The Evolvement And Characterization Of Low Dimension Materials Induced By Interface

One of the very basic concerns in mechanics, physics and chemistry of solids is that most properties of a solid vary depending on the microstructure, which is determined by the chemical composition, the arrangement of the atoms (the atomic structure) and the size of a solid in one, two or three dimensions. If one or several of these parameters is changed, the properties of the solids in mechanics, electronics, magnetism, optics, catalytics and thermodynamics are significantly altered from those of either the bulk solid. When the atomic structure of a solid deviates from equilibrium conditions, or the solid size is reduced to nanometer, or the solid dimensionality is smaller than three, apparent property change could be observed.Surface and interface is two-dimension area at which material physical and chemistry character breaks, many material physical and chemistry processes take place at surface and interface. At the same time, many physical and chemic phenomena are related to the surface and interface of materials. When the size of the materials is reduced to namoscale, the nature of size change is that interfaces are introduced to affect melting temperature, mechanical characters, interface morphology and so on. Lots of breakages and invalidations also start from surface and interface. It is very important that the study on microstructure of surface or interface and it's action to environment and physical or chemic phenomena related to surface or interface for control of surface physical and chemic processes and variety of surface character.In this thesis, the melting temperature, mechanical properties, the formation of monoatomic chains, interface stress and interface morphology are researched for nanocrystal.1. Summarized systematically the application on the size-dependent melting temperaturemodel for different low-dimensional crystals, the melting temperature, surface melting, melting enthalpy and melting entropy are investigated roundly in low dimensional In crystals. A unified model, free of any adjustable parameters, for the size-dependence and dimension-dependence of melting temperatures of nanocrystals is established. The predictions of the model for In crystals correspond to the results well. The size dependence of surface melting temperature Tsm(D) function is evidently weaker than that of Tm(D) function. This difference is induced by the fact that Cpm is only about one eleventh of Sm, which implies that the size of the driving force for the surface melting is much smaller than that for the melting. The size dependence of Hm(D) is stronger than that of Tm(D) because Hm(D) is a product of Tm{D) and Sm(D) where the both functions are size-dependent. The confirmation of model with experimental results indicates that the size-dependent melting is indeed a thermodynamic transition. The physical nature of the drop or increase of Hm(D) is related to the chenge of the internal energy of atoms in nanocrystals is stronger or weaker than that one of liquid with decreasing size under free surface or constrained surface respectively.2. The melting temperature is introduced to coefficients 0' kd' of Hall-Petch relationship. The effect of melting temperature on Hall-Petch relationship has been studied. As grain size decreases, the melting temperature of the nano-structured crystals decreases, the Hall-Petch relationship has its limitation and it is no longer sufficient when the grain size decreases to about 15-30 nm ranges. When the yield strength or hardness is taken as a function of reciprocal of the square root of the grain size, it has a numerical maximum omax that the interplay between dislocation and grain-boundary processes occurs during deformation. Namely, as D decreases to a certain location , grain-boundary weakening is present and the o(D) value drops gradually. Whose location depends on the size of the bulk melting enthalpy Hm of the crystals. The Hm and TA are two based parameters affecting the size-dependence of the yield stress. Since Td is typically the room temperature, a material with a...