Gas Sensing Response Principle of WO3-based Gas Sensors (II)

Details: Category: Tungsten Information; Published on Thursday, 05 February 2015 17:23; Written by cq; Hits: 1703

The sensitivity (S) could be defined as the ratio between the respective variation of resistance in the air (ΔRair) and in the target gases (ΔRgas) according to

S = ΔRair / ΔRgas

ΔR represents the difference between the maximum and minimum values of the resistance during one complete period of aerating cycling. However, we prefer to express the sensitivity with a more simple calculation route like this

S = Rgas / Rair

or S = Rair / Rgas

Rair and Rgas are the resistance of sensor in dry air and target gases, respectively. Usually, in oxidizing atmosphere, the oxide surface is covered by negatively charged oxygen adsorbate and the adjacent space charge region is electron-depleted: the oxide layer presents therefore a high resistance (Rgas＞Rair). Under reducing conditions, the oxygen adsorbate is removed by the reaction with reducing gas species and the electrons are re-injected into the space charge layers: as a result, the oxide layer resistance decreases (Rgas＜Rair). Besides, the voltage change of oxide may be used to illuminate the gas sensitivity at times. Therefore, the change of resistance, conductivity or voltage could be used as the parameters to scale the sensitivity.

In addition, the response time is often defined as the time required for the conductance to reach 90% of the equilibrium value after the test gas is injected, and the recovery time is the time necessary for the sensor to attain a conductance 10% above the original value in air.

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