In this thesis, the details of the design, fabrication, and characterization of an optical, integrated hydrogen gas and temperature sensor are explored. The hydrogen sensor is implemented by coating a ridge waveguide with a thin layer of palladium and shows very good response time and detection response for hydrogen concentrations ranging from 0.5-4%, both of which compare very favourably to similar existing technologies. Multiple film thicknesses were tested and it was found that thinner films give a faster response time at the expense of a reduced detection response. The temperature sensor is implemented with a multi-mode interferometer coupled ring resonator and has a sensing range of 100 K with good sensitivity. Both sensors are fabricated on a silicon-on-insulator platform and could easily be integrated together onto a single chip as part of an optical nose technology that would have the ability to sense multiple environmental factors simultaneously.

In this thesis, the details of the design, fabrication, and characterization of an optical, integrated hydrogen gas and temperature sensor are explored. The hydrogen sensor is implemented by coating a ridge waveguide with a thin layer of palladium and shows very good response time and detection response for hydrogen concentrations ranging from 0.5-4%, both of which compare very favourably to similar existing technologies. Multiple film thicknesses were tested and it was found that thinner films give a faster response time at the expense of a reduced detection response. The temperature sensor is implemented with a multi-mode interferometer coupled ring resonator and has a sensing range of 100 K with good sensitivity. Both sensors are fabricated on a silicon-on-insulator platform and could easily be integrated together onto a single chip as part of an optical nose technology that would have the ability to sense multiple environmental factors simultaneously.

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In this dissertation, three types of microfabricated solid-state sensors had been designed and developed on silicon wafers, aiming to detect hydrogen gas at elevated temperatures. Based on the material properties and sensing mechanisms, they were operated at 140°C, 500°C, and 300°C. The MOS-capacitor device working at 140°C utilized nickel instead of the widely-used expensive palladium, and the performance remained excellent. For very-high temperature sensing (500°C), the conductivity of the thermally oxidized TiO2 thin film based on the anodic aluminum oxide (AAO) substrate changed 25 times in response to 5 ppm H2 and the response transient times were just a few seconds. For medium-high temperatures (~300°C), very high sensitivity (over 100 times’ increment of current for H2 concentration at 10 ppm) was obtained through the reversible reduction of the Schottky barrier height between the Pt electrodes and the SnO2 nano-clusters. Fabrication approaches of these devices included standard silicon wafer processing, thin film deposition, and photolithography. Materials characterization methods, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), surface profilometry, ellipsometry, and X-ray diffractometry (XRD), were involved in order to investigate the fabricated nano-sized structures. Selectivities of the sensors to gases other than H2 (CO and CH4) were also studied. The first chapter reviews and evaluates the detection methodologies and sensing materials in the current research area of H2 sensors and the devices presented this Ph.D. research were designed with regard to the evaluations.

Hydrogen sensor technology is a crucial component for safety and many other practical concerns in the hydrogen economy. To achieve a desired sensor performance, proper choice of sensing material is critical, because it directly affects the main features of a sensor, such as response time, sensitivity, and selectivity. Palladium is well-known for its ability to sorb a large amount of hydrogen. Most hydrogen sensors use Pd-based sensing materials. Since hydrogen sensing is based on surface and interfacial interactions between the sensing material and hydrogen molecules, nanomaterials, a group of low dimensional systems with large surface to volume ratio, have become the focus of extensive studies in the potential application of hydrogen sensors. Pd nanowires and Pd-coated carbon nanotubes have been successfully used in hydrogen sensors and excellent results have been achieved. Motivated by this fact, in this dissertation, we perform theoretical modeling to achieve a complete and rigorous description of molecular interactions, which leads to the understanding of molecular behavior and sensing mechanisms.To demonstrate the properties of Pd-based sensing materials, two separate modeling techniques, but with the same underlying aim, are presented in this dissertation. Molecular dynamic simulations are applied for the thermodynamic, structural and dynamic properties of Pd nanomaterials. Ab initio calculations are utilized for the study of sensing mechanism of Pd functionalized single wall carbon nanotubes. The studies reported in this dissertation show the applications of computational simulations in the area of hydrogen sensors. It is expected that this work will lead to better understanding and design of molecular sensor devices.

Transistors are the fundamental components of computer processors. The dimensions of transistors used in microprocessors are decreasing every year and the challenge of maintaining this trend now requires nanoscale dimensions. A potential method of achieving nanoscale dimensions is using atomic clusters as building blocks. It is therefore desirable to investigate transistor-like behaviour in cluster devices. Traditionally, transistor devices are made from semiconducting materials. It was therefore proposed that gated behaviour would be observable in devices that are fabricated from germanium clusters. A germanium cluster source was designed and built. Field effects were successfully observed in films of germanium clusters. Immediately after deposition, the gate effect of germanium cluster films was insignificant. As the films slowly oxidized in vacuum, a decrease in the overall carrier concentration was observed which lead to an increase in the gate effect, with a maximum change in resistance observed of 12%. When films of germanium clusters were exposed to air, a resistance decrease was observed, attributed to water vapour adsorbing on the surface. The phenomenon was further investigated and the proposed resistance change mechanism involves water vapour creating surface defects which act as donors and cause the electron concentration in the film to increase. Films of germanium clusters were sensitive to hydrogen concentrations above 1% in air, with up to a factor of 25 decrease in resistance observed at room temperature for 5% hydrogen concentration. Thin films were found to be most sensitive. The higher sensitivity was attributed to the larger surface-to-volume ratio. The proposed mechanism for sensing is that defects are created on the surface of the film, which in turn act as donors which cause the electron concentration in the film to increase. Bismuth is a semimetal and gate effects have previously been observed in bismuth nanowires. Parallel bismuth nanowires of 300nm diameter were successfully deposited at a distance of 200nm apart allowing one of the wires to be used as a gate. The gate effects observed in bismuth cluster structures were weak and inconclusive, with a small gate effect (change in resistance of 0.1%) observed at 11K in some devices.

High speed separators from Alfa Laval are widely in use for processing flammable and non-flammable liquids. The following work is focusing on the case of non-flammable liquid as the process liquid in case the working area around the equipment may contain quantities of explosive gases. As stated by Alfa Laval documentation, the major risk is leaking of the explosive atmosphere into the separator from the surrounding environment which may result in producing zone 1 or zone 2 of hazardous area classification. Zone 1: Area in which an explosive gas-air mixture is likely to occur for short periods in normal operation.1 Zone 2: Area in which an explosive gas-air mixture is not likely to occur, and if it occurs it will only exist for a very short time due to an abnormal condition.1 According to Alfa Laval design package, there is a need of continuous inert gas injection into the separator during the process in order to reduce the oxygen concentration and keep it in the safe level (inert gas purging) and this policy is aimed to meet the requirements of ATEX-directive 94/9/EC/2003. The objective of the current thesis is a wish to have a better understanding of the potential risks, evaluating them and try to find ways to ease the process. The outcome can be useful to make a basic instruction for further tests and simplifications as well. The separator GTN 50 is selected and hydrogen (1% concentration) is used to simulate the explosive atmosphere. The result of the tests indicates that the cooling down stage after normal operation is the only period in which hydrogen will leak into the separator, frame top part and it should be cleaned up before the next start up. A number of recommendations -Ventilation to the fresh air, Water discharges, Pressurized air injectionare also being tested and discussed. Ventilation to the fresh air and injection of pressurized air seem to be applicable A Standard Testing Flow chart is suggested and calculation on real case is considered. A number of additional ideas are also included in the last section.

Detection of explosive gas leaks such as hydrogen (H2) becomes key element in the wake of counter-terrorism threats, introduction of hydrogen powered vehicles and use of hydrogen as a fuel for space explorations. In recent years, a significant interest has developed on metal oxide nanostructured sensors for the detection of hydrogen gas. Gas sensors properties such as sensitivity, selectivity and response time can be enhanced by tailoring the size, the shape, the structure and the surface of the nanostructures. Sensor properties (sensitivity, selectivity and response time) are largely modulated by operating temperature of the device. Issues like instability of nanostructures at high temperature, risk of hydrogen explosion and high energy consumption are driving the research towards detection of hydrogen at low temperatures. At low temperatures adsorption of O2- species on the sensor surface instead of O- (since O- species reacts easily with hydrogen) result in need of higher activation energy for hydrogen and adsorbed species interaction. This makes hydrogen detection at room temperature a challenging task. Higher surface area to volume ratio (resulting higher reaction sites), enhanced electronic properties by varying size, shape and doping foreign impurities (by modulating space charge region) makes nanocrystalline materials ideal candidate for room temperature gas sensing applications. In the present work various morphologies of nanostructured tin oxide (SnO2) and indium (In) doped SnO2 and titanium oxide (titania, TiO2) were synthesized using sol-gel, hydrothermal, thermal evaporation techniques and successfully integrated with the micro-electromechanical devices H2 at ppm-level (as low as 100ppm) has been successfully detected at room temperature using the SnO2 nanoparticles, SnO2 (nanowires) and TiO2 (nanotubes) based MEMS sensors. While sensor based on indium doped tin oxide showed the highest sensitivity (S =Ra/Rg= 80000) and minimal response time (10sec.). Highly porous SnO2 nanoparticles thin film (synthesized using template assisted) showed response time of about 25 seconds and sensitivity 4. The one dimensional tin oxide nanostructures (nanowires) based sensor showed a sensitivity of 4 and response time of 20 sec. Effect of aspect ratio of the nanowires on diffusion of hydrogen molecules in the tin oxide nanowires, effect of catalyst adsorption on nanowire surface and corresponding effect on sensor properties has been studied in detail. Nanotubes of TiO2 prepared using hydrothermal synthesis showed a sensitivity 30 with response time as low as 20 seconds where as, TiO2 nanotubes synthesized using anodization showed poor sensitivity. The difference is mainly attributed to the issues related to integration of the anodized nanotubes with the MEMS devices. The effect of MEMS device architecture modulation, such as, finger spacing, number and length of fingers and electrode materials were studied. It has been found that faster sensor response (~ 10 sec) was observed for smaller finger spacing. A diffusion model is proposed for elucidating the effect of inter-electrode distance variation on conductance change of a nano-micro integrated hydrogen sensor for room temperature operation. Both theoretical and experimental results showed a faster response upon exposure to hydrogen when sensor electrode gap was smaller. Also, a linear increase in the sensor sensitivity from 500 to 80000 was observed on increasing the electrode spacing from 2 to 20 μm. The improvement in sensitivity is attributed to the higher reactive sites available for the gaseous species to react on the sensor surface. This phenomenon also correlated to surface adsorbed oxygen vacancies (O-) and the rate of change of surface adsorbed oxygen vacancies. This dissertation studied in detail dimensionality aspects of materials as well as device in detecting hydrogen at room temperature.

Hydrogen sensors have obtained increased interest with the widened application of hydrogen energy in recent years. Among them, various chemiresistor based hydrogen sensors have been studied due to their relatively simple structure and well-established detection mechanism. The recent progress in micro/nanotechnology has accelerated the development of small-scale chemical sensors. In this work, MEMS (Micro-Electro-Mechanical Systems) sensor platforms with interdigitated electrodes have been designed and fabricated. Integrating indium doped tin dioxide nanoparticles, these hydrogen sensors showed improved sensor characteristics such as sensitivity, response and selectivity at room temperature. Design parameters of interdigitated electrodes have been studied in association with sensor characteristics. It was observed that these parameters (gap between the electrodes, width and length of the fingers, and the number of the fingers) imposed different impacts on the sensor performance. In order to achieve small, robust, low cost and fast hydrogen micro/nano sensors with high sensitivity and selectivity, the modeling and process optimization was performed. The effect of humidity and the influence of the applied voltage were also studied. The sensor could be tuned to have high sensitivity (105), fast response time (10 seconds) and low energy consumption (19 nW). Finally, a portable hydrogen instrument integrated with a micro sensor, display, sound warning system, and measurement circuitry was fabricated based on the calibration data of the sensor.