Size Effect On The Elasticity And Thermal Stability Of Nanomaterials

With miniaturization of a solid down to nanometer scale, the classical approaches such as continuum medium mechanics and the quantum mechanics approach encounter some difficulties. Therefore, it is highly demanded to find a theoretical model to predict and explain the fascinating phenomena of elastic enhancement and thermal stability depression of nanomaterials and understand their physical origin. Nanomaterials are different from the corresponding bulk and single atom due to low coordination surface atom and its interaction.This thesis focuses on the size dependence of the elasticity and thermal stability of nanostructured ZnO, TiO₂, Ag, and Au in various shapes. Major progress is summarised as follows:1. It has been clarified that: (i) the elastic modulus is intrinsically proportional to the sum of bond energy per unit volume, (ii) the elastic modulus of ZnO, TiO₂, Ag, and Au nanostructures increases with the inverse of feature size, (iii) the size induced trend is determined by the surface-to-volume ratio, (iv) only the outermost three atomic layers contribute to the elastic enhancement yet bonds in the core interior remain as they are in the bulk, and, (v) the broken-bond-induced local strain and skin-depth energy pinning and the tunable fraction of bonds between the undercoordinated atoms. Further, BOLS correlation mechanism also has been successfully predicted and explained the size dependence of the elasticity of the nanomaterials such as TiO₂, Ag, Au.2. Theoretical reproduction of the measured size dependence of the melting point of Ag and Au nanostructures affirms that the bond in the surface skin become shorter and stronger while the cohesive energy remnant per discrete surface atom decreases, which dominates the stability and hence the size-induced depression of the melting point. Further, BOLS correlation mechanism also has been successfully predicted and explained the size dependence of thermal stability of the nanomaterials such as Pb, In, Bi.Exceedingly good agreement between predictions and measurements confirmed that if one bond breaks, the remainder between the undercoordinated atoms becomes shorter and stronger. Then, local strain and quantum trapping are formed immediately at the sites surrounding the under-coordinated atoms. Consequently, binding energy density and cohesive energy are modified, which dominate the size dependence of mechanical and thermal properties for nanomaterials, respectively. Therefore, the BOLS correlation mechanism is able to link the macroscopic continuum approach and the quantum approach.