Enhancing the ignition and combustion performance of metal-based energetic materials

Energetic materials consisting of a fuel and oxidizer are capable of rapidly releasing large amounts of energy through their redox reaction. These materials have a broad range of applications ranging from solid rocket propellants and explosives to heat generators. Metal (e.g., silicon (Si), boron (B), and aluminum (Al))-based energetic materials are attractive due to their high volumetric and gravimetric energy densities. However, these materials exhibit slow energy release rates and are difficult to ignite, preventing them from being leveraged for their superior energy densities in these applications. In this dissertation, three approaches are developed for lowering the ignition threshold and improving the energy release rates of metal-based energetic materials. One approach utilizes the optical flash ignition method which can achieve distributed ignition of energetic materials to reduce ignition threshold and enhance energy release rates. Another approach is to add a metal oxide that supplies additional oxygen to improve oxygen diffusion rates to the fuel so to enhance energy release rates. Finally, creating a unique core/shell nanostructure increases intimate contact, along with a modified micro-emulsion method reduces particle agglomeration, increasing the energy release rates of metal-based energetic materials. First, the optical flash ignition of Si particles is examined, and complementary experimental and numerical studies unveil the impact of particle size and porosity on the ignition properties. Furthermore, a sensitivity analysis is presented, which details the role of the porosity of the Si particle bed in reducing the minimum radiant fluence for ignition. Five common metal oxides (CuO, Bi2O3, MoO3, Co3O4, and Fe2O3) are then surveyed as candidate oxidizers for improving the combustion characteristics of Boron particles, and it is found that binary oxide mixtures (CuO + Bi2O3) are more effective at promoting combustion characteristics of B particles than a single metal oxide. Though these metal oxide mixtures substantially improve the combustion performance of metal fuels, challenges in obtaining homogeneous mixtures and interfacial contact between fuels and oxidizers are prevalent. As a potential solution, we report synthesis and ignition studies of fuel and metal oxide systems (Si/Fe2O3) with a unique core/shell structure, which drastically lowers the ignition threshold compared to the traditionally mechanically mixed Si/Fe2O3. Finally, a modified micro-emulsion method is detailed here, which incorporates particle surface functionalization and pre-dispersion to achieve uniform distribution and contact between an aluminum fuel and polyvinylidene fluoride oxidizer (Al/PVDF). The fluoro-based oxidizer provides an advantage of higher oxidation potential, but Al fuel typically suffers from substantial agglomeration within fluoro-based oxidizers. The novel modified micro-emulsion method was found to reduce ignition delay and improve combustion efficiency. These approaches are applicable in preparing diverse metal-based energetic materials to improve their ignition and combustion properties.

Keywords: Mechanical Engineering.
Advisor: Zheng Xiaolin
Mitchell Reginald 
Tang Sindy (Sindy K.Y.)