Numerical analysis of response time for resistive oxygen gas sensors

The response time (t90) for resistive-type oxygen gas sensors based on thick films formed with cerium oxide (CeO2−δ) powder can be calculated as a function of the diffusion coefficient (DV), surface reaction coefficient (kV) and particle size (R). In the case of large particle size the kinetics of the sensors were controlled by diffusion, while in the case of small ones the kinetics were controlled by surface reaction. For medium particle sizes, sensor kinetics were controlled both by diffusion and by surface reaction. It could be confirmed that the response time was directly proportional to R/kV and R2/DV when surface reaction and diffusion acted as rate-limiting step, respectively. The degree of surface reaction was the rate-limiting step for oxygen gas sensors, based on cerium oxide thick film with a particle size not exceeding 1 μm. It is necessary to increase kV and/or to decrease R in order to shorten the response time of such sensors using cerium oxide powder.