Viscosity of Zirconium Dioxide at Its Melting Point

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Zirconium dioxide (ZrO2) is a hard, white or yellow-brown solid with a melting point of about 2,700 deg C. It is an important industrial commodity and is used as a gem-diamond simulant, an abrasive, a component of acid- and alkali-resistant glasses and of ceramics employed in fuel cells.

The crystalline structure of cubic ZrO2 and HfO2 is based on the fluorite structure (Fm3m) with 4 formula units per cell (Z = 4). Natural and synthetic uraninite, thorianite, and cerianite are present in this structure.

Toughening mechanism in ZrO2 is based on transformation toughening, which consists of a tetragonal-to-monoclinic phase change during crack formation, and it imparts higher fracture toughness to this compound. This toughening is a result of the repulsion of the monoclinic grains with their surrounding tetragonal grains during a crack-initiating stress in the grain.

Fusion enthalpies of ZrO2 and HfO2, volumetric thermal expansions during the transition from tetragonal to cubic, and oxygen diffusion coefficients were computed using ab initio MD computations for temperatures from 2000 to 3127 degC and measured by high temperature X-ray diffraction experiments. The computed fusion enthalpies agree within experimental uncertainties with values from drop and catch calorimetry on samples levitated in argon flow (Fig. 4) and are much lower than Kelley’s 87 kJ/mol value included in JANAF tables14, which was subsequently used by most thermodynamic assessments6,36.

In order to be able to predict molten core behavior, accurate physical properties of molten corium must be provided as input parameters for the governing equations. Viscosity is a key property to be considered, as it determines the flow behaviour of the liquid. This work provides the first experimentally obtained values for viscosity of liquid ZrO2 at its melting point.