Electrochemical Deoxidation of Solid Zirconium Dioxide in Molten Calcium Chloride

The reduction of zirconium dioxide pellets by electro-deoxidation in molten calcium chloride-calcium oxide (900 °C) has been studied. In this technique, the solid oxide is cathodically polarized against a graphite counter electrode under a constant applied potential. Unlike other metal oxides that have been reduced by this technique, only a small area around the cathodic current-collector wire was reduced to zirconium metal with zirconia pellets sintered at ~1100 °C; the rest of the sample was largely calcium zirconate. Pellets sintered above 1200 °C showed better reduction near the cathode wire and the reduction extended to the entire surface of the pellet with the passage of time. However, reduction of the inner core was found to be increasingly difficult, because the surface metal layer thickened on continuous electro-deoxidation. An analysis of the experimental results showed that the poor electrical conductivity of the intermediate compound, CaZrO₃ and its blocky morphology inhibited the electro-deoxidation process. The increase in the sintering temperature of the pellet made it better conducting. However, the pores formed in the thick zirconium metal layer in such samples were too small for an ideal contact between the inner core and the molten electrolyte and hence the reduction of the inner core remained incomplete. Within the scope of this study, it is concluded that preforms with good grain growth and porosity are necessary for the electro-deoxidation of solid zirconium oxide.