Thermal Dissipation and Variability in Electrical Breakdown of Carbon Nanotube Devices

We study high-field electrical breakdown and heat dissipation from carbon nanotube (CNT) devices on SiO2 substrates. The thermal "footprint" of a CNT caused by van der Waals interactions with the substrate is revealed through molecular dynamics (MD) simulations. Experiments and modeling find the CNT-substrate thermal coupling scales proportionally to CNT diameter and inversely with SiO2 surface roughness (~d/{\Delta}). Comparison of diffuse mismatch modeling (DMM) and data reveals the upper limit of thermal coupling ~0.4 W/K/m per unit length at room temperature, and ~0.7 W/K/m at 600 C for the largest diameter (3-4 nm) CNTs. We also find semiconducting CNTs can break down prematurely, and display more breakdown variability due to dynamic shifts in threshold voltage, which metallic CNTs are immune to; this poses a fundamental challenge for selective electrical breakdowns in CNT electronics

Dehydration-induced corrugated folding in Rhapis excelsa plant leaves

Plant leaves, whose remarkable ability for morphogenesis results in a wide range of petal and leaf shapes in response to environmental cues, have inspired scientific studies as well as the development of engineering structures and devices. Although some typical shape changes in plants and the driving force for such shape evolution have been extensively studied, there remain many poorly understood mechanisms, characteristics, and principles associated with the vast array of shape formation of plant leaves in nature. Here, we present a comprehensive study that combines experiment, theory, and numerical simulations of one such topic—the mechanics and mechanisms of corrugated leaf folding induced by differential shrinking in Rhapis excelsa. Through systematic measurements of the dehydration process in sectioned leaves, we identify a linear correlation between change in the leaf-folding angle and water loss. Building on experimental findings, we develop a generalized model that provides a scaling relationship for water loss in sectioned leaves. Furthermore, our study reveals that corrugated folding induced by dehydration in R. excelsa leaves is achieved by the deformation of a structural architecture—the “hinge” cells. Utilizing such connections among structure, morphology, environmental stimuli, and mechanics, we fabricate several biomimetic machines, including a humidity sensor and morphing devices capable of folding in response to dehydration. The mechanisms of corrugated folding in R. excelsa identified in this work provide a general understanding of the interactions between plant leaves and water. The actuation mechanisms identified in this study also provide insights into the rational design of soft machines

Separating Beads and Cells in Multi-channel Microfluidic Devices Using Dielectrophoresis and Laminar Flow

Microfluidic devices have advanced cell studies by providing a dynamic fluidic environment on the scale of the cell for studying, manipulating, sorting and counting cells. However, manipulating the cell within the fluidic domain remains a challenge and requires complicated fabrication protocols for forming valves and electrodes, or demands specialty equipment like optical tweezers. Here, we demonstrate that conventional printed circuit boards (PCB) can be used for the non-contact manipulation of cells by employing dielectrophoresis (DEP) for bead and cell manipulation in laminar flow fields for bioactuation, and for cell and bead separation in multichannel microfluidic devices. First, we present the protocol for assembling the DEP electrodes and microfluidic devices, and preparing the cells for DEP. Then, we characterize the DEP operation with polystyrene beads. Lastly, we show representative results of bead and cell separation in a multichannel microfluidic device. In summary, DEP is an effective method for manipulating particles (beads or cells) within microfluidic devices

Discrete differential geometry-based model for nonlinear analysis of axisymmetric shells

In this paper, we propose a novel one-dimensional (1D) discrete differential geometry (DDG)-based numerical method for geometrically nonlinear mechanics analysis (e.g., buckling and snapping) of axisymmetric shell structures. Our numerical model leverages differential geometry principles to accurately capture the complex nonlinear deformation patterns exhibited by axisymmetric shells. By discretizing the axisymmetric shell into interconnected 1D elements along the meridional direction, the in-plane stretching and out-of-bending potentials are formulated based on the geometric principles of 1D nodes and edges under the Kirchhoff-Love hypothesis, and elastic force vector and associated Hession matrix required by equations of motion are later derived based on symbolic calculation. Through extensive validation with available theoretical solutions and finite element method (FEM) simulations in literature, our model demonstrates high accuracy in predicting the nonlinear behavior of axisymmetric shells. Importantly, compared to the classical theoretical model and three-dimensional (3D) FEM simulation, our model is highly computationally efficient, making it suitable for large-scale real-time simulations of nonlinear problems of shell structures such as instability and snap-through phenomena. Moreover, our framework can easily incorporate complex loading conditions, e.g., boundary nonlinear contact and multi-physics actuation, which play an essential role in the use of engineering applications, such as soft robots and flexible devices. This study demonstrates that the simplicity and effectiveness of the 1D discrete differential geometry-based approach render it a powerful tool for engineers and researchers interested in nonlinear mechanics analysis of axisymmetric shells, with potential applications in various engineering fields.Comment: 36 pages, 11 figure

Interfacial cracks between piezoelectric and elastic materials under in-plane electric loading

Evaluation of crack tip driving force for interfacial cracks between piezoelectric actuators and elastic substrates is crucial to successful applications of smart materials and smart structures. Here the behavior of an interfacial crack between a piezoelectric material and an elastic material in under in-plane loading is studies. The displacement mismatch along a bonded interface due to electric potential loading on the piezoelectric material is modeled by an array of uniformly distributed dislocations along the interface. Using Fourier transformation method, the governing equations are converted to an integral equation, which is then converted to a standard Hilbert problem. A closed form solution for stresses, electric field, and electric displacements along the bonded interface is obtained. The results agree very well with that from numerical simulations using the finite element method. The results show that the closed form solution is not only accurate for far field distributions of stresses and electric variables, but also accurate for the asymptotic distributions near the crack tip. The solution also suggests the likelihood of domain switch in the piezoelectric material near the crack tip.published or submitted for publicationis peer reviewe

Experimental investigation of the bond coat rumpling instability under isothermal and cyclic thermal histories in thermal barrier systems

Reliable life prediction models for the durability of thermal barrier coatings require the identification of the relative importance of various mechanisms responsible for the failure of the coatings at high temperatures. Studies of these mechanisms in sub-systems of thermal barrier coatings can provide valuable information. In the present work, we undertake an experimental study of "rumpling", or progressive roughening of the bond coat surface in the bond coat-superalloy systems upon high temperature exposure. Thermal cycling and isothermal experiments are carried out on a platinum-aluminide bond coat and on a NiCoCrAlY bond coat deposited on a Ni-based superalloy in air and in vacuum. The cyclic experiments are conducted in air from 200°C to 1200°C for different levels of initial roughness of the bond coat surfaces. Isothermal experiments are carried out at various temperatures, ranging from 960°C to 1200°C. The bond coat surfaces in cyclic experiments rumple to a similar characteristic wavelength of about 60-100 µm and an amplitude varying from 2 µm to 5 µm. Additional small scale fluctuations are seen to develop between the thermally grown oxide (TGO) and the bond coat surface with a wavelength of about 3-5 µm. Smooth initial bond coat surfaces (fluctuations in tens of nanometers) are seen to have rumpled, indicating that significant initial flaws are not required for rumpling to occur. Observations of the rumpled bond coat edges are shown to indicate that bond coat stresses play a dominant role during the rumpling process. On comparing the experimental observations with existing rumpling models in literature, it is concluded that the TGO and the microstructural changes in the bond coat have a rather limited role in inducing rumpling. Diffusion driven by thermal mismatch stress in the bond coat is likely to be the dominant mechanism during rumpling.published or submitted for publicationis peer reviewe

Influence of surface morphology on the adhesive strength of aluminum/epoxy interfaces

Adhesively bonded aluminum joints have been increasingly used in automotive industry because of their structural and functional advantages. Interfacial debonding in these joints has become a major concern limiting their performance. The present work is focused on experimental investigation of the influence of surface morphology on the interfacial fracture behavior of aluminum/epoxy interface. The specimens used in this experimental study were made of an aluminum/epoxy bimaterial stripe in the form of a layered double cantilever beam (LDCB). The LDCB specimens were debonded by peeling off the epoxy layer from the aluminum substrate using a steel wedge. Interfacial fracture energy was extracted from the debonding length by using a solution for the specimen geometry based on a model of a beam on an elastic foundation. This model was validated by direct finite element analysis. The experimental observations establish a direct correlation between the surface roughness of aluminum substrate and the fracture resistance of the aluminum/epoxy interface. The results emphasize the importance of choosing surface features at an appropriate length scale in studying their effects on interfacial fracture resistance.published or submitted for publicationis peer reviewe

In situ X-ray diffraction study of electric field induced domain switching and phase transition in PZT-5H

In-situ x-ray experiments were conducted to examine the electric-field-induced phase changes in PZT-5H materials. The x-ray diffraction profiles at different electric field levels were analyzed by peak fitting and used to identify the occurrence of non-180?? domain switching and phase transition. We found that, in depolarized samples, there exists a threshold electric field for the phase changes; whereas in polarized samples, no such threshold exists. The profound difference in the diffraction profile changes under positive and negative electric fields in polarized samples is responsible for the asymmetry of piezoelectric effects. Peak fitting results show composition and transition of phases as well as domain switching at different electric field levels. These observations further indicate the importance of residual stresses in materials behaviors.published or submitted for publicationis peer reviewe

Rumpling instability in thermal barrier systems under isothermal conditions in vacuum

Bond coat (BC) surface rumpling has been identified as one of the important mechanisms that can lead to failure of the thermal barrier coatings. The driving force behind rumpling—whether the stresses in the thermally grown oxide over the BC or the stresses in the BC—remains to be clarified. Meanwhile, the mass transport mechanisms in the BC leading to rumpling are not clearly identified. In the present investigation, we subject two types of BC-superalloy systems, nickel aluminide and platinum aluminide BCs on a Ni-based superalloy, to isothermal exposure at temperatures ranging from 1150 to 1200°C in vacuum. The results show that the nickel aluminide BC rumples at 1200°C and at 1175°C in absence of significant oxidation. The wavelength of the rumpled surfaces was 60–100 µm, with an amplitude of 5–8 µm. The rumpling was insensitive to the initial BC surface morphology. At 1150°C, no clear rumpling was observed, but some surface undulations could be seen related to the BC grains. ...[more]...published or submitted for publicationis peer reviewe