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 Na3Zr2-(x/4)Si2-xP1+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.