Takada, T

Sensors and Actuators B: Chemical [Sensors Actuators B: Chem.], vol. B52, no. 1-2, pp. 45-52, 15 Sep 1998

Changes in sensor temperature Delta T and in resistance Delta R of SnO sub(2)- and In sub(2)O sub(3)-based sensors were simultaneously measured when they were exposed to different concentrations of H sub(2), CO, C sub(2)H sub(5)OH, alkanes and volatile organic compounds. A drop in sensor temperature was often observed on exposure to the reducing gases, together with a decrease in sensor resistance, despite the exothermic reaction of the gases with the surface oxygen adsorbates. As a plausible explanation for this phenomenon, it was considered that the temperature drop was caused by a change in thermal conductivity of the sensors as a result of an increase in the amount of conduction electrons due to consumption of negatively charged oxygen ad-ions. The combined utilization of the temperature drop and the resistance changes of a single sensor provided a new method for gas identification and determination of gas concentration. Any concentration of gas species i was represented by a point on a curve F sub(i), which was characteristic of certain species, in the 2-dimensional space ( Delta R, Delta T). Thus, the species could be identified as j without uncertainty from a measured point ( Delta R sub(m), Delta T sub(m)) on the characteristic curve F sub(j). In the present study, identification was successfully made for H sub(2) (10-1000 ppm), CO (10-500 ppm), C sub(2)H sub(5)OH (10-5000 ppm) and alkane such as CH sub(4) (10 ppm-2%), C sub(3)H sub(8) (10-2000 ppm) and i-C sub(4)H sub(10) (10-1000 ppm), and for volatile organic compounds of C sub(6)H sub(6), C sub(6)H sub(5)CH sub(3) and C sub(6)H sub(4)(CH sub(3)) sub(2) of 2-200 ppm.