Solid propellants are a class of energetic (combustible) materials that undergo a rapid exothermic chemistry when an activation barrier is overcome. Applications for such energetic materials can be found in space exploration technologies, automobile airbags, material synthesis, ordinance and rare-earth mining. To broaden their applications and capability, various studies have been conducted to safely and controllably release their stored potential energy. These studies have mainly focused on manufacturing techniques to tailor the properties of energetic materials to release their stored potential energy in a predictable fashion. Other studies have focused on the use of electromagnetic stimulation in ultraviolet (UV), visible (vis) and near-infrared (NIR) regions to control ignition and combustion of propellants in-operando; however, the UV-NIR electromagnetic stimulation was observed to be limited to surface level absorption due to the inherent high photon attenuations of the solid propellants. In contrast to UV-NIR electromagnetic radiation, microwave radiation has shown to display rapid, selective and volumetric heating to control and modulate energy release pathways in solid propellants.

In this work, we first explore mechanisms that control the ignition of energetic materials at microwave frequencies. High-speed color camera imaging, infrared pyrometry, temperature jump (T-jump) ignition and differential scanning colorimetry methods are used to understand the mechanisms driving ignition in 3D printed nanoscale titanium (nTi)/ polyvinylidene fluoride (PVDF) films. This work is further expanded to engineer material systems with controllable ignition under stimulation at 2.45 GHz, where manganese oxide (MnOx) was studied as an oxidizing microwave agent. Beyond research on microwave ignition, the effect of microwave heating during in-operando combustion of reactive materials is investigated. First, heat transfer mechanisms influencing the combustion of energetic materials were experimentally studied via microscope imaging and pyrometry tools. Subsequently, infrared thermometry and color ratio pyrometry were employed to study microwave heating rate, flame front and propagation velocity of gasless Al-Zr-C composites. The research explores mechanisms that drive and impede response of solid propellants to microwave energy prior to ignition as well as throughout rapid energy release.

Although sustainable thermal energy storage and conversion approaches are becoming increasingly popular, high-power requirements (~107 kWh) in certain energy applications such as spacecraft and jet propulsion, can only be accomplished through direct conversion of chemical to thermal energy through combustion. The current research objective in this field is to alleviate the kinetic and mass transfer limitations impeding rapid energy extraction from solid state fuels possessing high energy density. Part of the dissertation focuses on solving such challenges for boron particles and boron containing complexes, as elemental boron has the highest energy density. This dissertation provides mechanistic insights on selectively altering the thermochemical decomposition pathways of boron containing solid-state borane-ammonia complex with the help of a) ammonium-ion based oxidizing salts and b) polymeric carbonyl groups, to facilitate rapid and complete oxidation of boron in the borane-ammonia complex, which otherwise gets kinetically trapped into boron-nitrogen-hydrogen clusters. The dissertation also provides atomic scale understanding of the manipulation of boron oxide shell by implanting magnesium atoms, as a potential strategy to enhance reactivity of boron particles. Magnesium simultaneously reduces boron oxide creating dangling bonds on boron and induces a net tensile strain on the boron oxide surface, which respectively enhances the adsorption and diffusion rate of oxygen through the boron oxide shell, collectively enhancing the oxidation rate of boron. Significant part of the dissertation also focuses on tuning the reactivity of composites containing aluminum, another high energy density and widely explored solid fuel, by alteration of micro- and meso-structural features, as well as the incorporation of microwave absorbing sensitizers in aluminum-based composites for spatial confinement of the ignition zone when activated using microwave radiation. As a major breakthrough, this dissertation also demonstrates that certain non-flammable condensed phase materials with high energy density such as imidazolinium-ionic liquids can be electrochemically made inflammable and their flame can be dynamically extinguished simply by removing the voltage bias driving the electrochemical reactions. Since most energy dense fuels pose the danger of unintended fire and explosion, this concept paves the path for the development of potentially ‘safe’ fuels, thereby opening multiple avenues for future research.

Energetic materials convert chemically stored potential energy to kinetic energy through combustion. A current focus within this field is to address the constraints related to mass transfer, which hinder the rapid release of energy from solid-state energetic materials. Nanomaterials have emerged as a promising avenue for overcoming these limitations by reducing the distance between fuels and oxidizers, thereby increasing energy release rates. This dissertation focuses on understanding reaction mechanisms and combustion behavior of solid nanoenergetic composite materials by tuning chemical and physical properties of the components as well as manipulating heat transfer of the composites. Specifically, it sheds light on the crucial role of oxidizer physical properties in influencing the ignition behavior of nanoscale boron, as well as the dominant effect of physical properties of fuels and their corresponding oxides on the microscopic combustion characteristics of composites containing different fuels. A significant portion of this dissertation focuses on tuning the energy release rate of nanoenergetic composites by manipulating heat transfer through various approaches, including: a) Altering the equivalence ratio between fuel and oxidizer, b) Decreasing agglomerate surface tension using an additive, and c) Incorporating carbon fiber to intercept and retain hot agglomerates near the burning surface. These approaches share a common objective of controlling the residence time of agglomerates on or near the burning surface, with longer residence times resulting in increased heat feedback and, consequently, higher energy release rates. Additionally, this dissertation explores the development of energetic biocidal agents using a series of metal iodates by studying their decomposition mechanisms and combustion behavior of assembled composites with nanoscale aluminum as the fuel.