Design of Planar Type CO2 Sensor for Indoor Enviroment Measurement

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A compact C0_2 sensor has been in a great demand for monitoring or controlling C0_2 in various environments. Among the various types of C0_2 sensors so far proposed, the potentiometric type combining a solid electrolyte with an auxiliary phase of metal carbonate is particularly attractive from its excellent sensing capability. From a practical point of view, however, the devices still remain prototype in structure. In this study, we tried to design planar type C0_2 sensor using NASICON thick film for the lower fabrication cost and down sizing of sensor devices.(1) NASICON (Na_3Zr_2Si_2PO_<12>) powder was prepared by a sol-gel method and was added with α-terpineol. The resulting paste was screen-printed 30μm thick on the alumina substrate. NASICON formation in thick film was almost complete after calcination at 950 and 1000℃, although film densification was still poor. The resulting Planar device could work fairly well as a C0_2 sensor under dry conditions but was unable to work at all … More under humid condition.(2) It was attempted to improve the density and mechanical strength of NASICON thick film with addition of glass forming components. The EMF of the resulting device exhibited excellent response to C0_2 not only in dry but in humid air, although the slope of the Nernstian correlations tended to be slightly less steep than the theoretical one.(3) Densification of thick film of NASICON was explored for the use in a planar C0_2 sensor. Among the many kinds of dispersants of NASICON powder for screen printing method, ethylene glycol was found to exhibit the best densified NASICON thick film after calcination at 1000℃. The resulting planar device showed good C0_2 sensing properties under humid as well as dry conditions.(4) In order to reduce the disturbance by humidity, applicability of an oxide ionic conductor of BiCuVOx as a counter electrode material was examined. The oxide-incorporated devices exhibited stable C0_2 sensing properties under dry as well as humid conditions. In addition, the dependence of EMF on relative humidity could be reduced considerably with the oxide-incorporated devices. Less