Effect of Fe–O ReaxFF on Liquid Iron Oxide Properties Derived from Reactive Molecular Dynamics

As iron powder nowadays attracts research attention as a carbon-free, circular energy carrier, molecular dynamics (MD) simulations can be used to better understand the mechanisms of liquid iron oxidation at elevated temperatures. However, prudence must be practiced in the selection of a reactive force field. This work investigates the influence of currently available reactive force fields (ReaxFFs) on a number of properties of the liquid iron-oxygen (Fe-O) system derived (or resulting) from MD simulations. Liquid Fe-O systems are considered over a range of oxidation degrees Z O, which represents the molar ratio of O/(O + Fe), with 0 &lt; Z O &lt; 0.6 and at a constant temperature of 2000 K, which is representative of the combustion temperature of micrometric iron particles burning in air. The investigated properties include the minimum energy path, system structure, (im)miscibility, transport properties, and the mass and thermal accommodation coefficients. The properties are compared to experimental values and thermodynamic calculation results if available. Results show that there are significant differences in the properties obtained with MD using the various ReaxFF parameter sets. Based on the available experimental data and equilibrium calculation results, an improved ReaxFF is required to better capture the properties of a liquid Fe-O system. </p

Molecular Dynamics Simulations of the Oxidation of Aluminum Nanoparticles using the ReaxFF Reactive Force Field

We performed ReaxFF-molecular dynamics (MD) simulations of the oxidation of aluminum nanoparticles (ANPs) at three different temperatures (300, 500, and 900 K) and two different initial oxygen densities (0.13 and 0.26 g/cm³) to elucidate the mechanism of oxidation kinetics of the ANPs and to study the oxidation states in the oxide layer. Our result shows that the mechanism of the oxidation of the ANPs is as follows: hot-spots and high-temperature areas are created by adsorption and dissociation of oxygen molecules on the surface of the ANPs; void spaces are generated because of hot-spots and high-temperature areas; the void spaces significantly lower a reaction barrier for oxygen diffusion (by up to 92%) and make this process exothermic. Subsequently, an oxide layer is developed by this accelerated oxygen diffusion. Our results also indicate that the oxidation of the ANPs depends on combined effects of the temperature and the oxygen gas pressure because such conditions have effects on not only the oxide layer thickness but also the density of the oxide layer. These ReaxFF results are in good agreement with available experimental literature on aluminum oxidation kinetics

Extension of the ReaxFF Combustion Force Field toward Syngas Combustion and Initial Oxidation Kinetics

A detailed insight of key reactive events related to oxidation and pyrolysis of hydrocarbon fuels further enhances our understanding of combustion chemistry. Though comprehensive kinetic models are available for smaller hydrocarbons (typically C₃ or lower), developing and validating reaction mechanisms for larger hydrocarbons is a daunting task, due to the complexity of their reaction networks. The ReaxFF method provides an attractive computational method to obtain reaction kinetics for complex fuel and fuel mixtures, providing an accuracy approaching ab-initio-based methods but with a significantly lower computational expense. The development of the first ReaxFF combustion force field by Chenoweth et al. (CHO-2008 parameter set) in 2008 has opened new avenues for researchers to investigate combustion chemistry from the atomistic level. In this article, we seek to address two issues with the CHO-2008 ReaxFF description. While the CHO-2008 description has achieved significant popularity for studying large hydrocarbon combustion, it fails to accurately describe the chemistry of small hydrocarbon oxidation, especially conversion of CO₂ from CO, which is highly relevant to syngas combustion. Additionally, the CHO-2008 description was obtained faster than expected H abstraction by O₂ from hydrocarbons, thus underestimating the oxidation initiation temperature. In this study, we seek to systemically improve the CHO-2008 description and validate it for these cases. Additionally, our aim was to retain the accuracy of the 2008 description for larger hydrocarbons and provide similar quality results. Thus, we expanded the ReaxFF CHO-2008 DFT-based training set by including reactions and transition state structures relevant to the syngas and oxidation initiation pathways and retrained the parameters. To validate the quality of our force field, we performed high-temperature NVT-MD simulations to study oxidation and pyrolysis of four different hydrocarbon fuels, namely, syngas, methane, JP-10, and <i>n</i>-butylbenzene. Results obtained from syngas and methane oxidation simulation indicated that our redeveloped parameters (named as the CHO-2016 parameter set) has significantly improved the C₁ chemistry predicted by ReaxFF and has solved the low-temperature oxidation initiation problem. Moreover, Arrhenius parameters of JP-10 decomposition and initiation mechanism pathways of <i>n</i>-butylbenzene pyrolysis obtained using the CHO-2016 parameter set are also in good agreement with both experimental and CHO-2008 simulation results. This demonstrated the transferability of the CHO-2016 description for a wide range of hydrocarbon chemistry

Coupled Thermal and Electromagnetic Induced Decomposition in the Molecular Explosive alpha HMX; A Reactive Molecular Dynamics Study

We use molecular dynamics simulations with the reactive potential ReaxFF to investigate the initial reactions and subsequent decomposition in the high-energy-density material alpha-HMX excited thermally and via electric fields at various frequencies. We focus on the role of insult type and strength on the energy increase for initial decomposition and onset of exothermic chemistry. We find both of these energies increase with the increasing rate of energy input and plateau as the processes become athermal for high loading rates. We also find that the energy increase required for exothermic reactions and, to a lesser extent, that for initial chemical reactions depend on the insult type. Decomposition can be induced with relatively weak insults if the appropriate modes are targeted but increasing anharmonicities during heating lead to fast energy transfer and equilibration between modes that limit the effect of loading type

Effects of oxidation on tensile deformation of iron nanowires: Insights from reactive molecular dynamics simulations

The influence of oxidation on the mechanical properties of nanostructured metals is rarely explored and remains poorly understood. To address this knowledge gap, in this work, we systematically investigate the mechanical properties and changes in the metallic iron (Fe) nanowires (NWs) under various atmospheric conditions of ambient dry O2 and in a vacuum. More specifically, we focus on the effect of oxide shell layer thickness over Fe NW surfaces at room temperature. We use molecular dynamics (MD) simulations with the variable charge ReaxFF force field potential model that dynamically handles charge variation among atoms as well as breaking and forming of the chemical bonds associated with the oxidation reaction. The ReaxFF potential model allows us to study large length scale mechanical atomistic deformation processes under the tensile strain deformation process, coupled with quantum mechanically accurate descriptions of chemical reactions. To study the influence of an oxide layer, three oxide shell layer thicknesses of ∼4.81 Å, ∼5.33 Å, and ∼6.57 Å are formed on the pure Fe NW free surfaces. It is observed that the increase in the oxide layer thickness on the Fe NW surface reduces both the yield stress and the critical strain. We further note that the tensile mechanical deformation behaviors of Fe NWs are dependent on the presence of surface oxidation, which lowers the onset of plastic deformation. Our MD simulations show that twinning is of significant importance in the mechanical behavior of the pure and oxide-coated Fe NWs; however, twin nucleation occurs at a lower strain level when Fe NWs are coated with thicker oxide layers. The increase in the oxide shell layer thickness also reduces the external stress required to initiate plastic deformation.Scientific and Technological Research Council of Turkey (BIDEB 2219 1059B191400364