Interfacial structure of NASICON-based sensor attached with Li₂CO₃-CaCO₃ auxiliary phase for detection of CO₂

Potentiometric devices combining NASICON (Na⁺ conductor, Na₃Zr₂Si₂PO₁₂) with a series of Li₂CO₃-CaCO₃ auxiliary phases were investigated for CO₂ sensing properties as well as hetero-junction structure involved. It was revealed that an addition of proper amount of CaCO₃ to Li₂CO₃ auxiliary phase was effective for improving CO₂ sensing capability at lower temperatures; Li₂CO₃-CaCO₃ (1:2) allowed normal operation down to 350 °C, while Li₂CO₃ did down to 400 °C. The analysis of fractured surfaces by means of an electron-probe micro-analyzer (EPMA) and SEM indicated the formation of an interfacial layer between NASICON and carbonates, the thickness and structure of which depended on the carbonate composition used. As revealed by X-ray diffraction analysis, CaZrO₃ was formed between NASICON and Li₂CO₃-CaCO₃ (1:2), suggesting its relevance to the lower temperature sensing capability.