HYDROTHERMAL SYNTHESIS AND SINTERING OF ULTRAFINE CEO2 POWDERS

Undoped CeO2 and Y2O3-doped CeO2 powders, with particle sizes of almost-equal-to 10-15 nm, were prepared under hydrothermal conditions of 10 MPa at 300-degrees-C for 4 h. The compacted powders were sintered freely in air or in O2 at constant heating rates of 1-10-degrees-C/min up to 1350-degrees-C. The undoped CeO2 started to sinter at almost-equal-to 800-900-degrees-C and reached a maximum density of 0.95 of the theoretical at 1200-degrees-C, after which the density decreased slightly. Isothermal sintering at 1150-degrees-C produced a sample with a relative density of almost-equal-to 0.98 and an average grain size of almost-equal-to 100 nm. The samples sintered above 1200-degrees-C exhibited microcracking. The decrease in density and the microcracking above 1200-degrees-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 almost-equal-to 200 nm at 1400-degrees-C.