A POTENTIALLY SELECTIVE METHANE SENSOR-BASED ON THE DIFFERENTIAL CONDUCTIVITY RESPONSES OF PD-DOPED AND PT-DOPED TIN OXIDE THICK LAYERS

Derived from a concept suggested by E. M. Logothetis et al., Proc. 2nd Int. Meet. Chemical Sensors, Bordeaux, France, 1986, p. 175, two differential sensing devices, based on the different catalytic properties of palladium and platinum, are proposed. The first one consists in covering two screen-printed layers of pure tin dioxide by Pd and Pt-doped filters. No major difference is observed between these sensors, probably because of the lack of thickness of the screen-printed filters. In the second device, tin dioxide is directly doped with palladium for one sensor and with platinum for the other. The difference in the response to methane is now sufficient to consider the feasibility of a differential device. However, such sensors have no long-term stability which can only be achieved by covering the layers with inert filters. These filtered SnO2:Pd and SnO2:Pt sensors, now satisfactory as far as differential sensitivity and long-term stability are concerned, also exhibit an important differential response to ethanol, but fortunately reverse to that to methane. The latter result enables a sensing device to be constructed which is selective to methane with respect to ethanol, and insensitive to changes in relative humidity and/or gas flow.