Laser-Induced Trapping of Metastable Amorphous-AlO_x /C (2.5< x ≤3.5) Nanocomposites Stabilized by Carbon Interfaces—Implications for Solid-Phase Gas Generator Additives

Tailored interfacial and metastable structures can effectively circumvent the diffusion limitations<br/>due to oxide shell formation in Aluminum (Al) nanoparticle (NP)-based energetic nanomaterials<br/>(ENMs). Yet, rational design and synthesis of metastable nanostructures via non-equilibrium<br/>routes remain largely unexplored. Specifically, non-stoichiometric/amorphous Al-oxide (a-AlO_x)<br/>structures in metastable states, albeit, theoretically predicted, are rarely reported in<br/>experiments due to the inability to kinetically trap and phase-stabilize such exotic out-of-<br/>equilibrium phases at nanoscale. We report a pivotal advancement in addressing this challenge<br/>by employing reactive Laser Ablation Synthesis in Solution (r-LASiS) as a one-step, one-pot<br/>non-equilibrium route to synthesize unusual metastable a-AlO_x NPs in hyper-oxidized states<br/>(2.5&lt;x≤3.5), phase-stabilized by interfacial C monolayers and dispersed in pyrolyzed-C<br/>matrices. Detailed characterizations from bulk and surface spectroscopic analyses as well as<br/>electron microscopy imaging reveal highly disordered a-AlO_x NPs (&lt;5-8 nm sizes), remarkably<br/>stabilized by interfacial monolayer shells of ordered C-atoms. Using techniques such as Solid-<br/>state Nuclear Magnetic Resonance (SSNMR), X-ray Photoelectron Spectroscopy (XPS), and<br/>Electron Energy Loss Spectroscopy (EELS), we are able to establish the material’s structure<br/>and composition in the hyperoxidized state. Interestingly, the a-AlO_x NPs of sizes less than 10<br/>nm were stable even at elevated temperatures. At temperatures higher than 750 ^oC, the a-<br/>AlO_x structures undergo a solid-solid phase transition that culminates in the formation of stable<br/>α-Al₂O₃ while releasing excess trapped gases. This talk will also present results for the detailed<br/>kinetic measurements carried out for the solid-solid phase transformation from metastable a-<br/>AlO_x to stable α-Al₂O₃ crystalline structures by using time-dependent high-temperature XRD<br/>analyses. The ability to kinetically trap such metastable nanomaterials in localized low-energy<br/>troughs, until their precipitous plunge to their global lowest energy state, has wide-ranging<br/>implications for their potential applications as solid-state gas generator additives that can<br/>bypass the diffusion limitations.