effect of heat treatment in the temperature

Abstract

The properties of nanocrystalline powders property of compositions (mol.%) 97 ZrO₂–Y₂O₃, 95 ZrO₂–3 Y₂O₃–2 CeO₂, 92.5 ZrO₂–2.5 Y₂O₃–5 CeO₂, 90 ZrO₂–2 Y₂O₃–8 CeO₂, and 88 ZrO₂–12 CeO₂ were studied. The powders were produced by hydrothermal synthesis in an alkaline environment from a coprecipitated mixture of hydroxides with a residual moisture content of 15–20%. The powder properties were determined by X-ray diffraction (XRD), electron microscopy, BET, and petrography. Metastable F-ZrO₂ was found to form in the powders after hydrothermal synthesis. According to XRD, the F-ZrO₂→T-ZrO₂ phase transformation began at 700 °С and finished at 850–1000 °С. The crystal optical characteristics of the powders indicate that the F-ZrO₂→T-ZrO₂ phase transformation started at 400 °C. The variations in F-ZrO₂ and T-ZrO₂ unit cell volumes are associated with lattice distortions under the action of different mechanisms in the costabilization of the ZrO₂-based solid solution by Y₂O₃ and CeO₂ and with the ratio of Y₂O₃ and CeO₂ in the solid solution. The tetragonality of the powders increases in the ZrO₂ costabilization. The effectiveness of the transformation hardening mechanism for ceramic materials based on ZrO₂ (Y₂O₃, CeO₂) solid solutions increases with the formation of T-ZrO₂, whose capability to the T-ZrO₂→M-ZrO₂ phase transformation increases. The morphology of the powders varies topologically continuously, and the sizes of their primary particles hardly increase up to 1150 °C. The variation in the  specific surface area (from 153 m²/g to 2 m²/g) of the powders is determined by the F-ZrO₂→T-ZrO₂ phase transformation and their activity in sintering above 1000 °C.