On the role of built-in electric fields on the ignition of oxide coated nanoaluminum: Ion mobility versus Fickian diffusion

Abstract

Using the classical molecular dynamics method we simulate the mechanochemical behavior of small ͑i.e., core diameterϽ 10 nm͒ oxide coated aluminum nanoparticles. Aluminum nanoparticles with core diameters of approximately 5 and 8 nm are simulated with 1 and 2 nm thick oxide coatings or shells. In addition to thickness the shells are parametrized by varying degrees of crystallinity, density, and atomic ratios in order to study their effect on the ignition of nanoparticle oxidation. The oxide shells are parametrized to consider oxide coatings with the defects that commonly occur during the formation of an oxide layer and for comparison with a defect free crystalline oxide shell. Computed results include the diffusion coefficients of aluminum cations for each shell configuration and over a range of temperatures. The observed results are discussed and compared with the ignition mechanisms reported in the literature. From this effort we have found that the oxidation ignition mechanism for nanometer sized oxide coated aluminum particles is the result of an enhanced transport due to a built-in electric field induced by the oxide shell. This is in contrast to the currently assumed pressure driven diffusion process. This induced electric field accounts for approximately 90% of the mass flux of aluminum ions through the oxide shell. The computed electric fields show good agreement with published theoretical and experimental results.

Consolidation of Micro- and Nano-Sized Al Powder

“…[38] The amorphous alumina layer becomes metastable when the thickness exceeds a threshold value [10,15]. Besides, it is largely accepted that the chemical reactivity of aluminum and other metals at the nanometer scale differs from that measurable at the macroscopic level; indeed different transformations and reactions may happen at the nano-scale, which are not favored at the micro-scale [16][17][18][19][20]. The small size and large surface area provide a significantly high chemical reactivity.…”

Correlating ignition mechanisms of aluminum-based reactive materials with thermoanalytical measurements

“…For the material milled for 97% of the time at which the material would react as a result of mechanical initiation, the powder primarily included Al and Ni nanolaminate layers that were less than 30 nm thick. At shorter milling times (75,50, and 25% of the critical time), the fine nanolaminate structure is mixed with a respectively increased fraction of coarser, micronsized Ni and Al layers. The material studied in Ref.…”

“…In more recent studies of oxidation of nano-sized aluminum particles, it was suggested that the slow oxidation at temperatures below the aluminum melting point is dominated by the inward diffusion of oxygen, whereas the formation of hollow nanoparticles above melting point indicates the significance of outward diffusion of aluminum ions [49]. The molecular dynamics simulation efforts [50,51] investigating nano-aluminum particles heated at a very high rate suggest that outward diffusion of aluminum controls the reaction rate in a broad range of temperatures, including temperatures well above those expected for the amorphous to g-alumina phase change. Thus, an experimental validation is necessary for any assumption regarding the location of the reactive interface for aluminum oxidation models.…”

New Method for Extracting Diffusion-Controlled Kinetics from Differential Scanning Calorimetry: Application to Energetic Nanostructures

“…Although prior studies suggest that thermite reactions, for example, are often regulated by oxygen diffusion, [33][34][35] Eq. (5) does not discriminate with respect to reacting species or diffusion 7 mechanisms.…”