Jun-Woo Lim

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.

Sensing characteristics of tin dioxide/gold sensor prepared by coprecipitation method

Abstract High selective and sensitive thick-film methane sensors were fabricated and their sensing characteristics for various hydrocarbon gases were investigated. SnO2/Au was prepared by coprecipitation method. SEM, TEM, BET and XRD analyses were carried out for investigation of surface morphology and crystalline structure. Pd-added SnO2/Au thick-film devices exhibited high sensitivity to methane gas at operating temperature of 400°C. The sensitivity of SnO2/Au+Pd thick-film devices to methane gas was higher than that of the other gases. The optimal adding amounts of Au and Pd were 1 and 3 wt.%, respectively.

Explosive gas recognition system using thick film sensor array and neural network

Abstract A sensor array with nine discrete sensors integrated on a substrate was developed for recognizing the species and quantity of explosive gases such as methane, propane, and butane. The sensor array consisted of nine oxide semiconductor gas-sensing materials with SnO 2 as the base material plus a heating element based on a meandered platinum layer all deposited on the sensor. The sensors on the sensor array were designed to produce a uniform thermal distribution and show a high and broad sensitivity and reproductivity to low concentrations through the use of nano-sized sensing materials with high surface areas and different additives. Using the sensitivity signals of the array along with an artificial neural network, a gas recognition system was then implemented for the classification and identification of explosive gases. The characteristics of the multi-dimensional sensor signals obtained from the nine sensors were analyzed using the principal component analysis (PCA) technique, and a gas pattern recognizer was implemented using a multi-layer neural network with an error back propagation learning algorithm. The simulation and experimental results demonstrate that the proposed gas recognition system is effective in identifying explosive gases. For real time processing, a DSP board (TMS320C31) was then used to implement the proposed gas recognition system in conjunction with a neural network.

Fabrication and characterization of micro-gas sensor for nitrogen oxides gas detection

Abstract WO 3 -based thin film micro-gas sensor was fabricated and the NO x gas sensing as well as electrical properties have been investigated. To obtain the optimal heat distribution, the structure of micro-hot plate was designed from the result of finite element simulation and was prepared by backside etching with KOH solution. The micro-hot plate was made out of N/O/N diaphragm with the thickness of 0.6 μm and area of 1.5×1.5 mm 2 . The power consumption to maintain the device temperature of 300°C was about 60 mW. WO 3 thin film was thermally evaporated on micro-diaphragm. The film exhibited a fast response to NO 2 gas and the relationship between sensitivity and NO 2 concentration showed good linearity in the gas concentration range of 0–30 ppm NO 2 . The sensitivity of micro-gas sensor to NO x was correlated with the microstructure of the thin film.

Sensing behavior of the polypyrrole and polyaniline sensor for several volatile organic compounds

The sensing characteristics of conducting polymers to several volatile organic compounds were investigated with a UV-Vis-NIR spectrophotometer, dynamic contact angles measurement and scanning probe microscopy (SPM). When gases were absorbed, the polypyrrole (PPy) and polyaniline (PANi) exhibited positive and negative sensitivity, respectively. The PPy-based sensor demonstrated decreasing conductivity while the PANi sensor exhibited increasing conductivity when the polarity of the molecules absorbed increased. PPy film has an hydrophilic property while the PANi film a hydrophobic one. These changes in polymer conductivity, it is speculated, are due to the interruption of free carrier movement or the generation of polarons by the absorbed molecules.

The effect of a migration barrier between tungsten oxide and indium tin oxide thin films in electrochromic devices

Abstract Electrochromic devices are based on the reversible insertion of guest atoms into the structure of the host solid. However, after cyclic operation, the tungsten in a WO 3 film and the indium in an ITO (indium tin oxide) film were migrated with each other and the electrochromic property of the device was decreased. The trapped lithium is associated with the indium (diffuse out from ITO to WO 3 film). In order to block migration, a thin tungsten barrier film was deposited on the ITO film. With the tungsten barrier, the indium and tungsten migration was effectively blocked and the decrease of the maximum current of cyclic voltammogram was reduced to 1/10. We can now manufacture electrochromic devices that have long lifetimes and high transmittance variance.

Thermal Optimization of a Novel Micro Gas Sensor Array With Different Operating Temperatures

A sensor array (3×5mm2 in diaphragm dimension) of 12 sensing elements with different operating temperatures was optimized with respect to thermal operation. This sensor array with single heater on a glass diaphragm over back-etched silicon bulk realizes a novel concept of a sensor array: an array of sensor elements operated at different temperatures can yield more information than single measurement. The proposed micro sensor array could provide well-integrated array structure because it has only single heater at the center of the diaphragm and used the various sensing properties of two kinds of metal oxide layers with various operating temperatures.

A Novel Micro Gas Sensor Array Using Temperature Gradient on the Single Glass

A micro gas sensor array was fabricated by anodic bonding of insulating membrane of 125µm-thick corning 7740 glass and the already-micromachined silicon substrate, as a support of the membrane. On the insulating membrane, two sensing films of SnO2 and SnO2+Pt on six pairs of electrodes located symmetrically from the center part, were deposited. The heating temperature-controlled method as an operating mode of sensor array was proposed to obtain the diverse sensing patterns. By controlling the temperature of a heater with two steps of 450 °C and 200 °C, 24 sensitivity patterns were obtained. From principal component analysis results, we confirmed that the classifying ability of the sensor array was enhanced by adding the 12 sensing signals obtained with controlled temperature.