Abstract
Structure and thermodynamics of pure cubic ZrO2 and HfO2 were studied computationally and experimentally from their tetragonal to cubic transition temperatures (2311 and 2530 °C) to their melting points (2710 and 2800 °C). Computations were performed using automated ab initio molecular dynamics techniques. High temperature synchrotron X-ray diffraction on laser heated aerodynamically levitated samples provided experimental data on volume change during tetragonal-to-cubic phase transformation (0.55 ± 0.09% for ZrO2 and 0.87 ± 0.08% for HfO2), density and thermal expansion. Fusion enthalpies were measured using drop and catch calorimetry on laser heated levitated samples as 55 ± 7 kJ/mol for ZrO2 and 61 ± 10 kJ/mol for HfO2, compared with 54 ± 2 and 52 ± 2 kJ/mol from computation. Volumetric thermal expansion for cubic ZrO2 and HfO2 are similar and reach (4 ± 1)·10−5/K from experiment and (5 ± 1)·10−5/K from computation. An agreement with experiment renders confidence in values obtained exclusively from computation: namely heat capacity of cubic HfO2 and ZrO2, volume change on melting, and thermal expansion of the liquid to 3127 °C. Computed oxygen diffusion coefficients indicate that above 2400 °C pure ZrO2 is an excellent oxygen conductor, perhaps even better than YSZ.
Highlights
The latest review of experimental data and assessment of the Gibbs free energy functions for all HfO2 and ZrO2 phases was performed by Wang, Zinkevich and Aldinger in 20066
The difficulties lie in thermal gradients unavoidable in conditions of uniaxial laser heating of aerodynamically levitated samples used for calorimetry[17,18] and X-ray diffraction[19,20,21]
Unit cell parameters of the tetragonal and cubic ZrO2 and HfO2 at transition temperatures were refined from X-ray diffraction (XRD) patterns containing both phases (Fig. 2), giving volume change upon transition
Summary
The latest review of experimental data and assessment of the Gibbs free energy functions for all HfO2 and ZrO2 phases was performed by Wang, Zinkevich and Aldinger in 20066 (referred further as the WZA assessment). It was adopted by most researchers for Calphad modeling for ZrO2- and HfO2- containing systems[2,7]. Measurements of enthalpy increments for cubic ZrO2 and HfO2 phases were performed by Pears et al in 196313 Their samples were exposed to carbon vapor in a graphite furnace and their data were not used in the WZA assessment[6]. The agreement between computed and measured values for fusion enthalpies and for thermal expansion supports the validity of the heat capacities, diffusion coefficients, and volume change upon melting obtained from the computation
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