PREPARATION OF TETRAGONAL ZIRCONIA POWDERS BY A SOLID-STATE REACTION - KINETICS, PHASES AND MORPHOLOGY

Powders of tetragonal (t)ZrO2 have been prepared by a solid state reac tion between sodium metazirconate and sodium metaphosphate, The reacti on temperatures and times have been varied between 450 and 550 degrees C and 5 and 75 h, respectively, Zirconia powder, mostly in the t and t' phases, is obtained, The yield of ZrO2 powder increases monotonical ly with time at all reaction temperatures according to a phase boundar y controlled kinetics, The fraction of t phase also increases with tim e at 450 degrees C and 500 degrees C but goes through a maximum at 550 degrees 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 degrees C/75 h, The ZrO2 powder consist s of crystallites of size 9-25 nm agglomerated into particles having a verage size between 2 and 4 mu m. The agglomerates have a breaking str ength of 100 MPa, A hydrothermal treatment is found to break the agglo merates into smaller sizes. Grinding the powder in a mortar and pestle converts only 12% of the t phase into monoclinic, indicating that sub stantial fraction of the tetragonal phase is the non transformable var iety t'. Heating experiments also confirm this.