Showing papers by "Xiaoding Wei published in 2011"

Failure mechanisms in composite panels subjected to underwater impulsive loads

Abstract: This work examines the performance of composite panels when subjected to underwater impulsive loads. The scaled fluid-structure experimental methodology developed by Espinosa and co-workers was emp ...

Robust carbon-nanotube-based nano-electromechanical devices: understanding and eliminating prevalent failure modes using alternative electrode materials.

Abstract: The International Technology Roadmap for Semiconductors (ITRS [ 1 ] ) identifi es emerging technologies with the potential to sustain Moore’s Law. A necessary succession from planar CMOS (complementary metal-oxide semiconductors) to nonplanar/dual-gate CMOS, and ultimately to novel architectures such as carbon nanotube (CNT)-based nano-electromechanical systems (NEMS) is envisioned. The ITRS also identifi es critical roadblocks currently precluding advances beyond CMOS. Primary among the roadblocks to NEMS are poor reliability and manufacturing challenges. Here we investigate the prevalent failure modes of CNT-based NEMS that hamper reliability through a combined experimental–computational approach. We fi rst identify their point of onset within the design space through in situ electromechanical characterization, highlighting the extremely limited region in which failure is avoided. We use dynamic multiphysics models to elucidate the underlying causes of failure, then return to the experimental characterization to show that the usable design space expands dramatically when employing novel electrode materials such as diamondlike carbon. Finally, we demonstrate the effi cacy of this solution through 100 successive actuation cycles without failure and applications to volatile memory operations. The immense potential of CNT-based NEMS is emergent in theoretical and experimental demonstrations of up to 100-GHz switching, [ 2 ] low leakage, and high ON–OFF ratios, [ 3 ] and outstanding current-carrying capacity. [ 4 , 5 ] To date however, individual demonstrations of performance such as these have been a primary focus, with limited reports of repeated actuation beyond a few cycles. [ 2 , 3 , 6 , 7 ] This is due

Substrate stiffness regulates extracellular matrix deposition by alveolar epithelial cells

Abstract: AIM: The aim of the study was to address whether a stiff substrate, a model for pulmonary fibrosis, is responsible for inducing changes in the phenotype of alveolar epithelial cells (AEC) in the lung, including their deposition and organization of extracellular matrix (ECM) proteins. METHODS: Freshly isolated lung AEC from male Sprague Dawley rats were seeded onto polyacrylamide gel substrates of varying stiffness and analyzed for expression and organization of adhesion, cytoskeletal, differentiation, and ECM components by Western immunoblotting and confocal immunofluorescence microscopy. RESULTS: We observed that substrate stiffness influences cell morphology and the organization of focal adhesions and the actin cytoskeleton. Surprisingly, however, we found that substrate stiffness has no influence on the differentiation of type II into type I AEC, nor does increased substrate stiffness lead to an epithelial-mesenchymal transition. In contrast, our data indicate that substrate stiffness regulates the expression of the α3 laminin subunit by AEC and the organization of both fibronectin and laminin in their ECM. CONCLUSIONS: An increase in substrate stiffness leads to enhanced laminin and fibronectin assembly into fibrils, which likely contributes to the disease phenotype in the fibrotic lung.

Design and identification of high performance steel alloys for structures subjected to underwater impulsive loading

Abstract: Abstract Martensitic and austenitic steel alloys were designed to optimize the performance of structures subjected to impulsive loads. The deformation and fracture characteristics of the designed steel alloys were investigated experimentally and computationally. The experiments were based on an instrumented fluid–structure interaction apparatus, in which deflection profiles are recorded using a shadow Moire technique combined with high speed imaging. Fractographic analysis and post-mortem thickness reduction measurements were also conducted in order to identify deformation and fracture modes. The computational study was based on a modified Gurson damage model able to accurately describe ductile failure under various loading paths. The model was calibrated for two high performance martensitic steels (HSLA-100 and BA-160) and an austenitic steel (TRIP-120). The martensitic steel (BA-160) was designed to maximize strength and fracture toughness while the austenitic steel (TRIP-120) was designed to maximize uniform ductility, in other words, to delay necking instability. The combined experimental–computational approach provided insight into the relationships between material properties (strength, uniform ductility, and post-necking ductility) and blast resistance of structures. In particular, the approach allowed identification of material/structure performances by identifying impulse-center deflection behavior and the impulse leading to panel fracture.

Electrodes to improve reliability of nanoelectromechanical systems

Abstract: The present invention provides for replacement of conventionally- used metal electrodes in NEMS devices with electrodes that include non-metallic materials comprised of diamond-like carbon or a dielectric coated metallic film having greater electrical contact resistance and lower adhesion with a contacting nanostructure. This reduces Joule heating and stiction, improving device reliability.

Arrays of Robust Carbon Nanotube-Based NEMS: A Combined Experimental/Computational Investigation

Abstract: We present an investigation of electrostatically-actuated carbon nanotube-based nanoelectromechanical switches. The primary goal of this study is to create a metric for design of robust, high-cycle devices. Methods for fabricating arrays of freestanding carbon nanotubes are discussed. Parametric studies, both experimental and computational, are then conducted to elucidate the failure mechanisms common to this class of carbon nanotubebased nanoelectromechanical systems, and to identify their point of onset within the design space. Experiments are performed in situ the scanning electron microscope, enabling direct imaging of device operation and the mode of eventual failure. Complimentary dynamic multiphysics finite element simulations of device operation are also presented to investigate the underlying mechanisms of the experimentally-observed failure modes.