EFFECTS OF HIGH-TEMPERATURE SINTERING ON SNO2 SENSOR RESPONSE TO REDUCING GASES

Sensors based on undoped polycrystalline tin(IV) oxide sintered at different temperatures in the range 800-1600-degrees-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-degrees-C show that increasing the sintering temperature leads to a gradual decline in methane response, but enhanced H-2 and CO sensitivity, which reaches a maximum after firing at 1375-degrees-C. If the sensors are maintained at a lower temperature (280-degrees-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-degrees-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 H-2-containing environments while remaining insensitive to methane. A further reduction in operating temperature to 175-degrees-C leads to a similar p-type sensor action in the presence of hydrogen.