De‐densification mechanisms of yttria‐doped cerium oxide during sintering in a reducing atmosphere

Abstract

The presence of residual carbon in oxides in which the valence state can change during sintering may lead to de‐densification or swelling phenomena during the last stage of sintering. This was demonstrated by sintering a Ce0.85Y0.15O2‐x powder compact with or without added graphite carbon in a reducing atmosphere (Ar/5 vol.% H2) at 1450°C. The shrinkage behavior was studied with a dilatometer combined with an oxygen probe and a gas chromatograph to analyze the composition of the released gases. Oxide reduction during sintering leads to a significant release of oxygen. This oxygen can react with carbon to form gaseous species such as CO. These gases can be released during the second stage of sintering, that is, when the porosity is still open, but they can be trapped in the closing pores during the final stage of sintering. This causes the pressure to increase in the pores, resulting in irreversible swelling, cracking and eventually pellet fracture.

Hydrothermal Conversion of Uranium(IV) Oxalate into Oxides: A Comprehensive Study

“…The presence of impurities coming from the hydrothermal degradation of U(C2O4)2.nH2O was finally checked in the oxide samples. Indeed, previous works showed that the presence of residual carbon in the oxide powders could impact negatively the densification step 9,11,12 . Powders were then first characterized thanks to a carbon analyzer ( 9 .…”

Direct sintering of UO2+x oxides prepared under hydrothermal conditions

“…This difference can be explained by the presence of smaller crystallites which react more quickly to sintering [19]. Above 1500°C, a phenomenon of de-densification, sometimes called "solarisation", was also observed, which could have been be caused by coalescence of pores or exaggerated grain growth [15], [25]- [28]. 1).…”

“…This morphological feature can limit the powder flowability when shaping pellets for ceramic fabrication, and then induce some mechanical defects after sintering. Also, oxide based powders coming from oxalate precursors generally incorporate residual traces of carbon, typically from around 100 ppm when calcined under air [13] up to several thousands of ppm for thermal conversion performed under inert or reducing atmosphere [12], which have been associated to dedensification processes [14], [15].…”

Synthesis of size-controlled UO2 microspheres from the hydrothermal conversion of U(iv) aspartate

“…Finally, it is worth noting that a slight weight loss signal was recorded between 500 and 1000°C, and associated with the detection of small amounts of CO and CO 2 , which could be assigned to the elimination of residual carbon in the sample. Indeed, the thermal decomposition of MO 2 precursors, including carboxylates such as oxalates, was systematically associated with the presence of amorphous carbon within the final oxides 39,40 .…”