Chemical synthesis of Ca-doped CeO2—Intermediate temperature oxide ion electrolytes

Nano-crystalline fluorite-like structure CeO2 and Ca-doped CeO2 compounds were prepared in the temperature range of 220–400 °C using a precursor method which involves coprecipitation of Ca2+ and Ce4+ ions using oxalic acid from the aqueous calcium chloride and ammonium cerium nitrate solutions. The precipitated products were characterized by employing thermogravimetric analysis (TGA), powder X-ray diffraction (PXRD), infrared spectroscopy (IR), laser particle size analysis (LPSA) methods and scanning electron microscopy (SEM). TGA studies show two step weight loss in the temperature range: (i) room temperature to 200 °C and (ii) 200–400 °C for all the investigated precursors. The former loss is attributed to loss of water while the latter is due to decomposition of oxalates. The XRD study reveal a complex pattern for the as-precipitated powders, and surprisingly we see the formation of single-phase fluorite-like structure at about 220 °C for Ce1−xCaxO2−x (0 < x < 0.20). However, XRD peaks were found to be very broad that sharpen with increasing temperature. The cubic fluorite-type lattice constant increases with increasing Ca-content, which is consistent with literature, and also follows the expected trend based on the ionic radii consideration. For purpose of comparison, Ce1−xCaxO2−x (0 < x < 0.25) samples were also prepared by solid-state reaction using CeO2 and CaCO3, and lattice parameter is consistent with precipitation method samples within the experimental error. This result suggesting that doping of Ca is successful by coprecipitation. The particle size of parent and Ca-doped CeO2 samples prepared by precipitated method was found to be in the range 10–85 nm (from PXRD) in the temperature range 400–1000 °C, while about order of higher size was observed for the ceramic method synthesized samples. The presently employed wet chemical method could be used to prepare ceria and doped materials with nano-sized particles for a large scale production at mild temperature.