparticles (MP) before suspending with surfactant in Kerosene. These MP-based nanofuels boast increased dispersibility and energetic loadings and thusly higher achievable burn rates (maximum 26.5% increase over control) than physically mixed analogs. A mechanism whereby NC addition promotes gas ejections which can disrupt agglomerate shell formation and transport nanoparticles from the droplet to the flame zone to increase the burn rate is proposed.