Phase Equilibria in the Refractory Oxide Systems of Zirconium, Hafnium and Yttrium with Rare-earth Oxides

Institute of Materials Science Problems, National Ukrainian Academy of Sciences

E.R. Andrievskaya

The systematic study of phase equilibria in the ternary systems HfO₂(ZrO₂)–Y₂O₃–Ln₂O₃ has been first carried out. Phase reactions and crystallization of ceramic alloys in the binary systems ZrO₂–Ln₂O₃, HfO₂–Ln₂O₃, Y₂O₃–Ln₂O₃ and phase equilibria in the series of ternary systems HfO₂–Y₂O₃–Ln₂O₃ and ZrO₂–Y₂O₃–Ln₂O₃ have been developed at high temperature. The most general regularities of the phase reactions in liquid and solid states inherent in these systems have been considered dependent on lanthanide ion radii. Taking into account literature data and newly developed results in binary and ternary systems, the analysis of the main regularities revealed in the constitution of phase diagrams, particularly its dependence on lanthanide ionic radius, was carried out. It was shown that temperature and composition of eutectic reaction, temperature of the pyrochlore phase decomposition, lattice parameters of solid solutions and other parameters of the binary phases linearly depend on ionic radius of lanthanide. For the first time it has been found that the affiliation of lanthanide oxides to cerium or yttrium subgroups predetermines phase relations in the systems and topology of the ternary phase diagrams. The data obtained are the basis for the novel prospective ceramic materials for both structural and functional applications in energetic, medicine, nuclear industry, thermal barrier coatings, solid oxide fuel cells, etc.