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A Colloidal Method to Ensure Phase Homogeneity in β‐Al 2 O 3 /ZrO 2 Composite Systems
Author(s) -
LANGE F. F.,
MILLER K. T.
Publication year - 1987
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.1151-2916.1987.tb04913.x
Subject(s) - slurry , thixotropy , materials science , homogeneity (statistics) , colloid , mineralogy , phase (matter) , composite number , analytical chemistry (journal) , chemical engineering , composite material , chemistry , chromatography , statistics , mathematics , engineering , organic chemistry
A colloidal processing method that ensures phase homogeneity has been developed for β″‐Al 2 O 3 /ZrO 2 composites. High volume fractions (<55 vol%) of β″‐AI 2 O 3 and limited volume fractions (5 vol%) of ZrO 2 powders were dispersed in 2‐propanoI. It is suspected that ions associated with the powders (e.g., Na + ) and, possibly, trace amounts of water produce the coulombic repulsive forces needed for dispersion in the alcohol. All particles larger than a predetermined size were eliminated by sedimentation. To concentrate the fine solids of the slurry and to prevent mass segregation during storage, the ZrO 2 powder was flocced with small additions of nitric acid. The water in nitric acid, however, leaches Na + from the β″‐Al 2 O 3 , creating large NaNO 3 crystals in the slurry that volatilize upon firing, leaving voids in the composite material. The β″‐AI 2 O 3 powder, therefore, was sedimented from a concentrated (35 vol%) dispersed 2‐propanol slurry and was not flocced. The two‐phase slurry, which exhibited the thixotropic rheology indicative of a flocced system, was mixed by passing through an ultrasonic mixing chamber. The viscosity and mixing uniformity, quantified by high spatial resolution phase analysis of sintered materials, increased with utrasonification time. These observations suggest that when the flocced ZrO 2 globules were sheared apart, they recombined with the dispersed β″‐Al 2 O 3 phase, causing two‐phase heteroflocculation. Viscosity measurements can therefore be used as a measure of phase homogeneity. The two‐phase slurries could be consolidated by slip casting or pressure filtration.