Examining Hydroxyl-Alumina Bonding toward Aluminum Nanoparticle Reactivity

The stabilizing, amorphous alumina (Al₂O₃) passivation layer surrounding aluminum (Al) particles participates in reactions that lower barriers to bulk Al oxidation. The behavior has been observed in thermites comprised of nanoscale Al particles (nano-Al) dispersed within a fluoropolymer matrix. Studies reported herein show the oxide passivation shell on nano-Al particles is affected by the polarity and hydrogen bonding properties of the solvent employed for thermite dispersal, resulting in enhanced thermal energy propagation during Al combustion in nano-Al + poly(tetrafluoroethylene) (PTFE) mixtures. Relative to conventional treatments that employ hexane for thermite dispersal, the speed of flame front movement measured in a Bockmon Tube apparatus under steady-state conditions increased more than 2-fold following treatments in acetone or 2-proponal. Differential scanning calorimetry and infrared spectroscopy measurements indicate contact with the polar solvents increases the amount and accessibility of hydroxyl species on the nano-Al oxide shell, which in turn participates in a preignition reaction (PIR) that activates PTFE and likely weakens the Al passivation layer. A molecular-scale mechanism is proposed for the PIR that derives from catalytic reactions of alumina and halocarbon fluorinating reagents. Additionally, infrared spectra show evidence for a greater fraction of disordered, liquid-like hydrogen-bonded water molecules within the alumina layer of nano-Al particles after treatment in the polar solvents studied. The O-H vibrational features suggest solvent treatment may affect the structure of nano-Al surface oxides and PIR kinetics. This study reveals potential strategies for optimizing fuel particle reactivity that include modification of the Al particle passivation shell using polar solvents to promote early preignition exothermic reaction.