Influence Of Interface Effect On Cohesive Energy And Phase-diagram Of Nanostructured Materials

With the development of material science,nanomaterials are paid more attention by a large number of researchers owing to their unique physical and chemical properties.Nanomaterials are divided into nanocrystal materials and nanostructured materials in light of the structure.Nanocrystals mean single nano particles,nanowires and nanofilms with free surfaces,while nanostructured materials are nanocrystalline materials composing nano grains with solid-solid interface.As the most commonly used materials,nanostructured materials have attracted more attention due to their applications in the fields of super large integrated circuits,thermal power generation and magnetic data storage.Size effect is one of the characters of nanomaterials.In order to boost the application of nanomaterials,the theoretical problem of size effect of nanomaterials need to be discussed.The most effective and intuitive method to resolve the size effect of nanomaterials is the thermodynamic approach.Based on the Lindemann melting criterion,the thermodynamic function of melting temperature of nanocrystal has been established.Further,the functions of cohesive energy and surface/interface energy have also been established.Accordingly,thermodynamics of materials has been expanded to the nanoscale.It is found that,as a material with free surface,the ratio of atoms locating on the surface to total atoms in nanocrystals is related to the size of particles.With the reducing of the size D,the specific surface area becoming larger.Besides,the number of bonds of atoms on the surface is insufficient compared with interior atoms,namely coordination defects.Moreover,the vibration amplitude of atoms locating on the surface is larger than that of interior atoms.Those above reasons lead to the specific properties of nanocrystals.On the other hand,the fraction of atoms at the interfaces of the nanostructured materials with solid-solid interfaces increases rapidly as the grain size reducing to nanoscale.Similarly,the atoms at the interfaces possess the coordination defect which is weaker than that of atoms on the surfaces.We predict that the change of properties induced by the interface effect is weaker than that induced only by surface effect,but there is still short of relevant theoretical work.On the basis of the model of size-dependent melting temperature of nano particles(NPs),this work established a theoretical model of size-dependent cohesive energy considering the interface effect and surface effect for nanostructured materials(NSs).The prediction results manifest the cohesive energy of NSs decreases with size reducing,similar to NPs.The difference is that the change of cohesive energy of NSs caused by interface energy is weaker than NPs,which is attributed to the smaller interface energy than surface energy.Based on this,the models of other thermodynamic properties of NSs or nanofilms(NFs)of metals and semiconductors have also been explored,including melting temperature,thermal expansion coefficient and bandgap.There is a similar change tendency of these parameters with cohesive energy.The prediction results of the model are in good agreement with the experimental results within a small error range.Secondly,with the help of interface effect,this work established the T-P nano-phase diagram of nanostructured C and BN,also considering the surface effect.For the phase boundary between solid-state phases,these two effects are introduced by considering the size-dependent additional pressure induced by the surface stress and grain boundary stress,respectively.The results show that the additional pressure induced by the surface and grain boundary effects rises with decreasing D,which is a favor of the G(graphite)-to-D(diamond)and h BN(hexagonal boron nitride)-to-c BN(cubic boron nitride)transitions.As D declines,moreover,the phase boundaries of solid-to-solid and solidto-liquid transitions displace toward low T and low P area.The prediction results of the model are in good agreement with the experimental results within a small error range.