Thermodynamic Modelling and Phase Equilibria in ZrO2-RE2O3-Y2O3-Al2O3 Materials Systems
Final Report Abstract
Phase relations in binary and ternary constituent of the ZrO 2 -RE 2 O 3 -Y 2 O 3 -Al 2 O 3 systems (RE = La, Nd, Sm, Gd, Yb) were experimentally studied in this project. Phase equilibrium studies at 1523, 1673 and 1873 K were performed by prolonged heat treatment of samples obtained by co-precipitation. XRD and SEM/EDX measurements were used for sample characterisation. DTA investigation was used to determine temperature of phase transformations. Based on experimental data obtained in present project the thermodynamic parameters for the ZrO 2 -Nd 2 O 3 , Nd 2 O 3 -Al 2 O 3 and Sm 2 O 3 -Al 2 O 3 binary systems were assessed and phase diagrams were calculated by CALPHAD approach. The thermodynamic description of the ZrO 2 -La 2 O 3 -Y 2 O 3 -Al 2 O 3 system was derived based on experimental data available in literature and results obtained in this project. The experimental studies indicated reaction between pyrochlore phase and Y 2 O 3 resulting in formation of fluorite phase in the La 2 Zr 2 O 7 -Y 2 O 3 system. Solubility of Y 2 O 3 in pyrochlore phase and extended homogeneity range in fluorite phase were experimentally established. Therefore an advanced sublattice model taking into account solubility of Y 2 O 3 in pyrochlore and a model accounting ternary interactions in fluorite were used to describe our experimental results. The thermodynamic description for the ZrO 2 -Nd 2 O 3 -Y 2 O 3 - Al 2 O 3 system was derived based on literature data and experimental data obtained in this project. The experimental investigation of the Nd 2 O 3 -Y 2 O 3 -Al 2 O 3 system was performed for the first time and phase equilibria were established in the temperature range up to 2000 K. The invariant reaction in solid phases at temperatures slightly below 1873 K was indicated experimentally and reproduced in calculations. The thermodynamic description for the ZrO 2 -Sm 2 O 3 -Y 2 O 3 -Al 2 O 3 system was derived based on literature data and experimental investigations performed in this project. The phase relations in two ternary systems ZrO 2 -Sm 2 O 3 -Y 2 O 3 and Sm 2 O 3 -Y 2 O 3 -Al 2 O 3 were established experimentally for the first time. The thermodynamic parameters for the ZrO 2 -Gd 2 O 3 -Y 2 O 3 -Al 2 O 3 system were reassessed because of changes in descriptions of zirconia, yttria and gadolinia polymorphs. The phase relations in binary and ternary systems were very well reproduced. The systematic changes with ionic radius of Rare Earth were observed for diffusionless transformations of tetragonal phase to monoclinic and fluorite structures. It was shown that the driving force for the partitioning to equilibrium assemblage increases with ionic radius of Rare Earth element. Several inconsistencies were revealed between experimental results obtained in different research groups and calculations in the Yb 2 O 3 -Al 2 O 3 , ZrO 2 -Yb 2 O 3 and ZrO 2 -Yb 2 O 3 -Al 2 O 3 system. Based on computational modeling performed in present project new experimental studies (both phase equilibria studies and thermodynamic measurements) were recommended to resolve existing contradictions. The experimental data on the Yb 2 O 3 -Y 2 O 3 -Al 2 O 3 system are not available. Therefore calculations based on binary extrapolation and assumption of ideal solid solutions should help to limit amount of future experiments. The development of thermodynamic database of the ZrO 2 -Yb 2 O 3 - Y 2 O 3 -Al 2 O 3 system and its combinations with databases including oxides from large rare earth ions (RE 2 O 3 with RE=La, Nd, Sm) developed in present project will be important for materials design for thermal barrier coating. Materials obtained by multiple co-doping of TBC, typically with one small and one large cation are perspective because of expected decrease of thermal conductivity due to formation of nano-clusters responsible for phonon scattering. Co-doping of YSZ by Gd also provides substantial decrease of thermal conductivity. The Gd 2 O 3 -Y 2 O 3 -Al 2 O 3 system is a constituent of the ZrO 2 -based four oxide system and it has not been experimentally studied so far. The experimental study of the Gd 2 O 3 -Y 2 O 3 -Al 2 O 3 system is necessary to improve thermodynamic description of quaternary system ZrO 2 -Gd 2 O 3 -Y 2 O 3 -Al 2 O 3 important for evaluation of stability of TBC co-doped by Gd and its compatibility with thermally grown oxide (Al 2 O 3 ). The review of recent literature indicated that the ZrO 2 -Y 2 O 3 -Ta 2 O 3 system presents interest for searching of perspective materials for the TBC application. The tetragonal phase is stable up to high temperatures and non-transformable to monoclinic phase. Thermodynamic data for the binary compounds in this system are missing. Therefore the development of thermodynamic database for the ZrO 2 -Y 2 O 3 -Ta 2 O 5 based on new experimental measurements of thermodynamic properties would be important for design of new candidate material for TBC. A major issue is also the reaction of TBCs with dust particles from arid environments or volcano ash clouds during turbine operation. We would like to propose research work addressing the interaction of TBC materials with relevant silicates and other oxides. Conditions of operating stationary and mobile gas turbines should be taken into account for this research. With regard to extensive flight cancellations during the past years because of volcano ash clouds in Europe and worldwide and the related massive economic damages it seems to be of highest interest to carry out research work to improve the resistance of TBCs against environmental impacts. Such work certainly would very much push and strengthen the research areas in thermal barrier coatings.
Publications
- Thermodynamic assessment of the ZrO2-Yb2O3-Al2O3 system, CALPHAD, 34, 206-214
O. Fabrichnaya, H.J. Seifert
- Assessment of thermodynamic functions in the ZrO2-Sm2O3-Al2O3 system, J. Alloys Comp. 475 (2009) 86-95
O. Fabrichnaya, H.J. Seifert
- Experimental investigation and thermodynamic modelling in the ZrO2-La2O3-Y2O3 system. J. Alloys Comp. 493 (2010) 263-271
O. Fabrichnaya, G. Savinykh, G. Schreiber, M. Dopita, H.J. Seifert
- Calorimetric investigation of the La2Zr2O7, Nd2Zr2O, Sm2Zr2O7 and LaYO3 compounds and CALPHAD assessment of the La 2O3-Y2O3 system, Thermochimica Acta 526 (2011) 50-57
O. Fabrichnaya, M.J. Kriegel, J. Seidel, G. Savinykh, L.P. Ogorodova, I.A. Kiseleva, H.J. Seifert
- Phase relations in the ZrO2-Nd2O3-Y2O3 system: experimental studies and advanced thermodynamic modelling, J. Phase Equilibria and Diffusion 32 (2011) 284-297
O. Fabrichnaya, G. Savinykh, G. Schreiber, H.J. Seifert
- Up-date of a thermodynamic database of the ZrO2-Gd2O3-Y2O3-Al2O3 system for TBC applications, J. Phase Equilibria and Diffusion 32 (2011) 2-16
O. Fabrichnaya, H.J. Seifert
