RESPONSE-TIMES OF RESISTIVE THICK-FILM OXYGEN SENSORS

Resistive oxygen sensors based on SrTiO3 show a reproducible change in the sensor resistance as a function of the oxygen partial pressure (pO2) of the atmosphere surrounding them. The sensor's response behaviour after a step in pO2 is determined by two consecutive processes, the transfer reactions of the oxygen at the surface and the diffusion of oxygen vacancies in the interior of the solid. Previous investigations showed that the kinetics of (200-1000-mu-m thick) densely sintered SrTiO3 ceramics is controlled by diffusion and the surface reactions are negligibly fast. Thus, the response times can be reduced by the square of the sample thickness, which determines the diffusion length. In porous thick films, however, the diffusion length is determined by the grain size (phi less-than-or-equal-to 1-mu-m) and is therefore extremely short. A variation in grain size here leads to a linear change in the response time. Thus it can be assumed that the kinetics are no longer controlled by diffusion but by the rate of the surface reactions, and the expected fast response behaviour cannot be achieved. In spite of this rate-limiting effect, response times much less than one second are possible with thick-film sensors.