Periodic table's 13th element, aluminum, is broadly used in energetic applications, functioning as plasmonics, nanofluids, explosives, solid propellants, and firefighting materials [1–4].
The widespread use of aluminum is due to its high combustion enthalpy, low toxicity, abundance, and good stability [5, 6].
A thin layer of oxide is automatically shaped on aluminum by placing it in any place, which produces an intricate amorphous structure during uncontrolled steps.
Researchers have been trying to understand and search for the steps of aluminum oxidation for many years. There are numerous experimental and theoretical studies investigating species and factors affecting the properties of the oxide layer and its quality.
Most studies on aluminum oxidation involve explaining the stages of thermal oxidation of aluminum in terms of temperature [7–9].
Aluminum oxide has developing applications as well as aluminum. For instance, thin aluminum-oxide films are employed in numerous microelectronic devices as a dielectric, diffusion, or tunneling barrier [10–12].
In order to optimize the application of aluminum-oxide films, it is required to consider layer thickness, morphology, physical and chemical properties [13].
Thus, researchers have done hundreds of theoretical and practical studies from micron-sized aluminum surfaces in older studies [14] to its nanoparticles [15, 16].
During these years, in addition to studying the structure of aluminum oxide and the factors affecting the morphology of the structure, the mechanism of this process has also received much attention [17–20].
These studies mainly aim to describe the successive stages of oxygen attachment to aluminum, oxide layer formation, and kinetics understanding is performed.
By increasing the oxidation temperature up to 1073 K, it has been reported [17] that approximately all the nanoparticles turn into hot solid spheres, and when reaching the temperature of 1373K, they are found to be hollow.
The oxidation steps could be explained in terms of temperature as follows: Aluminum is initially coated with a thin layer of amorphous oxide, which significantly reduces the oxidation rate. At temperatures > 873 K, the oxidation rate increases due to the conversion of the amorphous layer to a crystalline state. If the temperature increases up to 973 K, the oxidation rate decreases due to the destruction of the diffusion paths. The re-oxidation rate increases at temperatures above 1273 K because the molten aluminum protrudes out of the shell [21].
In fact, before the melting temperature, oxidation occurs through the diffusion of O2 into the aluminum oxide shell, and above the melting point, aluminum and oxygen diffuse through the oxide shell to raise the oxidation rate [17].
Henz et al. studied the simulation of aluminum nanoparticles which were covered with an oxide layer by a thickness of 1 and 2 nm and showed oxidation inception occurs with the swift propagation of aluminum ions to the surface of the nanoparticles, and this was in agreement with the experimental endeavors that have beholden the formation of hollow [17–18].
Jeurgens and colleagues described a mechanism in the incipient Al oxidation that the oxide-film evolvement is finite at temperatures up to 573 K due to the low locomotion of the oxygen species. However, at higher temperatures, while the growth of the layer is not restricted, a phase transition "amorphous-to-gamma-Al2O3" occurs [22]. Chu et al. studied simulation of Core-Shell Al/Al2O3 nanoparticles in an oxygen atmosphere. In addition to presenting a 4-step mechanism, they noted that during the melting phase, aluminum atoms propagate out of core Al atoms, while in the fast oxidation stage, inward diffusion of shell O prevails [19].
Along with increasing studies and growing applications of aluminum and aluminum oxide, a group of researchers took an interest in the oxidation of substances with ozone. Since then, these researches have constantly been expanding [23–27].
Kuznetsova and colleagues oxidized aluminum via an ultra-high vacuum apparatus using previously produced ozone in a generator at a temperature of 300 K.
They showed that the average resistance for oxide layers produced with ozone was ~ 10 times higher than that of oxygen-oxidized layers. The oxidized layer using 97% pure ozone was also superior in terms of corrosion passivation compared to the oxygen-oxidized layer [28].
Silicones oxidation by ozone has been prevalent for many years. However, ozone production has sometimes been a problem [27 and 29]. Oxidation of silicones with ozone requires lower temperatures and exhibits the Si/SiO2 interface with higher quality, but this entails further studies in the oxidation of metals. For instance, the effects of system temperature on the oxidation of metals by ozone have not been completely investigated.
On the other hand, when reviewing the published studies of the last decades, one would realize that there have been numerous experimental studies in metal oxidation from which notable conclusions have been drawn [30–33]. However, complexity, high laboratory costs, lack of reproducibility, and not providing a detailed explanation of the oxide layer formation in metal oxidation studies have caused a growing tendency towards molecular dynamics simulations.
Molecular simulation methods using empirical force fields have been widely employed in researching atomic systems up to 10,000 atoms simulation.
For instance, the embedded atom method and the modified one are precise in physical properties' calculations of various metals and alloys such as melting points, viscosities, elastic, and diffusivity constants [34–35]. The reactive force field (ReaxFF) method, originally developed by van Duin et al., has been widely applied in simulations of different oxidation states of metal, reactive crosslinking of polymers, and oxidation and pyrolysis of hydrocarbon fuels [36–38].
ReaxFF utilizes empirically specified interatomic potentials inside a bond-order formalism. In this manner, it can model events that involve connecting or breaking chemical bonds without explicitly considering quantum mechanics (QM), which entails very high costs for large atomic systems [39].
The present study investigates the oxidation of aluminum with ozone and oxygen at different temperatures using ReaxFF molecular dynamics.
The main purpose of this work is to provide an accurate atomic-level insight into the study of oxide films formed in the thermal oxidation of aluminum (100) surface with oxygen and ozone molecules and to examine the effect of temperature on this oxidation.
The article proceeds in the following order to achieve this goal: First, we briefly explain the chosen reactive force field (ReaxFF) to examine Al/O interactions. Then we explore the incorporation of oxygen molecules into the surface of aluminum (100) in the three different temperatures 400, 600, and 800 K and elucidate the effect of temperature on metal oxidation with regard to earlier studies. In the next part of this study, we investigate the aluminum oxidation through ozone molecules in conditions similar to the oxidation of aluminum with oxygen. Finally, we present our results and findings on the oxide layers produced in simulated conditions.