Thermodynamic investigation of transition metal oxides via CALPHAD and first-principles methods

Open Access
Author:
Zhang, Lei
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 10, 2013
Committee Members:
  • Zi Kui Liu, Thesis Advisor
Keywords:
  • CALPHAD
  • transition metal oxides
  • phase stability
  • first-principles
  • defect
Abstract:
This thesis describes the thermodynamic modeling of the La2O3-TiO2 system, SrCoO3-δ perovskite with extension to Sr-doped LaCoO3-δ. By using CALPHAD (CALculation of PHAse Diagram) method, the thermochemical properties gained by first-principles calculations are put together with phase equilibrium data for the optimization and phase stability prediction. Thermodynamic modeling of oxides, especially transition metal oxides are not as common as metallic systems. In the CALPHAD approach for oxides, the ionic compound energy formalism is adopted for thermodynamic model construction. In first-principles calculations, GGA+U method is used to account for the strong-correlation of d electrons in transition metal ions. By fitting energy-volume curve for a certain oxide, the 0 K enthalpy is obtained. The fitting parameters in energy-volume curve can then be utilized in the Debye-Grüneisen model to further predict the Gibbs energy at finite temperature, which can be optimized in CALPHAD method to predict the phase stability. In the La2O3-TiO2 system, the thermodynamic properties of ternary oxides are calculated by first-principles along with Debye- Grüneisen model. The phase diagram is then predicted with an optimized liquid phase thermodynamic description. The thermodynamic database constructed is crucial for ceramic processing involving lanthanum titanates. The SrCoO3-δ, when doped into the LaCoO3-δ, can be applied as the ionic transport membrane for gas separation and purification. The defect behavior in Sr-doped LaCoO3-δ along with phase stability in the service condition then becomes significant. The defect calculations in cubic SrCoO3-δ provide precious thermochemical data for the phase stability and defect concentration predictions.