Investigation On The Size Shape Effect Of Thermal Stability And Phase-transition Of Nanocrystalline

Nanocrystal materials,which are widely used in the fields of medicine,national defense and nano-ceramics,have many excellent properties,such as high strength,good plastic deformation ability,and high specific heat,and so on.In recent years,many researchers have carried out thorough research on nanocrystal materials from all around the world.Because of the surface effect and small size effect in the structure of nanocrystal materials,the surface atoms have higher activity.Under certain conditions,the nano particles grow up or undergo phase transition,thus losing the unique properties of the nanocrystal materials.Lu research group established the size model for critical temperature of nano particles growth and pressure induced phase transition in GaAs nanocrystals.In the above model,the shape of the nano particles is spherical,however,the actual nano polycrystal materials are polyhedron.Start from the practical facts,in this work,taking into account the size and shape effects on the thermodynamic properties according with the surface effect,we study the pressure induced phase transition,the dependence of nanocrystal growth critical temperature on size and shape effects underthe influence of thermodynamic behavior of nanocrystal atoms.The purpose of this study is to provide a possible way to explore the new nano-materials under high pressure in the field of materials,and to provide theoretical guidance for the interpretation and prediction of nanocrystal reconstruction type pressure induced phase transition.Meanwhile,the study on the nanocrystal growth critical temperature is an effective and practical model for how to maintain the thermal stability and other superior performance,and to improve the working temperature range of nano-materials.Under the external condition of heating up,the crystal grains of nanocrystal grow up into ordinary coarse crystalline materials,and the material will lose its excellent physical and chemical properties.In order to study the dependence of nanocrystal growth critical temperature on size and shape effects,starting from the existing microscopic mechanism of grain growth,based on size model that the critical temperature of crystal growth is proportional to the product of vacancy formation and spreading activation energy,we build a critical temperature model depended on the size and shape effects considering the effect of nanocrystal shape on the binding energy.Using this model,the thermal stability of different shapes vanadium and gold nanocrystals are predicted.Molecular dynamics simulation and experimental results show that the nanocrystal growth critical temperature decreases with the decrease ofgrain size,implying the clear size dependence;besides,at the same grain size,the critical temperature decreases with the increase of grain shape factor,showing a certain shape dependence.The results show that the nanocrystal growth critical temperature depends on the size and shape of the grain,and also illustrate that the thermal stability of nanocrystals is related to the energy state and activity of the internal,edge and vertex atoms.Based on the relationship between the pressure induced phase transition and grain binding energy,considering the influence of grain shape on the binding energy,we establish a thermodynamic model to describe the nanocrystal size and the shape of the pressure induced phase transition.The change of binding energy due to nanocrystal size and shape,the bond strength between atoms in the nanocrystal changes,which affects the transition pressure.According to the model,with the nanocrystal size decreasing the transition pressure decreased significantly,showing significant size dependence;moreover,at the same grain size,the phase transition pressure decreases with the increase of grain shape factor,showing obvious shape effect.The results are consistent with the molecular dynamics simulation in predicting the size and shape dependence of the phase transition pressure of GaAs nanocrystals.The study on the size and shape effects of nano-materials can provide theoretical guidance for the further exploration of materialsscience,the preparation and application of high performance materials,and have potential application value.