Fabrication and Characterization of Novel Core-shell Structured Metastable Intermolecular Composites

Abstract

The advancement in micro-electromechanical systems (MEMS) and other engineering applications demands highly efficient, responsive, and controllable energetic materials. Metastable Intermolecular Composites (MICs) consisting of an energetic blend of reducing and oxidizing particles in either a nano or microscale, have thus gained an immense limelight in the past few years. The reducing and oxidizing component of an MIC is also referred to as a fuel and an oxidizer, respectively. MICs belong to the broader category of thermites. Within the MIC family, classifications include conventional metal-based nanothermites, innovative core–shell configurations, 3D ordered macroporous structures (3DOM), layer-by-layer nanolaminates and ternary nanocomposites. Through specialized fabrication methods, it is possible to create any of the above-mentioned architecture to realize enhanced combustion performance while allowing precise control over ignition characteristics and safety measures. Amongst different geometries, the core-shell arrangement, in particular, stands out as a promising microstructure, offering a self-contained reactive system comprising fuel and oxidizer housed within a single assembly. It is however challenging to construct a perfect core-shell assembly wherein each fuel particle is uniformly and completely covered by the oxidizer particles. Therefore, this thesis puts forward, three wet-chemistry synthesis methods for fabricating three novel core-shell structured MICs, namely Al@CuO, Al@NiO and Al@Fe3O4, whereupon Al forms the core and CuO, NiO and Fe3O4 particles form the shell inside the core-shell unit respectively. Wet-chemistry synthesis is shown to overcome the obstacles associated with traditional manufacturing processes, such as incomplete mixing of fuel and oxidizer and phase separation across samples to some extent. These core-shell structured MICs are shown to exhibit significantly enhanced thermochemical behaviors, reduced ignition delay, and homogeneous combustion, which are critical for applications requiring precise energy delivery and minimal disturbance, such as in micro-initiators and welding. The first project embodying this research work was the development of the fabrication process and establishing the properties of spherical core-shell Al@CuO MICs. This study revealed that synthesis parameters, especially ammonia content, critically influenced the structure of the final product. This manipulation allowed the transition between a well-mixed nanocomposite and individual nanosized core-shell spheres at NH3/Cu ratio of 6.0. Notably, these Al/CuO core-shell nanoparticles demonstrated a reduction in both onset and peak combustion temperatures by 8 ℃ and 20 ℃ respectively, alongside a decreased activation energy by 20 kJ/mol when compared to physically mixed counterparts, indicating improved efficiency and reactivity due to the optimized proximity between fuel and oxidizer. The second project focused on application of a similar wet-chemistry based one-pot synthesis process to Al/NiO duo. The Al@NiO MIC was found to be relatively easier to fabricate since the core-shell structure was not found to be as sensitive to the synthesis parameters and showcased the initiation temperature and the content of energy release from the reaction between Al and NiO was in the same ballpark for various samples of different equivalence ratios and NH3/Ni ratios. These composites showed exceptional ability to be combusted without a significant delay to ignition after the laser was triggered. This could be attributed to the efficient thermite and subsequent alloy-formation reactions facilitated by the core-shell configuration. This structure not only reduced the activation energy for the thermite reaction but also enabled a rapid and complete combustion, outperforming physically mixed composites. For both Al@CuO and Al@NiO, electrostatic force was deduced to be the driving force behind the formation of these core-shell assemblies. Then the research was further advanced to Al@Fe3O4 MICs since magnetite (Fe3O4) was hypothesized to serve a dual role as an oxidizer and a functional ferrimagnetic component, thereby imparting an extra degree of freedom to the resulting composite that could be leveraged in unique applications. The previous wet-chemistry route was found to be inapplicable in this case since Fe did not form a coordination complex ion with NH3 to trigger a core-shell assembly around the negatively charged Al particles. A novel fabrication process leveraging the process of crystallization was invented. Fe3O4 shell was constructed by initiating the decomposition of Fe-salt into Fe3O4 crystallites that used Al nanoparticles as seed to nucleate upon. The fabricated core-shell structure resulted in a significant decrease in activation energy and a shorter ignition delay compared to physically mixed samples. The magnetic nature of these composites allowed for controlled transport and delivery, enhancing their application scope. Collectively, these studies highlighted the potential of core-shell structured MICs in refining the performance of energetic materials for industrial as well as engineering utilizations. The findings demonstrated that wet-chemistry synthesis routes can effectively produce advanced energetic materials with superior combustion properties, offering a promising avenue for the development of more efficient and reliable energetic systems.