Additive Manufacturing and Combustion of Graphene Based Nanothermite Aerogels

Abstract

Nanothermites are a key material for engineering applications related to energy and heat release, such as welding, heating, and pyrotechnics. The ability to additively manufacture energetic nanothermite structures with desired combustion properties is severely limited by current technology. The reactions are often slow to propagate and release less energy than desired due to polymer binders that act as an energy sink for the reaction but are required to support the energetic material. Additionally, the controllability and tuning of nanothermite reactions are limited by the lack of a conclusive understanding of the reaction mechanisms that govern the reactions of these materials. Several different reaction mechanisms have been proposed without any conclusive evidence to support the claims. In this work we fabricate a polymer-free, reduced graphene oxide (rGO)-based nanothermite aerogel with a range of nanoparticle loadings using a novel additive manufacturing process. The nanothermites analyzed in this work are aluminum iron-oxide (Al/Fe2O3), aluminum copper-oxide (Al/CuO), and aluminum bismuth-oxide (Al/Bi2O3). SEM images demonstrate the unique porous structure formed by the thin rGO sheets which form a skeletal structure that wraps the nanoparticles into individual nanothermite clusters. EDS confirms the homogeneity of the overall structure of the aerogel and the nanothermite clusters, demonstrating that the particles are well dispersed when printing. DSC-TGA results and high-speed combustion videos confirmed the enhanced energetic performance of the printed specimen, suggesting the important role of rGO compared to conventional printing methods. A relatively high linear burn rate of 5.8m/s was demonstrated for rGO/Al/metal oxide samples with a diameter of 1.6mm at 95% nanothermite loading by mass. It was also shown that the propagation rate of the reaction was independent of the print direction. Thermal camera footage clearly indicated the generation of the pre-heating zone, reaction front, and cooling zone during the propagation. SEM, EDS, and TEM are used to analyze the post-combustion material, whose collection is enabled by the novel graphene structure of the material. By analysing the combustion product within the skeletal fragments of the sample the reaction mechanisms for three nanothermite pairings are found and discussed. It is found that in the small nanothermite clusters several previously proposed reaction mechanisms occur concurrently. Diffusion and melt dispersion, which appear to be mutually exclusive in literature, are shown to be occurring concurrently. Overall, this method allows for complex 3D printing fabrication of various rGO/nanoparticle aerogels, while giving insights as to how the reactions propagate via a series of reaction mechanisms depending on the metal oxide used for the reaction.