Tailoring Material Properties: Synthesis, Characterization, and Interface Engineering of Composite Structures and Metal Fuels for Energy Applications

Aluminum (Al) particles are a highly efficient source of potential chemical energy with widespread applications ranging from solid fuels to batteries. The core-shell structure of Al particles with a highly passive alumina (Al2O3) shell restricts the oxidation diffusion mechanism and limits its reactivity. Shell modification by replacing the passive shell with a favorable alternative is a highly promising approach to overcome this limitation. Hydrating the Al particles allows the dissolving of the original Al2O3 shell and forming a bayerite (Al(OH)3) layer with a fuzzy surface and lower energy barrier. The goal of this study is to modify the shell properties of composites and metal fuels such as Al as a function of particle size to increase reactivity in power generation applications. To achieve this goal, a solid understanding of the reaction mechanism is necessary which will dictate favorable modification methods for the shell. Different types of shell modification approaches can provide advantages for different types of applications. For this study, the first objective is to identify the surface reaction mechanism of hydrated nano vs micron-size metal particles. The second objective is to explore the influence of surface hydration on metal-core oxidation. The last objective is to introduce a new oxygen-rich shell passivation strategy for power generation applications. The hydration kinetics for nano Al (nAl) are different from micron-sized Al (µAl) because of a 91% difference in their surface energy. While it is possible to hydrate nAl particles with a pH alteration method, the process does not extend to micron-sized particles (µAl). Instead, the µAl powder achieves similar hydration by only controlling solution temperature and time. The surface reaction mechanism as a function of particle size is explored in this study using XRD, SEM, TEM, EDS, DSC, and TGA. Accelerated aging tests, combined with thermal analysis, were performed to evaluate the reactivity of surface-hydrated aluminum particles. Additionally, safety assessments, including electrostatic discharge (ESD), impact, and friction sensitivity tests, were conducted to determine their viability for energy generation applications. The hydration mechanism was regulated to control the hydration concentration from 3 to 91% to explore the thermochemistry underlying (de)hydration and oxidation. The oxidation-melting enthalpy data from DSC reveals an 89% increase in apparent oxidation efficiency by modifying the accessibility to the core for hydrated compared to standard Al. This change in oxidation kinetics can be attributed to the newly developed porosity-created voids and surface structure on the dehydrated surface of the previously hydrated shell. Upon dehydration, the energy barrier reduces and the apparent activation energy for oxidation is lowered by 29% for hydrated Al. Density functional theory calculations were performed in combination with experimental data for a deeper understanding of the reaction mechanism. The successful shell modification by hydration leads to the passivation strategy of halogen-based oxygen-rich aluminum iodate hexahydrate (AIH) shell on Al particles. The presence of abundant oxidizing species in close molecular proximity to the Al core, combined with the relatively low decomposition energy of AIH, created optimal conditions to enhance the energy output of Al particles. Recent studies on replacing the original Al2O3 shell with AIH on nAl were confirmed as highly reactive with a flame speed of 3200 m/s. However, this composite is highly ignition sensitive and unsafe to handle as nAl itself has a low ignition threshold due to its high surface energy. Also, the limited control of AIH concentration on nAl restricts its intended application. The µAl powder with 91% lower surface energy is a suitable alternative to extend the AIH-based shell modification approach. This study utilized µAl particles to synthesize a controlled concentration of the AIH shell (µAl@AIH) by controlling three variables—H2O to I2O5 solution ratio, solution temperature, and time in solution. The oxidation-melting enthalpy data from DSC shows better core accessibility with a 24% increase in apparent oxidation efficiency compared to standard µAl. Accelerated aging studies reveal both physical and chemical transformation of the AIH shell that leads to a 1992% higher apparent oxidation efficiency than the standard µAl. The apparent activation energy for unaged and aged µAl@AIH powder was found to be 16–34% lower than standard µAl, corresponding to the key thermodynamic changes in equilibrium and non-equilibrium conditions. The findings presented in this study open new doors for developing advanced metal fuels with enhanced energy release capabilities. Unlike traditional particles restricted by a nearly impenetrable passivation layer that limits reaction kinetics, this approach overcomes those limitations, enabling rapid and sustained power generation.