Ceramics based on the Sm2O3-Y2O3 and Sm2O3-HfO2 systems at high temperatures: Thermodynamics and modeling

Ceramics based on the Sm2O3-Y2O3 and Sm2O3-HfO2 systems is promising for development of advanced refractory materials including casting molds for gas turbine engine blades and thermal barrier coatings. Vaporization processes and thermodynamic properties of the Sm2O3-Y2O3 and Sm2O3-HfO2 systems were studied by the Knudsen effusion mass spectrometric method. The samples used in the present study were synthesized by the solid-state method. Composition of the vapor over these systems at the temperature 2373 K was the same as over pure oxides. Temperature dependences of the partial pressures of the SmO and Sm vapor species were obtained over pure Sm2O3 in the temperature range 2265-2668 K and over Sm2Hf2O7 in the temperature range 2222-2650 K. The partial pressures of the vapor species, the component activities, and the excess Gibbs energies were determined over a wide concentration ranges in the Sm2O3-Y2O3 and Sm2O3-HfO2 systems at the temperature 2373 K. These thermodynamic functions exhibited negative deviations from the ideality and were compared with the data available for the La2O3-Y2O3, Gd2O3-Y2O3, ZrO2-Y2O3 and La2O3-HfO2, Gd2O3-HfO2, Nd2O3-HfO2 systems. Thermodynamic properties of these binary systems containing yttria and hafnia with rare earth oxides were discussed in the frame of the Generalized Lattice Theory of Associated Solutions. Correlations were found between the observed changes in the thermodynamic behavior of the systems mentioned and the relative number of bonds in the condensed phases when the second coordination sphere was taken into consideration.