Enthalpy of formation of cubic yttria-stabilized zirconia
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I. Molodetsky Schlumberger–Princeton Technology Center, 20 Wallace Road, Princeton Junction, New Jersey 08550 (Received 15 November 2002; accepted 13 January 2003)
Oxide melt solution calorimetric measurements were made to determine the enthalpy of formation of cubic-yttria-stabilized zirconia (c-YSZ) with respect to the oxides m-ZrO2 and C-type YO1.5. The enthalpy of formation can be fit either by a quadratic equation or by two straight line segments about the minimum near x ⳱ 0.40. The quadratic fit gives a strongly negative interaction parameter, ⍀ ⳱ −93.7 ± 12.0 kJ/mol, but does not imply regular solution behavior because of extensive short-range order. In this fit, the enthalpy of transition of m-ZrO2 to c-ZrO2, 9.7 ± 1.1 kJ/mol, is in reasonable agreement with earlier estimates and that of C-type to cubic fluorite YO1.5, 24.3 ±14.4 kJ/mol, is consistent with an essentially random distribution of oxide ions and anion vacancies in the high-temperature fluorite phase. The two straight-line segments give 6.1 ± 0.6 kJ/mol and 5.5 ± 2.5 kJ/mol for these transitions, respectively. The latter value would imply strong short-range order in cubic fluorite YO1.5. Clearly more complex solution thermodynamic descriptions need to be developed. The enthalpy of transition from the disordered c-YSZ phase to the ordered ␦-phase at 25 °C was also measured and was 0.4 ± 1.6 kJ/mol. No energetic difference between the disordered c-YSZ phase and the ordered ␦-phase underscores the importance of short-range order in c-YSZ. Enthalpy data were combined with Gibbs free energy data to calculate entropies of mixing. Using the quadratic fit, negative excess entropy of mixing in the cubic solid solution, relative to a system with maximum randomness on cation and anion sublattices, was found and was another indication of extensive short-range order in c-YSZ.
I. INTRODUCTION
Three polymorphs of pure ZrO2 exist at 1 atm: monoclinic (m), tetragonal (t), and cubic (c). Monoclinic zirconia, m-ZrO2, the stable polymorph at room temperature, transforms to t-ZrO2 at about 1200 °C with an associated enthalpy of transition of 5.94 ± 0.4 kJ/mol.1 Tetragonal zirconia, t-ZrO2, remains stable to 2377 °C, where it transforms to cubic zirconia, c-ZrO2. The enthalpy of this high-temperature transition is difficult to measure directly, as c-ZrO2 is unquenchable. However, based on the structural similarities between the t- and c-ZrO2 polymorphs, Ackermann et al. have estimated the entropy of the t–c transition to be about 2.09 J/mol/K and, thus, have reported the enthalpy of transition to be 5.55 kJ/mol.2 Cubic zirconia has a high oxygen mobility providing significant ionic conductivity at high temperatures.3 Below the temperature of t–c phase transition, structural changes in c-ZrO2 decrease oxygen mobility. Doping stabilizes the cubic phase to much lower temperatures, and 908
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J. Mater. Res., Vol. 18, No. 4, Apr 2003 Downloaded: 13 Mar 2015
stabilized cubic phases are quenchable. In addition to the stabilizing effect, dopi
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