1,720,969 research outputs found
An investigation into separation enhancement methods for miniaturised planar capillary electrophoresis devices
No full textThe analytical capability of field-portable instruments and in-situ devices is often limited due to cost, complexity and spatial restrictions. With powerful analytical methods incorporated onto miniaturised sensor platforms, in-depth sample and environmental examinations become feasible. There has been significant research reported in the literature for improving miniaturised capillary electrophoresis systems alongside lab-on-a-chip applications. The work presented in this thesis discusses the development of novel methods to improve the separation resolution of capillary electrophoresis on microfluidic devices without compromising device size. Further to this, complex chemical buffering systems have been avoided since they tend to make the end device highly application specific.The separation resolution of capillary electrophoresis can be enhanced by dynamic control of the electroosmotic flow. This is achieved through control of the zeta-potential, which can be modified by applying a potential to an electrode close to the surface of the separation channel. The separation enhancement methods increase the effective channel length and therefore can be used to aid the incorporation of capillary electrophoresis into both portable field instruments and for in-situ systems.Computational models have been developed for an in-depth investigation into the proposed separation enhancement routines. Where possible, these models have been experimentally verified. The operation limits of the enhancement methods have been investigated, and related to the composition of the sample to infer design criteria
Modelling new techniques for improving separation in miniature capillary- and planar-based capillary electrophoresis systems
No full textThe capillary/channel length is an important factor in capillary electrophoresis (CE) systems since it is directly related to the amount of separation attainable. In this work we present methods to increase the effective channel length without the need to modify the physical channel length. Using an electrode located close to the capillary surface it is possible to dynamically modify zeta-potential and therefore the electroosmotic flow (EOF). By controlling the EOF, certain ionic species within a sample can be held in a short channel whilst other species migrate along the channel. Alternatively the sample can be transported back and forth along the active channel length until sufficient separation has been attained. CE enables detailed analysis of a sample’s composition and this is of interest to a range of applications
Evolution of voltage transients during the switching of a MEMS relay with Au/MWCNT contacts
No full textGold is commonly used for microelectromechanical electrical contacts due to its desirable electrical and mechanical properties; however, the lifetime of gold contacts is limited, particularly in the case of hot switching. To improve the lifetime of electrical contacts, we have developed a gold-coated multiwalled carbon nanotube bilayer composite. Experiments with these composites have shown that the switching dynamics vary over the lifetime of the switch. The change in potential across the switch contacts during the contact-break process, referred to as the transient opening voltage, has been monitored at a number of intervals throughout the switch life. The transient opening voltage shows behavior indicative of the molten metal bridge (MMB) phenomenon. While stable for most of the contact lifetime, the duration of this behavior increases sharply as the contacts approach failure. Throughout the switch lifetime, the contacts are required to survive a large number of opening and closing cycles and, therefore, it is important to understand the switching dynamics. A contact pair was investigated to failure where the experimental conditions were: load current and voltage of 50 mA and 4 V, respectively. Failure occurred after 28 million hot-switched cycles. The average energy of the MMB process was evaluated as 1.54 \mu text{J} per opening event during the stable region, this rose to 13.2 \mu text{J} shortly before failure. An experiment run under similar conditions but with a lower load current of 10 mA, which was switched for over 500 million cycles, showed a stable contact resistance and an average MMB energy of 5~\Omega and 57 nJ, respectively
The wear of hot switching Au/Cr-Au/MWCNT contact pairs for MEMS contacts
No full textA gold coated carbon nanotubes composite was used as a contact material in Micro-Electrical-Mechanical-System (MEMS) switches. The switching contact was tested under typical conditions of MEMS relay applications: load voltage of 4 V, contact force of 1 mN, and load current varied between 20-200 mA. This paper focuses on the wear process over switching lifetime, and the dependence of the wear area on the current is discussed. It was shown that the contact was going to fail when the wear area approached the whole contact area, at which point the contact resistance increased sharply to three times the nominal resistanc
Carbon nanotube (CNT) composite surfaces for electrical contact interfaces
No full textMEMS relays boast numerous advantages over PIN diode and FET devices, for example: lower on-resistance, higher isolation and cut-off frequency. There are two common implementations of MEMS switches: capacitively coupled and metal-contacting. Whilst the use of capacitive switches at low frequencies is limited, they tend to be capable of surviving high numbers (>500,000,000) of switching cycles without showing any signs of mechanical failure. For metal-contacting switches, the electrical contacts are mechanically brought into contact without the presence of a dielectric layer on the contacts, consequently enabling the transmission of DC to high frequency signals. A combination of electrical and mechanical factors result in degradation of the contact surfaces over consecutive opening and closing processes which ultimately result in switch failure.The use of gold-coated multi-walled carbon nanotube (Au/MWCNT) bilayer composites have been investigated as a method for improving the reliability of switch contacts. Using a gold-coated MEMS cantilever beam to test the composite contacts. With a load current of 50 mA (load voltage 4 V), the use of a composites contact resulted in a switching lifetime in excess of 44,000,000 hot switching cycles. With a load current of 10 mA, the lifetime is in excess of 500,000,000 cycles. The use of Au/MWCNT composites offer a promising solution to enhance the lifetime of MEMS switches.<br/
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A nano-indentation study of the contact resistance and resistivity of a bi-layered Au/multi-walled carbon nanotube composite
Full text linkA bi-layer metal-carbon nanotube composite has been developed as a potential low-force electrical contact surface, for application in micro-electromechanical systems switching devices. The samples consist of a vertically aligned forest of multi-walled carbon nanotube (MWCNT), sputter coated with a layer of Au. The effect of varying the components and composition are investigated by means of a modified nano-indenter. By measuring the contact resistance of the composites under various loading conditions, the electrical properties and performance can be evaluated. The composites are shown to have homogenous properties, with each of the layers influencing the total electrical characteristics of the samples. The internal structure of the sample, the MWCNT height and penetration of gold into the forest is shown to directly influence the performance and characteristics of the samples. By analyzing the samples as bulk, the effective resistivities of the composites are also determined to have values from 303 n? m down to 54 n? m, depending on the composition of the sample
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