INTEGRATED MASS SPECTROMETER-MICROREACTOR SYSTEM FOR THE IN-SITU INVESTIGATION OF OXIDE SEMICONDUCTOR SENSORS

This work describes the construction of an experimental setup - a mass spectrometer coupled microreactor - which makes possible the simultan eous monitoring of the electric resistance of a solid sensor sample an d the composition of the near surface gas atmosphere. The information obtained from the constant voltage resistance measurement is complemen ted by the information about of the near-sensor gas composition. The e valuation of both sets of results offers insight into the mechanism of the sensing process. As an example, the results obtained on an undope d, oven-heated CeO2 sensor sample is presented, in a CO and oxygen con taining atmosphere between 25 and 900 degrees C. It was found that bet ween 200 and 300 degrees C the decrease in the sensor resistance is ma inly due to the CD chemisorption. In this temperature range the oxidat ion of CO is negligible. The CO sensitivity decreases with the tempera ture in the whole interval examined. However, its value is the highest in this temperature range due to the relatively high surface coverage . In the 300-400 degrees C range the most important process is the gas phase oxidation of the CO; about 85% of it is transformed. The transf ormation of CO leads to a smaller surface coverage, and so to a decrea sing - but still high enough - sensitivity. A further temperature rise hinders the CO chemisorption and the lack of chemisorption results in no CO sensitivity. It is noted that even the catalytic effect of the CeO2 sample is important in the studied system; its effect on the near -sensor gas composition is easily detectable. The maximum catalytic ac tivity of CeO2 in the CO oxidation was found to be around 400 degrees C. (C) 1998 American Institute of Physics. [S0034-6748(98)00309-8].