A simple equivalent circuit model to represent microstructure effects on theresponse of semiconducting oxide-based gas sensors

Abstract

We show that the effects of microstructure on the response of gas-sensitiveresistors based on semiconducting oxides can be understood in a pragmatic andpractically useful way using a simple three-element resistance network, in whichonly one of the elements is gas-sensitive. This model, with the gas-sensitiveresistance showing a simple form of response consistent with surface reactionmodels, displays the power-law response to variation of gas concentration(Pg)shown by practical devices: G = Ag Pgβ, whereG is(R − R0)/R0for resistance increase or (σ − σ0)/σ0for conductance increase. The observations that βvaries widely between preparations, is different for different gases on thesame sensor, and changes with change of the relative humidity of the gas,are simply explained as being due to changes in the relative values ofthe resistors in the network, related to the microstructure. The modelpredicts that, for a range of sensor preparations responding to a given gas,Ag andβshould be correlated. The predictions are confirmed by measurements of theresponse of a wide range of microstructures of sensors of both tin dioxide andchromium titanium oxide to toluene, ethanol and carbon monoxide inatmospheres of varying relative humidity. We show that the correlation ofAg andβ isa powerful tool for discovering subtle effects on the sensor response. Theseinclude: effects due to gas concentration gradients within the sensing layer, effectsof variation in microstructure throughout the sensing layer, the extent of sinteringof the material in the finished sensor, and whether water vapour acts on thesensor surface synergistically or independently of the reactive gas being measured.