Viable alternatives for ammonium perchlorate (AP) are needed to address growing environmental concerns about soil and groundwater contamination and subsequent effects especially on human health. Finding a viable replacement for AP is a significant challenge, because this workhorse oxidizer has many tremendous physical and safety attributes and, more importantly, high delivered performance in many rockets and munitions.
The objective of this SERDP Exploratory Development (SEED) project was to design, synthesize, and characterize novel energetic oxidizers that pose less threat to human health and the environment than AP, yet have the potential to replace it in propulsion system applications. Specifically, novel salts based on quaternary ammonium cations were investigated.
This project investigated new classes of oxygen-rich, non-halogen, hydrolytically stable materials, focusing on several novel oxidizers. These compounds had relatively straightforward, short route syntheses from commercially viable materials. Reasonable thermal stabilities and sensitivities were found (impact values between RDX and PETN) for several materials. Research relied upon parallel and integrated synthetic and theoretical efforts focused on understanding how the molecular structure of new, energetic-charged quaternary ammonium species incorporating pendant groups affected the overall stability as well as the physical and inherent safety properties. These theoretical studies investigated many proposed structures to help prescreen potential synthetic target materials. Those found by calculations to be rather unstable were not investigated. Molecular dynamics simulations were used to study interactions between commonly used energetic materials and to determine how these mixtures responded to external stimuli. Density functional theory, ab initio, and composite methods were employed, based on the complexity of the system. The calculations were performed on Major Shared Resource Center platforms at the Air Force Research Laboratory and the Army Research Laboratory. In the experimental effort, synthesis and characterization were carried out at the Aviation and Missile Research, Development, and Engineering Center.
New classes of robust nitrate esters have been created from quaternary ammonium salts as well as several neutral nitro substituted heterocycles in a quest of obtaining state-of-the-art (SOTA)materials to potentially replace ammonium perchlorate. A method that could quickly, cost-effectively, and reliably predict the thermal stabilities of organic salts with nitroxy-functionalized cations based upon a hypothesis that a salt’s stability would correlate with the Gibbs free energy difference [ΔGr(298)] between it and expected products of its first decomposition step was examined. Observations of thermal stability were assembled for a number of relevant salts, and computationally based methods were employed to estimateΔGr(298) values for them. Salts were considered to be thermally stable if they had been observed to lose <1% of their mass after being held at 75°C for 24 h, or if their decomposition was not observed to begin in a differential scanning calorimetry experiment at a temperature below 150°C. Salts meeting these criteria usually have negative ΔGr(298) values calculated for them while those salts that do not meet the above criteria usually have positive ΔGr(298) values. The results indicate that it is not likely that nitrate salts with cations having more than two NOx groups will prove to be stable for 24 h at 75°C. Finally, the synthesis, characterization, safety properties, and ingredient compatibility and initial formulations involving some of these materials were investigated.
Successful completion of this project will open a large new class of oxygen-rich, less sensitive materials with a wide array of potential applications. This technology may potentially impact future fielded weapons that use AP, including those using high performance propellants.