Stability of ZrO ₂ Phases in Ultrafine ZrO ₂ ‐Al ₂ O ₃ Mixtures

Abstract

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH 4 OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat‐treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO 2 in these ultrafine particles was studied by X‐ray diffractometry. Growth of ZrO 2 crystallites at elevated temperatures was strongly inhibited by Al 2 O 3 derived from aluminum hydroxide. The monoclinic‐to‐tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al 2 O 3 , and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO 2 is lower than that of the monoclinic form. With increasing A1 2 O 3 content, however, the transformation temperature gradually increased, although the growth of ZrO 2 particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO 2 . The reverse tetragonal‐to‐monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO 2 particles was stabilized by the presence of water vapor, which probably decreases the surface energy.