Characterising changes in the methane response of a semiconductor-based metal oxide sensor over time

Abstract

The semiconductor-based Figaro Taguchi Gas Sensor (TGS) is sensitive to reducing gases, including methane. TGS methane response can be characterised by using the ratio between resistance in the presence of methane mole fraction ([CH₄]) enhancements and a reference resistance, representative of sampling under the same environmental conditions and with the same background gas composition, but at a reference [CH₄] level. Effects of environmental variables, including water mole fraction ([H₂O]), are expected to cancel in this resistance ratio, allowing for independent [CH₄] characterisation. This work seeks to examine the cause of changes in [CH₄] resistance ratio characterisation over time, including the hypothesis that resistance ratios are independent of [H₂O]. Precise gas blends were sampled under controlled conditions during sensor characterisation in synthetic air (SCS) tests, which showed [H₂O] to influence resistance ratio methane characterisation, although this effect's importance depends on the reference gas. Three SCS tests were also performed with gaps of 137 days followed by 295 days, all under similar environmental conditions and gas blends. [CH₄] resistance ratio response changed significantly during the first time gap, suggesting that something inherently changed sensor behaviour, but negligibly during the second time gap, suggesting that natural ageing is not otherwise a key driver of sensor behaviour. Additional SCS tests showed persistent changes in [CH₄] resistance ratio response following hydrogen sulphide exposure; this may have caused a change between controlled SCS tests conducted 137 days apart, although other atmospheric species may also have been responsible. This is an important consideration for laboratory testing and final sensor application. Meanwhile, power loss and sampling dry air negligibly affected a different TGS. In addition, a total of 147 successful sensor characterisation in ambient air (SCA) tests occurred irregularly over approximately 25 months, where small amounts of gas with a high [CH₄] were blended with ambient outdoor air. SCA tests showed a weaker correlation between time and [CH₄] response when restricted to the period covering the second (295-day) time window between the similar SCS tests. A residual observed SCA testing correlation with time could be attributed to changes in [H₂O] over time, supporting SCS testing conclusions.