Mass spectrometric study of thermodynamic properties in the Gd2 O3 -Y2 O3 system at high temperatures.

Abstract

The Gd2 O3 -Y2 O3 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, Gd2 O3 and Y2 O3 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 Gd2 O3 -Y2 O3 system is of great importance. High-temperature Knudsen effusion mass spectrometry was used to study the vaporization processes and to determine the thermodynamic properties of the Gd2 O3 -Y2 O3 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 Gd2 O3 as a reference substance. Electron ionization at an energy of 25eV was employed. At the temperature of 2630K, GdO, YO and O vapor species were identified over the samples in the Gd2 O3 -Y2 O3 system. The Gd2 O3 and Y2 O3 activities and the vaporization rates of samples as functions of composition in the Gd2 O3 -Y2 O3 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 2630K. The thermodynamic properties of the Gd2 O3 -Y2 O3 system, such as the activities of components and the excess Gibbs energy, obtained in the present study using Knudsen mass spectrometry at 2630K, demonstrated significant negative deviations from ideal behavior. The vaporization rates of the samples were found to decrease as the Y2 O3 content increased. Copyright © 2016 John Wiley & Sons, Ltd.