Minor Additives, Major Impact: How Small Quantities of Metal Additives Can Influence the Stability and Performance of Energetic Materials

Many promising energetic materials fall short of their maximum theoretical potential due to high ignition temperatures, molten agglomerates that promote two-phase flow losses, poor combustion stability, and so forth. Additive materials such as lithium in aluminum or hydrocarbons in ammonia can help improve performance, but these additives introduce challenges of their own. For example, lithium in an aluminum-lithium alloy increases the specific impulse of a rocket motor and reduces agglomerate sizes through micro-explosions, but large quantities of lithium make the propellant susceptible to premature oxidation and age poorly over time. Similarly, hydrocarbon additives to ammonia improve combustion performance and militate against flame extinction, but they also decrease some of ammonia’s more attractive properties, such as its high hydrogen density, heat capacity, and low carbon content. This suggests that a balance must be made between improving the energetic properties of a material while mitigating any negative side-effects of the additive itself.

Therefore, the objective of this research was to investigate the effects that small quantities of gallium, indium, and lithium have on aluminum alloys in solid propellant formulations and the effects that small quantities of lithium, sodium, and calcium have on certain unique liquid propellant formulations.

The first area of research discusses methods used to fabricate aluminum-gallium and aluminum-indium alloy powders using a ball-mill and then examines their morphology, composition, oxidation behavior, and onset temperature in a slow-oxidation environment. Building on this work, these alloy powders were introduced into an ammonium perchlorate composite propellant formulation where their theoretical performance was calculated, and their experimental burning rate and condensed combustion products were measured.

This first area of research examined how gallium and indium may make aluminum more reactive through active oxide-disruption mechanisms and how this improvement affects the performance of an energetic formulation. However, aluminum-lithium (Al-Li) alloys suffer from the opposite problem, where their thermal instability and pre-mature oxidation near ambient conditions are a concern for practical viability. Therefore, the second area of research focused on the low-temperature oxidation of Al-Li alloys and then developed a theoretical understanding of their passivation behavior. Combining the results and lessons learned aluminum-gallium/indium/lithium alloys, a more complete understanding of aluminum alloy oxidation can be made. Particularly, that even small quantities of metal additives can have a significant effect on the thermal stability, oxidation behavior, and energetic performance of the alloy in propellant formulations.

The final area of research deviates from solid propellant formulations. Learning from the significant impact that lithium can have on the stability and performance of aluminum metal in solid propellant formulations, this last area of research expands this application to liquid propellants. Specifically, the energetic effects of small amounts of metals additives dissolved into certain liquid fuels to form highly esoteric solvated metal solutions that are hypergolic with white-fuming nitric acid.