Jeung-Soo Huh

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.

Thermal and electrical properties of BaO–B2O3–ZnO glasses

Abstract Glasses in the BaO–ZnO–B2O3 system were examined as potential replacement for PbO glass frits with low firing temperature (500–600 °C) for the dielectric layer of a plasma display panel (PDP). The glasses were evaluated for glass transition temperature (Tg), thermal expansion coefficient (α) and dielectric constant e. The electrical and the thermal properties were also compared with theoretical data calculated by a known empirical equation. Tg of the glasses varied between 480 and 560 °C, and α was in the range of 7–9×10−6 K−1. The dielectric constant ranges from 14 to 19 and the theoretical data showed lower α and e than the experimental data. The results suggest that BaO–ZnO–B2O3 glasses would be suitable as an alternative to Pb-based dielectric layer in PDPs.

Tin oxide-based methane gas sensor promoted by alumina-supported Pd catalyst

Abstract In an attempt to promote the sensitivity of tin oxide-based sensors to methane gas, the parent tin oxide powder, pure or loaded with Ca and/or Pt (0.1 wt.%), was mixed with a fixed amount (5 wt.%) of alumina-supported Pd catalyst (net Pd loading 0.25 wt.%). The resulting sensor was found to exhibit excellent sensing properties to methane in the concentration range of 500–10 000 ppm at 658 K regardless of the difference in starting tin oxide powder. It gave higher sensitivity to methane than any other sensors for which the tin oxide powder was either mixed similarly with supported Pt, Rh or Ni catalyst or loaded with the same amount of Pd by conventional methods. The high dispersion of Pd (or PdO) particles appears to be responsible for the excellent promoting action of the supported Pd catalyst. At lower temperature of 573 K, however, the use of the Ca and/or Pt loaded powder of tin oxide gave higher sensitivity to methane than that of the unloaded powder. It is suggested that the mechanism of methane sensing consists of two steps, i.e. activation of methane molecules on the supported Pd catalyst and surface reaction of the activated species on the tin oxide particles. The first step is rate determining at 658 K, while the second step becomes also important kinetically at 573 K, allowing the promoting action of Pt to take place.

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.

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.

Novel microencapsulation of potential drugs with low molecular weight and high hydrophilicity: hydrogen peroxide as a candidate compound.

Microencapsulation of drugs into solid biodegradable polymeric microspheres via solvent evaporation technique remains challenging especially with those having low molecular weight and high hydrophilicity nature. This paper presents an efficient encapsulation protocol for this group of drugs, demonstrated using hydrogen peroxide as a model compound that is encapsulated into poly(lactic-co-glycolic acid) microspheres. Hydrogen peroxide can be employed as antiseptic agent or its decomposed form into oxygen can be useful in various pharmaceutical applications. The new encapsulation technique was developed based on the modification of conventional double emulsion and solvent evaporation protocol with a backward concentration gradient of hydrogen peroxide. This was achieved by adding and controlling the concentration of hydrogen peroxide at the continuous phase during the solidification stage of the microspheres. Parameters involved in the production and the formulation aspect were optimized to achieve the best protocol having controlled efficiency of encapsulation that is simple, safe, practical, and economical. Evaluation on the encapsulation efficiency and the release profile has been made indirectly by monitoring the dissolved oxygen level of the solution where the microspheres were incubated. Morphology of the microspheres was investigated using scanning electron microscopy. This proposed method has successfully used to prepare batches of microspheres having different encapsulation efficiencies and its potential applications have been demonstrated accordingly.

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.

Gas sensing characteristics of SnO2 thin film fabricated by thermal oxidation of a Sn/Pt double layer

SnO2 thin film with homogeneously dispersed nano-crystallite Pt particles was reliably prepared via simple thermal oxidation of a Sn/Pt double layer on Si substrate oxidized. Its surface phase and morphology were probed using some surface sensitive tools such as X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The electrical response of SnO2 thin films to the i-C4H10 gas was systematically investigated at a wide range of temperatures and gas concentrations. In particular, a long-term stability test of the fabricated Pt/SnO2 thin films for the i-C4H10 gas proved its applicability as a reliable gas sensor because of its higher sensing stability than the conventional Pt/SnO2 films over a long period of run time.