Fabrication of thin film potentiometric CO₂ sensors on differentiate substrate surfaces and their characteristics

Two kinds of planar-type potentiometric CO₂ gas sensors using thermal evaporated Li₃PO₄ thin film as solid electrolyte were fabricated. Alumina plates with rough and smooth surfaces were used as the substrates of the sensors. X-ray diffraction analysis, atomic force microscopy and scanning electron microscopy were used to characterise the Li ₃PO₄ films. The sensing properties were investigated in the range of 500-5000 ppm CO₂ concentrations at 480°C. Both the rough substrate-based sensor (r-sensor) and the smooth substratebased sensor (s-sensor) were sensitive to CO₂ gas and showed a good Nernst behaviour. The output electromotive force (EMF) of the s-sensor showed a more stable signal than the r-senor. The ΔEMF/decade values obtained from the r-sensor and the s-sensor were 45 and 55 mV/decade, respectively. The response and recovery time were not primarily influenced by the electrolyte film. It was found that the sensitivity of the s-sensor was closer to the theoretical value. The results revealed that the substrate surface roughness may influence the characteristics of Li₃PO₄ film and the response properties of the sensors to CO₂.