Wave Propagation in Elastic Media with Micro/Nano-Structures

Abstract

Microand nano-scale materials and structures such as plateor beam-like structures with submicron or nano thicknesses have attracted considerable interest from the scientific community due to the increasingly strong demands of miniaturization in the fields of microelectronics and nanotechnology. More and more nano-structures, e.g. ultra-thin films, nanowires and nanotubes, have been fabricated and served as the basic building blocks for microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) (Jin et al., 1998; Craighead, 2000; Husain et al., 2003; Feng et al., 2007). For long-term stability and reliability of various devices at nanoscale, researchers should possess a deep understanding and knowledge of mechanical properties of nano materials and structures, especially for dynamic properties. Among many techniques, high-frequency acoustic wave technique has been regarded as one of very efficient nondestructive methods to characterize elastic media with microor nanostructures. (Hernaandez et al., 2002) used high-frequency laser-excited guided acoustic waves to estimate the in-plane mechanical properties of silicon nitride membranes. Mechanical properties and residual stresses in the membranes were evaluated from measured acoustic dispersion curves. The mean values of the Young’s modulus and density of three nanocrystalline diamond films and a free standing diamond plate were determined by analyzing the dispersion of laser-generated surface waves (Philip et al., 2003). Recently, growing interest of using terahertz (THz) waves in nanoscale materials and nano-photonic or nano-phononic devices has opened a new topic on the wave characteristics of nanomaterials (Schneider et al., 2000, Vollmannn et al., 2004; Ramprasad & Shi, 2005; Sampathkumar et al., 2006). As dimensions of the material become smaller, however, their resistance to deformation is increasingly determined by internal or external discontinuities (such as surfaces, grain boundary, strain gradient, and dislocation). Although many sophisticated approaches for predicting the mechanical properties of nanomaterials have been reported, few addressed the challenges posed by interior nanostructures such as the surfaces, interfaces, structural discontinuities and deformation gradient of the nanomaterials under extreme loading conditions. The use of atomistic simulation may be a potential solution in the long run. However, it is well known that the capability of this approach is