Duk-Dong Lee

Environmental gas sensors

Due to the dramatic growth in industrial development and population, the natural atmospheric environment has become polluted and is rapidly deteriorating. Thus, the monitoring and control of such pollutants is imperative to prevent environmental disasters. Conventional analytic instruments for this purpose are time consuming, expensive, and seldom used in real-time in the field. As such, a solid-state gas sensor that is compact, robust, with versatile applications and a low cost, could be an equally effective alternative. Accordingly, this paper presents a brief overview of solid-state gas sensors, which can be classified into semiconductor, capacitor, and solid-electrolyte type sensors, based on their sensing mechanisms and a simple NDIR instrument. Furthermore, the sensing properties of solid-state gas sensors to environmental gases, such as NO X , SO X , CO 2 , volatile organic compounds (VOCs), plus certain other gases, are also classified and summarized.

Nitrogen oxides-sensing characteristics of WO3-based nanocrystalline thick film gas sensor

Abstract TiO 2 added tungsten trioxides with grain size of about 3 nm, were synthesized by sol-coprecipitation of WCl 6 and TiCl 4 solution with ammonium hydroxide and surfactant. Tungsten trioxide and titanium oxides were also synthesized by sol-precipitation from WCl 6 and TiCl 4 , respectively. After calcining these materials, the thick film sensors were fabricated by the screen-printing technology. These materials were investigated about the characteristics of their structural properties by means of X-ray diffraction (XRD) measurements, the surface morphology by the SEM photography and specific surface area by the BET method. The grain size of precursor obtained by coprecipitation of TiCl 4 (4 wt.%) and WCl 6 was about 3 nm and that of WO 3 precursor by simple precipitation was about 8 nm. TiO 2 added WO 3 nanocrystalline thick films showed higher sensitivity to NO x over wide temperature ranges, than those of pure WO 3 or the simple mixture of WO 3 and TiO 2 powder. It was shown that the particle size and surface area of sensing materials make an important role of the sensing characteristics of WO 3 -based thick film sensor. The TiO 2 added WO 3 nanocrystalline sensor showed excellent sensitivity to low level NO x concentrations (0.5–30 ppm), fast response, recovery reaction, and good selectivity at 350°C.

Recognition of volatile organic compounds using SnO2 sensor array and pattern recognition analysis

Abstract A sensor array with 10 sensors integrated on a substrate was developed to recognize various kinds and quantities of volatile organic compounds (VOCs), such as benzene, toluene, ethyl alcohol, methyl alcohol, and acetone. The sensor array consists of gas-sensing materials using SnO 2 as the base material, plus a heating element based on a meandered platinum layer, all deposited on the substrate. The sensors on the sensor array are designed to produce a uniform thermal distribution and show a high and broad sensitivity and reproductivity to low concentrations through the usage of nano-sized sensing materials with high surface areas and different additives. By utilizing the sensing signals of the array with an artificial neural network, a recognition system can then be implemented for the classification and quantification of VOCs. The characteristics of the multi-dimensional sensor signals obtained from 10 sensors are analyzed using the principal component analysis (PCA) technique, and a gas pattern recognizer is implemented using a multi-layer neural network with an error-back-propagation learning algorithm. Simulation and experimental results demonstrated that the proposed gas recognition system is effective in identifying VOCs. For real-time processing, a DSP board can be used to implement the proposed VOC recognition system in conjunction with a neural network.

Effect of substrate on NO2-sensing properties of WO3 thin film gas sensors

Abstract A WO 3 thin film with a 1.2 μm thickness was deposited onto several substrates, including unpolished alumina, polished alumina, and silicon, using a thermal evaporating method with Pt meander electrodes and a heater. The WO 3 thin films were annealed at 600°C in air for 2 h, thereafter, the microstructure and NO 2 -sensing properties of the thin films grown on the different substrates were investigated. The WO 3 thin film on the unpolished alumina exhibited the highest sensitivity of 10–70 ppm NO 2 at an operating temperature of 300°C, whereas the film on the silicon substrate showed the lowest sensitivity of 3 ppm. The crystal structures of the thin films grown on the various substrates were nearly the same, however, their surface roughnesses were very different according to the kind of substrate. The sensitivity of WO 3 to NO 2 is highly dependent on the roughness of the substrate since this is also the main cause of modifications to the surface morphology of the sensing film.

Tin oxide microsensor for LPG monitoring

Abstract A silicon-based SnO 2 gas sensor has been fabricated for monitoring liquified petroleum gas (LPG), commonly used as town gas. The gas sensor is made by silicon IC technology together with SnO f2 thin-film processing. The whole chip with a size of 9 mm x 9 mm consists of nine sensors (three by three array). each sensor is supported by a thin membrane of SiO 2 /Si 3 N 4 /SiO 2 layers that provides a low thermal mass and prevents heat conduction through the surrounding substrate material. Tin oxide thin film is prepared by thermal evaporation of metallic tin granules and subsequent thermal oxidation of the metallic film at 600 °C. To form the SnO 2 (Pt) thin film, a layer of Pt with a thickness of several tens of angstroms is sputtered onto the tin oxide film and heat treated at 500 °C in air for several hours in order to stabilize its electrical response. The fabricated SnO 2 (Pt) microsensors exhibit about 85 and 92% sensitivities to 5000 ppm C 3 H 8 and 5000 ppm C 4 H 10 (the main components of LPG) at 250 °C, respectively, and show a rapid response time of less than 5 s.

Gas-sensing characteristics of SnO2—x thin film with added Pt fabricated by the dipping method

Abstract SnO 2— x thin films with added platinum are studied. The SnO 2— x thin films are prepared by reactive electron beam evaporation of sintered pellets of SnO 2 powder. The deposited films are dipped into an aqueous solution of H 2 PtCl 6 ·6H 2 O and heat treated. The surface structure and composition of the prepared films are investigated by electron spectroscopy for chemical analysis, Auger electron spectroscopy and X-ray diffractometry. The electrical properties of the thin films and the effect of Pt addition on the gas-sensing characteristics have also been investigated.

Fabrication and characteristics of SnO2 gas sensor array for volatile organic compounds recognition

Abstract Ten different gas sensors were integrated as an array on a substrate to identify various kinds and quantities of volatile organic compounds (VOCs), such as benzene, toluene, ethyl alcohol, methyl alcohol, and acetone. The sensor array consists of gas-sensing materials with SnO2 as the base material and a platinum heater and is fabricated using silk printing methods on an alumina substrate. The sensors show a high and broad sensitivity and reproducibility to low concentrations based on the use of nano-sized sensing materials with different additives. Utilizing the sensing signals of the array, an artificial neural network with an error-back-propagation learning algorithm is then implemented as a recognition system for classifying and quantifying the VOCs. Simulation and experimental results demonstrated that the proposed gas sensor array with the neural network was effective in recognizing various kinds and quantities of VOCs.

Low-power micro gas sensor

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.

Thick-film hydrocarbon gas sensors

Abstract γ-Fe 2 O 3 thick-film hydrocarbon gas sensors have been fabricated and their characteristics investigated. The sensitivity of a γ-Fe 2 O 3 thick film without any other catalysts heat treated at 400 °C for 2 h is 70% in 1000 ppm C 4 H 10 ambient at an operating temperature of 300 °C. The addition of Pd(1 wt.%) to γ-Fe 2 O 3 enhances the sensitivity to butane gas. The sensitivity of γ-Fe 2 O 3 /SnCl 4 ·5H 2 O(0.5wt.%)/Pd(1wt.%) film heat treated at 400 °C for 2 h to 1000 ppm C 4 H 10 is 80% at an operating temperature of 300 °C.

Characteristics of α-Fe2O3 thick film gas sensors

Abstract Haematite (α-Fe 2 O 3 ) with SO 4 2− and tin, as raw material, was prepared by a co-precipitation method. The α-Fe 2 O 3 -based thick film gas sensors were made by a screen printing technique. The microstructure of the raw material was affected by the hydrogen ion concentration which was controlled during the process of raw material preparation. Also, the pH value exhibited an effect on the sensitivity to gases for the thick film. It was thought that the sulphate ions localized in the thick film played an active role in hydrocarbon gas adsorption phenomena. Thick films of α-Fe 2 O 3 /α-Al 2 O 3 /Pd (78:20:2 by weight) had good adhesion to alumina substrates and exhibited relatively high sensitivity to CH 4 at 350°C.