Laser-induced trapping and stabilization of nanostructured metastable metal-oxide phases for energetic applications

Synthesis of kinetically “frozen” metastable nanostructures remains elusive. This limitation has severely restricted the current paradigms in materials discovery for metal/metal oxide-based energetic nanomaterials (ENMs) that can circumvent the diffusion limitations. Specifically, non-stoichiometric/amorphous Al-oxide (a-AlO_x) structures in metastable states, albeit theoretically predicted, are rarely reported in experiments due to the inability to kinetically trap and phase-stabilize such exotic out-of-equilibrium phases at nanoscale. We overcame this challenge by phase-stabilizing unusually hyper-oxidized and metastable amorphous-Aluminum oxide (a-AlO_x; 2.5x NPs, remarkably stabilized by an interfacial monolayer of ordered carbon (C) atoms. Both, structure and chemical composition were confirmed with disparate characterization methods at different length-scales. The nanoparticles of sizes less than 10 nm were stable even at elevated temperatures. Only at temperatures higher than 750^oC, the metastable a-AlO_x structures undergo solid-solid phase transition that culminates in the formation of stable a-Al₂O₃ while releasing excess trapped gases. Detailed chemical kinetic analyses for the solid-solid phase transformation is investigated via time-dependent high-temperature XRD, PDF analysis and in-situ high-temperature TEM imaging. The ability to kinetically trap such metastable nanomaterials in localized low-energy troughs, has wide-ranging implications for their potential applications as solid-state gas generator additives that can bypass the diffusion limitations. Furthermore, this thesis presents results for diverse Fe/Al-based carbides, oxides and oxycarbides that can be deployed as functional additives/oxidizers for solid fuels. Results are presented for the synthesis of Fe-oxide/carbide-based NPs with magnetic responses that have the potential for magnetic field-induced heating. To this end, laser and solvent parameters for LASiS are tuned to enable tailored synthesis of ultra-small (2-10 nm) amorphous Fe-oxide NPs, as well as core-shell Fe@Fe₂O₃ (20-40 nm) and Fe₃C@C (20-40 nm) NPs. Detailed structure-composition characterizations are presented that reveal that the crystalline phases and C-shell coatings in the aforesaid composite NPs could be tuned via LASiS under heated toluene. The research described paves the way for future investigations into other such metastable nanomaterials awaiting discovery in the non-equilibrium phase space.