Thermodynamic modelling and experimental study of the HfO2-Y2O3-ZrO2 system

Yttria-stabilized zirconia (YSZ) has a wide range of applications, such as oxygen sensors, solid‑fuel cells, oxygen pumps and thermal barrier coatings. Since HfO2 is a natural impurity of ZrO2, phase relations in the HfO2‑Y2O3‑ZrO2 system are important. Therefore, a consistent thermodynamic database for this system is necessary to prevent the formation of undesirable compounds in YSZ materials with HfO2 addition. There is a wide literature data of phase equilibria and thermodynamic properties for the HfO2‑Y2O3‑ZrO2 system, except for the δ‑Zr3Y4O12 phase, of which the stability and heat capacity are not well known. In this work, a δ‑Zr3Y4O12 single-phase sample was produced by co‑precipitation reaction followed by heat treatment at 1573 K for 90 days. Its heat capacity was experimentally measured by differential scanning calorimetry (DSC) and slightly deviate of the Neumann-Kopp rule. The results obtained, as well as the literature data on phase equilibria and experimental thermodynamic values have been used to assess thermodynamically the HfO2‑Y2O3 and HfO2‑Y2O3‑ZrO2 systems by applying the CALPHAD approach. The calculated isothermal sections at 1523, 1573, 1873 and 2173 K, vertical sections (Hf/Zr=40/60, 50/50 and 80/20) and liquidus projection of the HfO2‑Y2O3‑ZrO2 system reproduce well phase equilibria data and thermodynamic properties within uncertainties.