Hydrothermal synthesis and sintering of ultrafine CeO₂ powders

Abstract

Undoped CeO₂ and Y₂O₃-doped CeO₂ powders, with particle sizes of ≍10–15 nm, were prepared under hydrothermal conditions of 10 MPa at 300 °C for 4 h. The compacted powders were sintered freely in air or in O₂ at constant heating rates of 1–10 °C/min up to 1350 °C. The undoped CeO₂ started to sinter at ≍800–900 °C and reached a maximum density of 0.95 of the theoretical at 1200 °C, after which the density decreased slightly. Isothermal sintering at 1150 °C produced a sample with a relative density of ≍0.98 and an average grain size of ≍100 nm. The samples sintered above 1200 °C exhibited microcracking. The decrease in density and the microcracking above 1200 °C are attributed to a redox reaction leading to the formation of oxygen vacancies and the evolution of O₂ gas. Doping with Y₂O₃ produced an increase in the temperature at which measurable sintering commenced and an increase in the sintering rate, compared with the undoped CeO₂. Sintered samples of the doped CeO₂ showed no microcracks. The CeO₂ doped with up to 3 mol% Y₂O₃ was sintered to almost full density and with a grain size of ≍200 nm at 1400 °C.