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

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.% H-2) at 1450 degrees 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.