Masayoshi Nitta

Thick-film CO gas sensors

A gas sensor of SnO2-based materials has been made by thick-film technology utilizing hydrophobic silica as a binder. The technology can achieve a high productivity as well as a reduced humidity dependence and a sufficient mechanical strength of sensors. A thick-film sensor of SnO2incorporated with ThO2shows highly selective detection for CO gas, separated from H2gas; i.e., sensitivity to CO is 42 times higher than to H2at both gas concentrations of 50 ppm. An excellent humidity-independent sensitivity is also achieved.

Oscillation phenomenon in ThO2‐doped SnO2 exposed to CO gas

In the ThO2‐doped SnO2 a new self‐oscillation phenomenon has been found only when it is exposed to CO gas. This phenomenon is related to the environmental CO gas concentration, substrate temperature, and applied voltage. The oscillation is extremely senstivie to the concentration of CO gas, especially in the region of 0.2‐0.3%.

New oscillation phenomena in VO2 crystals

New oscillation phenomena in VO2 crystals were observed. The oscillation is generated in the temperature region of the crystal transition, and its voltage and frequency depend upon the ambient temperature. The waveform of the oscillation is a rectangular pulse.

Oscillation phenomenon in thick-film CO sensor

An oscillation phenomenon in ThO2-doped SnO2prepared by thick-film technology has been studied. The oscillation consists of two components at high sample temperature (210-230°C). When the sample temperature is kept at 210°C, a single-component oscillation with a sawtooth waveform starts to appear as CO-gas concentration increases. The activation energies of the surface reaction are 28.9 kcal/mol and 11.9 kcal/mol at the sample temperatures of 180-200°C and 210-230°C, respectively. From these experiments, it has been found that the coverage of oxygen on the surface plays an important role in establishing the oscillatory states.

High Sensitivity H 2 S Gas Sensors

H2S gas sensors have been fabricated by the admixture of SnO2-based powders and alcohol, hydrophobic silica, or trimethylchlorosilane. Their responses to H2S gas have been investigated. It is found that the resistivity of the sensor whose surface is covered by trimethyl-silyl groups strongly depends on the H2S gas concentration. A 10 ppm H2S gas concentration changes its resistivity to a value 1000 times lower than that in air, in spite of there being small changes in the sensor resistivity in other reducing gases. By covering the sensor surface with organic materials, development of sensitivity and selectivity for H2S gas has been achieved. Also, the mechanism of reaction between the organic materials on the surfaces of sensors and H2S gas is discussed.

H/sub 2/S gas detection by ZrO-doped SnO/sub 2/

The authors report on the fabrication of H/sub 2/S gas sensors made of SnO/sub 2/ mixed with ZrO/sub 2/. Their sensitivities, responses, and selectivities to H/sub 2/S are presented. A 10-p.p.m. concentration of H/sub 2/S gas changes the sensor resistivity to a value 300 times lower than that for air around 175 degrees C. The sensors exhibit highly selective detection of H/sub 2/S in an atmosphere containing H/sub 2/S and H/sub 2/. The mechanism of the reaction between the sensors and H/sub 2/S are also discussed. >

High sensitivity H2S gas sensors

H2S gas sensors have been fabricated by the admixture of SnO2-based powders and alcohol, hydrophobic silica, or trimethylchlorosilane. Their responses to H2S gas have been investigated. It is found that the resistivity of the sensor whose surface is covered by trimethyl-silyl groups strongly depends on the H2S gas concentration. A 10 ppm H2S gas concentration changes its resistivity to a value 1000 times lower than that in air, in spite of there being small changes in the sensor resistivity in other reducing gases. By covering the sensor surface with organic materials, development of sensitivity and selectivity for H2S gas has been achieved. Also, the mechanism of reaction between the organic materials on the surfaces of sensors and H2S gas is discussed.

Some unique aspects on ThO2‐doped SnO2 exposed to H2 gas

In ThO2‐doped SnO2 exposed to H2 gas, the remarkable phenomenon like the increase of sample resistivity has been found in spite of n‐type semiconductors. This phenomenon depends on the gas concentration, the sample temperature, and also the history of the samples. In samples sintered at 600 °C, it appears remarkably at the sample temperature below 240 °C and above 250 ppm of the gas concentration.