Engineering metal-based energetic materials for combustion applications
Keywords: Mechanical Engineering.

Metal-based energetic materials are attractive for combustion applications due to their high energy density, low cost, and low carbon emission. However, the slow and incomplete combustion of metal particles may prevent them from being widely used. The presence of a native oxide shell on the metal particle surface also slows down the oxygen diffusion, resulting in a high ignition threshold for boron (B) and micron-sized aluminum (Al) particles. Additionally, the incompatibility between metal particles and polymers leads to safety concerns and poor combustion performance of metal/polymer composites. To address the above problems, several strategies have been applied in this thesis, including interface engineering of the metal/polymer composites, addition of novel additives such as functionalized graphene, and morphology tuning, which is believed to enable more efficient applications of metal-based energetic materials in space exploration, hypersonic vehicles, thermites, and micro- and nano-devices. Specifically, the following works have been completed: First, interface engineering of B/HTPB composites. B particles are promising fuels for solid propulsion, which can be added into HTPB to form a solid fuel composite. However, the opposite polarities of the B surface and HTPB lead to B agglomerations and weak particle/matrix interfaces within the HTPB matrix. To address these problems, hydrophobic B particles are obtained via surface functionalization with nonpolar oleoyl chloride, which could improve the dispersion and interfacial interaction of B particles within HTPB. The dispersion and distribution of B particles within HTPB matrix are quantitatively imaged by X-ray CT, and the interfacial interaction is evaluated by DMA. As a result, surface-functionalized B/HTPB composites with particle loadings up to 40 wt% exhibit greatly enhanced Young's modulus, toughness, and heat of combustion. Second, functionalized graphene materials are used to lower the ignition threshold and facilitate the energy release of metal combustion. The effects of graphene oxide (GO) and graphene fluoride (GF), and/or their mixtures with Al and B particles are investigated by thermochemical analysis, reactive molecular dynamics (RMD) simulation, and optical ignition and combustion tests. The heat and reactive radicals generated from the disproportionation and dissociation reactions of GO and GF could facilitate the optical ignition of Al and B particles, leading to self-sustained combustion and long thermal and over-pressure events. The gaseous products of the aforementioned reactions reduce the agglomeration of condensed-phase products formed during metal combustion, resulting in higher combustion efficiency. Thermochemical analysis and RMD also suggests that GO and GF have thermal and chemical interactions at elevated temperature, which can further benefit the metal combustion. Third, morphology tuning of Al/CuO thermite composites. Compared to nano-sized particles, micron-sized Al is still desired for applications due to its higher effective Al content and fewer safety risks. However, the high ignition temperature and low reaction rate limit the use μ-Al in energetic applications such as thermite. Two facile synthesis methods are developed to modify the morphology of Al/CuO thermite. As a result, the precipitation method generates porous CuO networks attaching Al surface, which shows the shortest ignition delay time and highest energy release at optimal equivalence ratio. While Al/CuO core/shell particles are synthesized via a displacement method. This core/shell thermite has distinct ignition properties in that its ignition delay is rather insensitive to the equivalence ratio. We believe that similar synthetic strategies can be applied to other thermite and metal-based energetic materials for the tuning of morphology, microstructure, and combustion performance