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The selective detection of CO gas by stannic oxide incorporated with ThO2(l − 10 wt%), in the presence of H2 and petroleum gases such as C3H8 and iso-C4H2 has been studied. Materials mixed with 5 wt% ThO2 showed high selectivity to CO gas at a sample temperature of 200−250‡C. The effects of hydrophobic or hydrophilic silica present in the samples on the detection sensitivity to CO gas have been investigated. From the results it is apparent that the removal of the hy-droxyl radical from the surface of SnO2 enhances the sensitivity to CO gas.

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. >

The temperature dependence of resistivities of gas sensors made of SnO2, Pd-doped SnO2, and ThO2-doped SnO2 with Pd has been investigated in air containing reducing gases such as CO, H2, and C3H8. The curves for ThO2-doped sensors were significantly influenced by the reducing gases as compared to the sensors without ThO2. From these results, it is found that in Pd-doped SnO2 sensors the dopant plays an important role in oxidizing the surface of SnO2 above 170°C, and that the addition of ThO2 to Pd-doped SnO2 enhances the effects of Pd by removing the adsorbed hydroxyl on SnO2. It is also apparent that the interactions between reducing gases in air and SnO2-based sensors depend upon the oxidizing rates of the surface of SnO2, as well as the amounts of the adsorbed hydroxyl on SnO2.

The microstructure and electrical properties of RuO 2 thick film resistors have been studied with special emphasis on the effects of material parameters and processing variables such as size of glass frit particles, RuO 2 -glass mixing ratio, firing conditions, and so forth. A network of RuO 2 develops in a glass matrix on firing, and the increase in the glass particle size enlarged the cell in the network without any significant change in the microstructure. It is shown that a decrease in particle size of glass frit increases the resistance and causes the temperature coefficient of resistor (TCR) to take negative values.

An oscillation phenomenon in ThO 2 -doped SnO 2 prepared 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.

A gas sensor of SnO 2 -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 SnO 2 incorporated with ThO 2 shows highly selective detection for CO gas, separated from H 2 gas; i.e., sensitivity to CO is 42 times higher than to H 2 at both gas concentrations of 50 ppm. An excellent humidity-independent sensitivity is also achieved.

The particle sizes of Ru0 2 and glass powder have direct effects upon the resistance value, TCR, and current noise.