Investigations on irreversible- and reversible-type gas sensing for ZnO and Mg0.5Zn0.5Fe2O4 chemi-resistive sensors

Semiconducting metal oxides are attractive material candidates for combustible gas sensors. Little or marginal base resistance drift of these metal oxide sensors is desirable during repeated response and recovery cycles. However, due to the partial recovery, often a significant drift in base resistance is observed. The gas sensing is termed irreversible when there is a partial recovery of base resistance, whereas for reversible sensing the base resistance is fully recovered. For reducing gas (hydrogen and carbon monoxide) sensing we have reported reversible and irreversible resistance transients for magnesium zinc ferrite and zinc oxide sensing elements, respectively. For a wide range of gas concentrations and operating temperatures, the response transients for these sensing elements are modelled using the Langmuir-Hinshelwood reaction mechanism. It is revealed that for irreversible-type sensing, the response time is reduced with the increase in test gas concentration. On the other hand, for reversible-type sensing, the response time is found to be independent of the gas concentration. Based on the estimation of pore size, pore size distribution and specific surface area of the calcined powder together with the analyses of the surface morphology of the sensing elements we have argued that due to the porous morphology of the magnesium zinc ferrite sensing element the oxidized product can easily desorb from the sensor during recovery. Therefore, irrespective of the test gas concentration, the base resistance of the magnesium zinc ferrite sensor recovers fully during the recovery process.