ReaxFF-based Molecular Dynamics Study On Surface Oxidation Behavior Of Cu And Ni At Nanoscale

The widespread use of metals has contributed to the development of human civilization and the progress of society,and has become an indispensable part of community life.When metal is exposed to air and some complex oxidizing media,its surface will be oxidized and corroded,which will affect the structure and properties of metal.Corrosion is common in petrochemical,metallurgy,marine equipment and other industrial fields,which seriously affects the safe service of metal equipment and causes a lot of economic losses.However,on the other hand,metal oxides and oxide-based composites have emerged in the fields of catalysis,optoelectronics and new electronic equipment.Controllable oxidation corrosion can be used to prepare oxide matrix composites for special applications.How to correctly understand the mechanism of metal oxidation on the atomic scale will provide important theoretical guidance for inhibiting metal oxidation corrosion and designing the synthesis process of metal/oxide composites.However,the initial oxidation behavior of metals is rapid and complex,and it is difficult to observe the process of gas adsorption,dissociation and the formation of oxidation reconstruction structure in situ by experimental equipment,while the computationsl simulation can reveal the atomic mechanism of metal oxidation in a smaller time scale.In this paper,the molecular dynamics simulation based on ReaxFF is used to investigate the oxidation process and the evolution of oxidation structure of copper nanoparticles,as well as the oxidation behavior of nickel under the influence of surface defects and applied strain.The main contents of this paper are as follows:(1)The oxidation mechanism of copper nanoparticles and the structural changes in the oxidation process were explored,and the effects of oxygen content and temperature on the oxidation process were analyzed.It is found that due to the size effect,the surface of copper nanoparticles can be seen as consisting of many flat crystal faces and uneven surface steps.A large number of uncoordinated active atoms located at the edges and corners of the steps will become the initial sites of oxidation reaction,and the oxygen adsorbed here tends to dissociate first and combine with copper atoms to form oxidation nuclei.A plurality of oxide nuclei are expanded and mutually connected to form an oxide film,and finally a core-shell oxide structure is formed.There are many kinds of oxidation structures in the oxide film,among which the number of four-coordination hollow structures is the largest,and it is proven that oxidation is a process from low coordination oxides to high coordination oxides.The number of active atoms on the surface limits the oxidation scale to a certain extent.The number of active atoms on the surface is small at low temperatures,and excessive oxygen has little effect on the oxidation degree.However,high temperatures can activate the surface of copper nanoparticles and effectively promote oxidation.(2)The oxidation behavior of nickel under the influence of surface groove defects and applied strain was investigated.It is found that due to the lower coordination,the atoms at the edge of the groove defect are more likely to leave the surface at the initial stage of oxidation and form cation vacancies.The vacancy expansion leads to oxidation,and it extends from the defect to both sides of the plane.At the same time,the atomic arrangement of the groove surface will affect the oxidation of nickel:the atomic arrangement of the Ni(110)surface is thinner than that of the Ni(111)surface,which will lead to stronger oxidation at the initial stage of oxidation.The high temperature will promote the oxidation reaction,and the whole oxidation process is characterized by layer-by-layer expansion from defects to both sides and from surface to interior.The participation of water molecules makes the structure of oxide film sparse,and the oxidation tends to internal expansion along the groove.The applied tensile strain and compressive strain will make the oxidation start earlier,and improve the oxidation degree by promoting the inward diffusion of oxygen atoms and the outward diffusion of nickel atoms,respectively.In addition,the promoting effect of compressive strain is stronger than that of tensile strain.In this paper,the influence of atomic structure of metal surface and different oxidation conditions on oxidation kinetics and oxide structure evolution is systematically explored,and the complex oxidation mechanism in the process of metal oxidation is revealed at the atomic scale,which provides theoretical guidance for improving the service stability of metal materials and the preparation of oxide-based nanomaterials.