Gibbs energy of formation of Ca₃Ti₈Al₁₂O₃₇ and phase relations and chemical potentials in the system Al₂O₃-TiO₂-CaO

Abstract

The quaternary oxide in the system Al₂O₃-CaO-TiO₂ is found to have the composition Ca₃Ti₈Al₁₂O₃₇ rather than CaTi₃Al₈O₁₉ as reported in the literature. The standard Gibbs energy of formation of Ca₃Ti₈Al₁₂O₃₇ from component binary oxides is measured in the temperature range from 900 to 1250 K using a solid-state electrochemical cell incorporating single crystal CaF2 as the solid electrolyte. The results can be represented by the equation: ΔG°_f(ox)(±70)/Jmol⁻¹=−248474−15.706(T/K). Combining this information with thermodynamic data on calcium aluminates and titanates available in the literature, subsolidus phase relations in the pseudo-ternary system Al₂O₃-CaO-TiO₂ are computed and presented as isothermal sections. The evolution of phase relations with temperature is highlighted. Chemical potential diagrams are computed at 1200 K, showing the stability domains of the various phases in the chemical potential-composition space. In each chemical potential diagram, chemical potential of one component is plotted against the cationic fraction of the other two components. The diagrams are valid at relatively high oxygen potentials where Ti is present in its four-valent state in all the oxide phases.