Reactive 10 mol% RE2O3 (RE = Gd and Sm) doped CeO2 nanopowders: Synthesis, characterization, and low-temperature sintering into dense ceramics

Nanoparticles of CeO2 doped with 10 mol% of RE2O3 (RE = Sm and Gd) have been synthesized by the homogeneous precipitation method, employing nitrates as the starting salts and hexamethylenetetramine as the precipitant. Characterizations of the powders were achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), field-emission scanning electron microscopy (FE-SEM), and Brunauer–Emmett–Teller (BET). Sinterability of the powders was studied in air via dilatometry and isothermal sintering. Lightly hydrated crystalline solid solutions with the pre-determined stoichiometry were formed directly during precipitation, as were shown by the above characterization techniques and a leaching test with hydrochloric acid. The maximum doping level achieved by this synthetic procedure was found to be ∼22 at.% for Sm3+ and ∼24.4 at.% for Gd3+. The best annealing temperature for the powders intended for sintering is 400 °C, at which major dehydration has occurred without significant crystallite growth and hard-aggregates formation. Fully dense ceramics (>99%) with ultrafine microstructures (∼117–134 nm) have been fabricated via pressureless sintering for 4 h at a low temperature of 1000 °C.