CHEMICAL ASPECTS IN THERMAL-TREATMENT OF ZRO2-CEO2-Y2O3 ALLOY

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 25.5CeO2-2.5Y2O3-72ZrO2 (percentages by weight) fine powder, suitable for producing sintered components, have been studied as a function of temperature (up to 1550°C) and atmosphere (flowing N2(90%)-H2(10%) and static air). 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. In particular, the analysis of the characteristic satellite structure, binding energy and peak lineshape in the Ce3d spectra show that cerium in the starting powder is confirmed to be Ce(IV) with a small amount of Ce(III) and that after the thermal treatments cerium is either unaffected, partially or fully reduced to Ce(III). DTA and TGA thermograms confirm this redox behaviour and show, for static-air treatment, the presence of a strong and sharp exothermic peak at about 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 range 630 to 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 Zr2O3, under reducing condition at 1050°C has been also monitored by XPS. Furthermore, starting from >= 900°C. XPS results show the occurennce of segregation phenomena of both bulk-dissolved impurities, such as silicon, sodium, iron and aluminium as well as of yttrium-stabilizing oxide. The Wagner two-dimensional chemical state plots for silicon and aluminium indicate that the chemical nature of the segregated phase is ascribable to an infinite chain silicate of sodium and yttrium with the presence of aluminium at higher temperatures. The effect of the aforecited chemical aspects on the final properties of sintered materials, is also discussed.