High-temperature mass spectrometric study of the vaporization processes and thermodynamic properties in the Gd₂ O₃...

The refractory properties of the Gd₂ O₃ -Y₂ O₃ -HfO₂ system are considered promising for the production of many high-temperature materials, e.g., thermal barrier coatings and casting molds for gas turbine engine blades. At high temperatures, components of the Gd₂ O₃ -Y₂ O₃ -HfO₂ system may vaporize selectively and this may significantly change the physicochemical properties of the materials. Therefore, information on vaporization processes and thermodynamic properties of the Gd₂ O₃ -Y₂ O₃ -HfO₂ system is of great importance.

The vaporization processes and thermodynamic properties of the Gd₂ O₃ -Y₂ O₃ -HfO₂ system were studied using high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using a tungsten twin effusion cell containing the samples under study and pure Gd₂ O₃ as a reference substance. Electron ionization at an energy of 25 eV was employed in the present study.

It was shown that at a temperature of 2500 K the vapor over the samples in the Gd₂ O₃ -Y₂ O₃ -HfO₂ system consisted of the GdO, YO and O vapor species. The Gd₂ O₃ and Y₂ O₃ activities in the samples in the Gd₂ O₃ -Y₂ O₃ -HfO₂ system as well as their vaporization rates were derived from the partial pressures of the vapor species. Using these data the HfO₂ activities, the Gibbs energy of mixing and the excess Gibbs energy in this system were calculated at 2500 K.

The thermodynamic properties of the Gd₂ O₃ -Y₂ O₃ -HfO₂ system, i.e., the component activities in the samples and the excess Gibbs energy, obtained in the present study at 2500 K, exhibited negative deviations from ideal behavior. The concentration dependence of excess Gibbs energy of the Gd₂ O₃ -Y₂ O₃ -HfO₂ system was approximated with an empirical equation. Copyright © 2017 John Wiley & Sons, Ltd.