Hisao Ohnishi

Study on the sensing mechanism of tin oxide flammable gas sensors using the Hall effect

Measurements were made on the Hall effect of SnO2 thin‐film inflammable gas sensors prepared by using the reactive sputtering method. The results revealed that when the electrical conductivity of the sensors changed with a change in the flammable gas concentration in air, the apparent carrier density changed significantly while apparent mobility hardly changed at all. However, detailed analysis revealed that these experimental results could not be explained by using the double Schottky model or the neck model that had been proposed. In order to provide satisfactory explanation of the experimental results, the authors clarified that for SnO2 thin films there exists no distinction between bulk and surface of the grain while the carrier densities of bulk and surface of the grain simultaneously change with a change in oxygen absorption of the SnO2 surface in the presence of flammable gas. This model was in good agreement with the results of transmission electron microscopy observations.

LIGHT-INTENSITY DEPENDENCE IN PHOTOCATALYTIC DECOMPOSITION OF WATER OVER K4NB6O17 CATALYST

We have measured, for the first time, the light-intensity dependence in photocatalytic decomposition of water over K4Nb6O17 In higher light-intensity region, the rate of reaction is proportional to the square root of the light intensity. In lower light-intensity region, on the contrary, the rate is almost linearly proportional to the light intensity. We propose a reaction model whose main path of the reaction loss is the recombination of the charges generated under b and -gap radiation. This model describes the light-intensity dependence of this reaction well.

NOx sensor using YBa2Cu3O7 − δ thin films

Abstract The aim of our study is to develop NOx sensors that can measure NOx concentrations in exhaust gas. The present study investigates the gas-sensing properties of YBa 2 Cu 3 O 7 − δ thin films, deposited on a SrTiO 3 substrate by the ion-beam sputtering method. The thin film we made exhibits high sensitivity to NO at 350 °C, and the NO sensitivity increases with NO concentration in the range 30–3000 ppm NO. At 350 °C, the YBa 2 Cu 3 O 7 − δ thin film shows hardly any sensitivity to CO, H 2 and CH 4 gases that are also present in exhaust gas, giving it excellent selectivity. It is generally considered that the resistance of YBa 2 Cu 3 O 7 − δ irreversibly increases due to the absorption of NO, but this irreversible increase of the YBa 2 Cu 3 O 7 − δ resistance is not observed in the thin films we made.

Model of the external magnetic field effect on the H2O2 reaction on an SnO2 surface

An H2D2 isotope effect on an external magnetic field effect on the hydrogen-oxygen reaction on an SnO2 surface at 775 K was observed. The rate of increase for the external magnetic field effect on the H2O2 reaction was 1.5 times as large as on the D2O2 reaction. This effect strongly suggests that the H2O2 reaction on the SnO2 surface is an Eley-Rideal type reaction. The present results strongly suggest that only the H2O2 reaction via the activated complex OH2− was accelerated by an external magnetic field and that the activated complex of this reaction has a structure in which there is only a weak interaction between undissociated hydrogen molecules and surface-adsorbed O2−. A model is proposed in which the magnetic field affects the electron spin of the quasi-radical pair H · · H in activated complexes, and the recombination of H · · H is inhibited by the singlet-triplet conversion caused by applying an external magnetic field.

Quantitative analysis of external magnetic field effects on the hydrogen-oxygen reaction on the tin dioxide surface

In a steady state, the dependence of an external magnetic field effect on the H 2 -O 2 reaction on the SnO 2 surface upon field direction, field intensity, temperatures, and hydrogen concentration was measured. Measurements made of the dependence upon the magnetic field direction confirmed that the magnetic field effects resulted not from changes in the mobility of the carrier electrons but from the increased rate of the reaction between hydrogen and surface-adsorbed oxygen. Magnetic field effects were found to be higher in the 573-623 K temperature range than in the 673-773 K range