Potentiometric devices for which a disc of NASICON (Na+ conductor, Na3Zr2Si2PO12) was attached with an auxiliary phase of Na2CO3 (I) or Na2CO3-BaCO3 (2 : 3 in molar ratio) (II) were investigated for CO2 sensing properties and the structure of hetero-junctions involved, SEM observation combined with EPMA analysis indicated that, for device (I), a corrosion layer (20-30 μm) rich in Na content was formed inside NASICON. The corrosion layer was estimated to consist mainly of Na2ZrSiO3 and Na2ZrO2 from separate experiments using powder samples. For device (II), in contrast, such a corrosion layer was hardly formed between the carbonate and NASICON. This difference in junction structure could account well for the difference in CO2 sensing capability at lower temperature between the devices.
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