A STUDY OF TIN-DIOXIDE GAS-SENSOR THERMOCHEMISTRY UNDER CONDITIONS OFVARYING OXYGEN PARTIAL-PRESSURE

Changes in sensor resistance and temperature observed upon exposure to reducing-gas mixtures have been monitored simultaneously for prototyp e devices comprising a platinum wire coil surrounded by a sintered tin -dioxide bead. Studies performed in a dry air ambient show that decrea ses in SnO2 resistance in the presence of either H-2 or CO are matched by small increases in temperature, which are measured by using the Pt coil resistance. Investigations into the effect of operating temperat ure on sensor characteristics show that the maximum resistance and tem perature changes do not always occur at the same point. Decreasing the oxygen partial pressure of the test environment to a level where the reducing-gas concentration exceeds that of oxygen causes a marked incr ease in resistance response and this is accompanied by a significant d ecrease in coil temperature. These findings support the notion that a change in the detection mechanism of tin-dioxide sensors occurs under non-equilibrium conditions, probably as a result of direct reduction o f the semiconductor lattice by the reducing gas.