ROLE OF OXOANIONS IN THE STABILIZATION OF TETRAGONAL ZIRCONIA

Zirconia, pure and modified by sulfate or molybdate was prepared in a molten nitrate medium. Sintering, crystallization and the tetragonal-m onoclinic (T-M) phase transition were studied by using differential th ermal analysis (DTA), X-ray diffraction (XRD) and surface area measure ments. Doping of the reaction mixture with oxoanions such as pyrosulfa te and heptamolybdate changes the morphology of samples, increases the ir specific surface and stabilizes the T variety of zirconia. Oxoanion s also improve the stability to calcination in air. It was suggested o n the base of thermal analysis data, that oxoanions are included in th e amorphous phase which is spread around the crystallites of zirconia. This amorphous phase induces stabilization of the T form of ZrO2 and causes the specimens to have a high surface area, This stabilizing eff ect persists up to the temperature of crystallization of the amorphous phase (610 and 800-degrees-C for sulfate- and molybdate-doped systems , respectively). The last process is accompanied by an exothermic peak in the DTA curve and subsequent sublimation of the corresponding oxid e (SO3, MoO3).