Molecular Dynamics Simulations Of The Initial Oxidation Process On Ferritic Fe-Cr Alloy Surfaces

The solid oxide fuel cell（SOFCs）is an efficient and clean energy conversion device that converts chemical energy from fuel directly into electrical energy.To achieve higher performance,SOFCs are often assembled and used in power stacks.The interconnect is one of the key components in the SOFCs stack and is used to connect adjacent single cells in series.Metallic materials,particularly Fe-Cr alloys,have replaced ceramic materials as the dominant interconnect.Fe-Cr alloys form a dense Cr2O3 oxide layer on the surface at high temperatures,which protects the substrate from further oxidation.However,in the SOFCs environment,the Fe-Cr alloy surface forms large particles of Fe-O compounds,which thicken the oxide layer on the surface and increase the resistance,resulting in increased internal resistance of the stack and lower output power.Therefore,understanding the oxidation mechanism of Fe-Cr alloy at the atomic level is an important guide for improving the performance of SOFCs.The complexity of the oxidation reaction at the alloy surface and the extremely fast reaction rate,the processes of adsorption,dissociation,and oxide formation that occur when oxygen interacts with the alloy surface atoms are not clear,and theoretical simulations are needed to reveal the microstructural evolution of the Fe-Cr alloy during the reaction process.Using a reaction force field based molecular dynamics simulation（Reax FF-MD）,we have investigated how the atomic arrangement and reaction environment on the surface of Fe-Cr alloy synergistically influence the oxidation reaction mechanism and oxide growth pattern.The local interaction of O ions with the surface of Fe-Cr alloy and the initial oxidation process of O2 molecules with the alloy surface are analyzed in this work,and the contribution of structural defects of polycrystalline alloys to the oxide film growth is further investigated.The main results are as follows:（1）Based on the Reax FF reaction potential,the interactions between the incident O ion and the（001）surface of the ferritic Fe-Cr alloy are simulated by classical molecular dynamics.When the incident O ion collides with the surface,adsorption,reflection,and sputtering of the O ion are observed,with adsorption being the dominant interaction.The incident O ions are adsorbed at tetrahedral interstitial positions between Fe and Cr atoms in the upper Fe-Cr mixed layer and adsorb tightly around Fe atoms in the deeper pure Fe layers when they are incident with different initial kinetic energies.Our results show that Fe-O compounds are easier to form with increasing kinetic energy of the incident O ion and increasing temperature.（2）Based on Reax FF reactive potential,the interaction of O₂ molecules with three types of surfaces（100,110,111）of ferritic Fe-Cr alloy has been studied by classical molecular dynamics at constant O₂ concentrations and temperatures.The initial oxidation process is systematically studied according to the analysis of O₂absorption rate,charge variations,charge distributions,mean squared distributions,and oxidation rate.The results present that the Cr atoms are easier and faster to lose electrons than Fe atoms during the oxidation process.The obtained oxidation rate of Cr atoms is larger and the formation of Cr₂O₃ takes precedence over that of FeO.And the thickness of oxidation layers of different surfaces could be determined quantitatively.We also find that the high O₂concentration accelerates the oxidation process and obviously increases the thickness of oxidation layers,while the temperature has weaker effect on the oxidation process than the O₂ concentration.Moreover,the（110）surface presents the best oxidation resistance comparing with the other two surfaces.And the（110）surface is efficient in preventing Fe atoms from being oxidized.（3）Solid alloys consist of irregularly arranged grains,and the atomic arrangement of polycrystalline alloys is very different from that of single-crystal alloys,which may lead to differences in their oxidation mechanisms.Based on Reax FF-MD simulations,we show that the oxidation resistance of polycrystalline alloys is worse under the same reaction conditions.The oxidation susceptibility of polycrystalline alloys is due to their numerous structural defects,such as coordination defects and structural distortions.First,the surface defects of polycrystalline alloys reduce the dissociation barrier for oxygen by a"selective adsorption"mechanism,increasing the initial oxidation rate;second,the surface defects increase the inhomogeneity of the oxide film;finally,the internal defects of polycrystalline alloys provide a more convenient path for the migration of O atoms,which accelerates the growth of the oxide film.In this paper,how the atomic structure on the surface of Fe-Cr alloy affects the evolution of oxides in SOFCs is systematically described.Our results provide a theoretical basis for the experiments and the application of metal connectors.