Effect of surface modification on NO2 sensing properties of SnO2 varistor-type sensors

NO2 sensing properties of SnO2-based varistor-type sensors have been investigated in the temperature range of 400-650 degrees C and in the NO2 concentration range of 15-30 ppm. Pure SnO2 exhibited a weak nonlinear I-V characteristic in air, but clear nonlinearity in NO2 at 450 degrees C. The breakdown voltage of SnO2 shifted to a high electric field upon exposure to NO2 and the magnitude of the shift was well correlated with NO2 concentration. Thus, SnO2 exhibited some sensitivity to NO2 as a varistor-type sensor. When SnO2 particles coated with a SiO2 thin film were used as a raw material for fabricating a varistor, the breakdown voltage in air was approximately the double that of pure SnO2 and the sensitivity to 15 ppm NO2 was enhanced slightly. However, the sensitivity to 30 ppm NO2 decreased. The Cr2O3-loading on SnO2 also led to an increase in the breakdown voltage in air, but the Cr2O3 addition was not effective for promoting the NO? sensitivity under the present experimental conditions. (C) 1999 Elsevier Science S.A. All rights reserved.