Oscillation waveforms of SnO2-based thick-film co sensors

An oscillation phenomenon in SnO₂-based sensors, incorporating hydrophobic silica, has been noticeable when they are exposed to CO gas in air. Its waveform is a stable saw-tooth for sensors with 2 wt.% silica. Sensors with more than 5 wt.% silica, however, exhibit an oscillation with a complex wave when they are heated at higher temperature than 200°C and exposed to an appropriate high CO gas concentration. The dependences of critical conditions to be complex upon the sensor temperature, the gas concentration, and the silica contents have been examined. Also, the mechanisms of the appearance of the complex wave are discussed.