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Stability of ZrO 2 Phases in Ultrafine ZrO 2 ‐Al 2 O 3 Mixtures
Author(s) -
MURASE YOSHIO,
KATO ETSURO,
DAIMON KEIJI
Publication year - 1986
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1986.tb04706.x
Subject(s) - monoclinic crystal system , tetragonal crystal system , cubic zirconia , hydroxide , materials science , crystallite , phase (matter) , mineralogy , surface energy , analytical chemistry (journal) , crystallography , chemical engineering , chemistry , inorganic chemistry , crystal structure , metallurgy , organic chemistry , composite material , ceramic , engineering
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.