PHASE-RELATIONS IN THE SYSTEM CAO-SIO2-ZRO2 AT 1573-K

Zirconia-based solid electrolytes with zircon (ZrSiO4) as the auxiliar y electrode have been suggested of sensing silicon concentrations in i ron and steel melts. A knowledge of phase relations in the ternary sys tem MO-SiO2-ZrO2 (M = Ca, Mg) is useful for selecting an appropriate a uxiliary electrode. In this investigation, an isothermal section for t he phase diagram of the system CaO-SiO2ZrO2 at 1573 K has been establi shed by equilibrating mixtures of component oxides in air, followed by quenching and phase identification by optical miroscopy, energy dispe rse analysis of X-rays (EDAX) and X-ray diffraction analysis (XRD). Th e equilibrium phase relations have also been confirmed by computation using the available thermodynamic data on condensed phases in the syst em. The results indicate that zircon is not in thermodynamic equilibri um with calcia-stabilized zirconia or calcium zirconate. The silica co ntaining phase in equilibrium with stabilized zirconia is Ca3ZrSi2O9. Calcium zirconate can coexist with Ca3ZrSi2O9 and Ca2SiO4.