On-a-chip microdischarge thruster arrays inspired by photonic device technology for plasma television

This study shows that the practical scaling of a hollow cathode thruster device to MEMS level should be possible albeit with significant divergence from traditional design. The main divergence is the need to operate at discharge pressures between 1-3bar to maintain emitter diameter pressure products of similar values to conventional hollow cathode devices. Without operating at these pressures emitter cavity dimensions become prohibitively large for maintenance of the hollow cathode effect and without which discharge voltage would be in the hundreds of volts as with conventional microdischarge devices. In addition this requires sufficiently constrictive orifice diameters in the 10µm – 50µm range for single cathodes or <5µm larger arrays. Operation at this pressure results in very small Debye lengths (4 -5.2pm) and leads to large reductions in effective work function (0.3 – 0.43eV) via the Schottky effect. Consequently, simple work function lowering compounds such as lanthanum hexaboride (LaB6) can be used to reduce operating temperature without the significant manufacturing complexity of producing porous impregnated thermionic emitters as with macro scale hollow cathodes, while still operating <1200°C at the emitter surface. The literature shows that LaB6 can be deposited using a variety of standard microfabrication techniques

Screen-printed platinum electrodes for measuring crevice corrosion: Nickel aluminium bronze as an example

Screen-printed platinum electrodes were used to monitor crevice corrosion processes. The electrodes, printed on an inert alumina substrate, formed the bottom of an artificial crevice when mechanically clamped to a rectangular block of nickel-aluminium bronze (NAB). Cyclic differential pulse voltammetry was used to detect corrosion products over time whilst the assembly was immersed in a 3.5% by weight aqueous solution of sodium chloride. Cupric (Cu2+), ferric (Fe3+) and ferrous (Fe2+) ions were detected with evolution profiles indicative of selective phase corrosion

Lifetime performance characteristics of screen-printed potentiometric Ag/AgCl chloride sensors

Ag/AgCl chloride sensors were fabricated using thick-film technology. A number of different formulations were prepared and chloride responses were investigated over time. Near Nernstian, identical responses were observed over the first 160 days with an average chloride sensitivity of -51.8 ± 0.4 mV per decade change in chloride concentration (pCl), irrespective of paste formulation. After 6- months continuous immersion in tap water, pastes formulated with a glass binder began to exhibit a loss in sensitivity whilst those formulated from a commercial thickfilm dielectric paste remained functional for the best part of a year. The difference is attributed to the inclusion of proprietary additives in the commercial paste aiding adhesion and minimising AgCl leaching

Screen-printed platinum electrodes for the detection of cupric and ferric ions in high chloride backgrounds

Screen-printed platinum electrodes developed for use in corrosion monitoring applications have been used to detect cupric and ferric ions both individually and as mixtures in a background of 3.5% by weight sodium chloride and in the presence of dissolved oxygen. In single species detection linear responses for the Fe3+/Fe2+ couple were observed over the concentration range 0.3 to 100mM. By contrast, the small size of the working electrode caused a current limiting response for cupric ions over the same concentration range. In mixtures of these ions, the sensors show good differentiation and are able to separate the individual metal ion responses

Screen-printed potentiometric Ag/AgCl chloride sensors: Lifetime performance and their use in soil salt measurements

Silver – silver chloride electrodes (Ag/AgCl) for the detection of chloride ions were fabricated using thick-film technology. Five different formulations were prepared and chloride responses were investigated over time. Almost identical and near Nernstian responses were observed over the first 162 days with an average chloride sensitivity for all formulations of -51.12 mV ± 0.45 mV per decade change in chloride concentration compared with a value of -50.59 mV ± 0.01 mV over 388 days for the best two formulations. After 6-months continuous immersion in tap water, pastes formulated with a glass binder began to exhibit a loss in sensitivity whilst those formulated from a commercial thick-film dielectric paste remained functional for the best part of a year. This difference in lifetime performance is attributed to the inclusion of proprietary additives in the commercial paste aiding adhesion and minimising AgCl leaching. The mechanical and chemical robustness of these electrodes has been demonstrated through their ability to detect changing levels of chloride when immersed in soil columns. This particular capacity will make them an invaluable tool in the fields of hydrology, agricultural science, soil science and environmental science

Novel fabrication method for rapid creation of channels using PDMS for microfluidic networks on planar substrates

A novel and simple method for the rapid fabrication of microfluidic networks is presented. A silicone elastomer (PDMS - poly(dimethylsiloxane)) is cured around formers, which are then removed post-cure, resulting in a microstructure suitable for fluidic applications. The limiting factors in the fabrication method are in the materials and tools used for the development of the formers. If the methods used cannot produce a structure of accurate dimensions then the microstructure formed will be limited. For creating very narrow fluidic channels, the material used needs to be strong so that even with narrow dimensions it can be removed without damage but the use of sacrificial materials has been investigated as this overcomes this requirement. The principle of the technique is demonstrated with an unusual material (caramelised sugar – which can be easily dissolved in water) to fabricate channels with diameters down to 16μm

A Closed-loop, Non-linear, Miniaturised Capillary Electrophoresis System Enabled by Control of Electroosmotic Flow

The miniaturisation of capillary electrophoresis (CE) systems makes separation of ionic species with similar electrophoretic mobilities challenging. We report on a novel closed-loop system that does not rely on migration time to identify ionic species unlike many conventional CE systems. To aid miniaturisation our method requires the sample undergoing separation to travel back and forth along the short channel multiple times. For each consecutive cycle the sample becomes increasingly separated until it is deemed sufficiently separated such that it can be reliably identified by any appropriate detection system. As the sample approaches either of the channel ends, contactless conductivity detectors detect the presence of the sample and trigger the modification of the electroosmotic flow (EOF) to reverse the direction of flow in the channel. After sufficient separation the identification is performed in-channel using, in our case, an electrochemical detection scheme. Incorporation of a closed-loop control system means that unpredictable variation in migration time does not present an issue for ionic species identification. This new method of non-linear CE is demonstrated in a microfluidic channel formed in PDMS (polydimethylsiloxane), reversibly sealed to a glass wafer on which metal electrodes are patterned in gold. The sample movement in both directions along the channel occurs without affecting the electrophoretic separation already achieved during each cycle by changing the EOF in magnitude and direction. The EOF is changed by modifying the zeta-potential along the channel wall through the application of a voltage on a zeta-potential modification (ZPM) electrode placed close to the channel surface. Depending on the magnitude and polarity of the voltage applied to the ZPM electrode our experiments have shown the ability to increase, decrease or reverse the EOF

An Investigation into Separation Enhancement Methods for Miniaturised Planar Capillary Electrophoresis Devices

Large structures such as buildings, ships and aircraft often contain numerous components which are exposed to a failure risk due to corrosion processes. Aside from the extra cost of repair and replacement, there are also potential health and safety issues which need to be addressed. Corrosion detection is not a new concept and there exists a variety of methods to detect and evaluate corrosion. The analysis method that is the focus of this work is capillary electrophoresis (CE); commonly used for a number of biological and chemical processes, such as drug, food and water quality analysis, DNA and protein separation and so on. An alternative method to high pressure (or performance) liquid chromatography (HPLC), CE boasts high analysis speeds as well as low limits of detection. Both are of crucial advantage for use in a pharmaceutical market; however the application of CE for in-situ or portable corrosion monitoring has not been investigated in great depth

Sensor array signal processing for cross-sensitivity compensation in non-specific organic semiconductor gas sensors

A fundamental limitation of many chemically sensitive organic semiconductor materials is their high susceptibility to cross-interference resulting from interactions with background species other than those actively being detected. Such cross-sensitivities often preclude their use in 'real' sensor applications, particularly where discrete and selective gas sensing systems are required. It has been hypothesised, however, that this lack of specificity can largely be overcome with the adoption of a multi-element sensor array, thereby allowing the compensation of unwanted sensitivities through suitable signal processing.This thesis describes how such a multi-element sensor array of different gas sensitive metallophthalocyanine films, constructed on a single substrate, was used as the sensing element in an 'intelligent' chemical sensor. Since the individual sensors show varying degrees of gas sensitivity, the individual responses of each to any particular analyte will give rise to a characteristic change in the output 'pattern' comprised of each of the sensor resistances. By monitoring the change in this pattern of responses on exposure to specific gases of pre-determined concentration and employing a suitable feature extraction algorithm, the characteristic responses to particular analytes was learnt, and a knowledge base, from which future inferences may be drawn, was constructed.The success of suitable signal processing techniques to accommodate the inherent cross-sensitivities exhibited by these materials is demonstrated. The results demonstrate the viability of pattern recognition methods to analyse gas mixtures by comparing particular features of the combined array response with those previously learnt during a gas recognition training phase