Energetic composites are mixtures of solid fuel and oxidizer particles that, when combined, offer higher calorific output than monomolecular explosives. The composites traditionally deliver energy as diffusion limited reactions and, thus, their power available from reaction is much smaller than any explosive.

Technology, however, has advanced particle synthesis, and nanoparticles have become more readily available. The advent of nanoparticle fuels enables traditionally diffusive controlled reactions to transition towards kinetically dominant reactions. This transition results in faster reacting formulations that show promise of harnessing the power equivalent to a monomolecular explosive but packaged as discretely separate fuel and oxidizer composites.

This research focuses on developing an understanding of fundamental reaction dynamics associated with particulate media, in general. Once this foundational understanding is established, new strategies for designing aluminum fuel particles toward greater reactivity and, thus, faster reacting formulations will be presented. In addition to synthesis, several combustion characterization techniques will be examined to quantify combustion performance. All of this information will provide a basis for future research and applications involving aluminum-based fuels in any energetic system (i.e., as an additive to liquid propellants or even explosive formulations).

Composite energetic materials with nanoscale aluminum particles play a significant role in nearly every sector of an energy generating economy from industrial to ordnance technologies. Nanoscale aluminum fuel particles hold numerous advantages over their micron scale counterparts. Fluoropolymers have been gaining popularity over the last decade as a favored oxidizer in these composite systems because of their unique ability to react with the passivating alumina shell present over aluminum particles.

This research investigates the tailorability of energetic composites made of nano aluminum (Al) combined with different fluoropolymers, by incorporating different additives into the reactive material. Diffusion controlled reactions are limited by the proximity (i.e., diffusion distance) of reactant particles. The effect of the proximity of the oxidizer was also investigated by performing flame propagation experiments on molybdenum trioxide (MoO3) combined with aluminum particles with and without surface functionalized perfluoro tetradecanoic (PFTD) acid. Results showed that the surface functionalization enhanced the burn rate twice that of non functionalized energetic composite.

In order to control the burn velocity by altering their surface functionalizations, three different energetic composites consisting of aluminum particles with and without surface functionalization, combined with molybdenum trioxide were performed. Perfluoro tetradecanoic (PFTD) and perfluoro sebacic (PFS) acids were used to form organic corona around the aluminum nanoparticles. Flame propagation studies revealed that energetic composites made of Al functionalized with PFTD (Al PFTD) displayed burn velocity 86% higher than Al/MoO3, whereas Al with PFS/MoO3 are almost half of Al/MoO 3. Results showed that the fluorine content in the acids and their structural differences contribute to difference in burn velocity.

This work was done by Michelle Pantoya of Texas Tech and Keerti Kappagantula of Ohio University for the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) below. ARL-0232

This Brief includes a Technical Support Package (TSP).
Fast Reacting Nano-Composite Energetic Materials: Synthesis and Combustion Characterization

(reference ARL-0232) is currently available for download from the TSP library.

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This article first appeared in the December, 2020 issue of Aerospace & Defense Technology Magazine.

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