Thermal and surface characterizations of 25.5 (wt.%) CeO2-2.5 Y2O3-72 ZrO2 fine powder

Abstract

By means of X-ray photoelectron spectroscopy (XPS), simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA), changes of valence state and surface chemical composition of a 25.5 (wt.%) CeO2–2.5 Y2O3–72 ZrO2 fine powder have been studied as a function of temperature (up to 1550°C) and atmosphere (flowing 90% N2-10% H2 and static air). The XPS results lead to the evaluation of the valence state of cerium, zirconium and yttrium, both in the starting powder and in thermal-treated powders. The analysis of the characteristic satellite structures, binding energies and peak shapes of the Ce 3d spectra show that cerium in the starting powder is confirmed to be Ce(IV) with a small amount of Ce(III), and that after thermal treatment the cerium is either unaffected or partially or fully reduced to Ce(III). The DTA and TGA thermograms confirm this redox behaviour and disclose, for air treatment, the presence of a strong, sharp exothermic peak at ∼410°C, which is interpreted as being due to the oxidation of a small amount of Ce2O3. On the contrary, under reducing conditions in the temperature range 630–730°C, the powder is affected by a remarkable weight loss owing to the Ce(IV) reduction to Ce(III) oxide. The change of valence state of zirconium, due to the reduction of ZrO2 to ZrO2−x under reducing conditions at high temperature, has also been monitored by XPS. Furthermore, on high temperature (≥900°C)-treated powders, XPS results show the occurrence of segregation phenomena of both bulk dissolved impurities, such as silicon, sodium, iron and aluminium, as well as yttrium stabilizing oxide. The Wagner two-dimensional chemical state plots for silicon, sodium and aluminium indicate that the chemical nature of the segregated phase can be ascribed to a silicate of sodium and yttrium with the presence of aluminium at higher temperatures. The effect of the above-mentioned chemical aspects on the final properties of sintered materials is also discussed.