Thick films of NASICON (Na+ conductor, Na3Zr2Si2PO12) were prepared for the fabrication of a planar CO2 sensor. The powder of a precursor of NASICON, derived through a sol-gel process, was screen-printed on an alumina substrate and converted into a NASICON thick film by calcination. The formation of crystalline NASICON was almost complete after calcination at 900 and 1000°C, but the films remained rather poor in densification. On the other hand, the films were densified after calcination at 1100°C or above, but NASICON phase disappeared by decomposition. The planar CO2 sensor device was thus fabricated by using the NASICON thick film (30μm thick) calcined at 1000°C. The device attached with a binary carbonate auxiliary phase (Li2CO3-BaCO3) showed fairly good CO2 sensing properties at 450-600°C, despite rather poor densification of the thick film used.
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