Nanocrysrtalline Powders in the A12O3-ZrO2-CeO2-Y2O3 system for high-performance ceramics
X-ray diffraction, BET measurements, scanning electron and optical microscopy were used to analyze the properties variation of hydrothermal nanocrystalline powder with a composition 90 wt% Al<inf>2</inf>O<inf>3</inf>-10 wt%ZrO<inf>2</inf> (CeO<inf>2</inf>, Y<inf>2</inf>O<inf>3</inf>) after thermal treatment in the temperature range 400–1300 °C. The composition of the ZrO<inf>2</inf>-based solid solution was (mol %) 90ZrO<inf>2</inf>-8CeO<inf>2</inf>-2Y<inf>2</inf>O<inf>3</inf>. It was found that α-Al<inf>2</inf>O<inf>3</inf> is formed at 850 °C. Phase transformation F-ZrO<inf>2</inf>⇆T-ZrO<inf>2</inf> occurred at 1000 °C. Produced powder was sintered at 1300 °C. The present investigation demonstrates that heat treatment of composite nanocrystalline powders gives rise to complex processes. Understanding of such processes makes it possible to optimize conditions for fabrication of advanced ceramics with tailored properties.
Effect of heat treatment on the properties of nanocrystalline 80 wt % Al2O3-20 wt % ZrO2〈CeO2, Y2O3〉 powder
We have studied the evolution of nanocrystalline 80 wt % Al2O3-20 wt % ZrO2〈CeO2, Y2O3〉 powder prepared through hydroxide coprecipitation followed by hydrothermal decomposition of the hydroxides and firing at temperatures from 400 to 1300°C. α-Al2O3 has been shown to form at 850°C. The metastable phase F-ZrO2 persists up to this temperature. The variation in the morphology of the powder is topologically continuous. The processes induced by heat treatment of the nanocrystalline powder are interpreted in terms of the evolution of an open system.
Effect of stress conditions on collapse deformation behavior of a rockfill material
A set of saturated and dry-saturated large-scale triaxial tests were conducted in order to explore the effects of stress conditions on the saturation-induced collapse deformation behavior of a rockfill material. Specimens were tested in different confining pressures and for dry-saturated tests the specimens were submerged in various shear stress levels. The results indicate that in all of the dry-saturated tests a sudden reduction in the volume of the specimens was observed during the submerging process. The ratio of the minimum axial strength of a submerged specimen (at the end of the saturation process) to the shear strength of the specimen before saturation is defined as the coefficient of stress recovery, Csr. Results of the dry-saturated tests showed that this ratio increases as the confining pressure increases and decreases by increasing the shear stress level. According to the results of dry-saturated tests the values of internal friction angle reduction caused by saturation (Δϕc), the ratio of the elasticity modulus of the material after saturation to its elasticity modulus in dry condition, i.e., Ewet/Edry, and also the saturation induced sudden volumetric strain (εvc) decrease as the confining pressure increases. However the shear stress level does not have any meaningful effect on the variation of Dfc, Ewet/Edry and (εvc).