The Structural And Mechanical Propertires Of Bimetallic Cluster And Nanowire With Core-Shell Structure

Recently, many attentions have been paid to metal cluster and nanowire. Comparing with pure metal cluster and nanowire, bimetallic cluster and nanowire shows much more properties, especially physical and chemical properties. Therefore, bimetallic cluster and nanowire are applied to many fields, such as chemical catalysis, energies, machines, electronics, optics and magnetism. Be different from pure metal, bimetallic cluster and nanowire present special properties due to the chemical composition and atomic arrangement. The special properties have attracted a lot of interests from experimental and theoretical studies. Therefore, it is very important to study the bimetallic cluster and nanowire.The study of bimetallic cluster and nanowire mainly focus on experimental and theoretical effects. In experiment, effects are mainly put into synthesis and characterization of bimetallic cluster and nanowire in order to investigate the structure and properties. However, it is only the theoretical studies can give the mechanism. There are three common methods:density functional theory (DFT), molecular dynamics (MD) and Monte Carlo (MC) simulations. DFT is very exact, however, its main disadvantage is the count of computation is expensive and can be only used to small system. MC and MD, which are based experiential potential, can be applied to bigger system, such as bimetallic cluster and nanowire.In this work, atomistic simulations including both MC and MD are carried out to obtain the bimetallic clusters and nanowires with different compositions and simulate the tensile process in order to determine the effects of composition and strain rate on the mechanical properties. After the structures of cluster and nanowire are selected, MC simulation can obtain bimetallic clusters and nanowires with different compositions, temperatures and structures by changing some parameters. Using the bimetallic nanowires with different compositions, MD simulations are carried out to simulate the tensile process. Besides, we developed a code for the atomic pair distribution function (PDF) technique to study the structural properties of bimetallic cluster under atomic scale. The conclusions are mainly showed as follows:1. The structural study of Co-Pt bimetallic cluster. The Co-Pt bimetallic clusters with icosahedron (Ico-type with 561 atoms), decahedron (Dec-type with 561 atoms) and truncated octahedron (TOh-type with 711 atoms) are all obtained by MC simulations. The Co-Pt bimetallic cluster show perfect core-shell structure. The simulation results show that Co atoms occupy firstly the central site, then diagonal lines or adjacent vertices positions, finally the vertices and edges as the increase of Co mole fraction.2. Developed an "APDFBC" code for obtain atomic pair distribution function for bimetallic clusters. The code is based the Debye equation I(Q). This code can obtain the structural function and atomic pair distribution function for metallic and bimetallic clusters without periodic boundary conditions. The atomic distance is also further studied. The structural function and atomic distance obtained by simulation in this work is very close to experimental data.3. Molecular dynamics simulations of Cu-Ni bimetallic nanowire with different compositions and strain rates. After obtaining the Cu-Ni bimetallic nanowires with different Cu mole fractions, MD simulations are carried out to simulate the tensile process. During the extending process, we observe a structural phase transition from FCC to hexagonal ring with one atom in the center in the Cu2oo2Ni1598, which is deformed at 300K and constant strain rate of 1.0% ps-1. It is found that the mechanical properties are strongly related to the composition and strain rate.4. Two-component Elastic Modulus Diagram and its mechanism. A dramatic change of elastic modulus is also observed in the diagram of Cu-Ni bimetallic clusters called Two-component Elastic Modulus Diagram with increasing Cu mole fraction in Cu-Ni bimetallic nanowries. The variation of the surface energy is proposed to understand the composition dependence of elastic modulus. A rapid decrease of the surface energy could result in a sudden arise of the elastic modulus. Our results clearly show that the mechanical properties of bimetallic nanowires are depended on the surface energy.