The Size-effect On Alloying Ability, Phase-transition And Grain Growth Of Nanocrystal

Nanocrystal is metastable comparable to the bulk result from the inner energy unequilibrium distribution in thermodynamics. A high percentage of atoms are located in the surface or the boundary of nanocrystal with the enormous surface area to volume ratio. The surface atoms of a particle occupy a higher energy state due to the fact that those atoms have a lot of dangling-bonds result from lower coordination number, which will lead to the atoms mobility increasing and then affect the cohesive energy, melting temperature, formation enthalpy, vacancy formation energy, diffussion activation energy and the critical grain growth temperature of nanocrystal. Considering the alteration of size-dependent thermal properties and the surface-effect, we investigated the size-effect of alloying ability, phase transition as well as thermal stability result from the thermal behavior variation of inner atoms in nanocrystal.The mobility changing of surface atoms in nanoparticle lead to the fluctuation in alloying ability, which presented as the strengthen and weaken for immiscible and miscible in bulk separately. We investigated the size-effect of alloying ability of them with different model and method.On basis of Surface-difference-area model and the energy variation, we presented a method to calculate the formation enthalpy of alloy nanoparticle and investigated the alloying ability of Au/Pt which is immiscible in bulk. The results illustrate that the formation enthalpy of Au-Pt alloy nanoparticle turns to positive from negative in nanoscale, which imply that it is possible to form stable alloy.For the miscible metal in bulk, the effects of grain size and composition on the formation enthalpy of nano binary Ti-based alloy are investigated by taking the surface effect into account within the Sub-Regular model. It is demonstrated that the formation enthalpy of binary Ti based alloy with nano grains is size-dependence and exhibits evident surface effects. The formation enthalpy increases with the size decrease, and its value turns from negative to positive at a critical size, which will weaken the thermal stability of the nano grains and lead to phase transition. Furthermore, the composition segregation taking place in the nano grains of the Ti based alloy is obvious when the grain size is less than 10 nm. The element and degree of segregation are dependent on the segregation driving energy, which is depend on the surface energy and the atom radius.Based on the ideal solution approximation, the model for size-dependent melting temperature of pure metal nanoparticles is extended to binary alloy systems. The developed model, free of any adjustable parameter, demonstrates that the melting temperature is related to the size and composition of alloy nanoparticles. When the component is fixed, the melting temperature reduces with the size decreasing. The melting temperature of Cu/Ni, Pb/Bi, Sn/In binary alloy nanocrystals are found to be consistent with the experiments and molecular dynamics simulations. It is revealed that alloy nanocrystals have similar melting mechanism as pure metal while the intrinsic characters of atoms are homologous and the ideal solution approximation is a valid approximate method in nanoscale.According to the intrinsic relationship between phase transition pressure and cohesive energy, a simple quantitative thermodynamic model is proposed to describe the size-dependence of phase transition pressure in nanocrystals. For pressure-induced phase transition, the band-strength of atoms in nanoparticle decreases result from the cohesive energy reducing, which will lead to the phase transition pressure decrease. In terms of this model, it is revealed that the stability of nanostructures drops and the phase transition pressure reduces with size decreasing. As the size further decreases, phase transition pressure exhibits stronger size effect. The predicted results are consistent with MD simulations for GaAs nanocrystals reasonably.The intrinsic thermodynamical factor to dominate the stability of nanocrystalline was investigated through the microcosmic process of grain growth. It is suggested that the nanocrystalline grow at a certain temperature and the critical temperature is determined by the vacancy formation energy and diffussion activation energy of nanocrystalline. Based on a series of hypothesis, a simple model is proposed to predict the size-dependent critical temperature of grain growth. Within this model, we investigated the thermal stability of nanocrystalline?and Au and compare with available results. The results illustrate that the critical temperature decreases with the size reducing, showing evident size-effect. It is revealed that the thermal stability is depend on the energetic state of nanocrystaline and the mobility of the inner atoms.Understanding the size-effct on physical and chemical natures of material behind the new properties may provide guides for the design and fabrication of new materials and their possible industrial applications.