Turbulence in Rotating, Stratified and Electrically Conducting Fluids

This book investigates how turbulence responds to rotation, stratification or magnetic fields, identifying common themes, where they exist, as well as the essential differences which inevitably arise between different classes of flow.

A highly conductive and transparent solution processed AZO/MWCNT nanocomposite

A solution processed aluminum-doped zinc oxide (AZO)/multi-walled carbon nanotube (MWCNT) nanocomposite thin film has been developed offering simultaneously high optical transparency and low electrical resistivity, with a conductivity figure of merit (σDC/σopt) of ~75-better than PEDOT:PSS and many graphene derivatives. The reduction in sheet resistance of thin films of pristine MWCNTs is attributed to an increase in the conduction pathways within the sol-gel derived AZO matrix and reduced inter-MWCNT contact resistance. Films have been extensively characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffractometry (XRD), photoluminescence (PL), and ultraviolet-visible (UV-vis) spectroscopy. © 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Fabrication and integration of horizontally aligned carbon nanotube C60 films for UV sensors

Carbon nanotubes (CNTs) are promising for microsystems applications, yet few techniques effectively enable integration of CNTs with precise control of placement and alignment of the CNTs at sufficiently high densities necessary for compelling mechanical or electrical performance. This paper explores new methods for scalable integration of dense, horizontally aligned (HA) CNTs with patterned electrodes. Our technique involves the synthesis of vertically aligned (VA) CNTs directly on a conductive underlayer and subsequent mechanical transformation into HA-CNTs, thus making electrical contact between two electrodes. We compare elasto-capillary folding and mechanical rolling as methods for transforming VA-CNTs, which lead to distinctly different HA-CNT morphologies and potentially impact material and device properties. As an example application of this novel CNT morphology, we investigate fabrication of electrically addressable CNT-C60 hybrid thin films that we previously demonstrated as photodetectors. We synthesize these assemblies by crystallizing C60 from dispersion on HA-CNT thin-film scaffoldings. HA-CNTs fabricated by rolling result in relatively low packing density, so C 60 crystals embed inside the HA-CNT matrix during synthesis. On the other hand, C60 crystallization is restricted to near the surface of HA-CNT films made by the elasto-capillary process. © 2013 IEEE.

Development of all metal electrothermal actuator and its applications

The in-plane motion of microelectrothermal actuator ("heatuator") has been analyzed for Si-based and metallic devices. It was found that the lateral deflection of a heatuator made of a Ni metal is about ∼60% larger than that of a Si-based actuator under the same power consumption. Metals are much better for thermal actuators as they provide a relatively large deflection and large force, for a low operating temperature and power consumption. Electroplated Ni films were used to fabricate heatuators. The electrical and mechanical properties of electroplated Ni thin films have been investigated as a function of temperature and plating current density, and the process conditions have been optimized to obtain stress-free films suitable for microelectromechanical systems applications. Lateral thermal actuators have been successfully fabricated, and electrically tested. Microswitches and microtweezers utilizing the heatuator have also been fabricated and tested. © 2005 Society of Photo-Optical Instrumentation Engineers.

ZnO film based surface acoustic wave micro-pump

In this study, a micro-pump unit based on surface acoustic wave (SAW) on piezoelectric ZnO film is designed and fabricated as a micro-fluidic device. It employs a mechanical wave, which is generated electrically using an aluminum interdigital transducer (IDT), and propagates on the surface of the ZnO film. The ZnO film was used in this study because it has a high electromechanical coefficient and an excellent bonding with various substrate materials, in particular silicon. The sputtering parameters for ZnO film deposition have been optimized, and the ZnO films with different thickness from 1 micron to 5.5 microns were prepared. The film properties have been characterized using different methods, such as scanning electron microscopy, X-ray diffraction and atomic force microscopy. Aluminum IDT with a finger width and spacing of 8 microns was patterned on the ZnO film using a lift-off process. The frequency generated was measured using a network analyzer, and it varies from 130 MHz to 180 MHz as a function of film thickness. A signal generator was used to generate the frequency with a power amplifier to amplify the signal, which was then applied to aluminum IDT to generate the surface acoustic wave. If a liquid droplet exists on the surface carrying the acoustic wave, the energy and the momentum of the SAW will be coupled into the fluid, causing the liquid to vibrate and move on film surface. The strength of this movement is determined by the applied voltage and frequency. The volume of the liquid drop loaded on the SAW device in this study is of several hundreds of nanoliters. The movement of the liquid inside the droplet and also on the ZnO film surface can be demonstrated. The performance of ZnO SAW device was characterized as a function of film thickness. © 2007 IOP Publishing Ltd.

Effects of mechanical and electrical coupling on the parametric sensitivity of mode localized sensors

We compare and contrast the effects of two distinctly different mechanisms of coupling (mechanical and electrical) on the parametric sensitivity of micromechanical sensors utilizing mode localization for sensor applications. For the first time, the strong correlation between mode localization and the phenomenon of 'eigenvalue loci-veering' is exploited for accurate quantification of the strength of internal coupling in mode localized sensors. The effects of capacitive coupling-spring tuning on the parametric sensitivity of electrically coupled resonators utilizing this sensing paradigm is also investigated and a mass sensor with sensitivity tunable by over 400% is realized. ©2009 IEEE.

Electron microscopy of polymer-carbon nanotubes composites

Characterization of polymer nanocomposites by electron microscopy has been attempted since last decade. Main drives for this effort were analysis of dispersion and alignment of fillers in the matrix. Sample preparation, imaging modes and irradiation conditions became particularly challenging due to the small dimension of the fillers and also to the mechanical and conductive differences between filler and matrix. To date, no standardized dispersion and alignment process or characterization procedures exist in the trade. Review of current state of the art on characterization of polymer nanocomposites suggests that the most innovative electron and ion beam microscopy has not yet been deployed in this material system. Additionally, recently discovered functionalities of these composites, such as electro and photoactuation are amenable to the investigation of the atomistic phenomena by in situ transmission electron microscopy. The possibility of using innovative thinning techniques is presented. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Transparent electrode with a nanostructured coating.

Using single-walled nanotubes as an example, we fabricated transparent conductive coatings and demonstrated a new technique of centrifuge coating as a potential low-waste, solution-based batch process for the fabrication of nanostructured coatings. A theoretical model is developed to account for the sheet resistance exhibited by layered random-network coatings such as nanofilaments and graphene. The model equation is analytical and compact, and allows the correlation of very different scaling regimes reported in the literature to the underlying coating microstructure. Finally, we also show a refined experimental setup to systematically measure the curvature-dependent sheet resistance.

A resonant micromachined electrostatic charge sensor

A micromachined electrometer, based on the concept of a variable capacitor, has been designed, modeled, fabricated, and tested. The device presented in this paper functions as a modulated variable capacitor, wherein a dc charge to be measured is up-modulated and converted to an ac voltage output, thus improving the signal-to-noise ratio. The device was fabricated in a commercial standard SOI micromachining process without the need for any additional processing steps. The electrometer was tested in both air and vacuum at room temperature. In air, it has a charge-to-voltage conversion gain of 2.06 nV/e, and a measured charge noise floor of 52.4 e/rtHz. To reduce the effects of input leakage current, an electrically isolated capacitor has been introduced between the variable capacitor and input to sensor electronics. Methods to improve the sensitivity and resolution are suggested while the long-term stability of these sensors is modeled and discussed. © 2006 IEEE.

Methods for enhanced electrical transduction and characterization of micromechanical resonators

This paper details the design and enhanced electrical transduction of a bulk acoustic mode resonator fabricated in a commercial foundry MEMS process utilizing 2.5 μm gaps. The I-V characteristics of electrically addressed silicon resonators are often dominated by capacitive parasitics, inherent to hybrid technologies. This paper benchmarks a variety of drive and detection principles for electrostatically driven square-extensional mode resonators operating in air via analytical models accompanied by measurements of fabricated devices with the primary aim of enhancing the ratio of the motional to feedthrough current at nominal operating voltages. In view of ultimately enhancing the motional to feedthrough current ratio, a new detection technique that combines second harmonic capacitive actuation and piezoresistive detection is presented herein. This new method is shown to outperform previously reported methods utilizing voltages as low as ±3 V in air, providing a promising solution for low voltage CMOS-MEMS integration. To elucidate the basis of this improvement in signal output from measured devices, an approximate analytical model for piezoresistive sensing specific to the resonator topology reported here is also developed and presented. © 2010 Elsevier B.V. All rights reserved.

Enhanced transduction methods for electrostatically driven MEMS resonators

Electrically addressed silicon bulk acoustic wave microresonators offer high Q solutions for applications in sensing and signal processing. However, the electrically transduced motional signal is often swamped by parasitic feedthrough in hybrid technologies. With the aim of enhancing the ratio of the motional to feedthrough current at nominal operating voltages, this paper benchmarks a variety of drive and detection principles for electrostatically driven square-extensional mode resonators operating in air and in a foundry MEMS process utilizing 2μm gaps. A new detection technique, combining second harmonic capacitive actuation and piezoresistive detection, outperforms previously reported methods utilizing voltages as low as ± 3V in air providing a promising solution for low voltage CMOS-MEMS integration. ©2009 IEEE.

Parasitic feedthrough cancellation techniques for enhanced electrical characterization of electrostatic microresonators

Capacitive parasitic feedthrough is an impediment that is inherent to all electrically interfaced micron scale resonant devices, resulting in increased challenges to their integration in more complex circuits, particularly as devices are scaled to operate at higher frequencies for RF applications. In this paper, a technique to cancel the undesirable effects of capacitive feedthrough that was previously proposed is here developed for an on-chip implementation. The method reported in this paper benefits from the simplicity of its implementation, and its effectiveness is demonstrated in this paper. This technique is demonstrated for two disk-plate resonators that have been excited in the wine glass mode at 5.4 MHz, though applicable to almost any electrically interfaced resonator. Measurements of the electrical transmission from these resonators show that the magnitude of the frequency response of the system is enhanced by up to 19 dB, while the phase is found to shift through a full 180° about the resonant frequency. This method is proposed as a useful addition to other techniques for enhancing the measured response of electrostatic micromechanical resonators. © 2009 Elsevier B.V. All rights reserved.

Carbon nanotube alignment via electrohydrodynamic patterning of nanocomposites

Electrohydrodynamic (EHD) pattern formation in carbon nanotube-polymer composite films yields well-defined patterns on the micrometer scale along with the alignment of carbon nanotubes (CNTs) within these patterns. Conductive pathways in nanotube networks formed during EHD patterning of nanocomposite films results in a substantial increase in the composites' conductivity at loadings exceeding the percolation threshold. The degree of nanotube alignment can be tuned by adjusting the EHD parameters and the degree of alignment is mirrored by the conductivity across the film. Using etching techniques or by embedding relatively long nanotubes, patterned surfaces decorated by CNT brushes were generated. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.