Size-dependent Segregate Energy Of Binary Nanoalloy And Phase Diagram With Segregation

There is another world between the macroscopic one and the microscopic one, the mesoscopic world of the nanosystem where many special phenomena occur. The research on such state changes of condensed matter as melting and freezing is very important. Takagi in 1954 demonstrated for the first time that ultrafine metallic particles melt below their corresponding bulk melting temperature. It is now known that the melting temperature of all kinds of low-dimensional crystals, including metals, semiconductors and organic crystals, depends on their sizes. The melting temperatures could be higher or lower than the corresponding bulk ones depending on the surface states of low-dimensional materials. The study on superheating and surface stress related to surface phenomena is significant to stability of nanosized device.Surfaces or interfaces are the two-dimensional regions where physical and chemical properties of materials change enormously. Many important physical and chemical processes of materials firstly happen at the surfaces. The interface phenomena, which affect the materials behaviors, are in nature produced by different energetic states of molecules on the interface in comparison with those within the materials. The atomic mismatch between interfaces and flaws possibly extended result in the differences of bond lengths between interface atoms and interior ones. Since the molecular interactions on the interfaces differ from those on the interior of phases, the excess specific Gibbs free energy or interface energy for molecules of unit interface area exists. Note that interface energy is defined as the reversible work per unit area to form a new solid surface and equals the difference between the total Gibbs free energy of interface molecules and that within the phases per unit area. Nanoscience and nanotechnology are one of the rapidly developed fields in materials science and engineering in recent years. As size of materials drops to nanometer size range, interface/volume ratio increases and thus interface effect on materials properties becomes evident.Nanomaterials are characterized by the fact that the ratio of the number of surface to volume atoms is not small. In that case, they are always unstable. In order to decrease the interface energy, segregation is present. The phenomena play a big role to the characters of nanomaterials. That's because segregation changed the alloy component and structure. There were a lot of works about the segregation, but most of them are bulk materials.Moreover, we should realize that the phase diagram is important to industry application. The study of nanophase diagram may deepen the understanding for phase transition theory and extend possible industry application fields. However, a systematic study on phase equilibria among nanometer sized components related to phase diagrams is limited. Since nanophase equilibrium is metastable in nature (when temperature approaches melting temperature, the growth of nanocrystals is quicker, which leads to a continuous change of the size), direct experimental measurements are difficult to obtain. Therefore, theoretical work may be an alternative method to study nanophase diagram. It is well known that some thermodynamic quantities related to melting Gibbs free energy, such as melting temperature and melting enthalpy of components, are basic quantities to describe phase diagram. And for regular solution, atomic interaction energy among components needs to be determined. Moreover, the surface segregation effect, i.e. an enrichment of the surface layer with one of the components of the alloy, is reported or predicred for most binary tradition-metal alloys. So segregation is also can't be omitted.To solve the above problem of size-dependent thermodynamic function, the essential way lies in the transition from microscopic properties of the material or the macroscopic ones to the mesoscopic ones. Thermodynamics is a simple method to study the transition from macroscopic world to the mesoscopic one. The application of thermodynamics to nanomaterials reveals that a new branch of thermodynamics appears, i.e., nanothermodynamics. Although there are relatively extensive investigations on the size-dependent melting of nanocrystals, it has not been accompanied by the necessary investigation of the size-dependent thermodynamics of nanocrystals. A clear cognizance of the size-dependent thermodynamic function will help us to know more about the size-dependent energy transition law in the mesoscopic world. And it will stimulate the application of thermodynamics to the small system composed of hundreds of or thousands of atoms as well.In spite of an increasing body of excellent computational materials science, the classic thermodynamics remain importance to model the above phenomena. This is because that the thermodynamics theory can be extended to sizes of nanocrystals having several hundreds of molecules. Thus, the thermodynamics can be employed to discuss the changes of interface energy with 102～1023 molecules while the computer simulations often deal with the behaviors of 100～102 molecules. Not only the size span studied in the computer simulations is much smaller than that in the thermodynamics, but also the computer simulations remain many uncertainties in this size range. As results, it is not outdated and even practical with the development of nanoscience and nanotechnology to study the interface phenomena in the light of the thermodynamics, especially for the size dependences of interface phenomena.From the thermodynamics theory, we established model for size-dependent segregate energy of FCC binary systems which based on the models for size-dependent interaction energy and interface energy. Using this model, Size-dependent solution phase diagrams of binary systems are established with segregation. The detailed contents are listed as follows:1. Though the reasonable hypothesis, Size-dependent segregate energy of FCCbinary systems are established based on the models for size-dependent atomic interaction energy and interaction energy. We calculated segregate energy of different size and component. The result shows that the liquid segregate energy is larger than the solid ones. Moreover, the segregate energy of liquid and solid both are decrease with decreasing grain size.2. Through the study the Wulff structure of Cu-Ni system, the effects of size-dependent surface segregation are also being treated; it leads to further modifications of the phase diagrams. The result show that as D decreases, both the solidus and liquidus drop, and the area of two-phase zone of"core"item and surface item are decrease. As D→2D₀, which is about several nanometers, the two-phase zone even approaches zero.