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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.

Thick film resistors have been prepared by adding glass frits to the conductive materials of the resistors. By this method, it is found that the resistance of the glass phases in the thick film resistors decreases, with a resulting decrease of voltage drift after applying high voltage pulses to the resistors. In addition, the electrical properties and stability of these resistors are improved over resistors made from some present commercially available pastes.

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.

The phenomena of high-frequency-discharge trimming of RuO 2 -basad glaze resistors was investigated as a function of the following: 1) the sintering condition of thick-film resistors, 2) the relation between the amount of additives to resistors and the rate of increase or decrease of resistance, 3) the effect of heat treatment before and after the trimming and effect of glass overcoating, 4) the influence of the trimming on neighboring resistors, and 5) the stability of resistors after the trimming. It has been found that the rate of change of resistance by this trimming is affected by fabrication conditions of the resistor, that its influence on nearby resistance is small, and that the stability of resistors after the trimming is satisfactory.

Accelerated life tests under elevated temperarure, high humidity, and vacuum have been conducted on Pd-Ag thick-film resistors It is found that the change of resistance under those accelerated life test conditions is mainly caused by oxidation and reduction of the electrically conductive components (Pd and Ag) of Pd-Ag thick-film resistors. Through life tests in vacuum, the effect of adsorption was found to be less important than the structural change inside the resistor.

The basic reactions, compositions, and electrical properties of glazed Pd-Ag sintered layers through a firing process have been examined by differential thermal analysis, thermograviometric analysis, and X-ray diffraction analysis. The evaporation and combustion of screening vehicle and solvents, the oxidation-reduction behaviors of Pd and Ag the creation of PdAg solid solution, and the electrical properties and stability of these resistors have been studied. From these tests it was found that the degree of process sensitivity for these resistors was high, and electrical properties and stability of these resistors were affected in much the same way by firing temperature and firing period. They were influenced most by temperature rise rate and least by temperature fall rate.

Mechanism of resistance change by high-frequency-discharge trimming has been investigated for RuO2 glaze resistors. As a result, it is found that the resistance of RuO2 glaze resistors eventually increases when subjected to the discharging over an extended period because of structural damage of the resistor films. As for RuO2 · glass resistors, the resistance increases with the increasing discharge time, but as for Ag- or Nb-doped RuO2 · glass resistors, the resistance decreases during the initial stage of the discharging. The main cause for the resistance reduction is the increasing concentration of Ag atoms in the trimmed area in the case of Ag-dopad resistors, and in the case of Nb-doped resistors the change in valence value of Ru, i.e., Ru4+ -> Ru3+.

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.