ENHANCING THE COMBUSTION CHARACTERISTICS OF ENERGETIC NANOCOMPOSITES THROUGH CONTROLLED MICROSTRUCTURES
Zachariah, Michael R
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Metastable Intermolecular Composites (MIC’s) are a relatively new class of reactive materials which, through the incorporation of nanoscale metallic fuel and oxidizer, have exhibited multiple orders of magnitude improvement in reactivity. Although considerable research has been undertaken, their reaction mechanism is still poorly understood, primarily due to the complex interplay between chemical, fluid mechanic and thermodynamic processes that happen rapidly at nanoscale. For my dissertation, I have attempted to tackle this problem by employing controlled nanomaterial synthesis routes and optical diagnostics to identify the dominant underlying mechanisms. I begin my investigation by examining the nature of metal nanoparticle combustion wherein, I employed laser ablation to generate size- controlled aggregates of titanium and zirconium nanoparticles and studied their combustion behavior in a hot oxidizing environment. The experiments revealed the dominant role of rapid nanoparticle coalescence, before significant reaction could occur, resulting in a drastic loss of nanostructure. The large-scale effects of sintering on MIC combustion was explored through a forensic analysis of reaction products. Electron microscopy was employed to evaluate the product particle size distributions and focused ion beam milling was used to expose the interior composition of the product particles. The experiments established the predominance of condensed phase reaction at nanoscale and the interior composition revealed the poor extent of reaction due to rapid reactant coalescence before attaining completion. In light of such limitations, the final part of my dissertation proposes a solution to counteract rapid, premature coalescence through the synthesis of smart nanocomposites containing gas generating (GG) polymers. The GG acts as a binder as well as a dispersant, which disintegrates the composite into smaller clusters prior to ignition, thereby avoiding large scale loss of nanostructure. High speed optical diagnostics including an emission spectrometer and a high-speed color camera pyrometer were developed to quantify the enhanced combustion characteristics which indicate an order of magnitude improvement in reactivity over counterparts using commercial nanomaterials. Moreover, thermal pretreatment as a possible bulk processing strategy to improve nanoaluminum reactivity in a MIC is examined, where a 1000% increase in reactivity was observed compared to the untreated case. Finally, composites of nanoaluminum and reactive fluoropolymers (PVDF) are examined as a possible candidate for energetic material additive manufacturing (EMAM) and its viability is demonstrated by 3D printing and characterizing reactive multilayer films.