Effect Of Interface And Size On The Structure Of Several Low-dimension Materials

One of the very basic concerns in mechanics, physics and chemistry of solids is that mostproperties of a solid vary depending on the microstructure, which is determined by thechemical composition, the arrangement of the atoms (the atomic structure) and the size of asolid in one, two or three dimensions. If one or several of these parameters is changed, theproperties of the solids in mechanics, electronics, magnetism, optics, catalytics andthermodynamics are significantly altered from those of either the bulk solid. When the atomicstructure of a solid deviates from equilibrium conditions, or the solid size is reduced tonanometer, or the solid dimensionality is smaller than three, apparent property change couldbe observed.Surface and interface is two-dimension area at which material physical and chemicalcharacter breaks, many material physical and chemical processes take place at surface andinterface. At the same time, many physical and chemical phenomena are related to the surfaceand interface of materials. When the size of the materials is reduced to namoscale, the natureof size change is that interfaces are introduced to affect melting temperature, mechanicalcharacters, interface morphology and so on. Lots of breakages and invalidations also startfrom surface and interface. It is very important that the study on microstructure of surface orinterface and it's action to environment and physical or chemic phenomena related to surfaceor interface for control of surface physical and chemic processes and variety of surfacecharacter.To solute the problem of nanocrystal material size-dependent properties, the essentialway lies in the transition from the material microscopic properties or the macroscopic ones tothe mesoscopic ones. Thermodynamics is a simple method to study the transition frommacroscopic world to the mesoscopic one. The application of thermodynamics tonanomaterial tells that a new branch of thermodynamics appears, nanothermodynamics.Although there are relatively extensive investigations on the size-dependent melting ofnanocrystals, it has not been accompanied by the necessary investigation of thesize-dependent thermodynamics of nanocrystals. To have a clear cognizance of thesize-dependent thermodynamic function will help us to know more about the size-dependentenergy transition law in the mesoscopic world. And it will stimulate the application ofthermodynamics to the small system composed of hundreds of or thousands of atoms as well. In this thesis, the size and interface dependent of the structure for low dimensionmaterials are discussed, listed as follows:1. Bi-phase transition diagrams and phase stability of metallic thin multilayers Based on the classical thermodynamics of the competition between bulk and interfacialenergies of the multilayers, as well as the Goldschmidt premise for lattice contraction, asimple model is established for structural stability of multilayer systems and is utilized toconstruct Co/Cr, Ti/Nb, Zr/Nb and Ti/Al multilayer systems. Reasonable agreements betweentheoretical predictions and experimental results support the model.2. Size and interface dependent of the structure for epitaxial growth thin films (1) The critical layer number of epitaxially grown metallic films with strained structure Through thermodynamic and elastic considerations, the critical layer number ofepitaxially grown Cu and Ni ultrathin films on metallic matrixes has been determined. Alarger nc value can be obtained by decrease of εi value and increase of Hm value of thesubstrate. It is found that the predicted values are in agreement with the experimental results. nc = 3(1?νf )γi 4(1? vf )SvibHm Efhsinθεi = 2 sinθVmREf εi 2 (2) Critical misfit of epitaxial growth metallic thin films The critical misfit of epitaxial growth metallic thin films fc is thermodynamicallyconsidered. It is found that there exists a competition between the energy of the misfitdislocation energy of film and non-coherent interface energy of film-substrate. Equilibriumbetween these energies is present at a critical atomic misfit fc. When the atomic misfit is largethan this critical value, epitaxial growth does not occur. The critical misfit of the epitaxialgrowth thin films can be predicted. The results show that fc is proportional to the non-coherentinterface energy of the film-substrate, and inversely proportional to the elastic modulus andthe thickness of the film. fc ≈SvibHm(1?νf )/VmEf[ln 2 t +1] h3. The stability of face structure for crystals...