The conditions for the solid state synthesis of solid solutions in zirconia and hafnia systems with the oxides of rare earth elements

Abstract

The goal of this work was to study the specific features of obtaining (Zr,Hf)1–xRxO2-0.5x solid solutions through solid-phase sintering and to analyse the correctness of the existing variants of phase diagrams for (Zr, Hf)O2-R2O3 zirconia and hafnia systems with the oxides of rare earth elements. We analysed the existing data on the duration of annealing used to study phase equilibria in zirconia and hafnia systems with the oxides of rare earth elements. The “annealing time logarithm – reciprocal temperature” dependences were constructed. It was shown that the effective diffusion coefficient upon annealing was at least 200 kJ/mol. The time of annealing required for the achievement of equilibrium at 1300 ºС was no less than 6 months. The annealings for one year did not allow receiving reliable information on phase equilibria in these systems with temperatures lower than 1250 ºС. All the data on phase diagrams presented in earlier studies for lower temperatures did not characterise the equilibrium state of systems. Apart from low-temperature phases of variable compositions presented in phase diagrams, among the characteristics of non-equilibrium states there were violations of the Hume-Rothery rule and observations of diffusionless processes of ordering of solid solutions, including those occurring upon “fluorite-pyrochlore” solid state transitions. Probable schemes of low temperature phase equilibria in the ZrO2-Er2O3 and HfO2-Eu2O3 systems were presented taking into account the third law of thermodynamics. The obtained results are fundamental and will be useful for the assessment of the stability of thermal barrier coatings and fuel cells based on zirconium and hafnium oxides with the oxides of rare earth elements.

Keywords

• zirconia
• hafnia
• zirconium oxide
• hafnium oxide
• oxides of rare earth elements
• solid solutions
• ordering
• phase diagrams
• sintering