A study on the structural arrangements and compositional changes in laser-stabilized metastable AlO _x /C...

Amorphous metal oxides hold promising and yet, untapped potential as new classes of functional engineering materials due to their long-range disordered structures. While such disruptive features are alluring, fundamental understanding of structure-composition properties during their disorder-to-order transitions hold the key to their rational design as novel functional materials. Recently, we employed laser ablation synthesis in solution (LASiS) to kinetically entrap amorphous and metastable Al-oxide (a-AlO _x ) nanostructures with non-stoichiometric compositions (x ∼ 2.5-3.0) that indicated remarkable stability from carbon interfaces. In this work we provide detailed insights into thermally induced phase change characteristics and structural evolution of these highly disordered a-AlO _x /C nanocomposites (NCs) using Atomic Pair Distribution Function (PDF) analyses from X-ray scattering experiments. Our results indicate that the ultra-small AlO _x nanostructures evolve from highly disordered to low-order polycrystalline structures while undergoing Al-O bond re-arrangements indicating coordination shifts during each step of high-temperature (>500-800 °C) phase transition. We also demonstrate our ability to tailor the composition (x = O/Al ratio) and percentage crystallinity in these NCs by varying the laser flux within accessible ranges by comparing two discrete nanosecond pulse durations (5 ns and 9 ns) during LASiS; this paves the path for ability to regulate their energy release and trapped oxidative gas contents in their design as solid-state phase change materials (SS-PCMs) in energetic applications. These studies provide critical insights into the stability and structural bond re-arrangements in metastable a-AlO _x /C NCs undergoing disorder-to-order transitions during high temperature isothermal phase change processes. Such studies pave a novel path for the future design and development of advanced SS-PCM based energetic additives as intrinsic initiators/oxidizers in next-generation solid propellant formulations.