Phase equilibria and thermodynamic properties of the ZrO 2 -GdO 1.5 -YO 1.5 system
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Basic and Applied Research: Section I
Phase Equilibria and Thermodynamic Properties of the ZrO2-GdO1.5-YO1.5 System O. Fabrichnaya, Ch. Wang, M. Zinkevich, C.G. Levi, and F. Aldinger
(Submitted March 12, 2005; in revised form August 16, 2005) The thermodynamic description of the ZrO2-GdO1.5-YO1.5 system was derived based on binary assessments. One of the binaries (GdO1.5-YO1.5) was assessed as part of this investigation, another (ZrO2-YO1.5) was reassessed, and the third one (ZrO2-GdO1.5) was adopted directly from a recent report in the literature. The assessments performed incorporated available experimental data on phase equilibria, calorimetric, and vapor pressure measurements. Ternary isothermal sections at 1200, 1400, and 1600 °C were calculated and agreed well with phase equilibria experiments performed as part of this work. The liquidus projection, selected isopleths and a projection of the T0 surface for the diffusionless tetragonal↔fluorite transformation in the ZrO2-GdO1.5-YO1.5 ternary are also reported.
1. Introduction Yttria-stabilized zirconia (YSZ) has multiple and diverse industrial applications, notably as thermal barrier coatings (TBCs) and electrolytes for solid-oxide fuel cells (SOFCs). Codoping of YSZ by rare earths such as Gd is of interest in thermal barrier systems due to concomitant benefits to the thermal insulating efficiency.[1,2] Compositions about the Gd2Zr2O7 pyrochlore compound are also attractive for TBCs because they combine lower thermal conductivity with enhanced microstructural stability upon high-temperature exposure.[3,4] A concern with the latter approach is that compositions with more than ∼32%GdO1.5 are not thermochemically compatible with the underlying alumina layer in the coating system[5,6] and tend to form interphases at high temperature, with significantly active kinetics at ∼1100 °C and above.[5] An approach to circumvent the problem is to add an interlayer of YSZ (∼8%YO1.5) between Gd2Zr2O7 and the underlying alumina.[7] Conversely, the codoped compositions can be designed to be thermochemically compatible, but those that exhibit higher cyclic lives tend to be supersaturated tetragonal solid solutions (t⬘) that are metastable at the temperatures of practical interest.[7] It has been shown that the phase stability of Y + Gd codoped ZrO2 compositions depends strongly on the relative Gd:Y ratio and the total amount of stabilizer.[8] It has also been suggested that the trends are related to the magnitude of the driving force for the precipitation process, but no experimental information or thermodynamic models are available to ascertain this hypothesis. Understanding of the ZrO2-YO1.5-GdO1.5 system is thus essential to understand the behavior of novel TBC systems based on Gd2Zr2O7/7YSZ bilayers or Y+Gd codoped compositions. The primary aim of this work is to help in develO. Fabrichnaya, Ch. Wang, M. Zinkevich, and F. Aldinger, MaxPlanck-Institut für Metallforschung, Heisenbergstr. 3, D-70569, Stuttgart, Germany; and C.G. Levi, Materials Department University of Califo
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