Mass spectrometric study of thermodynamic properties in the Gd ₂ O ₃ ‐Y ₂ O ₃ system at high temperatures

Abstract

Rationale The Gd 2 O 3 ‐Y 2 O 3 system possesses a number of practical applications, one of the most important of them being production of casting molds for gas turbine engine blades. The components of this system are often added to zirconia or hafnia to obtain high‐temperature ceramics which are used for the development of thermal barrier coatings. However, Gd 2 O 3 and Y 2 O 3 are more volatile than zirconia or hafnia and may vaporize selectively during synthesis or usage of high‐temperature materials which may lead to changes in their physicochemical properties. Therefore, information on the vaporization processes and thermodynamic properties of the Gd 2 O 3 ‐Y 2 O 3 system is of great importance. Methods High‐temperature Knudsen effusion mass spectrometry was used to study the vaporization processes and to determine the thermodynamic properties of the Gd 2 O 3 ‐Y 2 O 3 system. Measurements were performed with a MS‐1301 mass spectrometer. Vaporization was carried out using a tungsten twin effusion cell containing the sample under study and pure Gd 2 O 3 as a reference substance. Electron ionization at an energy of 25 eV was employed. Results At the temperature of 2630 K, GdO, YO and O vapor species were identified over the samples in the Gd 2 O 3 ‐Y 2 O 3 system. The Gd 2 O 3 and Y 2 O 3 activities and the vaporization rates of samples as functions of composition in the Gd 2 O 3 ‐Y 2 O 3 system were derived from the partial pressures of the vapor species mentioned. Using these data the Gibbs energy of mixing and excess Gibbs energy of the hexagonal solid solution in this system were calculated at 2630 K. Conclusions The thermodynamic properties of the Gd 2 O 3 ‐Y 2 O 3 system, such as the activities of components and the excess Gibbs energy, obtained in the present study using Knudsen mass spectrometry at 2630 K, demonstrated significant negative deviations from ideal behavior. The vaporization rates of the samples were found to decrease as the Y 2 O 3 content increased. Copyright © 2016 John Wiley & Sons, Ltd.