Ceramics with eutectic microstructure in the ZrO2–PrOx system

Abstract

Praseodymium oxides present redox properties analogous to those of Ce-based systems and have been proposed for catalytic applications in combination with CeO2, ZrO2, or both. However, uncertainties remain concerning the nature and redox behavior of Pr-rich mixtures, especially with ZrO2. Here we study the eutectic composites of the ZrO2–PrOx system, focusing on the sensitivity of their microstructure, phase symmetry, and composition to variations of the processing atmosphere from oxidizing to reducing. Mixed oxides have been produced by a laser-assisted directional solidification technique in O2, air, N2, or 5%H2(Ar) environment, and the resulting materials have been analyzed by scanning electron microscopy/energy-dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, and magnetic susceptibility. In air, N2, or 5%H2(Ar) atmosphere, a lamellar, eutectic-like microstructure forms, the major phase being the one with less Pr content. Both the Pr concentration in each phase as the PrOx molar percentage of the eutectic composites decrease as the atmosphere becomes more reducing. Both eutectic phases are fluorite-like when processing in air, whereas in N2 or 5%H2(Ar), the phase with high Pr content is of the A-R2O3 type, and the phase with low Pr content can be described as a fluorite phase containing C-R2O3-like short-range-ordered regions. The results obtained for samples processed in O2 suggest that for high enough pO2 no eutectic forms, in analogy with the ZrO2–CeO2 system. The evolution of the phase composition and symmetry is discussed in terms of the limited stability of the phases found in the ZrO2–Pr2O3 system, namely, A- or C-R2O3-like, beyond a certain Pr oxidation degree and oxygen content.