Effects of high-temperature sintering on SnO2 sensor response to reducing gases

Abstract

Sensors based on undoped polycrystalline tin(IV) oxide sintered at different temperatures in the range 800–1600 °C have been comprehensively characterised with respect to their response to mixtures of carbon monoxide, methane or hydrogen in air. Results obtained at an operating temperature of 400 °C show that increasing the sintering temperature leads to a gradual decline in methane response, but enhanced H₂ and CO sensitivity, which reaches a maximum after firing at 1375 °C. If the sensors are maintained at a lower temperature (280 °C), then the characteristics obtained differ considerably. A general decrease in sensitivity to all three reducing gases is observed upon increasing the firing temperature until a point is reached where a sensor sintered at 1500 °C displays a remarkable change in the mechanism of detection of carbon monoxide. This sensor exhibits a significant increase in resistance (i.e. p-type behaviour) upon exposure to a low concentration of CO in air but responds conventionally to H₂-containing environments while remaining insensitive to methane. A further reduction in operating temperature to 175 °C leads to a similar p-type sensor action in the presence of hydrogen.