Methane sensing: From sensitive thick films to a reliable selective device

This paper shows the long route between gas-sensitive tin oxide material and a reliable prototype for selective methane sensing. A good reproducibility of fabrication is a prerequisite to any development of a cheap sensing device. Hybrid thick-film technology appears well adapted for this purpose, on the condition that the rheological properties of the pastes are well controlled. Sensitivity was shown to be modified by the thickness of the tin oxide layer, the addition of palladium, the addition of a glass binder, the nature of the electrode metal and the presence of a coating. Methane selectivity was achieved by adapting a concept proposed previously, based on the differential catalytic activity of palladium and platinum versus the oxidation of methane at 400 degrees C and by combining two sensing principles on the same alumina substrate. As for long-term stability, much information could be gained by annealing at temperatures higher than that of operation. Metal interdiffusion in the sensor connections (Kirkendall effect) and between the catalysts and the other metallic parts of the sensor was demonstrated to be a major cause of instability. Moreover, the firing and annealing conditions used in the fabrication process of our sensors, including that of the Pt heating resistor, were also shown to influence the reliability.