Fast reaction of aluminum nanoparticles promoted by oxide shell

Abstract

Combustion of aluminum nanoparticles (AlNPs) has long been investigated experimentally because of their use in various energetic formulations for propellants and explosives. But the limited spatiotemporal resolution in experiments, in particular, makes it challenging to explore the microstructural evolution of AlNP oxidation and associated mechanisms. Here, we perform large-scale reactive molecular dynamics simulations to study the structural evolution of AlNPs with a 2–4 nm thick oxide shell in an oxygen environment. We find the temporal hollowing processes of AlNPs for both symmetrical and asymmetrical oxidations, in which the morphological evolution can be understood by a discrepant electric field and temperature distributions for different systems. In the early time, core aluminum atoms experience a fast reaction with an oxide shell. Environmental oxygen does not react with AlNPs until the surface O/Al ratio decreases to ∼1.2. Moreover, based on our simulation results, previous experimental data agree well with the proposed model, which can well describe the relationship between combustion efficiency and oxide shell thickness, confirming that the oxide shell promotes rather than hinders the combustion of AlNPs. The molecular insights obtained here would be significant for understanding the underlying mechanism and further modeling of AlNP combustion.

Ignition and Combustion of Hydrocarbon Fuels Enhanced by Aluminum Nanoparticle Additives: Insights from Reactive Molecular Dynamics Simulations

“…The first point to note is the void formation in the center of AlNP at 22 ps, which is attributed to the faster outward diffusion rate of Al atoms through the oxide shell than the inward diffusion rate of oxygen atoms. This phenomenon has been widely observed in experiments and previous simulations . As oxidation proceeds, the temperature of AlNP increases, followed by its melting and deformation.…”

“…This phenomenon has been widely observed in experiments 45 and previous simulations. 46 As oxidation proceeds, the temperature of AlNP increases, followed by its melting and deformation. The highest temperature of AlNP reaches ∼4500 K, exceeding the boiling point of alumina.…”

“…The number of atomic oxygen with AlNP addition is enhanced compared with that of pure methane in the early stage (as shown in the Supporting Information, Figure S9). The increase of the amount of atomic oxygen might be attributed to the decrease of the coordination number of particle surface atoms, 46 leading to easier detachment of atomic oxygen from AlNP at a high temperature of ∼4500 K. At 3000 K, the number of atomic oxygens in the whole process has not been significantly enhanced, which is due to the overall temperature control that causes the environmental gas temperature to be lower than 3000 K.…”

Transport of Shale Gas in Microporous/Nanoporous Media: Molecular to Pore-Scale Simulations

“…Fortunately, molecular dynamics (MD) simulations, originating from Newtonian mechanics to define the force and capture the corresponding motion trajectory of all the particles, make it possible to directly characterize and observe the dynamics behaviors of each atom or molecule at the nanoscale. − Typically for shale nanopores flow, the multiple phases (e.g., water and methane) and radical ambient conditions (e.g., high pressure and temperature) can be easily taken account into MD simulations system, − and, hence, a great deal of MD simulation studies have been performed to reveal the transport characteristics of shale gas within nanopores, providing important cognitions and fundamental frameworks for gas transport through shale nanopores. In this work, the authors attempt to present a comprehensive review on current advances of shale gas transport through microporous/nanoporous media from molecular perspectives, including molecular models, flow simulation strategies, and significant results.…”

Preparation of spherical α‐Al2O3 nanoparticles by microwave hydrothermal synthesis and addition of nano‐Al seeds

“…In order to further investigate the thermal behavior of nano-Al seeds at high temperatures, a thermal analysis was performed from room temperature to 900 • C, and the resulting Thermogravimetric analysis-Differential scanning calorimetry (TG-DSC) curves are shown in Figure 4. The TG curve shows that a slow weight gain starts from 300 • C, with a total weight gain of about 3.5% between 300 and 500 • C, which indicates that oxidation of 36 As oxidation process proceeds, the thickness of alumina layer on the surface increases, which hinders the further diffusion of O atoms, and thus the oxidation rate at this stage is slower. Between 500 and 645 • C, the sample gains 15.2% of its weight sharply, and the DSC curve shows that during this process, the sample releases a large amount of heat with an exothermic enthalpy of about 2331.4 J/g.…”