Microstructural Transformations in Alumina Gels
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MICROSTRUCTURAL TRANSFORMATIONS IN ALUMINA GELS
F. W. DYNYS, M. LJUNGBERG AND J. Case Western Reserve University,
W. HALLORAN Cleveland, Ohio 44106
ABSTRACT Microstructural development in alumina gels is dominated by a series of phase transformations between the hydrous oxides to transition alumina phases, between transition phases, and transformation to alpha-alumina. These microstructural transformations are illustrated in boehmite AIOO1 and bayerite AIO0)3 gels. INTRODUCTION The gel process naturally producesan exceedingly fine and uniform microstructure in the xerogel precursor of crystalline ceramics. The fine size of the microstructural features promise exceptional reactivity and sinterability. However the course of evolution from the desiccated xerogel to the final dense ceramic is not necessarily a uniform process of densification and grain growth. Rather there are important examples in which the evolution is punctuated by one or more phase transformations which can profoundly alter the microstructure. Indeed, microstructure development can be dominated by the processes of phase transformations. This situation occurs whenever there is a microstructural change caused by dehydroxylation of a hydrous oxide xerogel, or when the oxide xerogel is an amorphous or metastable crystalline phase. Many xerogels are in amorphous or metastable forms [1], and so these transformations are of general importance. In this paper we will discuss the case of alumina gels, in which the transformation behavior is especially rich. We will consider several phase transformations, in psuedoboehmite and bayerite
derived gels,
with emphasis on microstructural alterations.
Experimental Proceedures Psuedoboehmite gels (AlOOH) were prepared by hydrolysis of aluminum sec-butoxide using proceedures similar to those of Yoldas [2]. Hydrolysis was accomplished by mixing a 0.35 molar solution of the alkoxide in secbutanol with a 16.5 molar solution of water in sec-butanol, such that the molar ratio of water to alkoxide was 100. Nitric acid, present at 10-2 molar in the water solution, served to peptize the psuedoboehmite to form a clear sol. The psuedoboehmite sol was placed in a drier at 85'C. where it formed a translucent gel. Upon drying, the gel cracked into pieces about a centimeter in size. The X-ray diffraction pattern was consistent with poorly crystalline psuedoboehmite. Some gels were aged at room temperature to crystallize bayerite [3]. After hydrolysis the sol was allowed to age at room temperature for sixhours, during which a gel formed which occupied 2/3 of the original sol volume. Excess supernate was decanted and replaced with water coating 10-5 molar nitric acid. This proceedure was repeated after 24 hours. After six days the batch was a thixotropic opaque white gel. This gel was dried at 850C to produce 5 mm size fragments. X-ray diffraction showed well-crystallized Al(OH) 3 as bayerite. Boehmite and the trihydrate polymorphs nostradite and gibbsite were not detected.
Mat. Rems.Soc. Symp. Proc. Vol.
32 (1984) Published
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