Phase relations in the system CaO-SiO₂-ZrO₂ at 1573 K

Zirconia-based solid electrolytes with zircon (ZrSiO4) as the auxiliary electrode have been suggested of sensing silicon concentrations in iron and steel melts. A knowledge of phase relations in the ternary system MO-SiO2-ZrO2 (M = Ca, Mg) is useful for selecting an appropriate auxiliary electrode. In this investigation, an isothermal section for the phase diagram of the system CaO-SiO2ZrO2 at 1573 K has been established by equilibrating mixtures of component oxides in air, followed by quenching and phase identification by optical miroscopy, energy disperse analysis of X-rays (EDAX) and X-ray diffraction analysis (XRD). The equilibrium phase relations have also been confirmed by computation using the available thermodynamic data on condensed phases in the system. The results indicate that zircon is not in thermodynamic equilibrium with calcia-stabilized zirconia or calcium zirconate. The silica containing phase in equilibrium with stabilized zirconia is Ca3ZrSi2O9. Calcium zirconate can coexist with Ca3ZrSi2O9 and Ca2SiO4