Size Effects at Nano-Scale: Governing Mechanism and Efficient Simulations

Abstract

Materials at nano-scale exhibit interesting behaviors in mechanical and other aspects. For instance, the Young’s modulus of nano-scale samples demonstrates distinct size effects in both experiments and simulations, either increases or decreases with decreasing sample size. So, the challenging issue is: what mechanism underlies the opposite size effects and how to simulate the size effects correctly and efficiently in various possible applications In our work on size effect at nano-scale, it is found that the intermolecular potentials can significantly affect surface lattice and result in different size effects. For example, since L-J potential produces a repulsive equilibrium state in bulk, it would lead to a looser surface structure than bulk once a new surface is created, then results in the decrease of Young’s modulus when sample size decreases. While Buckingham pair potential (with Coulomb interaction) induces the opposite size effect, owing to a denser surface resulting from an attractive equilibrium state. These findings have been justified with 1D and 2D analysis or the comparison to experimental observations, [1-3]. For larger nano systems with more complex potentials, efficient simulations are badly needed. For this sake, a coupled method, MST/CST (Molecule and Cluster Statistical Thermodynamics) has been developed [3-4]. This method could bridge the gaps in both spatial and temporal scales involved in size effects. Practically, the MST/CST method is equal to MD to characterize the deformation of atomic lattice, phase transformation, etc, but it can beat the latter in computational efficiency.