Hae-Won Cheong

The role of additives in tin dioxide-based gas sensors

Abstract The role of additives (PdCl2, Al2O3 and Nb2O5) in SnO2 gas sensors has been examined by X-ray photoelectron spectroscopy (XPS) and analyses of the gas-sensing behavior. Al2O3-doped SnO2 shows a slight increase in sensitivity to CO and CH3CN as compared with pure SnO2. On the contrary, Nb2O5-doped SnO2 shows the opposite trend. The sensitivity of PdCl2-doped SnO2 to both CO and CH3CN in the low-temperature region below 300 °C is greatly increased. The major oxidation states of Pd in 0.5 wt.% PdCl2-doped SnO2 sintered at 800 °C and 650 °C are 4+ and 2+, respectively. The binding energies (BEs) of Sn 3d 5 2 and O 1s 1 2 remain almost constant up to 1.0 wt.% PdCl2 loading. When exposed to H2 at room temperature, the major oxidation state of Pd in 0.5 wt.% PdCl2-doped SnO2 sintered at 800 °C changes from 4+ to 2+. This change in the oxidation state of Pd, along with the enhanced sensitivity to CO below 200 °C, may suggest the electronic interaction of Pd with SnO2 at low temperature. The combustion products of CH3CN with 1.0 wt.% PdCl2-doped SnO2 powder are confirmed to be CO2, H2O and CO by gas chromatography (GC).

Surface Reaction Mechanism of Acetonitrile on Doped SnO2 Sensor Element and Its Response Behavior

The decomposition rate of toxic acetonitrile (CH3CN) in the presence of SnO2 sensor elements was greatly affected by the addition of a Pd dopant. When the PdCl2 was doped into the SnO2 sensor elements, the oxidation reaction was predominant owing to both the enhanced oxygen adsorption and the effective oxygen supply in the presence of Pd catalysts. To clarify such effects, we analyzed the mechanisms of the reaction between CH3CN gas and the PdCl2-doped SnO2 elements using gas chromatography. From the results of the analysis of the decomposition products of CH3CN gas in the presence of the doped SnO2 sensor elements, we suggested a possible surface reaction mechanism of acetonitrile gas decomposition.

Effects of Li 2 O Addition and Heat-Treatment on Formability of FeS 2 Powder for Cathode of Thermal Battery

has been widely used for cathode materials in thermal battery because of its high stability and current capability at high operation temperature. Salts such as a LiCl-KCl were added as a binder for improving electrical performance and formability of cathode powder. In this study, the effects of the addition of in LiCl-KCl binder on the formability of powder compact were investigated. With the increasing amount of addition to LiCl-KCl binder salts, the strength of the pressed compacts increased considerably when the powder mixture were pre-heat-treated above . The heat-treatment resulted in promoting the coating coverage of particles by the salts as was added. The observed coating as addition might be attributed to the enhanced wettability of the salt rather than its reduced melting temperature. The high strength of compacts by the addition and pre-heat-treatment could improve the formability of raw materials.

Carbon-Heteroatom Bond Formation by an Ultrasonic Chemical Reaction for Energy Storage Systems

The direct formation of CN and CO bonds from inert gases is essential for chemical/biological processes and energy storage systems. However, its application to carbon nanomaterials for improved energy storage remains technologically challenging. A simple and very fast method to form CN and CO bonds in reduced graphene oxide (RGO) and carbon nanotubes (CNTs) by an ultrasonic chemical reaction is described. Electrodes of nitrogen- or oxygen-doped RGO (N-RGO or O-RGO, respectively) are fabricated via the fixation between N2 or O2 carrier gas molecules and ultrasonically activated RGO. The materials exhibit much higher capacitance after doping (133, 284, and 74 F g-1 for O-RGO, N-RGO, and RGO, respectively). Furthermore, the doped 2D RGO and 1D CNT materials are prepared by layer-by-layer deposition using ultrasonic spray to form 3D porous electrodes. These electrodes demonstrate very high specific capacitances (62.8 mF cm-2 and 621 F g-1 at 10 mV s-1 for N-RGO/N-CNT at 1:1, v/v), high cycling stability, and structural flexibility.

Characteristics of Ceramic Separator Impregnated by Molten Salt for Thermal Batteries

Thermal batteries are primary power sources for military applications requiring high reliability, robustness and long storage life. Conventional electrodes for thermal batteries are prepared by compacting powder mixtures into pellets. Separator is composed of halide mixture, such as LiCl-KCl eutectic salt, blended with MgO to immobilize the molten salt. In order to increase the power density and energy density, the resistance of electrolyte should be reduced because the resistance of electrolyte is predominant in thermal batteries. In this study, wetting behaviors and impregnation weight of molten salts as well as the micro structures of ceramic felt were investigated to be applicable to thin electrolyte. Discharge performances of single cell with the ceramic separator impregnated by molten salt were evaluated also. Zirconia felt with high porosity and large pore outperformed alumina felt in wetting characteristics and molten salt impregnation as well as discharge performances. Based on the results of this study, ceramic felt separator impregnated with molten salt have revealed as an alternative of conventional thick MgO based separator with no conspicuous sign of thermal runaway by short circuit.

Enhancement of gas selectivity by additives and pattern recognition in SnO/sub 2/ sensors

Using gas chromatography (GC), the effect of additives on the decomposition reactions of both reducing gases (CH/sub 3/OH, C/sub 2/H/sub 5/OH, etc) and an oxidizing gas (CH/sub 3/CN) was analyzed for the selection of proper additives. The effect of additives was further investigated in terms of the sensing characteristics. Based on these studies, two sensors, PdCl/sub 2/-added SnO/sub 2/ and La/sub 2/O/sub 3/-added SnO/sub 2/, showing discriminative sensing behaviors to CH/sub 3/CN were selected. By combining the signal patterns obtained from the two sensors, selectivity of the SnO/sub 2/ sensors for CH/sub 3/CN was highly enhanced.