The Zirconia–Hafnia System: DTA Measurements and Thermodynamic Calculations

Abstract

The equilibrium temperature (T 0 ) at which the tetragonal and monoclinic phases of either ZrO 2 or HfO 2 coexist is generally defined by the middle temperature of A s (the onset transformation temperature on heating) and M s (the onset transformation temperature on cooling). It cannot be directly determined due to the athermal nature of the martensitic transformation. Practically, the determination of T 0 is important for the prediction of A s and M s in ZrO 2 or HfO 2 -based materials. In this work, the ZrO 2 -HfO 2 system was studied experimentally by differential thermal analysis (DTA) to obtain the martensitic tetragonal 3 monoclinic transformation temperatures in the temperature range of 1273-1973 K. The T 0 temperatures obtained for ZrO 2 and HfO 2 are 136775 and 205275 K, respectively. They are adopted for the assessments of the Gibbs energy parameters of these two oxides. A reasonably calculated ZrO 2 ÀHfO 2 phase diagram is presented.

Heat capacity of hafnia at low temperature

“…The heat capacity data were extrapolated from (2 to 0) K. The calculated standard entropy at 298.15 K is 56.15 ± 0.57 J Á mol À1 Á K À1 . The error is estimated by applying ±5% measurement error for experimental data below 20 K and ±1% for data above 20 K. The calculated entropy at 296.16 K (56.16 ± 0.57 J Á mol À1 Á K À1 ) is in moderate agreement with Todd's 59.33 ± 0.42 J Á mol À1 Á K À1 [6], although the latter was estimated with an extrapolation from (52.47 to 0) K. The entropy of hafnia at 298.15 K is greater than that reported for zirconia by Tojo et al [10] and Wang et al [24], which are (49.79 and 49.76) J Á mol À1 Á K À1 , respectively.…”

Effect of variant strain accommodation on the three-dimensional microstructure formation during martensitic transformation: Application to zirconia

“…According to Ref. [57], the equilibrium temperature is a temperature at which the Gibbs free energy of both tetragonal and monoclinic phases are the same; this temperature for T ! M is 1367 ± 5 K, and the Gibbs free energies for monoclinic and tetragonal zirconia are:…”

“…Wang et al [57] calculated the equilibrium temperature for the T ! M phase transformation for pure zirconia and adopted it to assess the Gibbs free energy of zirconia in different phases.…”

Phonon anharmonicity of monoclinic zirconia and yttrium-stabilized zirconia

“…Vibrational spectroscopy studies and theoretical work on zirconia have been performed, 1,[12][13][14][15] and other studies were focused on phase transitions under pressure.…”