In Situ Study Of Surface Effect On Mechanical Deformation Of Metallic Nanoparticles

The influence of surface effect on properties of metallic materials at nanometer scale is an important field and the mechanical properties and deformation process of metals at nanometer scale especially attracts much attention.It is of great significance to understand the deformation mechanism of metals for further understanding the mechanical properties of metallic materials and developing new devices.In order to deeply explore the mechanical deformation process of metals at nanometer scale,although many experimental and theoretical studies have already been carried out,the mechanism of surface effect and its influence on the deformation process of materials still need to be further studied because of the relatively difficult technology to carry out the research.In this dissertation,the mechanical deformation process of Pb and hcp-Ag nano-particles was studied using in-situ high-resolution transmission electron microscopy（TEM）.Specifically,the dynamic evolution of metal crystals subjected to extrusion or stretching was directly captured at the atomic scale level.Meanwhile,the effects of particle size and surface oxidation on mechanical behavours were also explored in detail.Main results of this dissertation are listed as follows:1.Controllable preparation of Pb and hcp-Ag nanoparticles.（1）Pb nano-particles with different sizes or surface structures were in-situ prepared on surface of PX-PbTiO₃ nanowires by controlling temperature and electron beam irradiation.Phase transition from body-centered tetragonal PX-PbTiO₃ to monoclinicTiO₂（B）was in-situ observed at the atomic scale.Electron beam irradiation and temperature are two necessary factors for the phase transition or growth of Pb particles;（2）Hexagonal hcp-Ag particles with different sizes are prepared on the surface of perovskite PbTiO₃ nanowires after chemical precipitation-light reduction process.2.In situ study of effect of particle size during mechanical deformation of Pb nano-particles.（1）Liquid-like pseudo-elastic deformation and plastic deformation（twinning,dislocation）were observed at the atomic scale during in-situ mechanical deformation of Pb nano-particles;（2）Effects of particle size on deformation mechanism of Pb nano-particles were analyzed quantitatively based on statistical data.As particle size decreases,the probability of observing displacive deformation decreases gradually.The smallest diameter of particles on which the displacive deformation can be observed was 7.25 nm;（3）As particle size decreases,proportion of surface atoms increases sharply while melting point of the particles decreases gradually.These make the surface diffusion of atoms easier.3.In situ study of effect of oxidation during mechanical deformation of Pb nano-particles.（1）Surface oxidation of Pb nano-particles was observed in-situ at the atomic scale;（2）Effect of oxidation during mechanical deformation of Pb nano-particles was in situ studied.Surface oxidation has no obvious effect on displacive deformation process such as twinning or dislocation slip while significantly limits the surface diffusion process of Pb atoms.The surface oxidation hinders the occurrence of diffusive deformation such as pseudo-elastic deformation and creep,so that nanoparticles can only show plastic deformation behavior.4.In situ study of effect of surface diffusion during mechanical deformation of hcp-Ag nano-particles.（1）The mechanical deformation processes of hcp-Ag nano-particles were studied using in-situ TEM.Some phenomenon,such as self-repair and spontaneous surface deformation,which is induced by atomic surface diffusion,was observed at the atomic scale;（2）Based on the experimental results,surface diffusion coefficient of hcp-Ag nanoparticles was measured semi-quantitatively.In order of magnitude,the D_s is calculated to be 10^-18 m²/s.In this dissertation,using in-situ high-resolution TEM methods,real-time and atomic-scale observation of the surface structure evolution under external field was carried out on different nanomaterials（PX-PbTiO₃ nanowires,Pb and hexagonal hcp-Ag nanoparticles）.It provides a theoretical basis for preparation of materials with complex surface structure and design of multifunctional devices.