Modeling the material structure and couple stress effects of nanocrystalline silicon beams for pull-in and bio-mass sensing applications

Abstract

Designed devices for pull-in and bio-mass sensing applications are usually composed of elastic micro/nano-sized beams made of nanostructured materials, such as nanocrystalline silicon (Nc-Si). In addition to the higher-order deformations that ever present in these beams, their material properties are highly affected with the heterogeneity features of their material structure. Neglecting one of these inherent parameters in the modeling of these systems can result in inaccurate predictions of their behaviors. In this research study, the effects of the material structure along with the microstructure couple stress of an electrostatically-actuated micro/nanocantilever beam, made of Nc-Si, on its pull-instability and sensitivity to bio-cells are investigated. Considering the material structure inhomogeneity of the Nc-Si and the beam׳s size, we propose a novel model that combines a couple stress based-Euler–Bernoulli beam theory and a micromechanical model. To account for the beam׳s size effect, the modified couple stress theory is used. The heterogeneity nature of the material structure is modeled using a size-dependent micromechanical model that includes the interface and the grain size effects. Performing a parametric study, the results show that including the effects of the material structure and couple stress strongly affects the pull-in voltage values and the estimated masses of the bio-cells. Unlike the couple stress effects, it is demonstrated that the inhomogeneity nature of the material structure softens the beam and hence decreases its natural frequency and pull-in voltage. It is also shown that the couple stress effect enhances the sensitivity of the bio-mass sensor unlike the interface and the grain size effects. The results show that using higher-order modes leads to a significant enhancement in the sensitivity of the bio-mass sensor.