Synthesis of NASICON with New Compositions for Electrochemical Carbon Dioxide Sensors*

Powders and pellets of new NASICON compositions have been synthesized using a mixed inorganic-organic sol-gel synthesis, by the preliminary formation of a pre-hydrolized TEOS xerogel. The investigated compositions can be described by the general formula Na₃Zr_2-(x/4)Si_2-xP_1+xO₁₂, obtained by keeping the Na concentration constant (= 3) at the optimum value reported for ionic conductivity, with x = 0 (the usual NASICON composition), 0.667, and 1.333. The xerogels were calcined at various temperatures in the range 400–1200°C. The powder samples were analyzed by TG/DTA, BET measurements, XRD, and SEM. The powders calcined at 500°C were sintered into pellets at 1100°C for 6 h. The sintering behavior of the pellets was investigated by dilatometric measurements and SEM observations. The sinterability increased with increasing x value. Dense samples of the new compositions were obtained by sintering at only 1100°C. This is attributed to the occurrence of liquid phase sintering. The electrical conductivity of the NASICON sintered bodies was measured by ac impedance spectroscopy. The conductivity decreased with decreasing c₀ lattice parameter of the hexagonal structure or increasing x value. The CO₂ gas sensors, using as electrolyte the dense samples of the new NASICON compositions, showed good EMF response that was very close to the theoretical value, even for the sample with x = 1.333 that showed much lower conductivity.