Preparation of tetragonal zirconia powde

Powders of tetragonal (Zr)ZrO₂ have been prepared by a solid state reaction between sodium metazirconate and sodium metaphosphate. The reaction temperatures and times have been varied between 450 and 550°C and 5 and 75 h, respectively. Zirconia powder, mostly in the t and f phases, is obtained. The yield of ZrO₂ powder increases monotonically with time at all reaction temperatures according to a phase boundary controlled kinetics. The fraction of t phase also increases with time at 450°C and 500°C but goes through a maximum at 550°C, the highest temperature employed. A maximum of 55% of the precursor monoclinic zirconia (used to prepare sodium meta zirconate) is converted to t phase at 500°C/75 h. The ZrO₂ powder consists of crystallites of size 9–25 nm agglomerated into particles having average size between 2 and 4 μm. The agglomerates have a breaking strength of 100 MPa. A hydrothermal treatment is found to break the agglomerates into smaller sizes. Grinding the powder in a mortar and pestle converts only 12% of the t phase into monoclinic, indicating that substantial fraction of the tetragonal phase is the nontransformable variety t'. Heating experiments also confirm this.