In this thesis, the size and temperature-dependent mechanical behaviors of nanomaterials were studied using molecular dynamics simulations (MD) and first-principles calculations.

First, the surface eigenstress model was adopted to extract the interested mechanical properties and the orientation effect on the size dependency of Young’s moduli of thin films was examined. MD and first-principles calculations were performed on Au (111) and Au (110) thin films to verify the model predictions and to obtain material properties. The origin of the strengthening effects was found by calculating the charge density of the films to be the electron redistribution, which enhances the bonding strength between surface atoms.

The surface eigenstress model was also further developed to study the su...[ Read more ]

In this thesis, the size and temperature-dependent mechanical behaviors of nanomaterials were studied using molecular dynamics simulations (MD) and first-principles calculations.

First, the surface eigenstress model was adopted to extract the interested mechanical properties and the orientation effect on the size dependency of Young’s moduli of thin films was examined. MD and first-principles calculations were performed on Au (111) and Au (110) thin films to verify the model predictions and to obtain material properties. The origin of the strengthening effects was found by calculating the charge density of the films to be the electron redistribution, which enhances the bonding strength between surface atoms.

The surface eigenstress model was also further developed to study the surface-induced size-dependent ultimate tensile strength of thin films. First-principles calculations and MD simulations were conducted on Au and Si (100) thin films with various thicknesses. The atomistic calculations show that the ultimate tensile strength of Au thin films increases when the film thickness decreases, a typical the thinner the stronger phenomenon, while the Si thin films exhibit the thinner the weaker behavior. Nevertheless, both behaviors were perfectly predicted by a nonlinear scaling law developed from the surface eigenstress model. Moreover, the first-principles calculations put insight into the surface induced strengthening or weakening mechanism. The surface strengthening and weakening phenomena were explained from an electronic structure point of view.

The temperature effect was then included in the surface eigenstress model by explicitly considering the temperature dependence of the physical parameters. Nanometer-thick thin films were taken as an ideal system to investigate the surface-induced temperature- and size-dependent young’s modulus and size-dependent thermal expansion coefficient. The theoretical modeling was conducted on the basis of the surface eigenstress model with the consideration of thermal expansion, leading to analytic formulas of temperature- and size-dependent Young’s modulus and size-dependent thermal expansion coefficient of thin films. To verify the model, MD simulations on Ag, Cu and Ni (100) thin films were conducted at temperatures ranging from 300 K to 600 K and the good agreement between the MD results and the theoretical predictions justify the validity of the model. The newly developed surface eigenstress model will be able to attack the similar problems in other types of nanomaterials.