Demonstrating an altered metal oxidation reaction mechanism correlated with variations in surface energy

Stress-Alteration Enhancement of the Reactivity of Aluminum Nanoparticles in the Catalytic Decomposition of exo-Tetrahydrodicyclopentadiene (JP-10)

“…This lowering of the surface energy leads to diminished adhesion of JP-10 molecules, affecting the residence time on the surface, hence producing reduced surface reactivity for the SA-AlNPs at the initial temperatures. The identical order of reduced reactivities has also been observed previously for the preignition reaction at the surface of SA-Al particles in the presence of molecular oxygen, where Shancita et al showed that a thicker oxide film was generated for the UN particles, indicating that a greater extent of heterogeneous reaction originated from a higher surface energy compared to that of the SA particles. , …”

“…After removing the container from the furnace, it is placed into a liquid quenching solution (a mixture of water, soap, surfactant, and sodium chloride) designed to rapidly reduce the powder temperature to ambientreferred to as superquenching. This pretreatment produces AlNPs with about 40% elevated stress, specifically increasing the stress at the core–shell boundary from −0.191 to −0.380 GPa, with significant delamination (∼80% delamination between the core and shell) and reduced surface energy up to 40% . Both the UN and pretreated AlNP samples have a “passivation” layer of aluminum oxide (Al 2 O 3 ) around the reactive metallic aluminum core (Scheme ); hence, they are suitable to preserve and use under atmospheric conditions.…”

“…This pretreatment produces AlNPs with about 40% elevated stress, specifically increasing the stress at the core−shell boundary from −0.191 to −0.380 GPa, with significant delamination (∼80% delamination between the core and shell) 31 and reduced surface energy up to 40%. 37 Both the UN and pretreated AlNP samples have a "passivation" layer of aluminum oxide (Al 2 O 3 ) around the reactive metallic aluminum core (Scheme 1); hence, they are suitable to preserve and use under atmospheric conditions.…”

“…The identical order of reduced reactivities has also been observed previously for the preignition reaction at the surface of SA-Al particles in the presence of molecular oxygen, where Shancita et al showed that a thicker oxide film was generated for the UN particles, indicating that a greater extent of heterogeneous reaction originated from a higher surface energy compared to that of the SA particles. 37,58 Given that the low-temperature dissociative oxidation of JP-10 is solely governed by the alumina shell of the AlNPs involving a heterogeneous oxygen atom transfer to the fuel molecule, it thus indicates that the SA-AlNPs do not have the requisite surface energy (or chemical potential) at 650 K, hence providing a less reactive or less energetically favorable alumina surface for the activation of the JP-10 molecule. The initial product formation and their quantitative distribution disclose diminished branching ratios of oxygenated products (80.2 and 17.8% for UN-and SA-AlNPs, respectively), and they decrease with the rise in temperature for the SA-AlNPs, unlike for the UN-AlNPs, where the oxygenated products collectively (including water) exceed the hydrocarbon products up to a temperature of 1050 K. This observation also substantiates the suppressed heterogeneous oxidation chemistry on the alumina shell and indicates other contributing reaction channels through thermocatalytic C−C/C−H bond cleavages, favoring the formation of hydrocarbon molecules and radicals.…”

Opportunities in Engineering Energetic Materials

“…There are numerous opportunities for these types of materials that have tailored microstructure: new alloys could be imagined for example. Coatings or surface treatments [9], core-shell geometries, foils (layered materials) [10], MA/ABM, alloys, and multicomponent crystals (not well-ordered co-crystals or amorphous multicomponent-crystals) are a few additional approaches. For example, ignition thresholds can dramatically be decreased, and reactions rates increased in some cases [11].…”