Synthesis, processing and characterization of nasicon solid electrolytes for CO2 sensing applications

Precursor powders of nasicon solid electrolytes were synthesized by both the colloidal method (CM) and sol–gel method (SGM) and characterized. The phase evolution of the sensor materials and sintered microstructures was analyzed by X-ray diffraction technique (XRD) and scanning electron microscopy (SEM). Zirconia-free nasicon phase was formed at relatively low temperature from the sol–gel precursor powder. Sintered pellets produced from calcined sol–gel powder exhibited single-phase (homogenous) microstructure while a silica-rich glass phase was present at the grain boundaries in the pellets sintered from calcined colloidal powders. Disc-shaped sensor elements were fabricated via uniaxial pressing of the calcined powders and subsequently sintered at 1150 °C for the CM powders and 1200 °C for the SGM powders, respectively. Platinum electrode was applied to the sensor elements to produce potentiometric/electrochemical carbon dioxide gas sensors. The electrical response of the sensors to CO2 was investigated at various operating temperatures and CO2 concentrations. Sensors made from sol–gel derived materials displayed better sensing response to CO2 gas at higher testing temperature as compared to sensors made from the colloidal powders.