Influence of substrate surface roughness on the properties of a planar-type CO₂ sensor using evaporated Li₃PO₄ film

Planar-type potentiometric CO₂ gas sensors using thermal evaporated Li₃PO₄ thin film as the solid electrolyte were fabricated. Al₂O₃ plates with rough and smooth surfaces were used as the substrates of the sensors. X-ray diffraction analysis, atomic force microscope and scanning electron microscope were used to characterize the Li₃PO₄ films. The sensing properties were investigated in the range of 500∼5000 ppm CO₂ concentrations at 480 °C. Both the rough substrate sensor (r-sensor) and the smooth substrate sensor (s-sensor) showed a good Nernst behavior. The output EMF of s-sensor showed a more stable signal than the r-senor. Response and recovery times of the r-sensor were 40 s and 75 s, and for the s-sensor they were 35 s and 60 s. The ΔEMF/decade values obtained from the r-sensor and s-sensor were 45 mV/decade and 55 mV/decade, respectively. It can be found that the Nernst's slop of the s-sensor was closer to the theoretically value. The results revealed that the substrate surface roughness may influence the characteristics of Li₃PO ₄ film and the response properties of the sensors to CO₂.