Kinetic behavior of resistive oxygen sensor using cerium oxide

In order to decrease exhaust gas emissions, oxygen gas sensors with fast response are required. We evaluated two kinds of fast response time (<1 s) for two oxygen sensors with different cerium oxide particle sizes and crystallite sizes, using two methods: the commonly used jump method and the so-called dynamic method. The dynamic method consists of comparing the amplitude of oxygen partial pressure with that of the sensor output, following the changes in oxygen partial pressure produced by periodic modulation of the hydrostatic pressure with the composition of the atmosphere kept constant. The response times obtained with the jump method and dynamic method are defined as t90 and tb, respectively. Further, we evaluated the relationship between the amplitude magnitude of the oxygen sensor output (An) and the frequency of the oxygen partial pressure (f), using the dynamic method. The results obtained were as follows. The value of tb for the oxygen sensor with a crystallite size and grain size of about 100 nm was 134 ms or less at 1173 K. The value of t90 was 20 and 1 ms when the oxygen partial pressure changed from high to low and from low to high, respectively. From a plot of log An versus log f, it was concluded that the kinetics of a sensor using cerium oxide with crystallite and grain sizes from 100 to 300 nm were controlled by diffusion when the oxygen partial pressure was periodically changed in the shape of a sine wave. It was found that the newly developed equipment was able to evaluate two kinds of response times less than 50 ms.