Mass spectrometric study of thermodynamic properties in the Gd2O3-Y2O3 system at high temperatures

RATIONALE: The Gd2O3-Y2O3 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, Gd2O3 and Y2O3 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 Gd2O3-Y2O3 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 Gd2O3-Y2O3 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 Gd2O3 as a reference substance. Electron ionization at an energy of 25 eV was employed. RESULTS: At the temperature of 2630 K, GdO, YOandOvapor species were identified over the samples in the Gd2O3-Y2O3 system. The Gd2O3 and Y2O3 activities and the vaporization rates of samples as functions of composition in the Gd2O3-Y2O3 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 Gd2O3-Y2O3 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 Y2O3 content increased. Copyright (C) 2016 John Wiley & Sons, Ltd.