A Survey on Gas Sensing Technology
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paper_title: Differential Absorption Lidar to Measure Subhourly Variation of Tropospheric Ozone Profiles

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A tropospheric ozone Differential Absorption Lidar system, developed jointly by The University of Alabama in Huntsville and the National Aeronautics and Space Administration, is making regular observations of ozone vertical distributions between 1 and 8 km with two receivers under both daytime and nighttime conditions using lasers at 285 and 291 nm. This paper describes the lidar system and analysis technique with some measurement examples. An iterative aerosol correction procedure reduces the retrieval error arising from differential aerosol backscatter in the lower troposphere. Lidar observations with coincident ozonesonde flights demonstrate that the retrieval accuracy ranges from better than 10% below 4 km to better than 20% below 8 km with 750-m vertical resolution and 10-min temporal integration.
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paper_title: Design and Construction of an Odour Sensor for Various Biomedical Applications

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This project deals with the construction of electronic device that senses odour and its application in Telesurgery. Of the body's five senses, the sense of Smell is the most mysterious. In the past, gas chromatography and mass spectrometry have been used as sensing systems, although these are usually expensive and time consuming. Today, the development of odour sensor helped to analyze odors and hence applied in various Medical applications such as, Telesurgery. The odour sensor is generally made of two main parts: a sensing system and a pattern recognition system. The sensing system of the "Odour Sensor" consists of individual thin-film carbon-black polymer homogeneously mingled throughout a non-conducting polymer. When the composite is exposed to a vapor-phase analyte, the resistance varies, which is unique for each odour. The change in resistance is converted into voltage and hence given to the Computer through a Data Acquisition Card. The analysis of Sensor output is done using MatLAB code called Smellware. The response of the Smellware is given to Artificial Neural Network, that is pattern recognition system and hence odour is determined. The main application of this developed odour sensor is Telesurgery. The odour sensor would identify odours in the remote surgical environment. These identified odours would then be electronically transmitted to another site where an odour generation system would recreate them. This developed "odour sensor" was successfully tested and was submitted as a Project Work.
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paper_title: Photoacoustic Spectroscopy with Quantum Cascade Lasers for Trace Gas Detection

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Abstract: Various applications, such as pollution monitoring, toxic-gas detection, noninvasive medical diagnostics and industrial process control, require sensitive and selectivedetection of gas traces with concentrations in the parts in 10 9 (ppb) and sub-ppb range.The recent development of quantum-cascade lasers (QCLs) has given a new aspect toinfrared laser-based trace gas sensors. In particular, single mode distributed feedback QCLsare attractive spectroscopic sources because of their excellent properties in terms of narrowlinewidth, average power and room temperature operation. In combination with these lasersources, photoacoustic spectroscopy offers the advantage of high sensitivity and selectivity,compact sensor platform, fast time-response and user friendly operation. This paper reportsrecent developments on quantum cascade laser-based photoacoustic spectroscopy for tracegas detection. In particular, different applications of a photoacoustic trace gas sensoremploying a longitudinal resonant cell with a detection limit on the order of hundred ppb ofozone and ammonia are discussed. We also report two QC laser-based photoacousticsensors for the detection of nitric oxide, for environmental pollution monitoring andmedical diagnostics, and hexamethyldisilazane, for applications in semiconductormanufacturing process.
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paper_title: Trace amount formaldehyde gas detection for indoor air quality monitoring

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Formaldehyde is not only a carcinogenic chemical, but also causes sick building syndrome. Very small amounts of formaldehyde, such as those emitted from building materials and furniture, pose great concerns for human health. A Health Canada guideline, proposed in 2005, set the maximum formaldehyde concentration for long term exposure (8-hours averaged) as 40 ppb (50 μg/m3). This is a low concentration that commercially available formaldehyde sensors have great difficulty to detect both accurately and continuously. In this paper, we report a formaldehyde gas detection system which is capable of pre-concentrating formaldehyde gas using absorbent, and subsequently thermally desorbing the concentrated gas for detection by the electrochemical sensor. Initial results show that the system is able to detect formaldehyde gas at the ppb level, thus making it feasible to detect trace amount of formaldehyde in indoor environments.
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paper_title: Response Speed Optimization Of Thermal Gas-flow Sensors For Medical Application

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Gas-flow sensors, for medical applications (respiration monitoring), require a relatively large bandwidth (10-300 Hz). Fast flow changes occur e.g. at the start of inspiration. Thermal gas-flow sensors are very favourable for medical applications. Due to the small size, they have a small influence on the total system (small obstruction in a tube). On the other hand, the response speed of these sensors is not automatically large enough: temperature changes take a relatively long time. Optimization of the response speed is necessary.
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paper_title: A study on NDIR-based CO2 sensor to apply remote air quality monitoring system

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Recently, various CO2 sensors reported in the literature may be classified in two major categories depending on measuring principle of sensors. Fist, the chemical CO2 gas sensors have the principal advantage of a very low energy consumption and small size. On the negative side, It is difficult to apply to a variety of practical fields because they have a short lifetime and as well as low durability. Second, NDIR-based CO2 sensor is commonly used in monitoring indoor air quality due to relatively high accuracy compared with that of a chemical CO2 gas sensor. In this paper, therefore, we evaluate NDIR-based CO2 sensors to verify applicable potentialities of them in a remote air quality monitoring system. In addition, the principle and structure of NDIR CO2 sensor is discussed, and then we analyze the CO2 concentration has been measured in the real platform of subway station for a month. Finally this paper proposes that the accuracy of the measuring values from NDIR CO2 sensor is enough for indoor air quality monitoring applications.
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paper_title: Workflow for High Throughput Screening of Gas Sensing Materials

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The workflow of a high throughput screening setup for the rapid identification of new and improved sensor materials is presented. The polyol method was applied to prepare nanoparticular metal oxides as base materials, which were functionalised by surface doping. Using multi-electrode substrates and high throughput impedance spectroscopy (HT-IS) a wide range of materials could be screened in a short time. Applying HT-IS in search of new selective gas sensing materials a NO2-tolerant NO sensing material with reduced sensitivities towards other test gases was identified based on iridium doped zinc oxide. Analogous behaviour was observed for iridium doped indium oxide.
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paper_title: Overview of automotive sensors

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An up-to-date review paper on automotive sensors is presented. Attention is focused on sensors used in production automotive systems. The primary sensor technologies in use today are reviewed and are classified according to their three major areas of automotive systems application-powertrain, chassis, and body. This subject is extensive. As described in this paper, for use in automotive systems, there are six types of rotational motion sensors, four types of pressure sensors, five types of position sensors, and three types of temperature sensors. Additionally, two types of mass air flow sensors, five types of exhaust gas oxygen sensors, one type of engine knock sensor, four types of linear acceleration sensors, four types of angular-rate sensors, four types of occupant comfort/convenience sensors, two types of near-distance obstacle detection sensors, four types of far-distance obstacle detection sensors, and and ten types of emerging, state-of the-art, sensor technologies are identified.
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paper_title: The Effects of the Location of Au Additives on Combustion-generated SnO2 Nanopowders for CO Gas Sensing

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The current work presents the results of an experimental study of the effects of the location of gold additives on the performance of combustion-generated tin dioxide (SnO(2)) nanopowders in solid state gas sensors. The time response and sensor response to 500 ppm carbon monoxide is reported for a range of gold additive/SnO(2) film architectures including the use of colloidal, sputtered, and combustion-generated Au additives. The opportunities afforded by combustion synthesis to affect the SnO(2)/additive morphology are demonstrated. The best sensor performance in terms of sensor response (S) and time response (τ) was observed when the Au additives were restricted to the outermost layer of the gas-sensing film. Further improvement was observed in the sensor response and time response when the Au additives were dispersed throughout the outermost layer of the film, where S = 11.3 and τ = 51 s, as opposed to Au localized at the surface, where S = 6.1 and τ = 60 s.
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paper_title: Highly sensitive and selective ammonia gas sensor

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We have fabricated and examined an ammonia gas sensor with high sensitivity using thick-film technology. The sensing material of the gas sensor is FeO/sub x/-WO/sub 3/-SnO/sub 2/ oxide semiconductor. The sensor exhibits resistance increase upon exposure to low concentration of ammonia gas. The resistance of the sensor is decreased, on the other hand, for exposure to reducing gases such as ethyl alcohol, methane, propane and carbon monoxide. We have proposed and investigated a novel method for detecting ammonia gas quite selectively by using a sensor array with two sensing elements which contains an ammonia gas sensor and a compensation element. The compensation element is a Pt-doped WO/sub 3/-SnO/sub 2/ gas sensor which shows an opposite direction of resistance change in comparison with the ammonia gas sensor upon exposure to ammonia gas. Excellent selectivity has been achieved using the sensor array with two sensing elements.
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paper_title: Airborne Chemical Sensing with Mobile Robots

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Airborne chemical sensing with mobile robots has been an active research area since the beginning of the 1990s. This article presents a review of research work in this field, including gas distribution mapping, trail guidance, and the different subtasks of gas source localisation. Due to the difficulty of modelling gas distribution in a real world environment with currently available simulation techniques, we focus largely on experimental work and do not consider publications that are purely based on simulations.
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paper_title: Odour measurement using conducting polymer gas sensors and an artificial neural network decision system

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The conventional way of assessing the magnitude of nuisance odours using an olfactometer and a sensory panel is costly. This paper describes experiments that have been conducted into matching the results from trained sensory panellists to those from a conducting polymer-based electronic nose. By taking the data from the electronic nose and applying them to a trained neural network, it has been shown that the data can be manipulated to give rise to results that are within a few percent of those from the sensory panellists. This is the first time that an electronic nose has been calibrated in terms of odour intensity measurements and it points the way forward to more objective measurements of nuisance odours.
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paper_title: Compact Raman lidar for hydrogen gas leak detection

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A compact Raman lidar system for hydrogen gas leak detection was constructed. Laser-induced fluorescence at a distance of 70 m and Raman scattering light from N 2 gas at short range could be detected.
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paper_title: Planar Zeolite Film-Based Potentiometric Gas Sensors Manufactured by a Combined Thick-Film and Electroplating Technique

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Zeolites are promising materials in the field of gas sensors. In this technology-oriented paper, a planar setup for potentiometric hydrocarbon and hydrogen gas sensors using zeolites as ionic sodium conductors is presented, in which the Pt-loaded Na-ZSM-5 zeolite is applied using a thick-film technique between two interdigitated gold electrodes and one of them is selectively covered for the first time by an electroplated chromium oxide film. The influence of the sensor temperature, the type of hydrocarbons, the zeolite film thickness, and the chromium oxide film thickness is investigated. The influence of the zeolite on the sensor response is briefly discussed in the light of studies dealing with zeolites as selectivity-enhancing cover layers.
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paper_title: Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

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A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented.
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paper_title: Measurement of CH4 by differential infrared optical absorption spectroscopy

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To solve the problem that the current use of optical methods used to detect various gases existing in sensors and signal processing was too complicated, detection sensitivity was not high, detection of the type of gases was single. We used optical phase-shifted grating laser window adjust the differential infrared absorption spectrometry and studied Spectrum adjustment and detection signal processing problem. CH4 in cigarette main smoke of different kind brands was measured continuously by differential optical absorption spectroscopy (DOAS). By using RM200 rotating disk smoking machine with 20 smoking flue, 20 kinds of gas were inserted into the rotating disk and ignited every 3s. By use of DOAS technique, the measurement was completed with in 6~7min after the gas was transported into a White cell with 31.5m path length which was connected to the smoking machine. This technique can improve the measuremental temporal resolution compared with traditional method. At the beginning, the press in White cell was 5.2×104 Pa and it increased to 1.03×105Pa while the measurement finished. The result shows the concentration of CH4 in main gas is between 0.89mg/m3 and 1.54mg/m. Pool in the White cell entrance placed wide infrared wavelength LED light source, light source after a large-diameter phase-shifted fiber grating access to White cell, Through the piezoelectric ceramic (PZT) phase-shift controlled Phase-Shifted Fiber Bragg Grating to infra-red LED into a laser modulation of narrowband continuous changes in light, wavelength corresponding to various gases such as CH4 absorption spectrum. White cell output pool signals entered into the photoelectric detector and ADC further, controlled by computer-related demodulation digitally. It can achieve high sensitivity digital lock-in amplifier function and be suitable for high-resolution detection of trace gases. Through simulation studies and practical tests, The results show that the piezoelectric ceramic (PZT) control Phase-Shifted Fiber Bragg Grating spectral scan can effectively suppress the background optical interference, the elimination of Rayleigh scattering and Raman scattering. Phase-Shifted Fiber Bragg Grating modulation and digital lock-in amplifier detection sensitivity of the combination improve three orders of magnitude. The methods solve the problem a wide range of gas detection by one kind of equipment. The device is small size, low cost, easy to carry, can measure a wide range of gas content, and has an important application value for environmental monitoring. If the White pool with underwater ocean can detect trace gases, for rivers and the marine environment and resources to detect environmental monitoring is also of great significance.
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paper_title: Membrane Based Measurement Technology for in situ Monitoring of Gases in Soil

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The representative measurement of gas concentration and fluxes in heterogeneous soils is one of the current challenges when analyzing the interactions of biogeochemical processes in soils and global change. Furthermore, recent research projects on CO(2)-sequestration have an urgent need of CO(2)-monitoring networks. Therefore, a measurement method based on selective permeation of gases through tubular membranes has been developed. Combining the specific permeation rates of gas components for a membrane and Dalton's principle, the gas concentration (or partial pressure) can be determined by the measurement of physical quantities (pressure or volume) only. Due to the comparatively small permeation constants of membranes, the influence of the sensor on its surrounding area can be neglected. The design of the sensor membranes can be adapted to the spatial scale from the bench scale to the field scale. The sensitive area for the measurement can be optimized to obtain representative results. Furthermore, a continuous time-averaged measurement is possible where the time for averaging is simply controlled by the wall-thickness of the membrane used. The measuring method is demonstrated for continuous monitoring of O(2) and CO(2) inside of a sand filled Lysimeter. Using three sensor planes inside the sand pack, which were installed normal to the gas flow direction and a reference measurement system, we demonstrate the accuracy of the gas-detection for different flux-based boundary conditions.
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paper_title: A wireless home safety gas leakage detection system

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A wireless safety device for gas leakage detection is proposed. The device is intended for use in household safety where appliances and heaters that use natural gas and liquid petroleum gas (LPG) may be a source of risk. The system also can be used for other applications in the industry or plants that depend on LPG and natural gas in their operations. The system design consists of two main modules: the detection and transmission module, and the receiving module. The detection and transmitting module detects the change of gas concentration using a special sensing circuit built for this purpose. This module checks if a change in concentration of gas(es) has exceeded a certain pre-determined threshold. If the sensor detects a change in gas concentration, it activates and audiovisual alarm and sends a signal to the receiver module. The receiver module acts as a mobile alarm device to allow the mobility within the house premises. The system was tested using LPG and the alarm was activated as a result of change in concentration.
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paper_title: Catalytic Fiber Bragg Grating Sensor for Hydrogen Leak Detection in Air

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The explosion risk linked to the use of hydrogen as fuel requires low-cost and efficient sensors. We present here a multipoint in-fiber sensor capable of hydrogen leak detection in air as low as 1% concentration with a response time smaller than a few seconds. Our solution makes use of fiber Bragg gratings (FBGs) covered by a catalytic sensitive layer made of a ceramic doped with noble metal which, in turn, induces a temperature elevation around the FBGs in the presence of hydrogen in air.
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paper_title: A wireless, passive carbon nanotube-based gas sensor

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A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO/sub 2/) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO/sub 2/), oxygen (O/sub 2/), and ammonia (NH/sub 3/). The absorption of different gases in the MWNT-SiO/sub 2/ layer changes the permittivity and conductivity of the material and consequently alters the resonant frequency of the sensor. By tracking the frequency spectrum of the sensor with a loop antenna, humidity, temperature, as well as CO/sub 2/, O/sub 2/ and NH/sub 3/ concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO/sub 2/ and O/sub 2/ is both linear and reversible. Both irreversible and reversible responses are observed in response to NH/sub 3/, indicating both physisorption and chemisorption of NH/sub 3/ by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO/sub 2/ coated, and MWNT-SiO/sub 2/ coated sensor, enables CO/sub 2/ measurement to be automatically calibrated for operation in a variable humidity and temperature environment.
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paper_title: Development of a protected gas sensor for exhaust automotive applications

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A /spl beta/-alumina-based gas sensor for automotive exhaust application (hydrocarbon, CO, NO/sub 2/ detection in 10-1000 ppm concentration range) has been developed by thick film technology (screen-printing) in the frame of a European project. The sensing device consists of a solid electrolyte (/spl beta/ alumina) and of two metallic electrodes having different catalytic properties, the whole system being in contact with the surrounding atmosphere to be analyzed. The detection principle is based on the chemisorption of oxygen which leads to a capacitance effect at the metal-electrolyte interface, resulting in a measurable difference of potential depending on nature and concentration of pollutants and on the sensor temperature. For application in exhaust pipe, a porous protective layer based on /spl alpha/-alumina for preserving the sensing material and the metal electrodes from contamination and deterioration was screen-printed on the sensing element. For limiting the possible interface interactions between the overlapped layers, a new concept of screen -printable ink was set up based on mixing the oxide powder and its gelly precursor without any inorganic binder addition. The performances of the sensor were tested both on laboratory and engine bench. The sensitivity is relevant for exhaust application, and the long-term stability is improved by the protective layer.
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paper_title: A MEMS-based Benzene Gas Sensor with a Self-heating WO3 Sensing Layer

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In the study, a MEMS-based benzene gas sensor is presented, consisting of a quartz substrate, a thin-film WO3 sensing layer, an integrated Pt micro-heater, and Pt interdigitated electrodes (IDEs). When benzene is present in the atmosphere, oxidation occurs on the heated WO3 sensing layer. This causes a change in the electrical conductivity of the WO3 film, and hence changes the resistance between the IDEs. The benzene concentration is then computed from the change in the measured resistance. A specific orientation of the WO3 layer is obtained by optimizing the sputtering process parameters. It is found that the sensitivity of the gas sensor is optimized at a working temperature of 300 °C. At the optimal working temperature, the experimental results show that the sensor has a high degree of sensitivity (1.0 KΩ ppm−1), a low detection limit (0.2 ppm) and a rapid response time (35 s).
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paper_title: Circuit and Noise Analysis of Odorant Gas Sensors in an E-Nose

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In this paper, the relationship between typical circuit structures of gas sensor circuits and their output noise is analyzed. By using averaged segmenting periodical graph and improved histogram estimation methods, we estimated their noise power spectra and optimal probability distribution functions (pdf). The results were confirmed through experiment studies.
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paper_title: Novel nano-hybrid gas sensor based on n-TiO2 functionalized by phthalocyanines via supersonic beam co-deposition: Performance and application to automotive air quality

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Supersonic beams of TiO2 clusters and metal phthalocyanines have been developed for the synthesis of hybrid for gas-sensing applications. This approach allows high degree of control on the properties of the synthesized materials and on the interface between clusters and organic molecules, so that new functional materials with novel and promising sensing properties are obtained. These materials can be synthesized in different architectures, shape and phases, and are constituted by nanocrystalline clusters of TiO2 functionalized during the growth by the co-deposited molecules. The outcome is a porous nanostructured material characterized by a diffused organic-inorganic interface at the nanoscale. The properties of the codeposited interfaces, where the functionalization of the hybrid material plays a crucial role, can be tailored directly acting on the beam parameters of the cluster (mass size distribution, phase) and of the organic molecules (kinetic energy, deposition rate).
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paper_title: Thermopile sensor array for an electronic nose integrated non-selective NDIR gas detection system

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The fabrication and characterization of a thermopile sensor array is described in this work. The device is intended to be part of a non-dispersive infrared (NDIR) gas detection system integrated in an electronic nose where each sensor is not oriented for the detection of a particular substance. Different designs were considered changing the size of the array (9 or 16 elements) and improving some specific detector performance parameters: responsivity and noise equivalent power (NEP). After device fabrication, the measurements of these parameters show a good agreement with the thermal one-dimensional model used for the design.
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paper_title: A Real-Time De-Noising Algorithm for E-Noses in a Wireless Sensor Network

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A wireless e-nose network system is developed for the special purpose of monitoring odorant gases and accurately estimating odor strength in and around livestock farms. This system is to simultaneously acquire accurate odor strength values remotely at various locations, where each node is an e-nose that includes four metal-oxide semiconductor (MOS) gas sensors. A modified Kalman filtering technique is proposed for collecting raw data and de-noising based on the output noise characteristics of those gas sensors. The measurement noise variance is obtained in real time by data analysis using the proposed slip windows average method. The optimal system noise variance of the filter is obtained by using the experiments data. The Kalman filter theory on how to acquire MOS gas sensors data is discussed. Simulation results demonstrate that the proposed method can adjust the Kalman filter parameters and significantly reduce the noise from the gas sensors.
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paper_title: Quartz Crystal Microbalance Coated with Sol-gel-derived Thin Films as Gas Sensor for NO Detection

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This paper presents the possibilities and properties of Indium tin oxide (ITO)-covered quartz crystal as a NOx toxic gas-sensor. The starting sol-gel solution was prepared by mixing indium chloride dissolved in acetylacetone and tin chloride dissolved in ethanol (0-20% by weight). The ITO thin films were deposited on the gold electrodes of quartz crystal by spin-coating technique and subsequently followed a standard photolithography to pattern the derived films to ensure all sensors with the same sensing areas. All heat treatment processes were controlled below 500°C in order to avoid the piezoelectric characteristics degradation of quartz crystal (Quartz will lose its piezoelectricity at ~573°C due to the phase change from α to β). The electrical and structural properties of ITO thin films were characterized with Hall analysis system, TG/DTA, XRD, XPS, SEM and etc. The gas sensor had featured with ITO thin films of ~100nm as the receptor to sense the toxic gas NO and quartz crystal with frequency of 10MHz as the transducer to transfer the surface reactions (mass loading, etc) into the frequency shift. A homemade setup had been employed to measure the sensor response under the static mode. The experimental results had indicated that the ITO-coated QCM had a good sensitivity for NO gas, ~12Hz/100ppm within 5mins. These results prove that the ITO-covered quartz crystals are usable as a gas sensor and as an analytical device.
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paper_title: Temperature Gradient Effect on Gas Discrimination Power of a Metal-Oxide Thin-Film Sensor Microarray

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The paper presents results concerning the effect of spatial inhomogeneous operating temperature on the gas discrimination power of a gas-sensor microarray, with the latter based on a thin SnO2 film employed in the KAMINA electronic nose. Three different temperature distributions over the substrate are discussed: a nearly homogeneous one and two temperature gradients, equal to approx. 3.3 °C/mm and 6.7 °C/mm, applied across the sensor elements (segments) of the array. The gas discrimination power of the microarray is judged by using the Mahalanobis distance in the LDA (Linear Discrimination Analysis) coordinate system between the data clusters obtained by the response of the microarray to four target vapors: ethanol, acetone, propanol and ammonia. It is shown that the application of a temperature gradient increases the gas discrimination power of the microarray by up to 35 %.
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paper_title: Development of laser based spectroscopic trace-gas sensors for environmental sensor networks and medical exposure monitors

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We report wavelength modulated TDLAS/QEPAS trace gas sensors with reduced size, efficiency, and cost for use in environmental sensor networks and medical exposure monitors. CO2 measurements with a 2 mum diode laser dissipate <1 W.
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paper_title: Vinegar Classification Based on Feature Extraction and Selection From Tin Oxide Gas Sensor Array Data

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Tin oxide gas sensor array based devices were often cited in publications dealing with food products. However, during the process of using a tin oxide gas sensor array to analysis and identify different gas, the most important and difficult was how to get useful parameters from the sensors and how to optimize the parameters. Which can make the sensor array can identify the gas rapidly and accuracy, and there was not a comfortable method. For this reason we developed a device which satisfied the gas sensor array act with the gas from vinegar. The parameters of the sensor act with gas were picked up after getting the whole acting process data. In order to assure whether the feature parameter was optimum or not, in this paper a new method called “distinguish index”(DI) has been proposed. Thus we can assure the feature parameter was useful in the later pattern recognition process. Principal component analysis (PCA) and artificial neural network (ANN) were used to combine the optimum feature parameters. Good separation among the gases with different vinegar is obtained using principal component analysis. The recognition probability of the ANN is 98 %. The new method can also be applied to other pattern recognition problems.
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paper_title: Flame-Spray-Made Undoped Zinc Oxide Films for Gas Sensing Applications

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Using zinc naphthenate dissolved in xylene as a precursor undoped ZnO nanopowders were synthesized by the flame spray pyrolysis technique. The average diameter and length of ZnO spherical and hexagonal particles were in the range of 5 to 20 nm, while ZnO nanorods were found to be 5–20 nm wide and 20–40 nm long, under 5/5 (precursor/oxygen) flame conditions. The gas sensitivity of the undoped ZnO nanopowders towards 50 ppm of NO2, C2H5OH and SO2 were found to be 33, 7 and 3, respectively. The sensors showed a great selectivity towards NO2 at high working temperature (at 300 °C), while small resistance variations were observed for C2H5OH and SO2, respectively.
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paper_title: Application research of laser gas detection technology in the analysis of sulphur hexafluoride

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The decision of latent fault in SF 6 gas electrical equipment can be made more timely and accurately through online monitoring of content change of HF, one of SF 6 gas decomposition. Tunable diode laser absorption spectroscopic (TDLAS) technology applies the wavelength tunable characteristics of diode lasers to gain the absorption spectroscopy of the selected absorption line of the target gas for the qualitative or quantitative analysis of the target gas. Because of its high sensitivity, high selectivity, short response time and without cross-interference, etc., we can achieve online monitoring and make the detection of HF more direct, accurate and timely through installing the technical equipment in the SF 6 electrical equipment monitoring site location. The analysis shows that the TDLAS technology is an effective and reliable method of SF 6 online monitoring technique.
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paper_title: Gas Sensors Based on Electrospun Nanofibers

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Nanofibers fabricated via electrospinning have specific surface approximately one to two orders of the magnitude larger than flat films, making them excellent candidates for potential applications in sensors. This review is an attempt to give an overview on gas sensors using electrospun nanofibers comprising polyelectrolytes, conducting polymer composites, and semiconductors based on various sensing techniques such as acoustic wave, resistive, photoelectric, and optical techniques. The results of sensing experiments indicate that the nanofiber-based sensors showed much higher sensitivity and quicker responses to target gases, compared with sensors based on flat films.
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paper_title: Cytochrome C Biosensor—A Model for Gas Sensing

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This work is about gas biosensing with a cytochrome c biosensor. Emphasis is put on the analysis of the sensing process and a mathematical model to make predictions about the biosensor response. Reliable predictions about biosensor responses can provide valuable information and facilitate biosensor development, particularly at an early development stage. The sensing process comprises several individual steps, such as phase partition equilibrium, intermediate reactions, mass-transport, and reaction kinetics, which take place in and between the gas and liquid phases. A quantitative description of each step was worked out and finally combined into a mathematical model. The applicability of the model was demonstrated for a particular example of methanethiol gas detection by a cytochrome c biosensor. The model allowed us to predict the optical readout response of the biosensor from tabulated data and data obtained in simple liquid phase experiments. The prediction was experimentally verified with a planar three-electrode electro-optical cytochrome c biosensor in contact with methanethiol gas in a gas tight spectroelectrochemical measurement cell.
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paper_title: The Multi-Chamber Electronic Nose—An Improved Olfaction Sensor for Mobile Robotics

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One of the major disadvantages of the use of Metal Oxide Semiconductor (MOS) technology as a transducer for electronic gas sensing devices (e-noses) is the long recovery period needed after each gas exposure. This severely restricts its usage in applications where the gas concentrations may change rapidly, as in mobile robotic olfaction, where allowing for sensor recovery forces the robot to move at a very low speed, almost incompatible with any practical robot operation. This paper describes the design of a new e-nose which overcomes, to a great extent, such a limitation. The proposed e-nose, called Multi-Chamber Electronic Nose (MCE-nose), comprises several identical sets of MOS sensors accommodated in separate chambers (four in our current prototype), which alternate between sensing and recovery states, providing, as a whole, a device capable of sensing changes in chemical concentrations faster. The utility and performance of the MCE-nose in mobile robotic olfaction is shown through several experiments involving rapid sensing of gas concentration and mobile robot gas mapping.
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paper_title: Gas-sensing properties of catalytically modified WO/sub 3/ with copper and vanadium for NH/sub 3/ detection

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Ammonia gas detection by pure and catalytically modified WO/sub 3/-based gas sensors was analyzed. Sensor response of pure tungsten oxide to NH/sub 3/ was unsatisfactory, probably due to the unselective oxidation of ammonia into NO/sub x/. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH/sub 3/ detection. The introduction of Cu and V as catalytic additives improved the sensor response to NH/sub 3/. Possible reaction mechanisms of NH/sub 3/ over these materials are discussed. Sensor responses to other gases like NO/sub 2/ or CO and interference of humidity on ammonia detection were also analyzed so as to choose the best sensing element.
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paper_title: Remote Moisture Sensing utilizing Ordinary RFID Tags

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The paper presents a concept where pairs of ordinary RFID tags are exploited for use as remotely read moisture sensors. The pair of tags is incorporated into one label where one of the tags is embedded in a moisture absorbent material and the other is left open. In a humid environment the moisture concentration is higher in the absorbent material than the surrounding environment which causes degradation to the embedded tag's antenna in terms of dielectric losses and change of input impedance. The level of relative humidity or the amount of water in the absorbent material is determined for a passive RFID system by comparing the difference in RFID reader output power required to power up respectively the open and embedded tag. It is similarly shown how the backscattered signal strength of a semi-active RFID system is proportional to the relative humidity and amount of water in the absorbent material. Typical applications include moisture detection in buildings, especially from leaking water pipe connections hidden beyond walls. Presented solution has a cost comparable to ordinary RFID tags, and the passive system also has infinite life time since no internal power supply is needed. The concept is characterized for two commercial RFID systems, one passive operating at 868 MHz and one semi-active operating at 2.45 GHz.
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paper_title: A Nanopore Structured High Performance Toluene Gas Sensor Made by Nanoimprinting Method

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Toluene gas was successfully measured at room temperature using a device microfabricated by a nanoimprinting method. A highly uniform nanoporous thin film was produced with a dense array of titania (TiO2) pores with a diameter of 70~80 nm using this method. This thin film had a Pd/TiO2 nanoporous/SiO2/Si MIS layered structure with Pd-TiO2 as the catalytic sensing layer. The nanoimprinting method was useful in expanding the TiO2 surface area by about 30%, as confirmed using AFM and SEM imaging. The measured toluene concentrations ranged from 50 ppm to 200 ppm. The toluene was easily detected by changing the Pd/TiO2 interface work function, resulting in a change in the I-V characteristics.
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paper_title: Dynamic thermal conductivity sensor for gas detection

paper_content:

A dynamic thermal conductivity sensor for gas detection based on the transient thermal response of a SiC microplate slightly heated by a screen-printed Pt resistance is described. This sensor is developed for specific applications such as the determination of carbon monoxide content in hydrogen for fuel cell, or that of methane in biogas applications. On the contrary of existing devices, the apparatus developed here does not need any reference cell, it operates in transient mode near room temperature (�T ≈ 5 K in air), so has a very low power requirements (≈5 mW) and keeps the gas near thermal equilibrium which simplifies the mathematical model and eases data processing. In test gases mixtures (N2 + He), absolute and precise measurement of the gas thermal conductivity have been achieved, leading to the exact molar fraction of the gas to detect with a good reproducibility. © 2003 Elsevier B.V. All rights reserved.
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paper_title: On the Electrooxidation and Amperometric Detection of NO Gas at the Pt/Nafion® Electrode

paper_content:

Abstract : The electrochemical oxidation of nitric oxide (NO) gas at the Pt/Nafion â electrode has been studied at a concentration of 500 ppm. The electrooxidation of NO taking place over a wide potential range can be described by a transcendental equation, from which the half-wave potential of the reaction can be determined. For NO oxidation with appreciable overpotentials but negligible mass-transfer effects, the Tafel kinetics applies. The obtained charge transfer coefficient (a) and the exchange current density (i o ) are 0.77 and 14 mA/cm 2 , respectively. An amperometric NO gas sensor based on the Pt/Nafion â electrode has been fabricated and tested over the NO concentration range from 0 to 500 ppm. The Pt/Nafion â electrode was used as an anode at a fixed potential, preferably 1.15 V (vs. Ag/AgCl/sat. KCl), which assures current limitation by diffusion only. The sensitivity of the electrochemical sensor was found to be 1.86 mA/ppm/cm 2 . The potential interference by other gases, such as nitrogen dioxide (NO
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paper_title: Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

paper_content:

Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.
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paper_title: A thin-film SnO2 sensor system for simultaneous detection of CO and NO2 with neural signal evaluation

paper_content:

Simultaneous CO and NO 2 measurements are of importance for the ventilation control of automobiles and other applications. For this purpose semiconducting SnO 2 sensors are often used. A well known disadvantage of SnO 2 sensors is the concurrent reaction of the oxidizing NO 2 and the reducing CO on the sensor surface, which causes a near zero sensor signal in the presence of both gases in a certain range of mixtures. A second disadvantage of SnO 2 sensors are the long rise and decay times of the sensor signal. The combination of different SnO 2 sensors, operated at different temperatures and combined with a signal evaluation system based on a specially trained neural forward network (artificial neural net (ANN)) solves this problem. The runtime version of the neural net is a small program, compatible with customary micro controllers. These signal evaluation techniques are applicable to similar problems using sensor arrays or single sensors in a non-stationary operating mode.
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paper_title: Electronic nose: A toxic gas sensor by polyaniline thin film conducting polymer

paper_content:

This paper presents study of toxic gas sensing property by polyaniline thin film conducting polymer for ammonia gas. In this work, the precipitate of the conducting polymer has been synthesized by chemical method. The conducting polymer film which acts as an active layer of sensor was prepared with the help of the precipitate (of the conducting polymer) by spin-coating technique. The deposited films were characterized by FTIR spectroscopy and four probe method. The study indicates that, the resistance of the film decreases with the increase in concentration of ammonia at the known volume.
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paper_title: Conductive polymer‐carbon black composites‐based sensor arrays for use in an electronic nose

paper_content:

Polymer‐carbon black composites are a new class of chemical detecting sensors used in electronic noses. These composites are prepared by mixing carbon black and polymer in an appropriate solvent. The mixture is deposited on a substrate between two metal electrodes, whereby the solvent evaporates leaving a composite film. Arrays of these chemiresistors, made from a chemically diverse number of polymers and carbon black, swell reversibly, inducing a resistance change on exposure to chemical vapors. These arrays generate a pattern that is a unique fingerprint for the vapor being detected. With the aid of algorithms these patterns are processed and recognized. These arrays can detect and discriminate between a large number of chemical vapors.
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paper_title: Gas Sensor Based on Photonic Crystal Fibres in the 2ν3 and ν2 + 2ν3 Vibrational Bands of Methane

paper_content:

Abstract: In this work, methane detection is performed on the 2 ” 3 and ” 2 + 2 ” 3 absorptionbands in the Near-Infrared (NIR) wavelength region using an all-ﬁbre optical sensor. Hollow-core photonic bandgap ﬁbres (HC-PBFs) are employed as gas cells due to their compactness,good integrability in optical systems and feasibility of long interaction lengths with gases.Sensing in the 2 ” 3 band of methane is demonstrated to achieve a detection limit one orderof magnitude better than that of the ” 2 + 2 ” 3 band. Finally, the ﬁlling time of a HC-PBF isdemonstrated to be dependent on the ﬁbre length and geometry.Keywords: gas detectors; methane; ﬁbre optic sensors; photonic crystal ﬁbres; hollow-corephotonic bandgap ﬁbres; absorption spectroscopy1. IntroductionMethane (CH 4 ) is a very common gas, found in many industrial applications where it carries a sig-niﬁcant explosion hazard, and in the environment where it can give rise to strong greenhouse effectcontributing to global warming. It is therefore of great interest to develop methane sensors to accurately
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paper_title: ZnO:Al Thin Film Gas Sensor for Detection of Ethanol Vapor

paper_content:

The ZnO:Al thin films were prepared by RF magnetron sputtering on Si substrate using Pt as interdigitated electrodes. The structure was characterized by XRD and SEM analyses, and the ethanol vapor gas sensing as well as electrical properties have been investigated and discussed. The gas sensing results show that the sensitivity for detecting 400 ppm ethanol vapor was ~20 at an operating temperature of 250°C. The high sensitivity, fast recovery, and reliability suggest that ZnO:Al thin film prepared by RF magnetron sputtering can be used for ethanol vapor gas sensing.
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paper_title: Application of ultrasonic to a hydrogen sensor

paper_content:

A fast responce hydrogen sensor using ultrasonic was developed and demonstrated. It uses the difference in sound velocity between hydrogen and the air. Thus, it is possible to measure hydrogen concentration by measuring the sound velocity changing when hydrogen is mixed in the air. The hydrogen sensor using ultrasonic in this study can be very fast in response time which was less than 84 msec. We also have shown that it is possible to detect hydrogen concentration as low as 100 ppm with the distance between ultrasonic probes as small as 20 mm. Temperature variation test was also demonstrated between −10 and 50°C and confirmed that it can match with calculation results.
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paper_title: Real-Time Gas Identification by Analyzing the Transient Response of Capillary-Attached Conductive Gas Sensor

paper_content:

In this study, the ability of the Capillary-attached conductive gas sensor (CGS) in real-time gas identification was investigated. The structure of the prototype fabricated CGS is presented. Portions were selected from the beginning of the CGS transient response including the first 11 samples to the first 100 samples. Different feature extraction and classification methods were applied on the selected portions. Validation of methods was evaluated to study the ability of an early portion of the CGS transient response in target gas (TG) identification. Experimental results proved that applying extracted features from an early part of the CGS transient response along with a classifier can distinguish short-chain alcohols from each other perfectly. Decreasing time of exposition in the interaction between target gas and sensing element improved the reliability of the sensor. Classification rate was also improved and time of identification was decreased. Moreover, the results indicated the optimum interval of the early transient response of the CGS for selecting portions to achieve the best classification rates.
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paper_title: Thermal and flow analysis of SiC-based gas sensors for automotive applications

paper_content:

Different block and tube mounting alternatives for SiC-based gas sensors were studied by means of temperature measurements and simulation of heat transfer and gas flow for steady state conditions. The most preferable tube mounting design was determined. Simulation-based guidelines were developed for designing tube-mounted gas sensors in the exhaust pipes of diesel and petrol engines, taking into account thermal constraints and flow conditions.
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paper_title: Bromocresol Green/Mesoporous Silica Adsorbent for Ammonia Gas Sensing via an Optical Sensing Instrument

paper_content:

A meso-structured Al-MCM-41 material was impregnated with bromocresol green (BG) dye and then incorporated into a UV-Vis DRA spectroscopic instrument for the online detection of ammonia gas. The absorption response of the Al-MCM-41/BG ammonia sensing material was very sensitive at the optical absorption wavelength of 630 nm. A high linear correlation was achieved for ppmv and sub-ppmv levels of ammonia gas. The response time for the quantitative detection of ammonia gas concentrations ranging from 0.25 to 2.0 ppmv was only a few minutes. The lower detection limit achieved was 0.185 ppmv. The color change process was fully reversible during tens of cycling tests. These features together make this mesoporous Al-MCM-41 material very promising for optical sensing applications.
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paper_title: Theory of power laws for semiconductor gas sensors

paper_content:

It has long been known empirically that the electric resistance of a semiconductor gas sensor under exposure to a target gas (partial pressure P) is proportional to Pn where n is a constant fairly specific to the kind of target gas (power law). This paper aims at providing a theoretical basis to such power laws. It is shown that the laws can be derived by combining a depletion theory of semiconductor, which deals with the distribution of electrons between surface state (surface charge) and bulk, with the dynamics of adsorption and/or reactions of gases on the surface, which is responsible for accumulation or reduction of surface charges. The resulting laws describe well sensor response behavior to oxygen, reducing gases and oxidizing gases.
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paper_title: Trace amount formaldehyde gas detection for indoor air quality monitoring

paper_content:

Formaldehyde is not only a carcinogenic chemical, but also causes sick building syndrome. Very small amounts of formaldehyde, such as those emitted from building materials and furniture, pose great concerns for human health. A Health Canada guideline, proposed in 2005, set the maximum formaldehyde concentration for long term exposure (8-hours averaged) as 40 ppb (50 μg/m3). This is a low concentration that commercially available formaldehyde sensors have great difficulty to detect both accurately and continuously. In this paper, we report a formaldehyde gas detection system which is capable of pre-concentrating formaldehyde gas using absorbent, and subsequently thermally desorbing the concentrated gas for detection by the electrochemical sensor. Initial results show that the system is able to detect formaldehyde gas at the ppb level, thus making it feasible to detect trace amount of formaldehyde in indoor environments.
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paper_title: MICROMACHINED THIN FILM SNO2 GAS SENSORS IN TEMPERATURE-PULSED OPERATION MODE

paper_content:

Abstract Gas detection measurements based on a micromachined SnO 2 gas sensor with periodically pulsed heater voltage are presented. Additionally, the field-effect-induced changes in resistivity of the sensitive layer caused by the heater voltage were investigated. The combination of both results leads to an improved design for low power SnO 2 gas sensors. In temperature-pulsed mode, the sensor resistances were measured at constant delays after the pulse edges. The measurements were carried out with the common test gases carbon monoxide and nitric dioxide in synthetic air with 50% humidity. In the cold pulse phase, the CO sensor response is higher and shows only a slow decrease with increasing pulse duration. The sensor sensitivity is related to the pulsed heated mode, on the one hand, and the continuously heated, on the other. The comparison of the measurement results reveals that the temperature-pulsed operation mode (TPOM) caused a significant reduction of power consumption and higher sensitivity.
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paper_title: A CMOS Single-Chip Gas Recognition Circuit for Metal Oxide Gas Sensor Arrays

paper_content:

This paper presents a CMOS single-chip gas recognition circuit, which encodes sensor array outputs into a unique sequence of spikes with the firing delay mapping the strength of the stimulation across the array. The proposed gas recognition circuit examines the generated spike pattern of relative excitations across the population of sensors and looks for a match within a library of 2-D spatio-temporal spike signatures. Each signature is drift insensitive, concentration invariant and is also a unique characteristic of the target gas. This VLSI friendly approach relies on a simple spatio-temporal code matching instead of existing computationally expensive pattern matching statistical techniques. In addition, it relies on a novel sensor calibration technique that does not require control or prior knowledge of the gas concentration. The proposed gas recognition circuit was implemented in a 0.35 μm CMOS process and characterized using an in-house fabricated 4 × 4 tin oxide gas sensor array. Experimental results show a correct detection rate of 94.9% when the gas sensor array is exposed to propane, ethanol and carbon monoxide.
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paper_title: Gas sensing properties of CNT-SnO2 Nanocomposite Thin Film Prepared by E-beam Evaporation

paper_content:

This work presents new results on the gas sensing study of CNT-SnO2 nanocomposite thin film prepared by e-beam evaporation process. The SnO2 gas sensing layers have been achieved with varying CNT concentration. Structural and morphological characterization by means of SEM TEM and XRD indicate that CNTs is incorporated in form of small fragments uniformly dispersed within SnO2 crystal and CNT inclusion does not modify surface morphology and slightly alter the crystal structure. Electrical characterizations highlight a very peculiar and interesting behavior for the layer tested. The SnO2 film's conductance decreases by more than two orders of magnitude with 1%CNT concentration. As CNT content increases to 1%, CO, ethanol, and particularly NO2 responses are improved and the working temperature tends to reduce. In addition, 1% CNT-SnO2 exhibit maximized response to NO2 with sensitivity of ~10 at very low NO2 concentration of 250 ppb at 200degC.
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paper_title: Highly sensitive and selective ammonia gas sensor

paper_content:

We have fabricated and examined an ammonia gas sensor with high sensitivity using thick-film technology. The sensing material of the gas sensor is FeO/sub x/-WO/sub 3/-SnO/sub 2/ oxide semiconductor. The sensor exhibits resistance increase upon exposure to low concentration of ammonia gas. The resistance of the sensor is decreased, on the other hand, for exposure to reducing gases such as ethyl alcohol, methane, propane and carbon monoxide. We have proposed and investigated a novel method for detecting ammonia gas quite selectively by using a sensor array with two sensing elements which contains an ammonia gas sensor and a compensation element. The compensation element is a Pt-doped WO/sub 3/-SnO/sub 2/ gas sensor which shows an opposite direction of resistance change in comparison with the ammonia gas sensor upon exposure to ammonia gas. Excellent selectivity has been achieved using the sensor array with two sensing elements.
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paper_title: Enhancement of MOS gas sensor selectivity by 'on-chip' catalytic filtering

paper_content:

Abstract A novel approach to enhancing the selectivity of a thick-film metal-oxide-semiconductor gas sensor is presented. The approach complements the efforts of many investigators who have tried to improve metal-oxide-semiconductor gas sensor selectivity by altering the sensing material alone. The discussion and data presented will focus on how 'on-chip' catalytic filtering is employed to build a selective fuel gas sensor without any adverse effects on the sensor's sensitivity or speed of response. Data are also presented showing the sensor's stable response to methane over an extended concentration range for over three years.
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paper_title: Semiconducting Metal Oxide Based Sensors for Selective Gas Pollutant Detection

paper_content:

A review of some papers published in the last fifty years that focus on the semiconducting metal oxide (SMO) based sensors for the selective and sensitive detection of various environmental pollutants is presented.
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paper_title: A wireless home safety gas leakage detection system

paper_content:

A wireless safety device for gas leakage detection is proposed. The device is intended for use in household safety where appliances and heaters that use natural gas and liquid petroleum gas (LPG) may be a source of risk. The system also can be used for other applications in the industry or plants that depend on LPG and natural gas in their operations. The system design consists of two main modules: the detection and transmission module, and the receiving module. The detection and transmitting module detects the change of gas concentration using a special sensing circuit built for this purpose. This module checks if a change in concentration of gas(es) has exceeded a certain pre-determined threshold. If the sensor detects a change in gas concentration, it activates and audiovisual alarm and sends a signal to the receiver module. The receiver module acts as a mobile alarm device to allow the mobility within the house premises. The system was tested using LPG and the alarm was activated as a result of change in concentration.
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paper_title: Advances in SAW Gas Sensors Based on the Condensate-Adsorption Effect

paper_content:

A surface-acoustic-wave (SAW) gas sensor with a low detection limit and fast response for volatile organic compounds (VOCs) based on the condensate-adsorption effect detection is developed. In this sensor a gas chromatography (GC) column acts as the separator element and a dual-resonator oscillator acts as the detector element. Regarding the surface effective permittivity method, the response mechanism analysis, which relates the condensate-adsorption effect, is performed, leading to the sensor performance prediction prior to fabrication. New designs of SAW resonators, which act as feedback of the oscillator, are devised in order to decrease the insertion loss and to achieve single-mode control, resulting in superior frequency stability of the oscillator. Based on the new phase modulation approach, excellent short-term frequency stability (±3 Hz/s) is achieved with the SAW oscillator by using the 500 MHz dual-port resonator as feedback element. In a sensor experiment investigating formaldehyde detection, the implemented SAW gas sensor exhibits an excellent threshold detection limit as low as 0.38 pg.
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paper_title: Thin-film SnO2 sensor arrays controlled by variation of contact potential—a suitable tool for chemometric gas mixture analysis in the TLV range

paper_content:

The selectivity of SnO 2 thin-film gas sensors can be modulated by changing the contact areas of the contact electrodes. The investigated SnO 2 arrays have been structured with two mask steps only. Gas mixture analysis in the German threshold limit value (TLV) range has been performed with an integrated thin-film sensor array using principal component regression (PCR) analysis. As an example, NO 2 , CO, CH 4 and H 2 O mixtures in air have been analyzed. In this case additional doping materials, catalysts, temperature gradients, etc., to enhance selectivity are not necessary.
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paper_title: Atomic layer deposition of tin dioxide sensing film in microhotplate gas sensors

paper_content:

We report the use of atomic layer deposition (ALD) to produce the gas-sensitive tin dioxide film in a microhotplate gas sensor. The performance of the device was demonstrated using ethanol, acetone and acrylonitrile as model analytes. Fast response times and low drift rates of the output signal were measured, indicating a structurally stable tin dioxide film and reflecting the capabilities of ALD in gas sensor applications. Fabrication of the microhotplate using tungsten metallization and plasma deposited silicon dioxide dielectrics is also detailed.
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paper_title: Methane and Carbon Monoxide Gas Detection system based on semiconductor sensor

paper_content:

One of the most important actual problems in the gas detection field is that there are strong demands for gas methane leak detection and CO (carbon monoxide) detection to prevent explosions or CO poisoning accidents. In this sense, the present paper describes technical characteristics, test results, and a concluding application for methane and carbon monoxide based gas detection using a sensor which can detect both CO and methane with a single sensing element. The paper presents the detection method as well as the apparatus functional sketch.
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paper_title: Metal Oxide Gas Sensors: Sensitivity and Influencing Factors

paper_content:

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.
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paper_title: Multi-layered thick-film gas sensor array for selective sensing by catalytic filtering technology

paper_content:

A C3H8 gas sensor array with high sensitivity and good selectivity by the combination of the catalytic filtering and the gas diffusion control has been achieved. The gas sensor array consists of a couple of catalytic filter layers, one with a Pd and the other with a Pt, a SiO2 insulating layer and two sensing layers on an alumina substrate. The sensor array shows high sensitivity to 500 ppm of C3H8 at temperature above 400°C and good selectivity for the interfering gases such as CO and C2H5OH by using a simple signal processing technique.
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paper_title: Low-power micro gas sensor

paper_content:

The stable and low-power heating characteristics of a microheater are very important for the micro gas sensor. Membrane-type gas sensors have been fabricated by silicon IC technology. Steady-state thermal analysis by the finite-element method is performed to optimize the thermal properties of the gas sensor. From the analysis, the desirable size of the microheater for low power consumption is determined. The heating properties of fabricated poly-Si and Pt microheaters have been tested. The sensing characteristics of the packaged microsensor are also examined.
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paper_title: Metal oxide nano-crystals for gas sensing

paper_content:

Abstract This review article is focused on the description of metal oxide single crystalline nanostructures used for gas sensing. Metal oxide nano-wires are crystalline structures with precise chemical composition, surface terminations, and dislocation-defect free. Their nanosized dimension generate properties that can be significantly different from their coarse-grained polycrystalline counterpart. Surface effects appear because of the magnification in the specific surface of nanostructures, leading to an enhancement of the properties related to that, such as catalytic activity or surface adsorption. Properties that are basic phenomenon underlying solid-state gas sensors. Their use as gas-sensing materials should reduce instabilities, suffered from their polycrystalline counterpart, associated with grain coalescence and drift in electrical properties. High degree of crystallinity and atomic sharp terminations make them very promising for better understanding of sensing principles and for development of a new generation of gas sensors. These sensing nano-crystals can be used as resistors, in FET based or optical based gas sensors. The gas experiments presented confirm good sensing properties, the possibility to use dopants and catalyser such in thin film gas sensors and the real integration in low power consumption transducers of single crystalline nanobelts prove the feasibility of large scale manufacturing of well-organized sensor arrays based on different nanostructures. Nevertheless, a greater control in the growth is required for an application in commercial systems, together with a thorough understanding of the growth mechanism that can lead to a control in nano-wires size and size distributions, shape, crystal structure and atomic termination.
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paper_title: The surface and materials science of tin oxide

paper_content:

Abstract The study of tin oxide is motivated by its applications as a solid state gas sensor material, oxidation catalyst, and transparent conductor. This review describes the physical and chemical properties that make tin oxide a suitable material for these purposes. The emphasis is on surface science studies of single crystal surfaces, but selected studies on powder and polycrystalline films are also incorporated in order to provide connecting points between surface science studies with the broader field of materials science of tin oxide. The key for understanding many aspects of SnO 2 surface properties is the dual valency of Sn. The dual valency facilitates a reversible transformation of the surface composition from stoichiometric surfaces with Sn 4+ surface cations into a reduced surface with Sn 2+ surface cations depending on the oxygen chemical potential of the system. Reduction of the surface modifies the surface electronic structure by formation of Sn 5s derived surface states that lie deep within the band gap and also cause a lowering of the work function. The gas sensing mechanism appears, however, only to be indirectly influenced by the surface composition of SnO 2 . Critical for triggering a gas response are not the lattice oxygen concentration but chemisorbed (or ionosorbed) oxygen and other molecules with a net electric charge. Band bending induced by charged molecules cause the increase or decrease in surface conductivity responsible for the gas response signal. In most applications tin oxide is modified by additives to either increase the charge carrier concentration by donor atoms, or to increase the gas sensitivity or the catalytic activity by metal additives. Some of the basic concepts by which additives modify the gas sensing and catalytic properties of SnO 2 are discussed and the few surface science studies of doped SnO 2 are reviewed. Epitaxial SnO 2 films may facilitate the surface science studies of doped films in the future. To this end film growth on titania, alumina, and Pt(1 1 1) is reviewed. Thin films on alumina also make promising test systems for probing gas sensing behavior. Molecular adsorption and reaction studies on SnO 2 surfaces have been hampered by the challenges of preparing well-characterized surfaces. Nevertheless some experimental and theoretical studies have been performed and are reviewed. Of particular interest in these studies was the influence of the surface composition on its chemical properties. Finally, the variety of recently synthesized tin oxide nanoscopic materials is summarized.
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paper_title: Gas sensors based on anodic tungsten oxide

paper_content:

Nanostructured porous tungsten oxide materials were synthesized by the means of electrochemical etching (anodization) of tungsten foils in aqueous NaF electrolyte. Formation of the sub-micrometer size mesoporous particles has been achieved by infiltrating the pores with water. The obtained colloidal anodic tungsten oxide dispersions have been used to fabricate resistive WO3 gas sensors by drop casting the sub-micrometer size mesoporous particles between Pt electrodes on Si/SiO2 substrate followed by calcination at 400 °C in air for 2 h. The synthesized WO3 films show slightly nonlinear current–voltage characteristics with strong thermally activated carrier transport behavior measured at temperatures between −20 °C and 280 °C. Gas response measurements carried out in CO, H2, NO and O2 analytes (concentration from 1 to 640 ppm) in air as well as in Ar buffers (O2 only in Ar) exhibited a rapid change of sensor conductance for each gas and showed pronounced response towards H2 and NO in Ar and air, respectively. The response of the sensors was dependent on temperature and yielded highest values between 170 °C and 220 °C.
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paper_title: A thin-film SnO2 sensor system for simultaneous detection of CO and NO2 with neural signal evaluation

paper_content:

Simultaneous CO and NO 2 measurements are of importance for the ventilation control of automobiles and other applications. For this purpose semiconducting SnO 2 sensors are often used. A well known disadvantage of SnO 2 sensors is the concurrent reaction of the oxidizing NO 2 and the reducing CO on the sensor surface, which causes a near zero sensor signal in the presence of both gases in a certain range of mixtures. A second disadvantage of SnO 2 sensors are the long rise and decay times of the sensor signal. The combination of different SnO 2 sensors, operated at different temperatures and combined with a signal evaluation system based on a specially trained neural forward network (artificial neural net (ANN)) solves this problem. The runtime version of the neural net is a small program, compatible with customary micro controllers. These signal evaluation techniques are applicable to similar problems using sensor arrays or single sensors in a non-stationary operating mode.
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paper_title: Feasibility of wireless gas detection with an FMCW RADAR interrogation of passive RF gas sensor

paper_content:

The feasibility of the remote measurement of gas detection from an RF gas sensor has been experimentally investigated. It consists of a Frequency-Modulated Continuous-Wave (FMCW) RADAR interrogation of an antenna loaded by the passive sensor. The frequency band of the RADAR [28.8–31GHz] allows the detection of the resonant frequencies of Whispering Gallery Modes that are sensitive to gas concentration. Reported experimental results provide the proof-of-concept of remote measurement of gas concentration fluctuation from RADAR interrogation of this new generation of passive gas sensors.
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paper_title: Detection of hydrogen fluoride using SnO2-based gas sensors : Understanding of the reactional mechanism

paper_content:

Tin dioxide-based gas sensors are very efficient devices for the detection of hydrogen fluoride in trace levels since amounts lower than 50 ppb can be detected. Considering the working temperatures of tin dioxide-based gas sensors which are included between 25 and 500 ◦C, the best sensitivity was obtained when the sensor's temperature was maintained at 380 ◦C. In order to explain why high sensitivity is obtained for this temperature and then to understand the reactional mechanism between HF molecules and tin dioxide surfaces, X-ray photoelectron spectroscopy investigations were performed on tin dioxide samples treated with HF vapors at temperatures ranging from 200 to 500 ◦C. For this temperature range, the comparison between the electrical response curves and the XPS characterization, led to the consideration of two separate temperature ranges, where the interaction mechanism between HF and SnO2 can be explained. For temperature lower than 380 ◦C, the adsorption of HF induces the formation of surface hydroxyl groups and SnF4 species. In that case, the electrical conductance of the sensitive material gradually increases. Beyond this temperature, water vapors desorb from tin dioxide surfaces and then the electrical conductance is lowered. Finally, for both ranges of temperatures, the interaction mechanism occurring at the gas/detector interface is proposed.
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paper_title: Gas Sensors Based on Conducting Polymers

paper_content:

The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.
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paper_title: CMOS single-chip gas detection system comprising capacitive, calorimetric and mass-sensitive microsensors

paper_content:

A single-chip gas detection system fabricated in industrial CMOS technology combined with post-CMOS micro-machining is presented. The sensors rely on a chemo-sensitive polymer layer, which absorbs predominantly volatile organic compounds (VOCs). A mass-sensitive resonant-beam oscillator, a capacitive sensor incorporated into a second-order /spl Sigma//spl Delta/-modulator, a calorimetric sensor with low-noise signal conditioning circuitry and a temperature sensor are monolithically integrated on a single chip along with all necessary driving and signal conditioning circuitry. The preprocessed sensor signals are converted to the digital domain on chip. An additional integrated controller sets the sensor parameters and transmits the sensor values to an off-chip data recording unit via a standard serial interface. A 6-chip-array has been flip-chip packaged on a ceramic substrate, which forms part of a handheld VOC gas detection unit. Limits of detection (LOD) of 1-5 ppm n-octane, toluene or propan-1-ol have been achieved.
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paper_title: Design of conducting polymer gas sensors : modelling and experiment

paper_content:

Abstract The use of conducting polymers as active materials in chemical sensors is growing rapidly; for example they have been used in the place of metal oxides to sense gases and vapours, such as NH 3 , NO 2 and alcohols. Here we model a polymer gas sensor in terms of homogeneous diffusion coupled to simple adsorption within a bounded layer. From the model we present analytical expressions of the adsorbate profiles for the diffusion-rate limited, reaction-rate limited and intermediate cases in terms of fundamental dimensionless parameters. The model is then used to calculate the conductance of a typical chemiresistor which consists of a pair of co-planar electrodes below on electropolymerised thin polymer film and on an impermeable substrate. The analytical expression for the electric field is combined with the diffusion reaction equations by assuming an single carrier conduction model. Finally, the theoretical chemiresistor response is calculated in six limiting cases and compared with experimental data on pyrrole-based conducting polymers. In practice the gas-polymer interaction is likely to be much more complex and so we are extending our model to consider in more detail the conduction principles.
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paper_title: Electronic nose: A toxic gas sensor by polyaniline thin film conducting polymer

paper_content:

This paper presents study of toxic gas sensing property by polyaniline thin film conducting polymer for ammonia gas. In this work, the precipitate of the conducting polymer has been synthesized by chemical method. The conducting polymer film which acts as an active layer of sensor was prepared with the help of the precipitate (of the conducting polymer) by spin-coating technique. The deposited films were characterized by FTIR spectroscopy and four probe method. The study indicates that, the resistance of the film decreases with the increase in concentration of ammonia at the known volume.
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paper_title: Comparative studies on polymer coated SAW and STW resonators for chemical gas sensor applications

paper_content:

This paper presents and compares experimental data from performance tests on polymer coated 433 MHz surface acoustic wave (SAW) and 1 GHz surface transverse wave (STW) based two-port resonators for chemical gas sensor applications. The acoustic devices were coated with gas sensitive polymer films of different thickness' and viscoelastic properties as parylene C, poly-(2-hydroxyethylmethacrylate) (PHEMA) and poly-(n-butyl-methacrylate) (PBMA). Then they were gas probed using perchloroethylene and water. The SAW versus STW sensor sensitivities, insertion loss, loaded Q and distortion of the frequency and phase responses during gas probing were evaluated and compared. It was found that STW devices, when coated with thin sensitive polymer layers, retain low loss, high Q and low noise and feature substantially higher relative gas sensitivities compared to their SAW counterparts. Coated SAW sensors can stand substantially thicker soft polymer films than STW ones but at the expense of highly degraded electrical and noise performance and moderate increase in relative sensitivity.
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paper_title: CMOS single-chip gas detection system comprising capacitive, calorimetric and mass-sensitive microsensors

paper_content:

A single-chip gas detection system fabricated in industrial CMOS technology combined with post-CMOS micro-machining is presented. The sensors rely on a chemo-sensitive polymer layer, which absorbs predominantly volatile organic compounds (VOCs). A mass-sensitive resonant-beam oscillator, a capacitive sensor incorporated into a second-order /spl Sigma//spl Delta/-modulator, a calorimetric sensor with low-noise signal conditioning circuitry and a temperature sensor are monolithically integrated on a single chip along with all necessary driving and signal conditioning circuitry. The preprocessed sensor signals are converted to the digital domain on chip. An additional integrated controller sets the sensor parameters and transmits the sensor values to an off-chip data recording unit via a standard serial interface. A 6-chip-array has been flip-chip packaged on a ceramic substrate, which forms part of a handheld VOC gas detection unit. Limits of detection (LOD) of 1-5 ppm n-octane, toluene or propan-1-ol have been achieved.
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paper_title: Study on selectivity enhancement of tin dioxide gas sensor using non-conducting polymer membrane

paper_content:

A non-conducting polymer membrane can be used as a molecular sieve to enhance the selectivity of a tin dioxide gas sensor. A commercial polymer, polyimide XU218 of the Eiba-Geigy company, is coated on the surface of a tin dioxide gas sensor FIGARO TGS842 which was designed to detect methane. The polymer coated sensor is calibrated in a high accuracy testing system which is controlled by a PC with GPIB and mass flow controllers (MFCs). Three gases, hydrogen, methane, and ammonia, are used to investigate the response of the sensor. It is found that the sensor shows a characteristic change in response to ammonia and an almost negligible change in response to hydrogen and methane.
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paper_title: Conductive polymer‐carbon black composites‐based sensor arrays for use in an electronic nose

paper_content:

Polymer‐carbon black composites are a new class of chemical detecting sensors used in electronic noses. These composites are prepared by mixing carbon black and polymer in an appropriate solvent. The mixture is deposited on a substrate between two metal electrodes, whereby the solvent evaporates leaving a composite film. Arrays of these chemiresistors, made from a chemically diverse number of polymers and carbon black, swell reversibly, inducing a resistance change on exposure to chemical vapors. These arrays generate a pattern that is a unique fingerprint for the vapor being detected. With the aid of algorithms these patterns are processed and recognized. These arrays can detect and discriminate between a large number of chemical vapors.
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paper_title: Gas sensing properties of CNT-SnO2 Nanocomposite Thin Film Prepared by E-beam Evaporation

paper_content:

This work presents new results on the gas sensing study of CNT-SnO2 nanocomposite thin film prepared by e-beam evaporation process. The SnO2 gas sensing layers have been achieved with varying CNT concentration. Structural and morphological characterization by means of SEM TEM and XRD indicate that CNTs is incorporated in form of small fragments uniformly dispersed within SnO2 crystal and CNT inclusion does not modify surface morphology and slightly alter the crystal structure. Electrical characterizations highlight a very peculiar and interesting behavior for the layer tested. The SnO2 film's conductance decreases by more than two orders of magnitude with 1%CNT concentration. As CNT content increases to 1%, CO, ethanol, and particularly NO2 responses are improved and the working temperature tends to reduce. In addition, 1% CNT-SnO2 exhibit maximized response to NO2 with sensitivity of ~10 at very low NO2 concentration of 250 ppb at 200degC.
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paper_title: A wireless, passive carbon nanotube-based gas sensor

paper_content:

A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO/sub 2/) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO/sub 2/), oxygen (O/sub 2/), and ammonia (NH/sub 3/). The absorption of different gases in the MWNT-SiO/sub 2/ layer changes the permittivity and conductivity of the material and consequently alters the resonant frequency of the sensor. By tracking the frequency spectrum of the sensor with a loop antenna, humidity, temperature, as well as CO/sub 2/, O/sub 2/ and NH/sub 3/ concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO/sub 2/ and O/sub 2/ is both linear and reversible. Both irreversible and reversible responses are observed in response to NH/sub 3/, indicating both physisorption and chemisorption of NH/sub 3/ by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO/sub 2/ coated, and MWNT-SiO/sub 2/ coated sensor, enables CO/sub 2/ measurement to be automatically calibrated for operation in a variable humidity and temperature environment.
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paper_title: Dynamic Control of Adsorption Sensitivity for Photo-EMF-Based Ammonia Gas Sensors Using a Wireless Network

paper_content:

This paper proposes an adsorption sensitivity control method that uses a wireless network and illumination light intensity in a photo-electromagnetic field (EMF)-based gas sensor for measurements in real time of a wide range of ammonia concentrations. The minimum measurement error for a range of ammonia concentration from 3 to 800 ppm occurs when the gas concentration magnitude corresponds with the optimal intensity of the illumination light. A simulation with LabView-engineered modules for automatic control of a new intelligent computer system was conducted to improve measurement precision over a wide range of gas concentrations. This gas sensor computer system with wireless network technology could be useful in the chemical industry for automatic detection and measurement of hazardous ammonia gas levels in real time.
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paper_title: Observation of dynamic behavior of PD-generated SF/sub 6/ decompositions using carbon nanotube gas sensor

paper_content:

The authors had proposed a new detection method for partial discharge (PD) occurring in sulfur hexafluoride (SF/sub 6/) gas using a carbon nanotube (CNT) gas sensor. In the previous study, we had investigated the dependency of gas sensor response on applied voltage and gas sensor position. In this paper, a series of experiments was performed to observe dynamic behavior of decomposition gas using the CNT gas sensor. The gas sensor responses to PD under several SF/sup 6/ gas pressures were measured. It was found that the sensor response normalized by PD power increased almost linearly with the gas pressure. The result is useful to understand the mechanism how the gas pressure influences the generation and diffusion of decomposition gas. Effects of absorbent placed inside the discharge chamber were also investigated. Finally, the gas sensor responses were measured with a time interval after PD was extinguished to check if an offline diagnosis was possible. Although the sensor response decreased with elapsed time after PD was extinguished, it was still possible to detect the residual decomposition gas using the CNT gas sensor.
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paper_title: Detection of individual gas molecules adsorbed on graphene

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The ultimate aim of any detection method is to achieve such a level of sensitivity that individual quanta of a measured entity can be resolved. In the case of chemical sensors, the quantum is one atom or molecule. Such resolution has so far been beyond the reach of any detection technique, including solid-state gas sensors hailed for their exceptional sensitivity1, 2, 3, 4. The fundamental reason limiting the resolution of such sensors is fluctuations due to thermal motion of charges and defects5, which lead to intrinsic noise exceeding the sought-after signal from individual molecules, usually by many orders of magnitude. Here, we show that micrometre-size sensors made from graphene are capable of detecting individual events when a gas molecule attaches to or detaches from graphene's surface. The adsorbed molecules change the local carrier concentration in graphene one by one electron, which leads to step-like changes in resistance. The achieved sensitivity is due to the fact that graphene is an exceptionally low-noise material electronically, which makes it a promising candidate not only for chemical detectors but also for other applications where local probes sensitive to external charge, magnetic field or mechanical strain are required.
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paper_title: The effect of sequence length on DNA decorated CNT gas sensors

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This article reports the effect of deoxyribonucleic acid (DNA) sequence length on the sensing characteristics of DNA decorated single-walled carbon nanotube (SWNT) devices. First, SWNTs were assembled on micro electrodes via a versatile, solution based Dielectrophoresis process. Then four single-stranded poly-G oligomers with lengths of 8, 16, 24 and 32 were decorated on SWNTs and the response of SWNT sensors to methanol and IPA vapors were measured. We found that the optimum DNA sequence length for sensing applications was 24. Sequence length of DNA had a dramatic impact on the response of DNA-SWNT sensors. This phenomenon can be explained by the difference in binding affinities of nucleotides on SWNTs and the conformations that DNA forms on SWNTs. These experimental results have significant implications for the interactions between DNA and SWNTs. They can facilitate the development of DNA functionalized SWNT sensors in analytical chemistry, biochemistry and environmental monitoring applications.
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paper_title: Remote Moisture Sensing utilizing Ordinary RFID Tags

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The paper presents a concept where pairs of ordinary RFID tags are exploited for use as remotely read moisture sensors. The pair of tags is incorporated into one label where one of the tags is embedded in a moisture absorbent material and the other is left open. In a humid environment the moisture concentration is higher in the absorbent material than the surrounding environment which causes degradation to the embedded tag's antenna in terms of dielectric losses and change of input impedance. The level of relative humidity or the amount of water in the absorbent material is determined for a passive RFID system by comparing the difference in RFID reader output power required to power up respectively the open and embedded tag. It is similarly shown how the backscattered signal strength of a semi-active RFID system is proportional to the relative humidity and amount of water in the absorbent material. Typical applications include moisture detection in buildings, especially from leaking water pipe connections hidden beyond walls. Presented solution has a cost comparable to ordinary RFID tags, and the passive system also has infinite life time since no internal power supply is needed. The concept is characterized for two commercial RFID systems, one passive operating at 868 MHz and one semi-active operating at 2.45 GHz.
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paper_title: Differential Absorption Lidar to Measure Subhourly Variation of Tropospheric Ozone Profiles

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A tropospheric ozone Differential Absorption Lidar system, developed jointly by The University of Alabama in Huntsville and the National Aeronautics and Space Administration, is making regular observations of ozone vertical distributions between 1 and 8 km with two receivers under both daytime and nighttime conditions using lasers at 285 and 291 nm. This paper describes the lidar system and analysis technique with some measurement examples. An iterative aerosol correction procedure reduces the retrieval error arising from differential aerosol backscatter in the lower troposphere. Lidar observations with coincident ozonesonde flights demonstrate that the retrieval accuracy ranges from better than 10% below 4 km to better than 20% below 8 km with 750-m vertical resolution and 10-min temporal integration.
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paper_title: Photoacoustic Spectroscopy with Quantum Cascade Lasers for Trace Gas Detection

paper_content:

Abstract: Various applications, such as pollution monitoring, toxic-gas detection, noninvasive medical diagnostics and industrial process control, require sensitive and selectivedetection of gas traces with concentrations in the parts in 10 9 (ppb) and sub-ppb range.The recent development of quantum-cascade lasers (QCLs) has given a new aspect toinfrared laser-based trace gas sensors. In particular, single mode distributed feedback QCLsare attractive spectroscopic sources because of their excellent properties in terms of narrowlinewidth, average power and room temperature operation. In combination with these lasersources, photoacoustic spectroscopy offers the advantage of high sensitivity and selectivity,compact sensor platform, fast time-response and user friendly operation. This paper reportsrecent developments on quantum cascade laser-based photoacoustic spectroscopy for tracegas detection. In particular, different applications of a photoacoustic trace gas sensoremploying a longitudinal resonant cell with a detection limit on the order of hundred ppb ofozone and ammonia are discussed. We also report two QC laser-based photoacousticsensors for the detection of nitric oxide, for environmental pollution monitoring andmedical diagnostics, and hexamethyldisilazane, for applications in semiconductormanufacturing process.
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paper_title: A study on NDIR-based CO2 sensor to apply remote air quality monitoring system

paper_content:

Recently, various CO2 sensors reported in the literature may be classified in two major categories depending on measuring principle of sensors. Fist, the chemical CO2 gas sensors have the principal advantage of a very low energy consumption and small size. On the negative side, It is difficult to apply to a variety of practical fields because they have a short lifetime and as well as low durability. Second, NDIR-based CO2 sensor is commonly used in monitoring indoor air quality due to relatively high accuracy compared with that of a chemical CO2 gas sensor. In this paper, therefore, we evaluate NDIR-based CO2 sensors to verify applicable potentialities of them in a remote air quality monitoring system. In addition, the principle and structure of NDIR CO2 sensor is discussed, and then we analyze the CO2 concentration has been measured in the real platform of subway station for a month. Finally this paper proposes that the accuracy of the measuring values from NDIR CO2 sensor is enough for indoor air quality monitoring applications.
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paper_title: Detection of gases by correlation spectroscopy

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This paper describes the detection of various common gases by means of Correlation Spectroscopy, employing a Complementary-Source-Modulation (CoSM) approach based on compact light-emitting diode (LED) sources. Theoretical results for the quantitative detection of O in air are presented, with the use of practical low cost LED sources.
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paper_title: Compact Raman lidar for hydrogen gas leak detection

paper_content:

A compact Raman lidar system for hydrogen gas leak detection was constructed. Laser-induced fluorescence at a distance of 70 m and Raman scattering light from N 2 gas at short range could be detected.
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paper_title: Measurement of CH4 by differential infrared optical absorption spectroscopy

paper_content:

To solve the problem that the current use of optical methods used to detect various gases existing in sensors and signal processing was too complicated, detection sensitivity was not high, detection of the type of gases was single. We used optical phase-shifted grating laser window adjust the differential infrared absorption spectrometry and studied Spectrum adjustment and detection signal processing problem. CH4 in cigarette main smoke of different kind brands was measured continuously by differential optical absorption spectroscopy (DOAS). By using RM200 rotating disk smoking machine with 20 smoking flue, 20 kinds of gas were inserted into the rotating disk and ignited every 3s. By use of DOAS technique, the measurement was completed with in 6~7min after the gas was transported into a White cell with 31.5m path length which was connected to the smoking machine. This technique can improve the measuremental temporal resolution compared with traditional method. At the beginning, the press in White cell was 5.2×104 Pa and it increased to 1.03×105Pa while the measurement finished. The result shows the concentration of CH4 in main gas is between 0.89mg/m3 and 1.54mg/m. Pool in the White cell entrance placed wide infrared wavelength LED light source, light source after a large-diameter phase-shifted fiber grating access to White cell, Through the piezoelectric ceramic (PZT) phase-shift controlled Phase-Shifted Fiber Bragg Grating to infra-red LED into a laser modulation of narrowband continuous changes in light, wavelength corresponding to various gases such as CH4 absorption spectrum. White cell output pool signals entered into the photoelectric detector and ADC further, controlled by computer-related demodulation digitally. It can achieve high sensitivity digital lock-in amplifier function and be suitable for high-resolution detection of trace gases. Through simulation studies and practical tests, The results show that the piezoelectric ceramic (PZT) control Phase-Shifted Fiber Bragg Grating spectral scan can effectively suppress the background optical interference, the elimination of Rayleigh scattering and Raman scattering. Phase-Shifted Fiber Bragg Grating modulation and digital lock-in amplifier detection sensitivity of the combination improve three orders of magnitude. The methods solve the problem a wide range of gas detection by one kind of equipment. The device is small size, low cost, easy to carry, can measure a wide range of gas content, and has an important application value for environmental monitoring. If the White pool with underwater ocean can detect trace gases, for rivers and the marine environment and resources to detect environmental monitoring is also of great significance.
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paper_title: Tunable erbium-doped fiber ring laser for applications of infrared absorption spectroscopy

paper_content:

Abstract We fabricate a low noise erbium-doped fiber ring laser that can be continuously tuned over 102 nm by insertion of the fiber Fabry–Perot tunable filter (FFP-TF) in the ring cavity with a novel cavity structure and the optimal gain medium length. As an application of this fiber ring laser, we performed the absorption spectroscopy of acetylene ( 13 C 2 H 2 ) and hydrogen cyanide (H 13 C 14 N) and measure the absorption spectra of more than 50 transition lines of these gases with an excellent signal to noise ratio (SNR). The pressure broadening coefficients of four acetylene transition lines are obtained using this fiber ring laser and an external cavity laser diode.
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paper_title: Investigation of Wavelength Modulation and Wavelength Sweep Techniques in Intracavity Fiber Laser for Gas Detection

paper_content:

Wavelength modulation technique (WMT) and wavelength sweep technique (WST) are introduced into intracavity fiber laser for both gas concentration sensing and absorption wavelength detection in this paper. The principle of gas sensing and spectral analysis using WMT and WST was studied. Polynomial fit was adopted to model the system nonlinear characteristic, based on which absorption wavelength can be detected. The system optimization and acetylene gas sensing were both realized, and the absolute detected error can be increased less than 75 ppm. The absorption wavelengths of the detected gas were calculated based on the polynomial fitting results of the system nonlinear. The absorption wavelengths of acetylene were detected using this method, with absolute error no more than 0.445 nm. The system has the ability of realizing both concentration sensing and gas-type recognition.
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paper_title: In situ FTIR investigations of reverse water gas shift reaction activity at supercritical conditions

paper_content:

Abstract In situ Fourier transform infrared spectroscopy was employed to investigate the formation of CO and other adsorbed species on Al 2 O 3 -supported catalysts exposed to a supercritical mixture of CO 2 and H 2 ( P = 138 bar ; T = 342 K ; molar CO 2 / H 2 = 19 ). On just the Al 2 O 3 support, surface carbonates are observed consistent with literature reports with some evidence of surface formates. On Pd / Al 2 O 3 , however, several surface species including carbonates, formates, and CO are unambiguously observed. On Pd / Al 2 O 3 , a peak at 1900 – 1920 cm - 1 corresponds to adsorbed CO. Based on the presence of water peaks at reaction conditions, the reverse water gas shift reaction is the most plausible mechanism for CO formation. The CO peak evolves with time on stream, gradually increasing in intensity from ∼ 20 min to 5 h. This suggests that short-residence time continuous reactors are preferred over batch reactors to minimize the effects of possible catalyst deactivation by CO. Interestingly, the CO peak was not observed on either Ru / Al 2 O 3 or Ni / Al 2 O 3 catalysts suggesting that either of these catalysts may be better suited than Pt- or Pd-based catalysts for hydrogenation in supercritical CO 2 .
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paper_title: Wavelength Sweep of Intracavity Fiber Laser for Low Concentration Gas Detection

paper_content:

Wavelength sweep technique (WST) is introduced into intracavity fiber laser (ICFL) for low concentration gas detection. The limitation induced by noise can be eliminated using this method, and the performance of the system is improved. The sensitivity of the system is reduced to less than 200 ppm. With WST, the sweeping characteristic of the ICFL can be described according to known gas absorption spectra.
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paper_title: Photonic MEMS for NIR in-situ Gas Detection and Identification

paper_content:

We report on a novel sensing technique combining photonics and microelectromechanical systems (MEMS) for the detection and monitoring of gas emissions in environmental, medical, and industrial applications. We discuss how MEMS-tunable vertical-cavity surface-emitting lasers (VCSELs) can be exploited for in-situ detection and NIR spectroscopy of several gases, such as O2, N2O, CH4, HF, HCl, etc., with estimated sensitivities between 0.1 and 20 ppm on footprints <<10-3 mm3. The VCSELs can be electrostatically tuned with a continuous wavelength shift up to 20 nm, allowing for unambiguous NIR signature determination. Selective concentration analysis in heterogeneous gas compositions is enabled, thus paving the way to an integrated optical platform for multiplexed gas identification by bandgap and device engineering. We will discuss here, in particular, our efforts on the development of a 760 nm AlGaAs-based tunable VCSEL for O2 detection.
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paper_title: Mobile robots with active IR-optical sensing for remote gas detection and source localization

paper_content:

While other robots use in-situ measurements for gas leak detection and localization, we propose to apply remote sensing. It is easier and safer to conduct, permits rapid scans and is applicable to leak sources high up. An IR-optical sensor is used, exploiting spectral absorption effects of gases. Tailored leak detection and localization strategies are proposed. A simulation environment with a 3D model of the gas concentration field is used for developing and testing the detection and localization strategies. The system performance is demonstrated in a case study with a chemical plant.
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paper_title: Detection of trace concentrations of helium and argon in gas mixtures by laser-induced breakdown spectroscopy

paper_content:

We report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary gas mixtures with nitrogen by laser-induced breakdown spectroscopy (LIBS). Although significant quenching of helium transitions due to collisional deactivation of excited species was observed, it was found that losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analyses were performed. In consequence, limits of detection of parts-per-million and tens of parts-per-million and linear dynamic ranges of several orders of magnitude in analyte concentration were obtained. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations.
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paper_title: Application research of laser gas detection technology in the analysis of sulphur hexafluoride

paper_content:

The decision of latent fault in SF 6 gas electrical equipment can be made more timely and accurately through online monitoring of content change of HF, one of SF 6 gas decomposition. Tunable diode laser absorption spectroscopic (TDLAS) technology applies the wavelength tunable characteristics of diode lasers to gain the absorption spectroscopy of the selected absorption line of the target gas for the qualitative or quantitative analysis of the target gas. Because of its high sensitivity, high selectivity, short response time and without cross-interference, etc., we can achieve online monitoring and make the detection of HF more direct, accurate and timely through installing the technical equipment in the SF 6 electrical equipment monitoring site location. The analysis shows that the TDLAS technology is an effective and reliable method of SF 6 online monitoring technique.
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paper_title: Non-selective NDIR array for gas detection

paper_content:

A micro component for a non-selective NDIR (non dispersive infrared) gas detection system is presented in this work. This device consist of an IR detection module composed of a thermopile and a thin film filter array. The thermopile arrays (up to 4x4) are built on a silicon substrate by bulk micro-machining processes. The whole matrix is built on a thin freestanding silicon oxide/silicon nitride membrane of 2100x2100μm2 defined by anisotropic wet etching. To ensure the existence of hot and cold junctions for each detector we define on the insulating membrane absorbers and ribs, 6μm thick, by heavy boron doping of the silicon underneath. The ribs crisscross the membrane contacting the silicon bulk acting as a heat sink. Absorbers are located in the centre of each individual pseudo-membrane defined by the ribs intersection. Incident radiation heats up the absorber creating a temperature difference that is measured by the thermocouples that are placed between the absorber and the ribs. On a second chip, the elements of the filter array are fabricated in a matching configuration. The filters are built on a silicon substrate alternating thin films of different refraction index acting like a Fabry-Perot structure with 2-8μm silicon oxide cores. The transmitted filter peaks are not tuned for the detection of any specific substance: they configure a non selective general purpose filter array (400-4000 cm-1), making signal processing and pattern recognition techniques necessary. Both dies have been fabricated and characterized and have been successfully attached using flip-chip techniques. The measurements on these devices have been used to build an optical simulation tool that allows the assessment of the whole NDIR system behaviour in operating conditions.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
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paper_title: Midinfrared sensors meet nanotechnology: Trace gas sensing with quantum cascade lasers inside photonic band-gap hollow waveguides

paper_content:

An integrated midinfrared sensing system for trace level (ppb) gas analysis combining a quantum cascade laser with an emission frequency of 10.3μm with a frequency matched photonic band-gap hollow core waveguide has been developed, demonstrating the sensing application of photonic band-gap fibers. The photonic band-gap fiber simultaneously acts as a wavelength selective waveguide and miniaturized gas cell. The laser emission wavelength corresponds to the vibrational C–H stretch band of ethyl chloride gas. This sensing system enabled the detection of ethyl chloride at concentration levels of 30ppb (v∕v) with a response time of 8s probing a sample volume of only 1.5mL in a transmission absorption measurement within the photonic band-gap hollow core waveguide, which corresponds to a sensitivity improvement by three orders of magnitude compared to previously reported results obtained with conventional hollow waveguides.
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paper_title: Comparison of infrared sources for a differential photoacoustic gas detection system

paper_content:

Abstract A prototype of the differential photoacoustic measurement system with an optical cantilever microphone has been developed. The system is based on gas filter correlation method. The proposed system allows real-time measurement of various IR-absorbing gases from the flowing sample or in the open air. Three setups with different kind of infrared sources were carried out to study selectivity and sensitivity of the prototype and applicability of the source types with differential method. The sources were a mechanically chopped blackbody radiator, electrically chopped blackbody radiator and mechanically chopped CO 2 -laser. A detection limit for C 2 H 4 was estimated with all three infrared sources. Cross sensitivity and detection limits of gases CH 4 , C 2 H 4 and CO 2 were measured with the mechanically chopped blackbody radiator. This crossinterference matrix was also modeled using HITRAN database and completed with CO and H 2 O. The measurements indicate that at least ppb-level detection of ethylene using CO 2 -laser, sub-ppm level with mechanically chopped blackbody and ppm-level with electrically modulated blackbody is possible with a proposed differential system.
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paper_title: A CMOS Single-Chip Gas Recognition Circuit for Metal Oxide Gas Sensor Arrays

paper_content:

This paper presents a CMOS single-chip gas recognition circuit, which encodes sensor array outputs into a unique sequence of spikes with the firing delay mapping the strength of the stimulation across the array. The proposed gas recognition circuit examines the generated spike pattern of relative excitations across the population of sensors and looks for a match within a library of 2-D spatio-temporal spike signatures. Each signature is drift insensitive, concentration invariant and is also a unique characteristic of the target gas. This VLSI friendly approach relies on a simple spatio-temporal code matching instead of existing computationally expensive pattern matching statistical techniques. In addition, it relies on a novel sensor calibration technique that does not require control or prior knowledge of the gas concentration. The proposed gas recognition circuit was implemented in a 0.35 μm CMOS process and characterized using an in-house fabricated 4 × 4 tin oxide gas sensor array. Experimental results show a correct detection rate of 94.9% when the gas sensor array is exposed to propane, ethanol and carbon monoxide.
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paper_title: Catalytic Fiber Bragg Grating Sensor for Hydrogen Leak Detection in Air

paper_content:

The explosion risk linked to the use of hydrogen as fuel requires low-cost and efficient sensors. We present here a multipoint in-fiber sensor capable of hydrogen leak detection in air as low as 1% concentration with a response time smaller than a few seconds. Our solution makes use of fiber Bragg gratings (FBGs) covered by a catalytic sensitive layer made of a ceramic doped with noble metal which, in turn, induces a temperature elevation around the FBGs in the presence of hydrogen in air.
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paper_title: Olfactory detection of methane, propane, butane and hexane using conventional transmitter norms

paper_content:

Abstract A Si based four element integrated calorimetric sensor was constructed for olfactory detection of hydrocarbon mixtures up to their lower explosion limit (LEL) concentration. In order to construct a four-wire, inherently explosion-proof transmitter, the power consumption of each sensor was reduced below 50 mW and a sequential read-out scheme was adopted. Olfactory detection is based on the principal component analysis (PCA) and partial least square (PLS) fitting.
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paper_title: An Analytic Model Of Thermal Drift In Piezoresistive Microcantilever Sensors

paper_content:

A closed-form semiempirical model has been developed to understand the physical origins of thermal drift in piezoresistive microcantilever sensors. The two-component model describes both the effects of temperature-related bending and heat dissipation on the piezoresistance. The temperature-related bending component is based on the Euler–Bernoulli theory of elastic deformation applied to a multilayer cantilever. The heat dissipation component is based on energy conservation per unit time for a piezoresistive cantilever in a Wheatstone bridge circuit, representing a balance between electrical power input and heat dissipation into the environment. Conduction and convection are found to be the primary mechanisms of heat transfer, and the dependence of these effects on the thermal conductivity, temperature, and flow rate of the gaseous environment is described. The thermal boundary layer value that defines the length scale of the heat dissipation phenomenon is treated as an empirical fitting parameter. Using t...
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paper_title: Dynamic thermal conductivity sensor for gas detection

paper_content:

A dynamic thermal conductivity sensor for gas detection based on the transient thermal response of a SiC microplate slightly heated by a screen-printed Pt resistance is described. This sensor is developed for specific applications such as the determination of carbon monoxide content in hydrogen for fuel cell, or that of methane in biogas applications. On the contrary of existing devices, the apparatus developed here does not need any reference cell, it operates in transient mode near room temperature (�T ≈ 5 K in air), so has a very low power requirements (≈5 mW) and keeps the gas near thermal equilibrium which simplifies the mathematical model and eases data processing. In test gases mixtures (N2 + He), absolute and precise measurement of the gas thermal conductivity have been achieved, leading to the exact molar fraction of the gas to detect with a good reproducibility. © 2003 Elsevier B.V. All rights reserved.
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paper_title: Evaluation of two gas chromatography-olfactometry methods: the detection frequency and perceived intensity method.

paper_content:

Two gas chromatography-olfactometry methods were evaluated in terms of repeatability, range of sensitivity and discriminating properties. Six volatile flavour compounds at various concentration levels were analysed by a panel of eight assessors using the detection frequency method and the perceived intensity method. The coefficient of variance, averaged over the individual compounds for three replicate samples, was 16% for the detection frequency method and 28% for the intensity method. The average correlation coefficient of the individual compounds with concentration was 0.93 (range 0.88-0.99) for the intensities. They were slightly higher than those for the detection frequencies (0.91, range 0.81-0.97). The detection frequency method was more accurate in terms of repeatability, and the intensity method was more accurate with regard to discrimination between concentration levels. The range of sensitivity was similar for both methods.
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paper_title: Comparison of various detection limit estimates for volatile sulphur compounds by gas chromatography with pulsed flame photometric detection.

paper_content:

This paper addresses the variations that presently exist regarding the definition, determination, and reporting of detection limits for volatile sulphur compounds by gas chromatography with pulsed flame photometric detection (GC-PFPD). Gas standards containing hydrogen sulphide (H(2)S), carbonyl sulphide (COS), sulphur dioxide (SO(2)), methyl mercaptan (CH(3)SH), dimethyl sulphide (DMS), carbon disulphide (CS(2)), and dimethyl disulphide (DMDS) in concentrations varying from 0.36ppb (v/v) up to 1.5ppm (v/v) in nitrogen were prepared with permeation tubes and introduced in the gas chromatograph using a 0.25-ml gas sampling loop. After measuring the PFPD response versus concentration, the method detection limit (MDL), the Hubaux-Vos detection limit (x(D)), the absolute instrument sensitivity (AIS), and the sulphur detectivity (D(s)) were determined for each sulphur compound. The results show that the MDL determined by the US Environmental Protection Agency procedure consistently underestimates the minimum concentrations of volatile sulphur compounds that can be practically distinguished from the background noise with the PFPD. The Hubaux-Vos detection limits and the AIS values are several times higher than the MDL, and provide more conservative estimates of the lowest concentrations that can be reliably detected. Sulphur detectivities are well correlated with AIS values but only poorly correlated with MDL values. The AIS is recommended as a reliable and cost-effective measure of detection limit for volatile sulphur compounds by GC-PFPD, since the AIS is easier and faster to determine than the MDL and the Hubaux-Vos detection limit. In addition, this study confirmed that the PFPD response is nearly quadratic with respect to concentration for all volatile sulphur compounds.
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paper_title: A prototype acoustic gas sensor based on attenuation

paper_content:

Acoustic attenuation provides the potential to identify and quantify gases in a mixture. We present results for a prototype attenuation gas sensor for binary gas mixtures. Tests are performed in a pressurized test cell between 0.2 and 32atm to accommodate the main molecular relaxation processes. Attenuation measurements using the 215-kHz sensor and a multiseparation, multifrequency research system both generally match theoretical predictions for mixtures of CO2 and CH4 with 2% air. As the pressure in the test cell increases, the standard deviation of sensor measurements typically decreases as a result of the larger gas acoustic impedance.
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paper_title: Ultrasonic Nondestructive Evaluation Systems: Models and Measurements

paper_content:

Ultrasonic Nondestructive Evaluation Systems: Models and Measurements provides the latest information and techniques available for ultrasonic nondestructive evaluation (NDE) inspections. Using a systems level approach, this book employs aspects of Fourier analysis, linear system theory, and wave propagation and scattering theory to develop a comprehensive model of an entire ultrasonic measurement system. The book also describes in detail the measurements needed to obtain all the system model parameters. This integrated approach leads to a new model-based engineering technology for designing, using and optimizing ultrasonic nondestructive evaluation inspections. Practicing engineers, teachers, and students alike will learn about the latest developments in NDE technology, including a recently developed pulse-echo method for measuring the sensitivity of an ultrasonic transducer, and the use of Gaussian beam theory in simulating the wave fields generated by ultrasonic transducers. In addition, this unique book incorporates MATLAB examples and exercises which allow readers to conduct simulated inspections and implement the latest modeling technology. Written by recognized experts in NDE research, Ultrasonic Nondestructive Evaluation Systems: Models and Measurements is designed to combine well-developed techniques with the latest advances in technology.
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paper_title: Gas density metering in ultrasonic gas flowmeters using impedance measurements and chemometrics

paper_content:

Measurements using ultrasonic transducers confirm that the density of the gaseous medium can be predicted from impedance measurements on the same transducers. Tests were performed with the following gases under different pressure: SF6, N2, He, and air. These gases were selected to achieve a large span of densities. Using chemometric techniques can circumvent any possible dual sensitivity of the transducers on temperature and density. Sensitivity analysis of the transducers as a function of density is also performed and compared with experimental results. The densitometer under discussion uses impedance variation as an intermediate variable. Impedance variations can be converted to variations in frequency. Finally, the scenario of ultrasonic mass flowmetering is discussed with plausible models for realising computer integrated mass measurements using ultrasonic transducers in dual mode, viz. Contra-propagating transit time and impedance modes.
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paper_title: Gas concentration detection using ultrasonic based on wireless sensor networks

paper_content:

In this article, a low power consumption wireless sensor for binary gas mixtures real-time quantitative analysis is presented. This small size wireless gas sensor, which is developed based on an improved time-of-flight (TOF) method, can evaluate target gas in gas mixtures with high resolution. And the wireless sensor network system reduce power consumption using a power control IEEE802.15.4 standard, which can provide an effective mechanism for improving energy efficiency by decreasing the transmitting power directly. In contrast with some other gas analysis techniques, it avoids the problem of the secondary pollution and the short sensor life of using chemic sensor, and the high power consumption problem of traditional TOF methods, makes ultrasonic wireless gas sensor available. Currently, this gas quantitative analysis system consists of several Wireless Ultrasonic Gas Sensor nodes (WUGS) and a master node. The WUGS hardware consists of a microcontroller for obtaining measuring data from the ultrasonic channel, and a Zigbee transceiver for transmitting the data sets to a master sensor node. Furthermore, an environmental monitoring chamber was designed for testing this WUGS using sulfur hexafluoride (SF 6 ) and hydrogen (H 2 ) mixed with air respectively. The resolutions of the tests were within 30µV/V and 500µV/V, respectively. The power cost of the WUGS during the detection period is lower than 90mW. The main advantages of this gas concentration measurement are high accuracy to the gas of which molecular weight is different largely from air, low power consumption and ease of network setup. The sensors have shown good stability for more than three months in a high-voltage substation, for an SF 6 leak alarm system test.
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paper_title: A Sonar-Based Technique for the Ratiometric Determination of Binary Gas Mixtures

paper_content:

Abstract We have developed an inexpensive sonar-based instrument to provide a routine on-line monitor of the composition and stability of several gas mixtures having application in a Cherenkov Ring Imaging Detector. The instrument is capable of detecting small (
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paper_title: Application of ultrasonic to a hydrogen sensor

paper_content:

A fast responce hydrogen sensor using ultrasonic was developed and demonstrated. It uses the difference in sound velocity between hydrogen and the air. Thus, it is possible to measure hydrogen concentration by measuring the sound velocity changing when hydrogen is mixed in the air. The hydrogen sensor using ultrasonic in this study can be very fast in response time which was less than 84 msec. We also have shown that it is possible to detect hydrogen concentration as low as 100 ppm with the distance between ultrasonic probes as small as 20 mm. Temperature variation test was also demonstrated between −10 and 50°C and confirmed that it can match with calculation results.
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paper_title: Whispering gallery dielectric resonator modes for W-band devices

paper_content:

The utilization of planar millimeter-wavelength whispering gallery dielectric resonator modes for the design of W-band directional filters and power combiners is studied. The device presented combines the output power of several millimeter-wavelength devices in a single step by means of whispering gallery dielectric resonator (DR) modes. At millimeter wavelengths the cylindrical DRs used on their conventional TE, TM, or hybrid modes are impractically small. When used in their whispering gallery modes (WGMs) these cylindrical DRs have dimensions larger than normal for millimeter wavelength. After a description of the WGM phenomena, both electromagnetic and circuit parameters of these resonators are defined when they are coupled with transmission lines. The analysis based on the ring resonator model makes it possible to predict the theoretical responses of any devices. Experimental results obtained for directional filters and for power combiners in W-band are given. >
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paper_title: Hand-Held Miniature Chemical Analysis System (μChemlab) for Detection of Trace Concentrations of Gas Phase Analytes

paper_content:

A miniature, integrated chemical laboratory (μChemLab) is being developed that utilizes microfabrication to provide faster response, smaller size, lower power operation, and an ability to utilize multiple analysis channels for enhanced versatility and chemical discrimination. Improved sensitivity and selectivity are achieved with three cascaded components: (1) a sample collector/concentrator, (2) a gas chromatographic (GC) separator, and (3) a chemically selective surface acoustic wave (SAW) array detector. Prototypes of all three components have been developed and demonstrated both individually and when integrated on a novel electrical and fluidic printed circuit board. A hand-held autonomous system containing two analysis channels and all supporting electronics and user interfaces is currently being assembled and tested.
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paper_title: Photoacoustic Spectroscopy with Quantum Cascade Lasers for Trace Gas Detection

paper_content:

Abstract: Various applications, such as pollution monitoring, toxic-gas detection, noninvasive medical diagnostics and industrial process control, require sensitive and selectivedetection of gas traces with concentrations in the parts in 10 9 (ppb) and sub-ppb range.The recent development of quantum-cascade lasers (QCLs) has given a new aspect toinfrared laser-based trace gas sensors. In particular, single mode distributed feedback QCLsare attractive spectroscopic sources because of their excellent properties in terms of narrowlinewidth, average power and room temperature operation. In combination with these lasersources, photoacoustic spectroscopy offers the advantage of high sensitivity and selectivity,compact sensor platform, fast time-response and user friendly operation. This paper reportsrecent developments on quantum cascade laser-based photoacoustic spectroscopy for tracegas detection. In particular, different applications of a photoacoustic trace gas sensoremploying a longitudinal resonant cell with a detection limit on the order of hundred ppb ofozone and ammonia are discussed. We also report two QC laser-based photoacousticsensors for the detection of nitric oxide, for environmental pollution monitoring andmedical diagnostics, and hexamethyldisilazane, for applications in semiconductormanufacturing process.
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paper_title: Trace amount formaldehyde gas detection for indoor air quality monitoring

paper_content:

Formaldehyde is not only a carcinogenic chemical, but also causes sick building syndrome. Very small amounts of formaldehyde, such as those emitted from building materials and furniture, pose great concerns for human health. A Health Canada guideline, proposed in 2005, set the maximum formaldehyde concentration for long term exposure (8-hours averaged) as 40 ppb (50 μg/m3). This is a low concentration that commercially available formaldehyde sensors have great difficulty to detect both accurately and continuously. In this paper, we report a formaldehyde gas detection system which is capable of pre-concentrating formaldehyde gas using absorbent, and subsequently thermally desorbing the concentrated gas for detection by the electrochemical sensor. Initial results show that the system is able to detect formaldehyde gas at the ppb level, thus making it feasible to detect trace amount of formaldehyde in indoor environments.
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paper_title: Enhanced toxic gas detection using a MEMS preconcentrator coated with the metal organic framework absorber

paper_content:

Widespread and timely sensing of explosives, toxic chemicals and industrial compounds needs fast, sensitive detection technology that is affordable and portable. Many gas detectors developed for portable applications are based on sensing a change in resistivity or other non-selective material properties, often leading to low sensitivity and selectivity. In this paper, we report a new generation of the UIUC MEMS gas preconcentrator (muGPC) and its integration into a microfluidic M-8 (muM8) detector to demonstrate an enhanced overall detection limit and selectivity in detecting a toxic gas simulant. The integration creates a portable sensor to sniff an analyte of interest at concentration of 10 ppb or below.
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paper_title: Microfabricated preconcentrator-focuser for a microscale gas chromatograph

paper_content:

The design, fabrication, and testing of a preconcentrator-focuser (PCF), consisting of a thick micromachined Si heater packed with a small quantity of a granular adsorbent material are described. The PCF is developed to capture and concentrate vapors for subsequent focused thermal desorption and analysis in a micro gas chromatograph. The microheater contains an array of high-aspect-ratio, etched-Si heating elements, 520 /spl mu/m (h)/spl times/50 /spl mu/m (w)/spl times/3000 /spl mu/m (l), bounded by an annulus of Si and thermally isolated from the remaining substrate by an air gap. This structure is sandwiched between Pyrex glass plates with inlet/outlet ports that accept capillary tubes for sample flow and is sealed by anodic bonding (bottom) and rapidly annealed glass/metal/Si solder bonding (top). The large microheater surface area allows for high adsorption capacity and efficient, uniform thermal desorption of vapors captured on the adsorbent within the structure. The adsorbent consists of roughly spherical granules, /spl sim/200 /spl mu/m in diameter, of a high-surface-area, graphitized carbon. Key design considerations, fabrication technologies, and results of performance tests are presented with an emphasis on the thermal desorption characteristics of several representative volatile organic compounds as a function of volumetric flow rates and heating rates. Preconcentration factors as high as 5600 and desorbed peak widths as narrow as 0.8 s are achieved from 0.25-L samples of benzene at modest heating rates. The effects of operating variables on sensitivity, chromatographic resolution, and detection limits are assessed. Testing of this PCF with a micromachined separation column and integrated sensor array is discussed briefly.
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paper_title: Methane and Carbon Monoxide Gas Detection system based on semiconductor sensor

paper_content:

One of the most important actual problems in the gas detection field is that there are strong demands for gas methane leak detection and CO (carbon monoxide) detection to prevent explosions or CO poisoning accidents. In this sense, the present paper describes technical characteristics, test results, and a concluding application for methane and carbon monoxide based gas detection using a sensor which can detect both CO and methane with a single sensing element. The paper presents the detection method as well as the apparatus functional sketch.
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paper_title: Metal Oxide Gas Sensors: Sensitivity and Influencing Factors

paper_content:

Conductometric semiconducting metal oxide gas sensors have been widely used and investigated in the detection of gases. Investigations have indicated that the gas sensing process is strongly related to surface reactions, so one of the important parameters of gas sensors, the sensitivity of the metal oxide based materials, will change with the factors influencing the surface reactions, such as chemical components, surface-modification and microstructures of sensing layers, temperature and humidity. In this brief review, attention will be focused on changes of sensitivity of conductometric semiconducting metal oxide gas sensors due to the five factors mentioned above.
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paper_title: Comparison of infrared sources for a differential photoacoustic gas detection system

paper_content:

Abstract A prototype of the differential photoacoustic measurement system with an optical cantilever microphone has been developed. The system is based on gas filter correlation method. The proposed system allows real-time measurement of various IR-absorbing gases from the flowing sample or in the open air. Three setups with different kind of infrared sources were carried out to study selectivity and sensitivity of the prototype and applicability of the source types with differential method. The sources were a mechanically chopped blackbody radiator, electrically chopped blackbody radiator and mechanically chopped CO 2 -laser. A detection limit for C 2 H 4 was estimated with all three infrared sources. Cross sensitivity and detection limits of gases CH 4 , C 2 H 4 and CO 2 were measured with the mechanically chopped blackbody radiator. This crossinterference matrix was also modeled using HITRAN database and completed with CO and H 2 O. The measurements indicate that at least ppb-level detection of ethylene using CO 2 -laser, sub-ppm level with mechanically chopped blackbody and ppm-level with electrically modulated blackbody is possible with a proposed differential system.
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paper_title: Feasibility of wireless gas detection with an FMCW RADAR interrogation of passive RF gas sensor

paper_content:

The feasibility of the remote measurement of gas detection from an RF gas sensor has been experimentally investigated. It consists of a Frequency-Modulated Continuous-Wave (FMCW) RADAR interrogation of an antenna loaded by the passive sensor. The frequency band of the RADAR [28.8–31GHz] allows the detection of the resonant frequencies of Whispering Gallery Modes that are sensitive to gas concentration. Reported experimental results provide the proof-of-concept of remote measurement of gas concentration fluctuation from RADAR interrogation of this new generation of passive gas sensors.
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paper_title: Design and Characterization of Micropost-Filled Reactor to Minimize Pressure Drop While Maximizing Surface-Area-to-Volume-Ratio

paper_content:

Micropost-filled reactors are commonly found in many micro total analysis system applications because of their high surface area for the surrounding volume. Design rules for micropost-filled reactors are presented here to optimize the performance of the micro-preconcentrator, which is a component of a micro gas chromatography system. The dimensionless figure of merit is proposed to be used to minimize the pressure drop while maximizing the surface-area-to-volume-ratio for a given overall channel geometry of the micropost-filled preconcentrator. Two independent models from the literature are used to predict the pressure drop across the micropost-filled channels for low Reynolds number flows. The pressure drop can be expressed solely as a function of a design parameter, β = a/s, a ratio of a radius of each post and a half-spacing between two adjacent posts. Pressure drop measurements are performed to experimentally corroborate the pressure drop model and the optimization using the dimensionless figure of merit. As the number of microposts; for a given β increases in a given channel size, a greater surface-area-to-volume-ratio will occur for a fixed pressure drop. Therefore, increasing the arrays of posts with smaller diameters and spacing will optimize the microreactor for higher surface area for a given flow resistance, at least until Knudsen flow begins to dominate.Copyright © 2006 by ASME
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paper_title: Vertically Aligned ZnO Nanorod Arrays Coated with $\hbox{SnO}_{\bf 2}$/Noble Metal Nanoparticles for Highly Sensitive and Selective Gas Detection

paper_content:

Mimicking the biological olfactory receptor array that possesses large surface area for molecule capture, vertically aligned ZnO nanowire arrays, used as structural templates, were coated with SnO2/noble metal nanoparticles as active materials for fabrication of 3-D gas sensors. The gas sensors showed room-temperature responses to environmental toxic gases, such as NO2 and H2S, down to ppb level, which can be attributed to the large surface area of their 3D structure and catalytic behaviors of noble metals. A sensor array composed of three sensors with the different noble metals decoration (Pd, Pt, and Au) has shown capability to discriminate five different gases (H2S, NO2, NH3, H2, and CO) when using principal component analysis (PCA) incorporated the response speed as a discrimination factor. This study demonstrates a rational strategy to prepare sensing devices with 3-D structures for selective detection, which can be readily extended to other sensing materials that can be hardly grown as 3-D nanowire arrays.
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paper_title: Evaluation of Multitransducer Arrays for the Determination of Organic Vapor Mixtures

paper_content:

A study of vapor recognition and quantification by polymer-coated multitransducer (MT) arrays is described. The primary data set consists of experimentally derived sensitivities for 11 organic vapors obtained from 15 microsensors comprising five cantilever, capacitor, and calorimeter devices coated with five different sorptive−polymer films. These are used in Monte Carlo simulations coupled with principal component regression models to assess expected performance. Recognition rates for individual vapors and for vapor mixtures of up to four components are estimated for single-transducer (ST) arrays of up to five sensors and MT arrays of up to 15 sensors. Recognition rates are not significantly improved by including more than five sensors in an MT array for any specific analysis, regardless of difficulty. Optimal MT arrays consistently outperform optimal ST arrays of similar size, and with judiciously selected 5-sensor MT arrays, one-third of all possible ternary vapor mixtures are reliably discriminated fr...
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paper_title: Enhanced gas sensing by individual SnO2 nanowires and nanobelts functionalized with Pd catalyst particles.

paper_content:

The sensing ability of individual SnO2 nanowires and nanobelts configured as gas sensors was measured before and after functionalization with Pd catalyst particles. In situ deposition of Pd in the same reaction chamber in which the sensing measurements were carried out ensured that the observed modification in behavior was due to the Pd functionalization rather than the variation in properties from one nanowire to another. Changes in the conductance in the early stages of metal deposition (i.e., before metal percolation) indicated that the Pd nanoparticles on the nanowire surface created Schottky barrier-type junctions resulting in the formation of electron depletion regions within the nanowire, constricting the effective conduction channel and reducing the conductance. Pd-functionalized nanostructures exhibited a dramatic improvement in sensitivity toward oxygen and hydrogen due to the enhanced catalytic dissociation of the molecular adsorbate on the Pd nanoparticle surfaces and the subsequent diffusion ...
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paper_title: Detection of gases with arrays of micromachined tin oxide gas sensors

paper_content:

Abstract A good detection of NO 2 , CO and toluene at low concentrations has been carried out by using a micromachined gas sensor array composed of three devices working at different temperatures. The structure is fabricated using standard microelectronic technologies and tin oxide layers as sensitive material. The total power consumption of the array is in the range of 150 mW and a good uniformity of temperature is achieved, thanks to a silicon plug placed under the active area of each sensor. With this device type, it is possible to discriminate gases in a mixture when each array microsensor is heated at a proper temperature.
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paper_title: A Gradient Microarray Electronic Nose Based on Percolating SnO2 Nanowire Sensing Elements

paper_content:

Fabrication, characterization, and tests of the practical gradient microarray electronic nose with SnO2 nanowire gas-sensing elements are reported. This novel device has demonstrated an excellent performance as a gas sensor and e-nose system capable of promptly detecting and reliably discriminating between several reducing gases in air at a ppb level of concentration. It has been found that, in addition to the temperature gradient across the nanowire layer, the density and morphological inhomogeneities of nanowire mats define the discriminating power of the electronic nose.
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paper_title: CMOS single-chip gas detection system comprising capacitive, calorimetric and mass-sensitive microsensors

paper_content:

A single-chip gas detection system fabricated in industrial CMOS technology combined with post-CMOS micro-machining is presented. The sensors rely on a chemo-sensitive polymer layer, which absorbs predominantly volatile organic compounds (VOCs). A mass-sensitive resonant-beam oscillator, a capacitive sensor incorporated into a second-order /spl Sigma//spl Delta/-modulator, a calorimetric sensor with low-noise signal conditioning circuitry and a temperature sensor are monolithically integrated on a single chip along with all necessary driving and signal conditioning circuitry. The preprocessed sensor signals are converted to the digital domain on chip. An additional integrated controller sets the sensor parameters and transmits the sensor values to an off-chip data recording unit via a standard serial interface. A 6-chip-array has been flip-chip packaged on a ceramic substrate, which forms part of a handheld VOC gas detection unit. Limits of detection (LOD) of 1-5 ppm n-octane, toluene or propan-1-ol have been achieved.
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paper_title: Application of electronic nose technology in detection of combustible gas

paper_content:

Based on the study of the theory and constituent of the electronic nose systems, a set of combined gas sensor array, microcontroller and PC for detection of gas mixture is designed and constructed. Four kinds of gas (hydrogen, methane, acetylene, propane) are tested by the system. The feature parameters are picked up from each curve of the gas sensor's reaction. The experimental samples are analyzed by the method of PCA (principal component analysis) and BPNN (BP neural network). The result shows that the four kinds of gas could not be distinguished well by the method of PCA. While BPNN can solve the problem of sensor cross-sensitivity, and four kinds of gas can be identified qualitatively, the recognition probability of BPNN is more than 90%.
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paper_title: A wireless, passive carbon nanotube-based gas sensor

paper_content:

A gas sensor, comprised of a gas-responsive multiwall carbon nanotube (MWNT)-silicon dioxide (SiO/sub 2/) composite layer deposited on a planar inductor-capacitor resonant circuit is presented here for the monitoring of carbon dioxide (CO/sub 2/), oxygen (O/sub 2/), and ammonia (NH/sub 3/). The absorption of different gases in the MWNT-SiO/sub 2/ layer changes the permittivity and conductivity of the material and consequently alters the resonant frequency of the sensor. By tracking the frequency spectrum of the sensor with a loop antenna, humidity, temperature, as well as CO/sub 2/, O/sub 2/ and NH/sub 3/ concentrations can be determined, enabling applications such as remotely monitoring conditions inside opaque, sealed containers. Experimental results show the sensor response to CO/sub 2/ and O/sub 2/ is both linear and reversible. Both irreversible and reversible responses are observed in response to NH/sub 3/, indicating both physisorption and chemisorption of NH/sub 3/ by the carbon nanotubes. A sensor array, comprised of an uncoated, SiO/sub 2/ coated, and MWNT-SiO/sub 2/ coated sensor, enables CO/sub 2/ measurement to be automatically calibrated for operation in a variable humidity and temperature environment.
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paper_title: Remote Moisture Sensing utilizing Ordinary RFID Tags

paper_content:

The paper presents a concept where pairs of ordinary RFID tags are exploited for use as remotely read moisture sensors. The pair of tags is incorporated into one label where one of the tags is embedded in a moisture absorbent material and the other is left open. In a humid environment the moisture concentration is higher in the absorbent material than the surrounding environment which causes degradation to the embedded tag's antenna in terms of dielectric losses and change of input impedance. The level of relative humidity or the amount of water in the absorbent material is determined for a passive RFID system by comparing the difference in RFID reader output power required to power up respectively the open and embedded tag. It is similarly shown how the backscattered signal strength of a semi-active RFID system is proportional to the relative humidity and amount of water in the absorbent material. Typical applications include moisture detection in buildings, especially from leaking water pipe connections hidden beyond walls. Presented solution has a cost comparable to ordinary RFID tags, and the passive system also has infinite life time since no internal power supply is needed. The concept is characterized for two commercial RFID systems, one passive operating at 868 MHz and one semi-active operating at 2.45 GHz.
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paper_title: Detection and discrimination of pure gases and binary mixtures using a dual-modality microcantilever sensor

paper_content:

A new method for detecting and discriminating pure gases and binary mixtures has been investigated. This approach is based on two distinct physical mechanisms which can be simultaneously employed within a single microcantilever: heat dissipation and resonant damping in the viscous regime. An experimental study of the heat dissipation mechanism indicates that the sensor response is directly correlated to the thermal conductivity of the gaseous analyte. A theoretical data set of resonant damping was generated corresponding to the gas mixtures examined in the thermal response experiments. The combination of the thermal and resonant response data yields more distinct analyte signatures that cannot otherwise be obtained from the detection modes individually.
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Title: A Survey on Gas Sensing Technology

Section 1: Introduction
Description 1: This section introduces the growing importance of gas sensing technology, its applications, and the main areas of research and development in the field.

Section 2: Classification of Gas Sensing Methods
Description 2: This section classifies the various gas sensing technologies based on different principles and properties, providing a foundational overview for further discussion.

Section 3: Performance Indicators and Gas Sensor's Stability
Description 3: This section discusses the key performance indicators used to evaluate gas sensors, such as sensitivity and selectivity, and explores factors that affect sensor stability.

Section 4: Metal Oxide Semiconductor
Description 4: This section covers sensors based on metal oxide semiconductors, detailing their operating principles, advantages, challenges, and applications.

Section 5: Polymers
Description 5: This section describes polymer-based gas sensors, explaining their working principles, advantages, limitations, and usage in detecting specific gases.

Section 6: Carbon Nanotubes
Description 6: This section explores the utilization of carbon nanotubes (CNTs) in gas sensing, highlighting their properties, performance, and potential applications.

Section 7: Moisture Absorbing Material
Description 7: This section details how moisture-absorbing materials are used in gas sensing, particularly in RFID-based systems for humidity detection.

Section 8: Classification Based on Sensors' Operating Modes
Description 8: This section classifies sensors based on their operating modes, such as direct contact and wireless transducers, and discusses their integration.

Section 9: Optical Methods
Description 9: This section gives an overview of optical methods used in gas sensing, discussing their principles, advantages, and limitations.

Section 10: Calorimetric Methods
Description 10: This section explains calorimetric methods for gas sensing, including catalytic and thermal conductivity sensors, and discusses their mechanisms and applications.

Section 11: Gas Chromatograph
Description 11: This section covers gas chromatography as a method for gas sensing, focusing on its analytical capabilities, sensitivity, and selectivity.

Section 12: Acoustic Methods
Description 12: This section describes gas sensing methods based on acoustic principles, such as speed of sound and attenuation, outlining their operational parameters and applications.

Section 13: Components of Wireless Sensor Networks
Description 13: This section introduces the components and functionalities of wireless sensor networks used in gas sensing technologies.

Section 14: Some Approaches to Improve Sensitivity and Selectivity
Description 14: This section discusses various approaches to enhance the sensitivity and selectivity of gas sensors, including modifying sensor materials and employing pre-concentration techniques.

Section 15: Conclusions
Description 15: This section summarizes the survey findings, compares different gas sensing technologies, discusses the factors affecting performance, and provides insights into future development trends in the field.